Battery Gauge Test?
 

Pulled the instrument cluster on my recently purchased 1970 C20 and the printed circuit was shorted going to the battery gauge. Like an idiot I assumed the problem had been fixed by the PO and I installed a new printed circuit. As soon as I hooked I up it immediately shorted again. Is there a test for the battery gauge or has anyone else had a similar experience and can shed a little light?

Re: Battery Gauge Test?
 

Welcome!

There should be a rectangular fiber board type insulator installed on the back of the cluster behind the battery gauge that prevents the mounting nuts from making electrical contact with the grounded metal case. And the gauge assembly has plastic grommets surrounding the studs. I believe those grommets protrude slightly from the back of the gauge to help center the studs through (and electrically isolate them from) the cluster case. However, I think it is still possible for the gauge to be installed slightly crooked and allow one or both studs to contact the case, creating a short to ground.

Checking to see if the gauge studs are accidentally shorted to ground can be done with a continuity tester or multimeter (set to the continuity/resistance scale). Simply connect one of the meter's leads to the metal gauge cluster case and then touch the other lead to each of the battery gauge terminals. You should get an open circuit (infinite resistance) reading on both if things are okay. If you get a low resistance reading, that indicates a short to ground.

I would also recommend checking to make sure your truck still has the two little 4 amp inline fuses that protect the battery gauge circuit. If a previous owner has eliminated, bypassed, or replaced them with larger fuses they won't adequately protect the battery gauge and it's wiring (including the printed circuit traces) if there is an overload or short circuit. One of those fuses should be located right near the junction block on the passenger side fender by the battery. And the other should be located at the front driver side of the engine compartment near the voltage regulator. The factory used small SFE-4 glass fuses in black rubber holders (see pic below) but any 4 amp fuse will work.

http://67-72chevytrucks.com/vboard/a...1&d=1482168208

Re: Battery Gauge Test?
 

Thank you! I'll give that a look tomorrow.

Re: Battery Gauge Test?
 

To follow up on Ray's excellent advice, in case you don't find the short in the gauge cluster, you can check the wires coming into the cluster the same way. The wires are connected to the cluster plug at pins 1 and 12, The wires are usually a solid black and a black with white stripe.

If you get low resistance on either of these wires, it indicates a short to ground. You'll have to check these pins to a good ground on the dash or the engine block that leads back to the battery negative post. If your wiring was intact you could check between the negative battery terminal and the small black wire on the fender terminal on the passenger fender where the positive lead goes from the battery shown here.

Attachment 1712001

Here's the one on the other side by the left headlight.



Attachment 1712000



These two wires should go into the firewall plug by the master cylinder and if they are shorted to ground then the small fuses will be blown. be sure to check both sides of the wires at the fuses for low resistance to ground. you should expect wires to be shorted on the firewall side and not the battery side of the fuse. Most likely the wire on the battery side of the fuse will be burnt to a crisp if it were shorted.


Here's a diagram of the dash plug and the wires to the battery gauge.

Re: Battery Gauge Test?
 

Hmmm, I don't have any of those parts under the hood. There is no external regulator or inline fuses. It looks like the PO installed an internally regulated alternator. Instead of back tracking to original equipment it may be easier for me to move forward with what's there. I'll probably have to add one under the dash or find a way to make one fit in the instrument panel. Thanks for all the help guys!

Re: Battery Gauge Test?
 

You have a couple of options depending on how much wiring you have and how the IR alternator is wired and where the large output wire from it goes.

If you don't have any wiring for the battery gauge you could just replace it with a volt meter, easy to wire in. Or you can run the output wire from the alternator to a junction in the engine compartment and run a length of 12 gauge wire from there back to the battery positive terminal to charge the battery. Then to get the battery gauge to work you'd just have to run the two black wires to the gauge. One runs from the main junction
where the alternator wire goes and the other would run from the positive battery terminal.

This is how the wiring would be run with the fusible link in the battery positive charge wire and the two black wires with their small 4 amp fuses. The diagram doesn't show the positive battery cable which would go to the starter solenoid.

Attachment 1712183




From your description you may have a one-wire alternator, which won't make any difference if it's wired like the diagram. you just won't have the two smaller wires to the alternator.

Re: Battery Gauge Test?
 

Perfect, thank you!