Comp II / SCX Performance Gauges

How can I troubleshoot my 2-1/16" Comp II Fuel Level Gauge?

Every Comp II LED fuel level gauge is checked multiple times during assembly and again before it ships. Because of the multiple checks we perform problems with the gauge are very rare. Issues with the Comp II fuel level gauge are typically caused by:
Not having a good common ground for both gauge and sender, often caused by the fuel level sender not having a good ground.
Not having a good signal to the gauge.

Step 1: Check Your Wiring and Grounds
Make sure your sending unit has a good ground. Recheck your wiring for the fuel level gauge and make sure that the gauge and the sender have a common ground. If in doubt ground them at the same location.
Step 2: Verify the sender is working
Disconnect the fuel level signal wire and measure the ohms on the signal wire with an ohm meter (multi meter). If possible confirm the ohms signal near empty and near full.

If you do not get a good reading your sender may be bad and require replacing.

Step 3: Confirm the fuel level gauge is good
Option 1: Check by Grounding the Signal
Hook up power and ground to the gauge but do not connect anything to the signal connector (S). On a 0-90 ohm fuel level gauge with no signal and power on the pointer should go all the way to the left. Now add a ground connection to the S terminal (use a jumper wire between G and S terminals) while keeping the power (I) and ground (G) connections in place. With a grounded signal connection (0 ohms input) the pointer on a 0-90 ohm gauge should move up towards the E marker.

Option 2: Check with a Resister
Another way to verify the gauge is working correctly is to "fake" a signal by putting a resistor across the S (signal) and G (ground) terminals. A 47 ohm resistor is ideal. On a 0-90 ohm fuel level gauge a 47 ohm "signal" will move the pointer to approx 1/2. The pointer will move to a different position for other ranges of fuel level gauges (i.e. 73-10 ohm, 20-150 ohm).

If you have a resister of a different value it will move the pointer to the corresponding dial position for your specific fuel level gauge. For example, if you use a 100 ohm resister on a 0-90 ohm fuel level gauge the pointer will go to Full +.

Option 3: Check with Another Known Good Signal
If you dont have access to a resister you can use the signal from your oil pressure gauge to test the fuel level gauge. Connect the power, ground and signal from the oil pressure gauge and connect them to the fuel level gauge.

If you have a 0-90 ohm fuel level gauge the pointer should move to a similar position as the pointer did on the oil pressure gauge. If you have another range fuel level sender the pointer will move to a different location but should still move. If the fuel level gauge responds with the oil pressure connections you know the gauge is good.

How can I troubleshoot my 2-1/16" Comp II LED Gauge?

Each Comp II LED 2-1/16" gauge is checked multiple times during production and again before each gauge ships. Most problems are caused by improper installation. You can confirm your gauge is OK by performing the following quick checks with a standard 9V battery (make sure battery is fully charged).
Confirm Gauge Powers Up Correctly
Volts: Place the + battery terminal on the gauge "I" terminal and the - battery terminal on the gauge "G" terminal. The gauge will move to 8-9V (depending on your battery voltage.
Pressure & Regular Fuel Level Ranges (0-90, 0-30, and 73-10 Ohm): Place the + battery terminal on the gauge "I" terminal and the - battery terminal on the gauge "S" terminal. The gauge will move to left side of scale.
Temp & Inverse Fuel Level Ranges (240-33, 73-10 Ohm): Place the + battery terminal on the gauge "I" terminal and the - battery terminal on the gauge "S" terminal. The gauge will move to right side of scale.
Confirm Gauge With Simulated Signal
To verify operation of the gauge with a simulated signal (not necessary for VOLTS gauge) you will need a 9V battery, some 18-22 gauge wire and a 100 ohm resistor (other ohm value is OK also but gauge output shown below will vary). Connection power and signal as follows:
9V+ to gauge "I"
9V- to gauge "G"
One end of resistor connected to 9V-, the other end connected to gauge "S"

For a 100 ohm resister, the gauge should go to the following approximate positions:
Pressure: 80+ PSI
Temp: 180-200F
Fuel Level (0-90, 0-30 Ohm): Full+
Fuel Level (20-150 Ohm): 1/2 - 3/4 FULL
Fuel Level (73-10 Ohm): EMPTY
Fuel Level (240-33 Ohm): 1/2 FULL

How can I troubleshoot my electric oil pressure gauge?

Most issues are due to incorrect wiring or sender hookup. The most common wiring issue is not having a good common ground between the gauge and sender. A common indicator of not having good wiring on your sender is the pointer on the Comp II LED oil pressure gauge going all the way to the right (open circuit on signal wire).
Check the Gauge with 0 Ohm Signal (Ground)
Connect the signal from the gauge to the gauge ground wire. This puts a signal of 0 ohms to the gauge. A Comp II LED gauge with 9200 sender will go to 0 PSI. A current SCX gauge with 9205 sender will go to 100 PSI.

Confirm the Gauge is OK by Using a Known Good Signal
Use a 47 ohm or 100 ohm resistor to verify the gauge with a known good signal other than 0 ohm (ground):

Install one end of the resistor to the gauge signal input and the other end to the gauge ground.
With power to the gauge and a resistor across the signal input and ground you will see the pointer move to to the corresponding dial position.
For gauges with 9200 sender: 47 ohms is approx 40 PSI, 100 ohms is approx 80 PSI.
For gauges with 9205 sender: 47 ohms is approx 90 PSI, 100 ohms is approx 70 PSI.
If you do not have access to a resistor but do have another working fuel level or temp gauge you can use the power, ground and signal connections from the known good gauge on the oil pressure gauge to verify that the oil pressure gauge is OK. Note, the pointer will move according to the ohm output from the sender you are using to test the gauge.
Confirm the Type of Sending Unit You Have
There are three types of oil pressure sending units. Confirm which sender you have and make sure that it is putting out a good signal and that it has a common ground with the gauge. We recommend grounding the gauge and sender together to eliminate any potential issues with not having a good common ground.

Type 2 is the most common sender (Our part 9200). It has a yellow label stating "WK = Ground". This type of sender is supplied with Comp II gauges.
Type 3 (our part 9205) is silver in color and has a white label stating "240-33 OHM (0-100 PSI)". This sender is supplied with most SCX gauges shipped after Jan 1, 2015.
Type 1 has no label. This sender was supplied with most SCX gauges shipped before 2015. This sender does not have a ground terminal. The sender is grounded through the body of the sender.
Verify Wiring and Common Ground
Verify your wiring is good and that the gauge and sender both have a good common ground. We recommend grounding the gauge and sender together to eliminate any potential issues with not having a good common ground.

If you have a Type 1 sender (no label) make sure you have not connected anything to the WK terminal. Verify the sending unit has a good ground connection on the body of the sender. If in doubt, install a metal clamp around the sender body and run a wire from the clamp to a good ground location on the vehicle chassis.

Verify Ohm Signal from Sending Unit
Turn your vehicle on and let it idle. Disconnect the gauge signal wire from the sender and check the ohms output with a multi-meter as follows:

For 9200 sender, the G terminal is the signal and the WK terminal is the ground.
For 9205 sender, - terminal is the ground and S terminal is the signal.
For old Type 1 sender, the G terminal is the signal and the body of the sender is ground.
Check the SCX or Comp II pressure/ohm data (found here) and confirm that the sender ohm output is reading as expected.

Verify Ohm Signal at the Gauge
Reconnect the gauge signal wire to your sender, disconnect the signal wire from the gauge and check the ohms between the signal wire and gauge ground wire (signal wire needs to be disconnected from the gauge). The ohm reading should be the same as you measured at the sender. If it is not, then you will need to check your wiring (verify common ground etc.).

How can I troubleshoot my electric temperature gauge?

Verify Wiring and Common Ground
Verify your wiring is good and that the gauge and sender both have a good common ground (ground both to the same place on the chassis).
Verify Ohm Signal from Sending Unit
Turn your vehicle on and let it warm up. Disconnect the gauge signal wire from the sender and check the ohms output between the sender and common ground/chassis with a multimeter. Compare the ohms output with the expected ohm output at warm temperature. Temp/Ohm data for the SCX and Comp II temperature senders can be found on the respective product pages:

SCX Temp/Ohm Data
Comp II Temp/Ohm Data
If the sender is not reading as expected contact us for a replacement.

Verify the Gauge
Check the SCX or Comp II temp/ohm data on the respective pages above and confirm that the gauge is reading at the expected temperature for the ohms reading at the gauge. Example: at 200 ohms, the SCX temp gauge should read approx 180F. If the gauge is not reading as expected contact us to return your gauge for checking/replacement.

How can I troubleshoot my speedo hall effect sender?

You can check the GM/Mopar (Item 9220) or FORD (Item 9222) hall effect senders as follows:
hook up 12V power (red) and ground (black) to the sender
connect a volt meter* + connector to the signal output wire from the sender (white). connect the volt meter - connector to the sender ground (black).
turn the sender key (the square part that turns) slowly by hand. If the sender is good you will see the VOLTS go from 0V to 12V and back to 0V. The voltage value will cycle from 0V to 12V sixteen times for a full rotation of the sender key (16 pulses per revolution).
*If you dont have a volt meter you can use a 12V bulb to indicate the 0V and 12V outputs. Make sure you hook the bulb up correctly before testing. The bulb will light up when the voltage output from the sender is 12V and will be off when the output from the sender is 0V.

How can I troubleshoot my speedometer?

Problems with the speedometer are very rare. If you are having issues it is most likely that your signal or ground is bad.
Verify You Have A Good Common Ground
Make sure that your speedo sender and the speedometer have a good common ground. If in doubt, ground them together on a good ground location.

Verify You Have A Good Signal
You can verify your signal by monitoring the pulses that get counted during calibration:

Enter calibration mode
If you see pulses being counted on the ODO while not moving then you most likely have signal noise. This can be due to a bad sender or wiring.
Start driving a measured mile at a steady speed and have someone monitor the pulses that are being counted on the ODO display.
If your signal is good you will see pulses count incrementally (not erratic). The pulses will increment at a faster rate the faster you go, so if you drive a steady speed (i.e. 25mph) you should see the pulse count increment at a steady rate. If the pulses increment erratically then you do not have a good signal. Erratic pulse count while driving a steady speed during calibration indicates you most likely have a bad sending unit.

How do I calibrate my electric speedometer?

Speedo Calibration
With the ignition off, press and hold the CAL/TRIP button. Turn the ignition on, then release the CAL/TRIP button.
The odometer/trip display will indicate CAL to verify that calibration mode has been accessed. The pointer will move to 50% scale.
Drive the vehicle EXACTLY one (1) measured mile then stop.
Press the CAL/TRIP button again to complete the calibration.
If the number of pulses is between 4,000 to 200,000 the odometer/trip display will indicate the actual pulses counted by the speedometer for five (5) seconds. This indicates a successful calibration. The speedometer will return to normal operation automatically.
If the number of pulses is below 4,000 at the end of one mile, the odometer/trip display will show zeros for five (5) seconds after the button is pressed. The calibration will not be updated, and the original calibration will be maintained. If this occurs you most likely have a problem with your sender and/or wiring. Correct the problem and recalibrate the speedometer.

If the number of pulses is above 200,000 at the end of one mile, the odometer/trip display will show zeros for five (5) seconds after the button is pressed. The calibration will not be updated, and the original calibration will be maintained. Correct the problem and recalibrate the speedometer.

What senders are included with the Comp II LED Six Gauge Set?

2-1/16" Gauges
The oil pressure and water temp sending units are included. They have 1/8" NPT male connections. The volts gauge does not require a sender, you get the signal from the 12V ignition circuit.
A fuel level sender is NOT included. The standard fuel level gauge is 0-90 ohm input (GM OEM sender output). We can also supply this set with a 240-33 ohm, 73-10 ohm (Most Ford and Chrysler vehicles), 20-150 ohm (most Ford vehicles after 1986) or 0-30 input range (Most Ford vehicles before 1965). Check your current fuel level sender output to verify the ohm output (Empty - Full) and let us know before ordering so we can confirm it is a range we can supply.

Tachometer
If you have a standard ignition coil, ignition box with tachometer signal output (i.e. MSD 6AL) or ECU 12V tachometer signal you do not need anything additional to hook up your tachometer. See the How do I hook up my tachometer? FAQ for more info.

Speedo
If you have a hall effect sender, GPS sender or magnetic speed sensor (i.e. vehicle speed sensor) already available the speedometer should work without any additional sending units. See the What will I need to make the electric speedometer work? FAQ.

What will I need to make the electric speedometer work?

f you have a hall effect sender, GPS sender or magnetic speed sensor (i.e. vehicle speed sensor) already available the speedometer should work without any additional sending units.
Any speed sender or electronic module that meets the following conditions can be used with our speedo:

Pulse range generated proportional to the vehicle speed
Signal with minimum 3V peak to peak signal - typical for 3-wire hall effect and 2 wire vehicle speed sensor outputs
4000-200,000 pulses per mile (16,000 is typical).

If you currently have a mechanical speedometer (cable driven) you will need a hall effect sender. The hall effect sender connects where the cable currently comes off. The signal from the hall effect sender provides the signal for the electric speedo. We sell hall effect senders for both GM/Mopar and FORD applications.
If none of the above options will work for your application the best option is to purchase a GPS based speedo sender like the one from Intellitronix. Google "Intellitronix-S9020-GPS-Speedometer-Sender" to find places to purchase.

Late Model Vehicles with PCM/Computer

This speedometer provides an output (OUT) terminal that can be used with late model vehicle installations using the existing Vehicle Speed Sensor (VSS). In these installations, the VSS signal wire should be connected to the SIG terminal of the speedometer only. The OUT terminal should then be connected to where the VSS output was originally wired. This will provide a buffered VSS signal to the PCM/computer in the vehicle.

Why won't the lighting work on my 3-3/8" speedo and tachometer?

The Comp II LED and SCX 3-3/8" speedo and tach feature integrated LED lighting. For the backlight to turn on you must also have 12V power to the gauges turned on (ACC).
When checking the lighting make sure the following are connected:

12V ACC
Ground
12V to light input

How Can I Reduce the Responsiveness of My SCX Fuel Level Gauge?

The SCX performance gauges are very responsive which can lead to a lot of pointer fluctuation on your fuel level gauge if your fuel tank has a lot of sloshing.
You can dampen the pointer response by adding a capacitor between your signal input and ground wires as shown in the diagram below.
It is important you use the correct capacitance (F) and Voltage rating. Suggested Farad and Voltage ratings for various signal input ranges:

For 73-10 ohm, 0-90 ohm, 20-150 ohm and 0-30 ohm ranges a 1F/2.7V capacitor gives good dampening results (Digikey Part # 493-3291-ND).
For 240-33 ohm range a 1.5F capacitor with a 5V rating gives good dampening results (Digikey Part # 589-1007-ND).

For increased dampening use a capacitor with higher Farad (F) value.

How do I hook up my tachometer?

Standard Ignition Coil
If you have a standard ignition coil you can use the negative terminal of the coil for the signal source. Use the SIG 2 (In-Dash Tach) or green wire (Pedestal Mount Tach) input (see "Signal Hookup" in the instructions). If you have a high output capacitive energy discharge type coil it is recommended that you install a tachometer filter to prevent damage to the tachometer.
Ignition Box
If you have an ignition box (i.e. MSD 6AL) you can use the "Tach Output" from the ignition box. Use the SIG 1 (In-Dash Tach) or purple wire (Pedestal Mount Tach) input (see "Signal Hookup" in the instructions).

12V ECU Tachometer Signal
If you have a modern ignition system and your ECU (computer) has a dedicated 12V tachometer signal output you can use this to drive the tachometer. Use the SIG 1 (In-Dash Tach) or purple wire (Pedestal Mount Tach) input (see "Signal Hookup" in the instructions).

5V ECU Tachometer Signal
Many ECU outputs are low voltage (4-5V) which will not drive the tachometer without modification. If you have a low voltage (4-5V) tachometer signal you can install a 680 ohm "pull up" resister from the 12V gauge power wire (ACC) to the signal wire to increase the signal strength. Connect one end of the resister to the 12V ACC wire and the other end of the resister to the signal input wire. A 680 ohm resister will work for signals in the 4V range. A higher value resister (i.e. 1000 ohm) can be used if your signal has a higher strength (i.e. 5-6V). NOTE: Make sure your tachometer has a common ground with the signal source (ECU).
HEI
If you have an HEI distributor with a "Tach Output" you can use the "Tach Output" from the distributor. Due to the high voltage of the signal from the HEI you should use a tachometer filter to prevent damage to the tachometer.

Coil Pack or Individual Coils (Distributorless Ignitions)
If you have a modern vehicle with "coil pack" or "individual ignition coils" distributorless igntion you will need a tach driver like the MSD 8913 to drive the tachometer. Connect the signal from the tach driver to SIG 1 (In-Dash Tach) or purple wire (Pedestal Mount Tach) input.

Troubleshooting

If your tachometer has no signal or an erratic signal (pointer jumping all over) you first need to verify that you have correct power, ground and signal connections.

Verify the gauge has 12V power. The gauge should perform a self calibration check when turned on (pointer moves to full scale, then back to zero).
Verify you have a good common ground. A bad ground connection is often the cause of many gauge issues. Make sure the gauge ground connection is connected directly to a good ground on the vehicle chassis. We recommend using the same location as the battery negative terminal ground.
Verify you are using a proper signal source. The most common locations for a tach signal are the negative terminal of the ignition coil or tachometer output terminal (HEI, Ignition Control Boxes or ECU).
No Pointer Movement
Verify 12V ACC, signal and ground connections (see above).
Verify you have a good signal that is at least 8V peak to peak in amplitude (requires an oscilloscope). Some ECU signals are not strong enough to drive the tachometer.
Try switching the signal input. Marshall tachometers come with 2 signal input options.
Erratic Pointer Movement
Verify correct signal and ground connections (see above).
Verify your signal wire is not routed close to any high voltage sources (i.e. spark plug wires, distributor). The signal wire can pick up electrical interference (signal noise) if routed close to a high voltage source.
Try switching the signal input. Marshall tachometers come with 2 signal input options, each input utilizes a different signal filter.
Install a tachometer signal filter.
Add a 10K Ohm resistor inline with the signal. Adding a resistor will increase the level of signal filtering. Try this on both signal inputs. Use a higher ohm resistor for more filtering, a lower value ohm resistor for less filtering. /ul>

How do I stop my MINI gauge lighting from blinking?

Problem: SCX stepper motor gauge backlight blinks periodically.
Cause: MINIs computer sends diagnostic pulses through its lighting circuit. The diagnostic pulses interrupt the 12V supply to the gauge LED light circuit causing the gauges to blink ON/OFF periodically.

Fix #1: Install a 12V relay between the light circuit and the gauge lighting wire. This will isolate the diagnostic pulses and provide a steady uninterupted 12V power to the gauge lighting circuit.

Fix #2: Purchase an inline dimmer from Steve at Custom Mini Shop. The inline dimmer module eliminates the blinking issue and makes lighting hookup a breeze and also provides dimming function to the gauges from the OEM dimmer.

My Comp II LED Gauge Light Doesn't Work, What's Wrong?

Each Comp II (C2) LED bulb and socket is checked for correct operation before it is packaged so the problem is most likely incorrect wiring, loose bulb, or the bulb is installed backwards.
Wiring
Make sure the white wire lead is connected to a 12VDC+ supply (verify voltage with a multimeter). The black wire lead should be connected to Ground.

Verify Bulb Is Tight
Remove the LED bulb from the socket and push the brass bulb clips closer together. Reinstall the bulb.

Verify Bulb Polarity
The Comp II LED bulbs are polarity sensitive. If the bulb is installed backwards it will not work. If your bulb does not light, remove the bulb, rotate 180 degrees and reinstall the bulb.

My mechanical boost gauge buzzes, how can I fix it?

his is a very common problem with mechanical boost gauges.
Causes
At certain pressures the fast moving air is at the resonance frequency of the gauge which causes the gauge to buzz. The location of the PVC supply line is also a major cause (see below).

Solution
All that is typically needed to eliminate the noise is a restrictor to dampen the air movement in the hose to the gauge.

Our mechanical boost gauge includes a restrictor on the tee that helps prevent this. Other companies call it an "anti-buzz" or "no-buzz" fitting.

Please see other suggested solutions below:

put a wad of cotton from a q-tip or cotton ball where the hose goes into the gauge
install a small fuel filter (i.e. for a lawn mower) inline
Some additional install tips:

use the included tee fitting with restrictor and/or additional vacuum restrictors in the tubing to the gauge
make sure your tubing is fastened at various points (so it is not loose)
it is better to locate the PVC tubing pressure source further from the turbo (i.e. in the manifold just before the engine).
Google boost gauge buzzing for more information on this subject.

My speedometer pointer is erratic, how can I fix it?

The most common causes of erratic pointer movement are:
Bad ground. Make sure the ground connection on the gauge is connected to the same spot as the car battery ground to ensure you have a good ground. Do the same for the sensor (the gauge and the sensor need to have a good common ground).
Externail noise. Noise introduced from routing the signal wire close to a high voltage electric source (i.e. spark plug wire, ignition). Make sure your signal wire isnt picking up any noise.
Bad or incorrectly installed sender. Ensure your sender is operating correctly and it producing a good signal.
If you are still seeing some noise after checking the above items, try adding a 0.05uF (micro Farad) or 0.1uF capacitor across the input to ground. If signal noise is the issue, this will help filter out any signal noise and smooth pointer response.

What are the dimensions of the gauges?

To verify if the gauges will fit your existing dash panel check the hole sizes and spacing against the gauge case od and bezel OD:
The 5 inch in-dash gauge case OD is approx 4.57" (4-9/16", 116mm) and the bezel OD is approx 5.0" (125mm). You want your hole size to be around 4.75" but anything between 4.6" and 4.9" is OK. Case depth is approx 1.75" not including the uclamp studs.

The 2-1/16" gauges have a case OD of approx 2-1/16" (52mm). The bezel OD is 2.25" (57mm). Case depth is approx 1.25" not including the uclamp studs. The uclamp studs are 0.75" long.

The 3-3/8" gauges have a case OD of approx 3-3/8" (86mm). The bezel OD is approx 3.75" (95mm). Case depth is approx 1.75" not including the uclamp studs. The uclamp studs are 1" long.

If you do not have an existing panel with these size holes you may need to modify your existing panel or purchase an aftermarket dash panel for your vehicle. We recommend checking with Classic Dash to see if they make a dash panel for your application. To find other manufacturers of after market dash panels do a Google search for "dash panel" plus the make and model of your vehicle, i.e. "dash panel for 1979 Camaro".

What is the BATT (Always On) Connection For?

The BATT (Always On) connection is required for the SCX based stepper motor gauge pointer to return to 0 when the ignition is turned off.
If you do not connect the BATT gauge connection to the battery + (always on) connect it to your 12V ignition source so the 12V BATT connection has full 12V power when the ignition is ON.

Without 12V power to the BATT connection the stepper motor gauge pointer will stay at its last reading (i.e. 30 PSI, 200F, 800 RPM etc) when the ignition is turned off. This does not affect the gauge (other than the pointer not returning to 0). When the ignition is turned back on, the gauge will perform its self calibration cycle and operate normally.

What is the warranty on the gauges?

If the gauge was purchased within the previous 12 months from an authorized distributor you are covered under warranty. Contact us with your item and proof of purchase for an RMA number and return instructions before returning your item. Sending units and accessories are not covered under warranty.
Return Address
Marshall Instruments
RMA: supplied RMA number
2930 East La Cresta Ave
Anaheim, CA 92806

Return Instructions
Return only the gauge. Keep any accessories that originally came with the item (u-clamp, light, sending unit etc).
Make sure you include your full contact information inside the package (name, email, phone and address) so we can contact you with questions.
Make sure you ship via a trackable method (USPS First Class is inexpensive and offers tracking). We are not responsible for lost items.
Return shipping charges may be applicable (see fees below).
Fees
Items less than 1 year old with proof of purchase:
Return shipping to a USA address via standard USPS service: No charge
Shipping to a non USA address or oversize package: Charge based on actual return shipping cost
Items over 1 year old or no proof of purchase:
Comp II LED 2-1/16" Gauges: $25.00ea
SCX 2-1/16" Gauges: $40.00ea
3-3/8" Speedometers and Tachometers: $50.00ea
5" Speedometers and Tachometers: $60.00ea
Prices above include standard first class USPS return shipping to a US address. An additional fee applies for oversized items, expedited shipping or shipping outside the US.

How can I dim my SCX gauges?

The SCX gauges feature integrated LED lighting and will not dim as desired when connected to a standard OEM dash dimmer.
To dim your SCX gauges we recommend installing one of our compact LED dimmer modules.

Alternate Option 1: Dimming Gauges With A Resistor
You can also dim the gauges by installing a resistor inline with the 12V ACC supply to each SCX gauge (not inline with the light connection). Installing the resistor lowers the voltage supply to the gauge which reduces the brightness of the lighting. A 0.5W 100 ohm resistor for each gauge works well however you can vary the resistance to your desired brightness.

If you dim the lighting using a resistor:

Make sure the 12V+ "Always On" connection to the SCX gauge is connected (see instructions). This connection must get a steady 12V supply.
Make sure you connect the resistor inline with the 12V ACC power to the gauge, NOT the lighting.
If your resistor value is too high, the gauge may not get enough power to operate correctly. If this happens, lower the ohm value of the resistor.
Alternate Option 2: Dimming Gauges With a Potentiometer
For adjustable levels of dimming, you can install a potentiometer to reduce the voltage supply to each gauge (which will dim the lighting). For 1-3 gauges we recommend a 5W 250 ohm potentiometer (Digikey Part CT3018-ND). For 4+ gauges we recommend a 5W 100 ohm potentiometer (Digikey Part 026T419S101A1A1-ND).
If you dim the lighting using a potentiometer:

Make sure the 12V+ "Always On" connection to the SCX gauge is connected (see instructions). This connection must get a steady 12V supply.
Make sure you connect the potentiometer output inline with the 12V ACC power to each gauges, NOT the lighting.
If your resistance value is too high, the gauge may not get enough power. If this happens, reduce the resistance.

How can I dim my Comp II LED gauges?

2-1/16" Gauges
The 2-1/16" Comp II LED gauges feature a single snap-in LED bulb. You can dim the lighting by reducing the voltage to the bulb (white wire).
A 1W 300ohm resistor installed inline on the white wire to each bulb gives a good level of dimming. Use a higher value resistor for dimmer light, a lower value resistor for brighter light.

3-3/8" Speedo and Tachometer
The 3-3/8" Comp II LED speedo and tachometer have integrated LED lighting. To dim the lighting on these gauges you need to reduce the voltage to the ACC connection (NOT the light wire). A 1W 100 ohm resistor installed inline on the ACC (12VDC) connection to the gauge gives a good level of dimming. Use a higher value resistor for dimmer light, a lower value resistor for brighter light.

Important notes when dimming the 3-3/8" Comp II LED gauges:

If your resistance value is too high, the gauge may not get enough power to operate. If this happens, reduce the resistor value.
When dimming the lighting with a resistor make sure the 12V+ "Always On" is connected to an unreduced voltage source, i.e. connect the "Always On" connection to either the 12V+ battery or to the main 12V ignition supply.
Can I use the Marshall dimmer switch to dim the Comp II LED gauges?
The dimmer switch is designed to work with the SCX gauge sets but can be adapted for use with the Comp II LED gauge sets. To use it with the Comp II gauges you will need to have the gauge backlight ON at all times (day and night) for the dimmer to work correctly on the 2-1/16" Comp II LED gauges. To use the dimmer switch with the Comp II LED gauge sets connect the output of the dimmer switch as follows:

2-1/16" Gauges: Connect output of dimmer to the positive light wire.
3-3/8" Gauges: Connect output of dimmer to the 12V ACC connection. Also, connect the white light wire to 12V ignition source so backlight is on when gauge is on. When dimming the 3-3/8" gauges make sure that the BATT+ connection has 12V+ connected. You can either use a direct connection to the battery 12V+ (always on) OR 12V+ power from the ignition circuit (only on when the vehicle is on).

How Does an Electric Gauge Work?

The Marshall Comp II short sweep electric gauges work via deflection of the pointer from a magnetic field that is directly proportional to the signal input being measured. The magnetic field is generated by a tightly wound coil. An increase or decrease in voltage through the coil creates a corresponding change in the magnetic field, which in turn moves the pointer.
Marshall SCX full sweep electric electric gauges utilize a patented high-end stepper motor movement to move the pointer to an exact position. The pointer movement is controller by a microprocessor that reads the input signal and moves the pointer accordingly.

How Does a Mechanical Gauge Work?

Mechanical gauges utilize an internal bourdon tube. One end of the bourdon tube is connected to a gear and shaft assembly that moves a pointer. When the pressure inside the bourdon tube increases, the bourdon tube uncoils slightly. The amount of uncoiling that occurs is proportional to the pressure inside the bourdon tube. As the tube uncoils, its motion activates the gear and shaft system that turns the pointer on the gauge. While all that you see when you look at the gauge is the pointer moving, you should understand that there is a small, bent tube (the bourdon tube) thats coiling and uncoiling with each change in the pressure inside that tube.

Mechanical pressure gauges are connected directly to the process fluid being measured (i.e. oil). As the process fluid pressure changes the pressure on the bourdon tube also changes which in turn moves the pointer on the gauge.

Mechanical temperature gauges also utilize a bourdon tube. They have a sealed capillary tube and bulb assembly that is filled with temperature sensitive liquid that produces a proportional vapor pressure on the bourdon tube. As the temperature changes, the pressure inside the bourdon tube changes, which in turn moves the pointer on the gauge.