Join Our Ford Truck Forum Today

Document your Ford truck project here and inspire others! Login/Register to view the site with fewer ads.

Ammeter = repost


California Chapter member
I think this all got lost in my "sending unit testing" thread, so now this is an electrical question:

I want to test the ammeter. I know that it's a shunt type from my shop manual and under the testing section it states to "check the battery to circuit breaker wire connections" For the life of me I cannot find that wire or a circuit breaker or the shunt. Someone mentioned it was off the alternator, but I've replaced that a number of times and didn't see a shunt wire/resistor there. I really want to try this before I go through all the trouble of replacing the gauge by taking out the dash. That's a pain to do. I know from replacing the instrument pod a number of years ago. I actually DID look further in the manual for the wiring diagrams, but all is see is a simple sketch for this that I can't find.... shouldnt' the wire for the shunt come after the alternator? I can't figure out how to trace the wiring from the gauge back to the battery... I'm just so confused now!!!
Oh, and I know the ammeter is not all that reliable, I just want to keep the original gauges working for historical sake.. :)

Thanks as always for your help!
IIRC, Try looking on the harness along the firewall behind the passenger side valve cover and by unravelling the tape will reveal a plastic/rubber Block with a fusible link and one of the wire runs to the fuse box the other one should be for the ammeter.


California Chapter member
Thanks Dennis!! I'm heading out camping and will check this when I get back. For the meantime, I have a temporary device that plugs in the lighter socket with leds that tells me if the battery is charging... Silly, but it works!

I'll post back when I get a chance to follow up
1973-1979 Ammeter Diagnostics

By Jim Barbera

I've seen a lot of late '70s Ford trucks with a non working ammeter. I sure wish mine was working when I broke a alternator brush wire out in the middle of nowhere and had the truck quit just after crossing a creek. I was able to fix the alternator and get a jump...after being stranded for about 3 hours. I immediately installed an aftermarket voltmeter, but still wanted the amp gauge to work.

When you turn on the headlights with the motor off, the gauge should deflect about -1/16 full scale. When you start the motor it may read as much as +1/4 full scale.

If your ammeter doesn't seem to move it could be bad or have a bad connection.

The gauge measures voltage across a heavy shunt wire in the harness between the alternator and starter relay.

It's really hard to identify a bad gauge because a bad gauge will respond to voltage in both directions. Don't put 12 volts across the gauge as this will peg it and probably wreck it. I'm pretty sure the gauge is a millivoltmeter, because they're using a shunt at the alternator.

To verify the gauge is receiving the proper voltage, pull the instrument cluster out and measure the voltage across the ammeter pins on the cluster connector, pins 2&3 from the upper right for a '76. Other years may be different.

These are the voltages you should see under the following conditions.

Headlights on, engine off -.032

Ignition Key on, engine off -.027 (with electric fuel pump running)

Ignition key ACC -.002

Engine on +.2

Engine on after a few minutes +.1

If you see those voltages, the gauge is bad or the connections from the studs to the PC board are oxidized. Don't forget to close the doors and hood so all of the lights are off. If you see any voltage with everything off, you've got something draining the battery that shouldn't be.

You can still get the gauge at the dealer for about $65. I was lucky. The instrument cluster I bought at the junk yard for $60 had a good gauge.

If you get an instrument cluster from the junk yard, there are subtle diffferences especially in the flexible PC board. Bring yours to the yard to ensure an exact match if you intend to replace the PC board. Also pay attention to the location of the high beam indicator light and the lights on either side of the signal arrows.

When rebuilding the cluster, be careful not to touch the needles of the gauges as they're really fragile. The speedometer needle protrudes a little past the housing so set it face down on a peice of cardboard with a hole cut out for the needle.

If you haven't pulled out the cluster before, it's really easy. The only tricky part is the headlight switch. DO NOT TRY TO PULL THE HEADLIGHT SWITCH KNOB OFF THE SAME WAY AS THE WIPER KNOB. You'll have to reach up under the dash and push a little button on top of the switch while pulling the knob. The lights will come on and with a little jockeying around the knob and shaft will come out of the switch. Then put it back in gently and turn off the lights, then pull it back out. The other tricky part is the speedometer cable. Just rock the white plastic connector to one side and it will come right out. Try to keep track all of the different length screws to make sure they go back in the same spot.
Last edited:
I really want to try {trouble shooting the circuit} before I go through
all the trouble of replacing the gauge by taking out the dash.
That's a good plan no matter what, :) since electrical problems can be
cleared-up while fooling around with 'em. Try to test 'em with just a meter
and find the electrical trouble (or that the electricity is getting-where-it's
supposed-to-go in-the-right-amount ;) without messing with it, any more
than you absolutely have too. If the electricity is getting to a part that's
not working, then it's the part that needs fixing or replacing. ;)


BTW, removing the instrument cluster is not a pain, danggit. ;)

But it took me like a week to work-out the "best way" to fix mine because
it ended up with a big hole in it to fill up...


The head of the screw presses against the black flanged bushing and the
small white nylon spacer on the screw keeps it centered. Kinda cool huh? :)

This large diameter white spacer goes against the metal and supports the
plastic dash panel (or "cover" as Ford calls it) and so takes the pressure
the screw exerts.

Same thing as that last one here, but these spots hadn't broke out yet...


All the nylon bushings/spacers/bearings were from the hardware store.
And the PlioBond contact cement was too. ;)

Here's the button Jim B. (by way of Dennis) mentioned...



I believe the ammeter in my '75 quit before it was out of warranty. :/
So, chances are, it's the meter itself that tends to go bad in these?

I've always wanted to re-wire it to turn the ammeter into a volt meter.
Just never got around to studying on it enough to figure out how to do
that. :/

Alvin in AZ
Last edited:


California Chapter member
Oh Alvin.... It IS a pain!! I've done it a couple of times! Yours really looks like a lot of fun though! The thing is, it takes so much time and you have to remove so many things just to get to a couple lousy screws holding the ammeter in!! I'll do it if I have to (and I probably will...) but I try to take the shortest route first!

Oh, and I SO agree that one can actually CAUSE electrical problems while trying to fix one!

Thanks again Dennis!