Ammeter hardly moves

[bigmal 220]

Hi all,

I am sure this has been discussed before. My ammeter movers only very slightly and would probably need a vernier to measure it. 

Is there any mod so it indicates more substantially?

I have been looking at the Voltmeter conversions by Rocketman's Classic Cougar Innovations which look good, but for the 69 it is still labelled Amps. If converted to a Voltmeter I would prefer it to say Volts. Is anyone aware of a Voltmeter conversion that refaces the gauge to Volts?

Thanks, Mal

[Mach1 Driver 529]

Hey Mal, yours actually moves? ;)

What we call an ammeter is actually a galvanometer, and yes it can be made to work a little better. Since it passes such a small amount of current you need to clean the connections at the "ammeter", at the flex circuit plug, at the firewall plug, and at the solenoid. Use something like Brasso in the areas marked in red.

For a big improvement others have lengthened shunt wire 38A. I've heard different lengths, but adding 22" of the same gauge wire would be a starting point. They all work the same way but the harnesses change from year to year and I've not heard of someone actually doing it to a 69. What they are doing is increasing the resistance of the shunt so that more current flows through the meter instead of around it through the shunt (in blue).

Other than that you would need to change it to a voltmeter as you mentioned, but I don't know of one that says "volts". I kinda remember them saying "alternator" but its early morning here and I don't want to wake people up.

[Mach1 Driver 529]

Here it is in schematic form- you can actually see what its doing here:

[bigmal 220]

22 hours ago, Mach1 Driver said:

Here it is in schematic form- you can actually see what its doing here:

Many thanks for this. I will give it a go 

[Mach1 Driver 529]

Mal, there is a post on VMF that is about this very subject, and may interest you: https://www.vintage-mustang.com/threads/66-ammeter-wiring-or-my-ammeter-doesn’t-work.1214659/page-3.

As I mentioned, the harnesses are a little different between the 66 and 69, but the "ammeter" works the same way. In post #41 rreitz mentioned that he added 22" of 12 ga wire to his wire #37 BK/Y which connects directly to the starter solenoid front terminal on a 66. On the 66 wire #37 BK/Y is the shunt wire.

The 69 is a little different and #38a Black is the shunt wire, and they added wire #38b Black to it, between it and the solenoid. This makes it more difficult to do what rreitz did. He only had to extend his #37 BK/Y by 22" and connect it back to the starter solenoid. On a 69 we would have to unwrap the harness and find the splice, cut 38a near where it connects to the splice and extend it 22" using 12ga wire. This makes the change much more difficult. Curse you wire 38b!!  ;)

BTW, you will still need to clean all the contacts that I previously marked in red. Below is the 69 wire diagram with the shunt 38a in blue and the dang nuisance 38b in yellow.

If you try this please let us know how it works. Lots of people complain about it, and it would be nice to find a solution for the 69s. Hmmm, maybe I'll ask Midlife if he knows how long wire 38B is. Then we would know how far back we would need to unwrap the harness. Randy (Midlife) knows these harnesses like the back of his hand.

[Midlife 806]

I don't know the length off the top of my head.  Each year seems to be different as well. 

This whole problem does not seem to have an easy solution at all.  Basically, one is measuring the difference in voltage across an approximately 2-3 foot section of #12 or #10 gauge wire, whose resistance is on the order of 0.05 ohms or so.  Corrosion at contact points are well above this level.  Unfortunately, this is not a precision system.  Attempts to make the ammeter respond in a more aggressive manner by adding extra length wires between alternator and battery (starter solenoid) is asking for reliability degradation as well. 

[Midlife 806]

The more I think about the issue, I believe the ammeter gauges were designed to let one know when either the battery or the alternator completely fails, and not for use during normal driving conditions.  It's intent was to act as a gauge equivalent to the alternator indicator lamp: if it deflects, it means something is dreadfully wrong.

[bigmal 220]

4 hours ago, Midlife said:

The more I think about the issue, I believe the ammeter gauges were designed to let one know when either the battery or the alternator completely fails, and not for use during normal driving conditions.  It's intent was to act as a gauge equivalent to the alternator indicator lamp: if it deflects, it means something is dreadfully wrong.

Thanks mate