No gauge lights

[Skweegle89]

Dome light works. Gauges work. Choke light works, but no gauge lights. Fuses are all good. What do I need to be looking for?


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[mtnmankev]

Headlight switch ?

 

[yeol1]

How are the bulbs in the gauges. I know my 79 has 8-10 bulbs

 

[yeol1]

If the bulbs are good then check grounds and check the circuit board on the back of the dash

 

[chengny]

Can you provide some history?

Did they all go out at once?
Did you get the truck with the dash unlit?
Did the dash start off well lit and then just get darker and darker?

An easy place to check for voltage in that circuit is at the gray wire in the radio power harness plug. Look up under the dash. Most modern aftermarket head units don't use an external lighting feed so it will most likely be just hanging loose around the back of the radio.

If you get a variable voltage (0 - 9) there, it would indicate that your dimmer is supplying power to the dash lights and your bulbs are all burnt out.

 

[Skweegle89]

It didn't work when I got it, but that might explain it. The dipshit hacked the dash up for a new stereo, but it doesn't work right. It doesn't go off with the key. You have to pop the faceplate off the get it to go off.


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[Skweegle89]

Ok coming out if the dash, he has grey hooked to green with black stripe.

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[Skweegle89]

Found a smaller grey wire that's cut off and there is no power to it at all


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[Skweegle89]

Is this the same grey wire that's numbered #2 on the main plug that egos into the back of the cluster? Because there's no power to that eother


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[chengny]

Other than the 8 speaker wires, there should be another group of 3. They originally went into the same gray harness plug but that is long gone.

The original 3 wire group was:

Yellow - switched power (hot only with ignition switch in RUN)
Black - just a normal ground lead
Gray - radio backlighting (part of the instrument panel circuit and controlled by the H/L dimmer switch)

he has grey hooked to green with black stripe

WTF - on an aftermarket head unit, green/ black is the LR negative speaker wire!

Anyway, that gray wire needs to be cut and wrapped with tape. But before you wrap it, strip the insulation off and check it for voltage. It is supposed to vary smoothly from 0 - 9 VDC as you twist the dimmer on the H/L switch. But don't be surprised if it just jumps from 0 right up to 9 volts at a certain point.

This is normal if a large number of the bulbs in the dash illumination circuit are burnt out. If you have gauges, there was originally a total of 7 bulbs in that circuit:

5 lamps behind the instrument cluster
1 lamp in the HVAC control panel
1 for radio backlighting

If you are down to say 3 bulbs, the dimmer won't smoothly increase the voltage as designed. It will put out an all-or-nothing power supply.

Back to the gray wire:

If you have voltage at that wire and no dash backlighting - the bulbs are burnt out.

If there is no voltage, we will have to look at the dimmer switch or the interconnecting wiring.

 

[chengny]

Is this the same grey wire that's numbered #2 on the main plug that egos into the back of the cluster? Because there's no power to that eother

Yep same circuit.

 

[Skweegle89]

No power to it at all. If the switch was bad, would it still turn the dome light on and off when turned all the way to the left? It still controls the dome light and head lights, but does nothing to the dash lights. The bulb filaments still seem to be in tact, so I'm going to rule those out until I get power the the #2 spot in the plug.


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[chengny]

Let's do it this way.

Check for voltage at the fuse socket labeled INST LPS (top fuse, 2nd column from right). The line side should be the left clip - but check both sides.

If you have no voltage there, read this:


The reason the fuse socket is dead for the panel lighting is because there is no power coming back from the rheostat in the HL switch.

The flow goes like this:

The red 3.0 wire from the firewall fusible link is tied into the line side of the far left column (looking forward). The actual connection is in the lower half, there is a shunt about halfway up that supplies the sockets in the upper half. These sockets are for various options (clock, cargo light, etc) that require constant battery power.

Power comes out of the TL/CTSY fuse and up to the HL switch on the .8 orange lead. The voltage is reduced as needed by the rheostat, and is lead out of the HL switch at pin 44. It goes back to the fuse block (from the HL switch) on the 1.0 green lead.

That 1.0 green lead is tied into the fuse block at the hot side of the 5A Inst Pnl fuse. The load side of the fuse is connected to the panel illumination lighting via a .5 gray wire.

Probably the easiest place you can start ringing this out at the HL switch:

Power in to the rheostat on the .8 orange at pin 40 but no power out on the 1.0 dk green at pin 44? Bad rheostat.

Power at pin 44 but not at the line side of the instrument light fuse? The green 44 wire is broken/disco'd.

If there is no power into the rheostat on the orange wire, check the load side of the TL/CTSY fuse. Power there? Broken orange lead from the fuse block to the HL switch.

 

[chengny]

Here is the dwg that goes with the description of the circuit (above):

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[Skweegle89]

Thanks I will get after that in the morning and let you know what I find. I wouldn't even worry about it being as how my speedo doesn't even work at the moment, but I get paranoid in the dark not being able to see my oil pressure and temp gauges.


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