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I had received a broken voltmeter for my 1969 free from a fellow member and took it apart. I found what looks like a blown diode but cannot see any markings to identify it. Does anyone know what value it could be or how to determine what to use?
Thanks
Ok. Zener diode. There's probably a cap in there across the + terminal of the gauge to the meter housing. Its there to protect the meter in the event of a reverse polarity condition. Like if a technician hooks up the wrong battery in the car and the posts are backwards or something like that.
How it broke in half is beyond me.
Anyway, does the meter needle move if you connect an old school 9V battery across the terminals? Not looking for accuracy, just life. And watch the polarity please. Dont want to damage anything
Thanks again for your help. I tried the 9volt battery but no response from the gauge. I jumped the blown diode and attached the battery and got full deflection on the gauge. I am thinking that if I replace the diode it might come to life. I have very little experience with electronics so not sure how to pick the right value of the diode.
don't diodes just keep elect going one way and resistors change
power levels?
With silicone diodes, in general yes. However zener diodes can be used forward or reverse bias. They are more commonly used to operate at breakdown in reverse bias. In other words, zener diodes conduct when hooked up backwards once the circuit reaches the rated voltage of the diode. Think of zener diodes as the stall speed on a torque converter. maybe not the best analogy but its all I could come up with
Since the 9V battery sends the meter into full deflection, we need to find the voltage that the meter goes to part deflection and use that voltage to determine what diode to use.
So the question now is, whats the happy voltage for the meter. Im going to dig some more and see what I can find out.
If anyone has any info on these C3 voltmeters it would help
I clumsily shorted the third post thats not used on that volt gauge with a test light while under the dash and blew the diode also. I couldn't locate a replacement p/n at the time either. So when the new gauge came in I cut that post off down to a 1/4". Man I was torqued to have to buy something twice.
I purchased an assortment of diodes and through trial and error I found that a 6.8-volt .05W diode seemed to work. My only concern is that the original diode seemed a little larger with a larger gauge lead. I am not sure if a higher wattage diode should be used. Thanks for all the help.
Tom
Sorry I didnt chime in earlier, I got punched in the face with covid24, been coughing my brains out the past day or so while trying to get my own carb/timing sorted out on my C3. (still doing that)
Anyway, your 6V full deflection test was THE clue. Nice trouble shooting Tex. You found the solution and for a lot less than replacing the entire meter.
Very cool, now anyone can reference this in the future and replace the failed diode instead of the entire meter.
The diode wattage "should" be OK long term. Voltmeters draw very small amounts of current to operate. Based on your findings, these later model C3 Corvette voltmeters are actually 6 volt meters but calibrated (or lets say scaled) for 18 volts. So when the meter is straight up and reads 12 (or13) volts, the voltage across the coil is actually half that.
No matter... It works, and Im glad it does.
Originally Posted by tkucala
I get partial deflection with a aa battery but full deflection with 6 volts. Here is a picture of the front of the gauge.
Voltmeter diode
