Gauge Lights

[dreavis]

I have read a number of threads on issues with gauge/instrument lights, but thought I would post this because the circumstances might provide a clue to the problem. (1977 1800)

I was driving at night and noticed the gauge lights were dim, but they've always seemed dim- not surprising given the age of the car. I turned the dimmer switch to the left and the lights dimmed and then back to the right to see if they would get any brighter than before. Not sure if they were any brighter, but I did confirm that the dimmer was working. After playing with the dimmer I got about a mile down the road and the gauge lights suddenly went out. All the other lights appear to be working normally - except the interior courtesy light, but I think that's a fuse because everything else associated with that fuse is inoperable.

I'm confident fiddling with the dimmer had something to do with the lights going out, but I wonder if anyone can think of how this information might help me diagnose and fix the problem.

Thanks for the advice!

PS - Can you get fuses for these cars at any auto parts store?

[baltobernie]

Fuses are available at auto parts stores. I'm sure you've heard by now that it is a good idea to occasionally rotate all fuses in their respective holders. This keeps oxidation of the terminals at bay.

It's quite possible that your ancient dimmer switch has indeed failed. You are correct in your observation regarding overall Spider instrument illumination. They are dim by today's standards. (I personally find them to be fine; but then .... I've got the dimmer on my new cars set to the lowest position ). The test for the dimmer would be to bypass it. You can do this by unscrewing the bezels on the dimmer and wiper delay controls and removing the section of "wood". That should give you enough room to jumper the dimmer switch, and replace (or bypass permanently).

[zachmac]

Another likelihood is that your trying to use the dimmer caused its "internal fuse" to blow. This us basically a very thin wire between the two halves of the dimmer switch internal wiring coils that is designed to melt if it sees to much current. Pull the switch from the dash and do a continuity check with a multimeter. if it has continuity but varies resistance then it is okay. If it shows open then the wire has popped. You can replace it if you open the switch up and solder a very tiny gauge wire back in; however, if you just jumper the two leads you'll basically be bypassing the dimmer with the dash lights set to their brightest (bot a bad thing).

[btoran]

bypass the dimmer to test it.

during the replacement of my dash (with a custom, home made one), I couldn't decide where I wanted the dimmer switch to go. ultimately, I decided having the ability to change gauge brightness from moderately dim to really dim just wasn't that necessary, so I left it out.

[dreavis]

I'll search the threads, but any guidance on bypassing the dimmer?

Thanks for the great advice!

[baltobernie]

There are only two wires on the dimmer. I think they are White and Yellow. Simply disconnect them from the dimmer and connect them to each other. You can temporarily "jumper" the wires before removing to test my diagnosis. A length of wire with alligator clips on each end would do nicely.

[TX82FIAT]

jump the dimmer switch, its pretty useless on these cars in my opinion. If you really want to get bright lights then convert the gauges to LEDs'. Did this a few years ago and I have been really happy with the bright instrument lights. The only dim light on my dash is the clock.

[dreavis]

I'm just getting around to fixing this and discovered that the dimmer has two connectors on the back. One has a red and white wire and the other has both a red and yellow and a black and yellow wire running into it.

I'm new to this so forgive me, but do I simply connect the ends of all three of these wires within one connector to bypass the dimmer?

Thanks!

[124JOE]

every other story about electrical problems have the problem of BAD GROUNDS so i would look there first

[dreavis]

Thanks for the reply - i'll look into that. But, if I am going to bypass the dimmer anyway, do I simply connect those three wires? I've been looking at the wiring diagram and can't figure out what else I would do.

Thanks again!

[dreavis]

Since I had everything apart, and I'm impatient, I went ahead and connected the three wires, reattached the battery and started the car. When I turned on the lights, voila!, the gauge lights are working as dimly as before the dimmer switch died..

If what I've done is incorrect, I'd appreciate it if someone would let me know.

Thanks!

[AriK]

All's good. I've done that minor surgery years ago and no troubles. I've got LED s in there now and a working rheostat would probably be useless anyway.

[76was124]

, I went ahead and connected the three wires,!

I have never seen the actual wiring diagram for the dimmer circuit, but in general it's a bit risky to connect three wires together from a device without knowing where they go.

In this case I am guessing the dimmer switch is acting as a rheostat vs. a potentiometer. A rheostat effectively is a two wire device that varies the resistance to vary the amount of current flow. A potentiometer is a 3 wire device that changes resistance between to cause a voltage change on the center tap ( wiper lead).

With a potentiometer one wire would be connected to +Voltage, one to Ground, and one is the variable voltage output with reference to Ground. With a potentiometer, you can see why connecting all three together would be a problem.

With a variant of Ohm's Law being I=V/R (Current =Voltage divided by Resistance), you can see changing the voltage or the resistance results in a change in Current. So in some cases either method ( Rheostat or Potentiomer) can be used a dimmer.

Why the third wire on a rheostat? It's probably one of the following:
1. It's really a potentiometer, with the wiper wired to one end of the other leads to create a rheostat see fig 1b at http://www.planetanalog.com/author.asp? ... e_number=1
2. The third wire (if it's on the same connection as one of the other wires) is probably feeding another circuit.
3. The dimmer is a potentiometer and the 3rd wire goes to ground, and may be an open circuit at the moment ( explaining why the lights don't work) which TEMPORARILY is preventing a bad situation when all three wires were connected.

[RRoller123]

On the 1980 wiring diagram (I will assume yours is the same, but this is always risky) there are indeed three wires at the back of the dimmer switch (potentiometer/switch, since the symbol indicates open until actuated).

"E" terminal gets: Yellow/Black goes to C33, and from there it goes to the fuse panel "8" or "H" for supply of power.

"A" terminal gets TWO White/Red wires, one goes to the "Optical Fiber Light 02120, and the other goes to "58D" on the back of the hazard switch.

According to this diagram, connecting all three together puts power to the optical fiber lights and panel lights any time that circuit "H" or "8" is powered. Unfortunately, the diagram I have doesn't show how "8" gets its power, but I suspect that it is activated switched by the Ignition switch?

[76was124]

Hi Pete,

Thanks for the wiring diagram... I don't have that one.

The dimmer looks like it is only a two terminal device. The third wire mentioned is as you said, on one of the same terminal as the other red/white wire ( vs. on a third terminal on the dimmer switch)...therefore it's really only jumpering a single circuit, no harm in this case.