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.