Author Topic: temp and oil Guage problem (Read 5373 times)

brads1978ta · « **on:** June 06, 2006, 10:20:57 PM »

I have a problem with my 78 TA. The oil pressure guage and the Temp guage quit. I pulled the cluster and checked the plastic/copper sheet on the back of the unit for a burned out spot. there was none. I replaced the bulbs with the new led lamps and they are alot brighter . But the guages are still inoperable. The fuse for the guages is ok checked it with a multimeter. Any suggestions?

Both the temp and the oil pressure units are plugged in.

this car is a 400 6.6 z engine

HELP :cry:

ta78w72 · « **Reply #1 on:** June 07, 2006, 12:18:31 AM »

It's the 12 volt circuit. Check the pink wire at pin number 5 (on the white connector that fits into the back of the cluster) for 12 volt. If you've got 12 volt there, then it's your circuit. You can't always see the breaks. I've even tested that circuit with an ohm meter and got connectivity, but under some magnification, I could see the break in the circuit.

The oil and temp gauges are on the same circuit. So when both go out, it's a good bet that you're not getting current.

The fuse that services that circuit is the fifth from the bottom on the right side of the fuse box.

MrMojoRisin681 · « **Reply #2 on:** June 07, 2006, 05:50:46 PM »

Also might be a ground issue.

ta78w72 · « **Reply #3 on:** June 07, 2006, 06:51:33 PM »

Quote from: MrMojoRisin681

Also might be a ground issue.

Probably not a ground issue because that circuit shares ground with the lights for the IP. He said his lights work. If you look at the ground circuit at the point where it hits the oil/temp gauges, it's a really big piece of copper and doesn't look like it is an area prone to spliting or breaking. It's possible it could have broken, but not probable. The more likely scenario is there is a break in the 12 volt circuit. That circuit is very thin and susceptible to splitting. My temp gauge went on on my previous 78. I couldn't even see the break in the circuit so I thought the gauge was bad. Got a new gauge and it didn't work either. Then I saw the break under magnification and fixed it. The new gauge didn't register correctly....turns out I pulled the gauge off a later year and they had changed the resistance spec. That's when I got interested in figuring out all I could about these things.

Funny, the first thing everyone thinks is the gauge has gone bad. I haven't seen one go bad yet. I'm sure they do....but they are pretty reliable.

brads1978ta · « **Reply #4 on:** June 09, 2006, 10:08:29 AM »

Thanks for the Input. I found the problem on the printed circuit. I took some computer solder and put a drop on each break, "there was two breaks in the same circuit. reassembled the cluster and it works fine. One other question though!! On the right side of the cluster as you look at it in the dash. There is a black fiberoptic wire that is inserted in one of the lights on the top right side of the circuit. Could you tell me where it
inserts into.
Thanks again.
Brad

Eagle 1 · « **Reply #5 on:** June 09, 2006, 12:46:50 PM »

Either the ash tray or I think the rear defroster.
Most of them came with the ashtray for sure.

Tin Indians Rule · « **Reply #6 on:** June 09, 2006, 08:11:04 PM »

Goes to the ashtray.

ta78w72 · « **Reply #7 on:** June 09, 2006, 08:29:28 PM »

The ash tray light would come off a gray wire and the bulb would ground to the ash tray. The rear defroster is part of the switch and is a diode not a bulb.

It's a stiff black wire with a bulb at the end of it...right? That has always stumped me. I plug it in to the top right slot on the passenger side of the cluster. It doesn't show up on the wiring diagram and it makes no sense to me. Was your car made in Van Nuys? The wire isn't fiberoptic, I think it's just stiff. It's probably a ground wire. You can easily check if it's ground with your ohm meter.

I've just checked two spare harnesses I have in my garage, one from 77 and one from 78, neither have that bulb. But two cars I have had, built in Van Nuys, have had that screwy wire/bulb.

That's something I'll have to research once I tear my car apart again. I guess it could be something outside the harness altogether.

Tin Indians Rule · « **Reply #8 on:** June 10, 2006, 01:38:22 AM »

The "wire" is not wire. Or at least not with strands of wire inside. It is in fact some sort of prehistoric fiber optic device. There is no bulb period and no electricity flows through it. Inside the stiff black "wire" you'll find strands of clear plastic or one single strand... one end plugs into a special bulb holder on the passengers side of the cluster and the other end snaps into the top of the ashtray. LIGHT is transferred via the clear plastic strands. Same principal as the clear wedge that transfers light to the headlight switch indicator on the dash bezel. I have the whole set up removed from the car if someone needs to see it all put together.

Eagle 1 · « **Reply #9 on:** June 10, 2006, 08:40:45 AM »

Exactly TIR.
I know for sure mine has one fiber optic line going to the ashtray, but I forgot where the other one goes. I was thinking the rear defrost, but maybe not.
I will have to look at it again, but I do have two on my car.

ta78w72 · « **Reply #10 on:** June 10, 2006, 09:27:42 AM »

I never could figure that one out. I guess because it's not part of the harness it doesn't show up on the wiring diagram. But the wiring diagram does show an ashtray light with one gray wire. Guess I never really thought about it because I've never used the ashtray. And, I never let anyone smoke in the car.

It makes sense now because the slot the bulb plugs into has both the power and ground circuit. I can't imagine how light can travel through that tube. I thought the tube was connected to the back of the bulb in a place where it wouldn't get any light but I must be wrong.

On newer models, there is a fiber optic light that lights up the symbol for the defroster. I think that's the one Eagle was referring to. I forgot about that one. The 78's didn't get a light to that spot.

Thanks for clearing that one up!

Tin Indians Rule · « **Reply #11 on:** June 10, 2006, 11:50:36 AM »

Quote from: Eagle 1

Exactly TIR.
I know for sure mine has one fiber optic line going to the ashtray, but I forgot where the other one goes. I was thinking the rear defrost, but maybe not.
I will have to look at it again, but I do have two on my car.

Are both of the "fiber optic" wires the same length?

Eagle 1 · « **Reply #12 on:** June 10, 2006, 11:17:18 PM »

No, one of them is longer than the other.
The fiber optic light has been around since the 60's. My 69 GTO has it, and it wasnt a special option, it just came standard on the car.

81BlackTA · « **Reply #13 on:** June 11, 2006, 09:55:29 PM »

I think my cig lighter lights up when the lighter is out. Wouldn't be for that would it? Can't remember if its a bulb or not. :oops:

ta78w72 · « **Reply #14 on:** June 11, 2006, 10:19:29 PM »

Quote from: 81BlackTA

I think my cig lighter lights up when the lighter is out. Wouldn't be for that would it? Can't remember if its a bulb or not. :oops:

No, it goes on when you turn your lights on. Eagle and Tin set me straight on that