DHow does the amp meter in a 66C20 work?

[StevenG]

I have a parasitic drain to the battery when the truck is off. When I connect a test light between the Neg terminal and the neg cable it lights. When I disconnect the blue and white wire to the alternator it goes off. If I disconnect the voltage Reg it will go off. When I test the voltage on the brown wire to the VR I get 12.6 V when the ignition key is in the off position. The brown wire is from the amp gauge in the dash cluster. The ignition key switch is new. The alt is new, the VR is new. The truck has all the wiring replaced under the dash, under the hood and to the rear lights as well with American auto wire harness. Their products are second to none. The amp gauge never moves, the needle stays in the center remove the battery no movement. Start the truck and the alternator sends 13.6v to the battery but the needle doesn’t move. It just stays in the middle. So how does the amp gauge know how many amps are being drawn? Does it have a shunt or sorting to test the load? Thanks in advance for your comments.
Steve

[franken]

The gauge is fed by a voltage close to the battery and one close to the alt. It moves left or right depending on which voltage is slightly higher than the other The gauge is just an indicator of whether the battery is being charged or not. Look for small 4A inline fuses at the fender core support corners. They're probably about 1.25" long and black rubber.
Sounds like the current draw is in the alt/regulator circuit, not the battery gauge.

[ray_mcavoy]

Originally Posted by StevenG

When I test the voltage on the brown wire to the VR I get 12.6 V when the ignition key is in the off position. The brown wire is from the amp gauge in the dash cluster.

You might want to double check that portion of the wiring. There should not be any connection between the brown wire and the battery gauge (ammeter).

The brown wire should run from the ACC terminal on the ignition switch, out through the firewall bulkhead connector, and end up at the #4 terminal on the VR. The portion of this wire in the under-dash harness is a special resistance wire and is typically brown with a white stripe.

This circuit should not have voltage present with the key off and whatever is causing that is likely the source of your battery drain.

The battery gauge uses black wires (one plain black, the other with a white stripe). The black one runs out through the engine harness bulkhead connector on the passenger side of the firewall and then down to the battery stud on the starter solenoid. The black wire with the white stripe runs out through the bulkhead connector on the driver side of the firewall and connects to the bus bar on the horn relay. No where along the way should either of these black wires be connected to the brown wire going to the VR.

Originally Posted by StevenG

So how does the amp gauge know how many amps are being drawn? Does it have a shunt or sorting to test the load?

Yes, there is a shunt. In these trucks, it takes the form of the (typically 12 gauge red) wire that runs from the positive battery terminal, across the radiator support, and connects to the bus bar on the horn relay.

You can think of it in terms of voltage difference at each end of that wire as franken described. Or you can think of it in terms of current flow. The vast majority of current flowing to or from the battery goes through that wire (just like it does on a truck without gauges). Only a small fraction of current takes the (higher resistance) parallel path through the gauge.

In any case, doing something that will cause the battery to discharge (such as turning the headlights on with the engine off) should cause the needle to deflect toward the "D" (discharge) side. After starting the engine, the needle should move toward the "C" (charge) side for a while until the battery is recharged. Then it's normal for the needle to stay near the center as long as the alternator is working and supplying enough current to power the truck's electrical loads.

Those 4 amp fuses that franken mentioned were standard equipment on 1967 & newer trucks. They are a good safety addition to 1966 & earlier. However, the AAW harnesses are usually exact copies of the GM originals so you probably won't have those fuses on your 66.

[StevenG]

Thank you for that explanation. I have replaced the key switch twice and no joy. Is it normal for the key switch to have 2 accessories positions? My past 2 key switches have an off position straight up then 1 click to the right is the run position. Another click to the right is the spring loaded start position. Turn left from straight up one click is accessory but then it will turn one more click to the left. I’ve never had a vehicle with 2 accessory positions. The can be removed in any of these positions. That seems odd.

[ray_mcavoy]

You're welcome!

The factory original key switch in these trucks only had 3 positions ... off, run, and start.

However, most all of the replacements seem to have 5 positions ... accessory, lock, off, run, and start. Normally, you can only remove the key in the lock position, but if the key and/or lock cylinder are worn, it's not unusual to be able to remove the key in any position.

Even though the original key switches didn't have an accessory position, they still had an ACC terminal. That should be the same on the replacement switch. And it's the ACC terminal that is connected to the brown wire that leads to the #4 terminal on the voltage regulator.

Have you tried temporarily unplugging the switch and checking to see if the brown wire on the VR still has power? That should let you know if it's an issue with the key switch or something else. Might also be the VR itself ... the red (voltage sensing) wire normally stays live all the time. So there is the potential for a faulty VR to be keeping power on the brown wire due to the field relay contacts being stuck closed.

[StevenG]

I will try what you suggest. I’ve had 3 regulators in this truck. First 2 started to smoke as soon as I connected it. I have a solid state regulator now so no points and no smoke but still the parasitic battery drain. I’ll get back to you.