OK so I installed some Autometer electronic gauges in my FD. These include a white illumination wire. Feeding it from 0 to +12 volts adjusts the gauge brightness from min to max. The FD illumination control however pulls the bulbs down to ground. In other words, one side of each bulb is at +12 volts, and the other side is varied by the illumination control from +12 to 0 volts, min to max. So the signal is the opposite of what the Autometer is expecting. I have this hooked up this way, and it works, but the levels are adjusted opposite to each other. Somewhere in the middle, you can adjust them to about the same (less than full brightness), but there is only that one level that you can select where they are both lit about the same.

So how can I invert the signal from the factory illumination control? Ideally it would be a simple transistor or IC to do the job and ideally would incorporate a potentiometer to adjust the middle of the range appropriately so zero to full brightness is roughly matched betwee the AM gauges and the OEM gauges. I tried searching the forum for answers but got none. Anybody know how to do this?

 

Me too, if you get this figured out holler...

 

Talked to my brother the Electrical Engineer and he gave me a simple cicuit which uses a P-Channel MOSFET power transistor to control the voltage to the AM gauges. The P-Channel is reverse polarity so it is ON when the gate signal is Off and vice versa. It does the job for $5 or less depending on how cheap you can find the part. See attached PDF for the circuit, part specification, and some construction notes.

Enjoy!

 

I just installed a pair of AM gauges and connected the gauge lights to one of the interior lights that dims with the panel lights. Works fine.

 

Out of curiousity which AM gauges are those? Do you get two wires for the lights? If so, then yeah, you can just hook them into any factory illumination wires and it should work.

If you have LED gauges like the Sport Comp-II, then there is an internal circuit that powers the LEDs and they only provide one extra wire (the WHITE one) which in combination with the ground wire (BLACK) is used to control the brightness. As described in my first post, you cannot correctly control this with the factory illumination wiring because of the reversed polarity. If you hook the white wire up to the factory illumination circuit, you will either get FULL brightness (non-adjustable) or reversed control (factory min = AM max and vice versa).

I am not familiar with the entire AM gauge line. So I don't know which models specifically need the circuit and which do not. If you have a white wire, you proabably need the circuit. If you have two wires that power the gauge lights directly (using incandescent bulbs), then you may not need the circuit.

 

While my original circuit worked, it didn't handle different electrical conditions very well. The transistor switches over a very small voltage range, so small fluctuations in system voltage would cause the required trimmer setting to vary. I updated the circuit by using a small variable voltage regulator as input to the transistor. I have been using the new circuit for several months and it works flawlessly now. Find updated circuit attached.

-- James