C2 Ammeter problems

[Randy S.]

Gentlemen,
For 3 years the ammeter has not worked on my 1966. I recently had the cluster redone and rebuilder says nothing wrong with ammeter. I checked the archives and the next likely culprit is the wiring thru the bulkhead connectors. Advice there was to check for voltage at both sides of the ammeter (battery) plug. I understand the ammeter is actually a sensitive voltmeter. I have 12 volts to ground on each side of the ammeter plug.

So where would be the next likely place to look.

Randy

[John H.]

Re: C2 Ammeter problems

Randy -

One wire from the plug goes to the large battery cable stud on the starter solenoid, and the other goes to a screw terminal on the horn relay; the ammeter (sensitive voltmeter) reads the voltage difference between those two points and displays it as "amps". With the engine running, a good digital VOM should show a slight difference in voltage between the two wires; if it does, and the ammeter needle shows no motion at all, the gauge is suspect.

[David K.]

Re: C2 Ammeter problems

I'll assume it's the same circuit as my 1967.
I had similar problems, and found them to be with the connector in the firewall.
I think this is what you are referring to as "bulkhead connector".
A simple way of saying what the "ammeter" does is that it measures the difference in voltage between what the alternator is putting out, and what the battery is putting out. With the engine off (alternator not running), the battery (positive post) is a higher voltage (12V). Current flows from the positive post of the battery to the alternator, and the gauge swings left. With the engine running, your alternator should supply about 14V, current flows from the alternator to the battery (charging it up), and the gauge swings to the right. The larger the difference in voltage, the more current flows, and the further the gauge swings.
Check the connectons at the back of the gauge. With the gauge, battery, and alternator disconnected, one wire should show no resistance to the connector for the positive battery post. The other should show no resistance to the connector for the positive post on the alternator.

[Randy S.]

Re: C2 Ammeter problems

Thanks John

I saw your earlier post re checking the voltage at the ammeter connector. If I read it correctly, even with key off I should get 12 volts (to ground)at both sides of the ammeter connector. I'll use my $30 Sperry VOM and with engine running see what voltage differential I get. For this test I assume both voltmeter probes should be connected to each leg of the ammeter connector. I am looking for the voltage differential between each conductor.
I would also assume with key on and engine not running I should get a big voltage differential when I turn headlights on.

Thanks for the help again

Randy

[Randy S.]

Re: C2 Ammeter problems

Thanks Dave,
I think I have continuity to both sides of the ammeter because of the 12 volt tests.
One side of ammeter (18 B) goes to the positive side of the battery via the B terminal on the starter solenoid. On the B terminal of the solenoid there is also a 10 R wire that goes to the B connection of the alternator. With engine not running and key off (ammeter is not in a switched circuit that I can determine)I get 12 volts to ground which is the battery voltage.

The other side of ammeter (18 B/W) goes to the horn relay and also connects to the B terminal of the alternator which is connected to the pos side of the battery too. I can also get 12 volts to ground, battery voltage, on this side of the connector also. Therefore I don't think I have a wiring problem thru the firewall connectors. As both you and Jack say with the alternator running I should see the voltage difference between the battery and the alternator.

I just had the cluster redone and vendor says nothing wrong with ammeter so I am looking for clues. Thanks for your input.

Randy

[David K.]

Re: C2 Ammeter problems

Just make sure you aren't fooling yourself, I know I got myself terribly confused.
That's why I suggested disconnecting the gauge, battery, and alternator, and measuring resistance. That isolates the connector in the firewall (or a broken wire somewhere). There are non-obvious paths to ground and/or 12V through the alternator, through the battery, and through the gauge.

[Dixon Green]

http://hyperphysics.phy-astr.gsu.edu/hbase/hframe.html

[Randy S.]

Re: C2 Ammeter problems

Dixon (and David and John)
Thanks for the input.
Here's the latest.

I confirmed the meter is working. I put a AAA battery across the terminals and "flashed" the meter to check for movement. meter moves (both directions).

I cleaned the terminal at the horn relay to reduce any additional resistance.

I don't have a very sophisticated digital VOM (no delta function) but I did measure these votages between the two ammeter wires.

.040 volts at idle
.014 volts engine off courtesy lights on
.098 volts engine running headlights on

At the last reading I get about a 1/16" deflection on meter to left.

I guess I can try cleaning the B terminal connection at the solenoid. As a last resort I will open the firewall connections but I am afraid of opening up another can of worms.

your thoughts

Randy

[Dixon Green]

[Randy S.]

Re: C2 Ammeter problems

Dixon,
Thanks for the suggestions. Update- I cleaned the firewall connectors and reconnected. No significant change. About 1/16" deflection with headlights on.
BTW I incorrectly reported previous voltage measurements, .098 volts with headlights on and engine OFF.

Would you suggest I measure the resistance of each leg of the ammeter circuit?

Randy

[William C.]

Re: C2 Ammeter problems

It might do well at this point to see if the gauge itself is functional. Take a 1.5 volt battery and attach test leads to it. A momentary touch of the wires to the terminals on the gauge will result in an immediate full scale deflection. It does not matter if it is to the + or - side, the deflection is all you need. It takes just a touch, do not leave the battery leads connected for more than a brief touch. You nay need a helper to either watch the gauge or administer the power, but the test is pretty easy.

[Randy S.]

Re: C2 Ammeter problems

Bill,

Yes I did that earlier. See complete thread a couple of weeks ago.
In summation cluster rebuilder said gauge OK, I verified with AAA battery check, I have 12 volt at each leg of the ammeter connector. That left the firewall connectors as the last suspect. I get only marginal ammeter deflection.
Randy

[William C.]

Re: C2 Ammeter problems

unfortunately, measuring resistance in the system requires the use of a milliohmeter, a standard unit will not measure the differences in the two legs of the harness.

[Dixon Green]