Battery gauge test

[Randy S.]

Is there a bench test for the battery gauge, i.e., a known voltage should produce a known deflection?

I've chased the usual suspects but no success. My 66 gauge only deflects about 1/16" even with headlights on and raise lower. No negative deflection on engine startup. Been this way for 15 years and 2 batteries

I've checked electrical connections including bulkhead connector, I have 12 volts on both sides of the gauge ( I know the gauge is a sensitive voltmeter) and the needle deflects to -40 or +40 when "flashing" the terminals with a AAA 1.5 volt battery.


I'd like to know the gauge is defective before going to the trouble of pulling the cluster and replacing it.

Randy

[Donald O.]

Re: Battery gauge test

Randy,
The gauge is fine. You checked its operation with the battery. Leave it alone and worry about something else.

Don

[Rich G.]

Re: Battery gauge test

My 66 does the same. I know things are ok because I've had it for 15 years and I'm on my second battery. I raised the same question years ago and a member at the time said his 66 does the same and he bought it new!
Having said that, my training as an electrical engineer tells me it should be different, but who knows?
You're correct that it's a voltmeter but it's measuring voltage across a shunt resistance and if that resistance is wrong then the readings are either too low or too high, I think. That's a general comment. I'm not familiar with the specific wiring in the car.
Rich

[Randy S.]

Re: Battery gauge test

Thanks Don
is this the Don Olson that makes the color coded wiring diagrams like the one I have in my garage?

randy

[Richard G.]

Re: Battery gauge test

Randy
I haven't dug into the wiring diagram for the 66 Corvette, however I can speak in general terms of what GM did in the 60's for amp gauges.

Typically the gauge is a milliamp gauge, it doesn't measure voltage but a small amount of amperage. The circuit is called a shunt circuit.
Here is a better explanation, than I can give here, if you are interested:

https://www.allaboutcircuits.com/tex...mmeter-design/

Usually the two wires from the gauge terminate at the same wire a known distance apart, I.E. a known resistance. The issue with this type of circuit is it is very susceptible to any small additional resistance.
I repaired one of these circuits with a new inline fuse and could not get the gauge to work correctly. I traced it back to the new fuse holder. I pulled the fuse and cleaned the fuse snap-in terminals with sand paper and added some petroleum jelly to keep any new corrosion from reappearing and the circuit worked fine. The fuse was new and looked fine. All the connections were soldered as crimp repairs are not usually acceptable. I am mentioning this to illustrate how particular this circuit can be. Trace both wire back to the source and double check every connection and if it is at a terminal take it apart and clean everything, use petroleum jelly or a product designed for this application. My bet is you can get it working again. The headlight check without the motor running is a very good check for the circuit function. The old method that dates to the 50’s is running large wires to the dash and going through a amp gauge. This was changed for costs and weight reasons, cost mostly as the wires can be very small. typically one end terminates at the voltage regulator, the other side varies.

Now, I just hope I am correct and the Corvette uses this type circuit! I am fairly confident it does.
Rick