Don Hanson wrote:
Does anyone know: The specific problem that is present when an Autometer electric temp gauge pegs the needle
immediately upon turning on the ignition? Is this a gauge failure, or is there a wire shorting out or something else?
I've got an oil temp gauge that behaves like described above. It started with the needle twitching from the actual
temp reading to full hot and back, now it just goes right over the top of the scale when I ignite the car.
I have another gauge that I could substitute, but I was planning on using that for my transmission oil temp for the
upcoming race at Laguna Seca and don't have time to get another one here. I do have an extra spare sender, could
that be the problem? (wasn't planning on changing oil, which I'd have to do to change the sender)
The 928's voltmeter has a cheapie potentiometer tacked on the back. The wiper of this pot wears a spot on the resistance substrate and when it wears too much and disconnects the needle pegs. I don't remember for sure but have little doubt the temperature gauge is the same way.
Nothing wrong with the sender or the needle. Replace the pot. I used fixed resistors for my voltmeter. Was a 100 ohm pot, IIRC used 42 ohms of fixed resistor's. Temperature gauge likely uses a different value pot.
David Kelly N4HHE, firstname.lastname@example.org
My car had voltmeter problems all the time, ever since I bought it 6 years ago. The voltmeter either showed 0 volts, 15 volts, or something else at random. I know the instrument is not too reliable but this is madness. I had the instrument cluster out some time ago and replaced the variable resister, assuming it would be corroded and was causing the defect. It worked.... For a while.... Well, just a few days really.
I took the instrument cluster out again. The voltmeter still did not work properly despite of my earlier attempt to make it "always" work and "always" indicate the right voltage. I took the instrument apart, cleaned connector contacts, and calibrated the new potentiometer to match my Fluke77 digital voltmeter. The main problem seemed to be the way the current is connected to the instrument. The fragile foil is connected to the studs of the instrument by fitting two nuts and two sort of rivets. I removed the gauge, carefully removed the oxidation from the contact spot, put some dielectric grease on, and gently put it back together. Put the instrument back in place, and after a struggle with a partially loose connector on the backplane all started to work again. I enjoy the voltmeter showing what's happening. It has never been like this, ever since I have owned the car. And its reliable too.
Took the car out for a fast and fun drive to test the voltmeter. I still love this car ;-)
'88 928s4 cherry red
The nuts connecting the gauges to the circuit board are a favorite place for corrosion. If this happens here in California where things are dry, I hate to think how much worse things get on the East Coast with year around humidity. Remove the connecting nuts, clean the contacts and re-cinch them to make a good contact after apply some more dialectic grease.
The Voltage gauge can be adjusted for accuracy. There is a small pot on the side. Be sure to fix the connections first to elimination any potential drop between the alternator and gauge. While you are in there be sure to clean the contacts for the bridge resistor.
Dan the Pod Guy