Oil Pressure and Temp Gauge not working

[chief poncho]

Oil pressure gauge is pegging to the negative and temp gauge is pegging high.  I checked both sending units and they seem to be working.  Also checked oil pressure with a mechanical gauge and oil pressure is great.  Looking at the schematic and circuit card, there are three connections to the oil pressure gauge and 2 connections to the temp gauge.  Not sure if one of the posts coming out is connected to both.  Anyway, looks like one side gets 12V, the other runs to the sending units and the third post???  Well that's the problem.  I can't tell if its supposed to go to ground, but it seems like it gets 12VDC when the lights are on.  It does run to some of the bulbs.  I'm very confused at this point.  Ideas anyone?

BTW, this is for a '71 TA.

[DeVilliers]

mmm, usually the sending unit itself is the ground, so I can't think that it would be ground.

[firebirdparts]

The third post is ground, and if you've lost that, then the symptoms do fit. 

The electromagnet on the sender side pulls the oil needle down and pulls the temperature needle up.  They're opposite.  These are pulling against a constant amount of magnetism pulling the other way, and that's what you've lost.

if you take the gauge apart,  you'll see two ballast resistors built into it that control power through that circuit (but don't do that, I guess).  So the ballast on one side competes with the sender on the other side (literally) of the gauge, but the sides are reversed on the oil pressure and the temperature.

Most of the bulbs are connected to that instrument panel ground circuit, and the tach is too.  It's the most widespread circuit on the whole thing.   

[chief poncho]

Ok, so I removed the gauge cluster and hooked the third post ground directly to a good chassis ground point.  I then ran jumpers from the gauge cluster connector for power and for the sending unit input directly to the posts on the oil pressure gauge.  Then I started the car.  IT WORKS!!  So it looks like its just a bad ground to the instrument cluster.  So I need to make sure the grounds are connecting properly.  Does the ground for the cluster come from the tab that attaches to the headlight switch or is it the other way around?  Regardless I may run a separate ground directly to chassis.

[chief poncho]

So here is a labeled image of the printed circuit for an early TA.  I believe this is correct for '70-72, but definitely for '71.  Some might find it helpful in the future.

[firebirdparts]

I guess I am missing a sticky on this.  I used to have a sticky that was titled "My lights make my gauges stop working"

The ground for the light switch is the cluster.  The cluster is grounded through the I.P. harness ground wire.  The IP harness ground wire is grounded to the dash frame on the left side, where the frame is only 12" long.  If the bolts holding that frame to the car are removed, presto, no gauge ground.  This happens a lot during restoration and repairs, and it's one of those electrical FAQ's for this car.

It was a dumb place to hook a ground wire.  The dash is plastic.

[chief poncho]

Thanks for putting that sticky up there.  Great information to share.  I wonder how many people have replaced gauges thinking they were bad, versus fixing the ground issue first.