Benchtest Volt meter: bad luck?

[jumpsoffrock]

I have two Voltmeters from a couple squarbodies in the junkyard, and I'm trying to bench test them.
I have read about a dozen threads regarding the matter and from what I can tell the ceramic resistors on BOTH are bad.
A couple threads addressed these gauges in detail and considered a ceramic resistor failure to be uncommon.
Both gauges are behaving the exact same way with every test. Because of this, and the alleged rarity of resistor failures, I feel like somehow I'm doing something wrong.


What I did:
I marked the three posts A(positive 12V), B (factory cluster ground), C(unused in factory vehicles;irrelevant)

Initially I hastily hooked a couple jumper wires and put 12v to post A and touched post B to a tested ground. Nothin.

I read a bunch of stuff; got a OHM meter and tested resistance between posts A-and-C, and for fun A-and-B, I was looking for ~127 ohms.
Nothin. there was no circuit, no resistance.


So now I got an old school 9V battery and wired it into an old Stewart Warner Voltmeter, and the needle showed 11 for some reason. Whatever.

Plugged 9V into both square body voltmeters(+ to A; - to B). Nothin.


Now I started finicking and had some "fun".
I hooked the 9v battery positive to the C post and left the negative on post B

Gauge doesn't necessarily peg, but it quickly shoots to about 18V. Both gauges do the exact same thing.


Am I somehow managing to screw up a simple test? Or do you think that this so-called "uncommon" event of resistor failure happened to both gauges?

Any help is appreciated.

[ray_mcavoy]

It looks like your test procedure is okay and your results are indicating that the gauge itself is okay but there is a problem with the ceramic resistor.

The problem could be a failure of the resistor or some tarnish / corrosion that is preventing the resistor from making good electrical contact with the nuts that secure it to the gauge assembly. Try loosening those nuts a little and carefully snugging them up again. That might restore the connection.

You can also remove the resistor, gently clean the contact surfaces if they appear tarnished or corroded, and test the resistor directly with your ohmmeter.

[jumpsoffrock]

If the resistor is bad, do you know where I could find a replacement? Or if a radio-shack like resistor could work as long as it has the same rating?

https://www.ebay.com/itm/273939772599
T5-125-2% RCL resistor 125 Ohm 5W 2% Wirewound

[ron350]

If you have two meters have you removed the resistors and tested the resistance with the meter?

[jumpsoffrock]

Yes, and they did the same thing as when they were installed on the gauges: no resistance or continuity.



So here's a twisted update with no pictures:
I went to the junkyard today and found 2 "blue" resistors , an orange resistor, and a yellow resistor out of a squarbody(same one I got one of the gauges from yesterday. All resistors were from the other gauges obviously.

I also was lucky enough to find another blue resistor from the voltmeter out of a 1972 chevy Van. I used a 9V battery and jumper wires to test the gauge as I found it in its cluster. appeared working.


The only two resistors that had continuity/resistance were the Vans blue, and the orange.
Out of five resistors, two "worked" and 3--after trying and trying--appear to be inoperable. Wow.


Ok, lets do some gauge testing. Working Blue resistor from a 1972 chevy van voltmeter had a resistance of 84.

Orange resistor had about 102 Ohms.

Installed both one blue and one orange resistor on the back of my two voltmeters.

hook it up to two different car battery's both tested at ~12.5v
Both gauges READ IDENTICAL, they shot far past the indicated 13 and almost touch the "red" zone near the 18.





Man this is getting ridiculous. And I wondered why most professional builds throw factory stuff in the garbage and opt for aftermarket.....

[ray_mcavoy]

Wow ... doesn't sound like you're having very good luck finding good used resistors for these gauges. I've hardly ever come across a bad one, but maybe they're finally getting old enough now to be more failure prone.

The ones originally used on these 76+ voltmeters should be around 127Ω so that explains why they read too high (near 18V) with the lower value (84 & 102Ω) resistors you tried.

Yes, you can replace them with regular axial lead resistors. The ebay example you linked to earlier should be pretty close at 125Ω but that particular one is a rather pricey option.

127Ω isn't a standard / common resistance, but you can easily make one by connecting a 27Ω resistor in series with a 100Ω. Resistors like these will work --> https://www.ebay.com/itm/324683051749

Crimp & solder some small ring terminals to the ends of the series resistor assembly and insulate it by covering it with some heat shrink tubing. Use some small nylon washers (or one of the old "open circuit" ceramic resistors) as an insulator between the back of the gauge housing and the ring terminals on your new resistor assembly.

[jumpsoffrock]

Ok I bought a some resistors and I'll report back in a few days when they come in and I put something together. At this point I'm on a wild goose chase and just having fun trying to make old stuff work.