71 Amps versus Volts

[Bob H.]

Good Evening. I just purchased a 71. When you first start the car the battery gage shows a slight positive and drops to zero or slightly below. Using turns signals you can see the gage go negative slightly with each pulse. When you turn on the lights or air it drops like to -15 regardless of revs. It does seem to read differently as to how negative on the same item each time you turn it on.

I put my meter on the battery and it read around 12.6 with everything off. Upon starting and running it shows slightly over 14 volts. This is true even turning the lights and the air on at the same time. The meter will show over -20. Do I have a bad gage or a ghost somewhere?

It scaring the hell out of me when I look at that gage.

[Terry M.]

Re: 71 Amps versus Volts

Good Evening. I just purchased a 71. When you first start the car the battery gage shows a slight positive and drops to zero or slightly below. Using turns signals you can see the gage go negative slightly with each pulse. When you turn on the lights or air it drops like to -15 regardless of revs. It does seem to read differently as to how negative on the same item each time you turn it on.

I put my meter on the battery and it read around 12.6 with everything off. Upon starting and running it shows slightly over 14 volts. This is true even turning the lights and the air on at the same time. The meter will show over -20. Do I have a bad gage or a ghost somewhere?

It scaring the hell out of me when I look at that gage.

The voltage readings you report at the battery with the car running or not are correct. Nothing to worry about. The ammeters in the cars are not accurate. Not to worry.

[Richard G.]

Re: 71 Amps versus Volts

Without having the wiring diagram for the 71 Corvette I will assume the circuit is typically of the mid to late 60 and early 70 GM vehicles.
The amp gauge is installed in a circuit called a shunt;
A shunt is a precision resistor that allows the safe measurement of potentially high electric current. Placed in series (inline) with a current-carrying conductor, a shunt’s low resistance creates very little voltage drop compared to other items in the circuit, so it consumes almost no energy. Measuring the voltage across the shunt gives the amount of voltage drop across that precise resistance. Those amounts are plugged into the Ohm’s law equation to give an accurate computation of current flow in amperes through the circuit containing the shunt.

GM used a length of wire as the resister in the shunt circuit described above. If you trace out the circuit you will find the gauge wires terminate to the same wire just some distance apart, typically 4 to 6 feet apart. This of course make no sense unless you understand the circuit. The shunt circuit requires absolutely perfect and clean connections to function correctly. Typical as the car ages this becomes a bigger and bigger issue. Often times you will find each leg of the gauge has an inline fuse attached. Typically, these can exit the harness and then have a fuse and then re-enter the same harness 5" apart. Fuses are required because both legs are hot all the time. Each leg will have 12V+ on it. The gauge is an mV gauge (I.E. A voltage gauge not amps). So, the resistance across the shunt is typically close to something like 0.0001 ohms. Don’t plan on measuring this value with your meter.

So, what does all this mean?
Clean connections are required. (Even the factory fuses, in this circuit, are protected inside rubber caps, that pull apart, for this reason)
Good place to start is the small black connection next to the battery with several wires connected on a post. (if your Corvette has this connection) One end could also be located at the starter?
Any bad connectors will have to be soldered, crimping will not do! Fuses have to be clean. I recommend using a little Scotch Brite on the fuse and connections and then covering them with Vaseline to reduce the chance of corrosion. A good wiring diagram should help you find the fuses and various connections. Any inspection should include the ones at the gauge but these are not as problematic.
I typically check them without the motor off and the headlights on. The gauge should go negative. The better the connections the lower it will go. If the gauge works correctly on the discharge it will work correctly when charging.
The manufactures subsequently went to Volt meters because of the above reliability issues. Amp circuits using a shunt just don’t work well over time. Early automotive amp gauges had the large wires traveling to the gauge terminals and were direct reading amp gauges. It just wasn’t a good idea to have this number of amps available at the gauge cluster. Plus, the expense of the large gauge wires needed to get to the meter was eliminated when using the shunt or volt meters. What manufacture doesn’t want to cut cost and weight at the same time?
I believe the gauge would as GM intended with a little effort.
Rick

[Joe R.]

Re: 71 Amps versus Volts

Bob,

Everything is working normally and just like every other 71 Corvette as GM intended it!
You have a normal working system.
Nothing but nothing is wrong!

JR

[Mark E.]

Re: 71 Amps versus Volts

Bob,

Everything is working normally and just like every other 71 Corvette as GM intended it!
You have a normal working system.
Nothing but nothing is wrong!

JR

I don't believe what Bob describes is normal. "When you turn on the lights or air it drops like to -15 regardless of revs." That's indicating a bad charging system, gauge or circuit (as Richard explains above).

[Bob H.]

Re: 71 Amps versus Volts

So it sounds like I am ok except for this gage reading wrong. I am no electrical guru but it looks like one side goes through the block to the horn relay (3 wires. The other side looks to go through the block and also ends up at the horn relay. Guess I should check the block connectors and the horn connections?

[Mark E.]

Re: 71 Amps versus Volts

Check the CSM for normal voltage range while running, but I think it's 13.8v - 14.8v measured at the alternator with the headlights on and all other accessories off. I believe the manual specifies RPM for this test... probably 1500rpm or so.

With 14.2v (at the alternator?), your charging system is probably doing its thing.

I would begin with the easy stuff like cleaning connections as Richard outlined.

[Bob H.]

Re: 71 Amps versus Volts

Thanks guys. after checking connections and it looking like the shunt circuit attaches to the horn relay (both side of the circuit) can the horn relay be causing this although the horn works?

[Mark E.]

Re: 71 Amps versus Volts

Yes.

The horn relay has three functions: horn relay, ignition key buzzer, and electrical block terminal.

You're concerned about the third function, which is the simplest. The block terminal is the exposed metal junction at the bottom of the device with two wire terminal screws . Attached to each of these two screws are several 10, 12, and maybe 14 gauge wires.

Remove these two screws and attached wires, and thoroughly clean the block terminal and each wire's connector. Then carefully check that each wire terminal is tightly attached and has no corrosion.

[Bob H.]

Re: 71 Amps versus Volts

Just wanted to report back what solved my problem. Richard, you were spot on. I removed and cleaned the terminals on the horn relay since it appears both sides of the gauge go back to that point. Once cleaned and reconnected, the problem went away. You guys saved me from chasing this down a different path such as replacing the gauge, etc.