I have read with interest most of the comments in the forums about ammeters for Fords as I had to rewire my 78 F250 completely. So I started with taking out all the wiring carefully ie. Take the plugs apart not just cutting the wires Took the dash out and checked all the gauges actually stripped the dash completely took out all the gauges cleaned the glass removed the printed circuit washed all the plastic etc. Reassembled everything then physically traced the printed circuit and this is the pin no. layout. I did the same with the indicator switch and the headlight switch.
Black / Red ————- Stop light switch
Light Green ———— Left rear indicator
Yellow / Black ———— Right rear indicator
Blue ———— 49A From flasher unit
White / Blue ———— Right front indicator
Green / White ———— Left front indicator
White / Red ———— 49A From hazards flasher unit
B ———— Battery positive
A ———— Battery positive to other
R———— Park / Tail lights
I ———— Dash lights
H ———— Headlights
D1 D2 ———— Interior light
4 ———— Temperature gauge
5 ———— Oil gauge
6 ———— Indicator right
7 ———— Amp meter
8 ———— Earth
9 ———— Amp meter
10 ———— Dash lights
11 ———— Bright indicator
12 ———— Indicator left
13 ———— Fuel gauge
14 ———— Ignition
Now as you can see the ammeter has 2 wires that connect to it, these wires are thin a red and a yellow as this ammeter has to have a shunt resistor connected to work . Having got all the harnesses out I traced these 2 wires to a 2-pin plug close to the battery found the wires corroded off. The other side of the plug was connected to a length of 4mm wire, this wire was folded a few times and taped in the harness. So I decided to make my own shunt resistor this is what I did.
The equipment used:
Fully charged battery
Analog ammeter with a scale 0-80 in increments of 1
Carbon pack battery tester
Sharp knife or a very sharp pointed steel pin
Two lengths of thin wire same as I would use to make harness approx 1.5 meters in length
1.5 meters 4mm wire the same wire that I used for the harness
Connect the 2 thin wires to the back of dashboard to the relevant pins ie. 7 & 9. Place the dashboard on workbench so that you can see the ammeter. If you have a steel bench put a piece of cardboard or rubber matt under the dashboard to avoid short circuit. Place the analog ammeter on bench so that you can see both ammeters.
Connect the one side of the 4mm wire to the battery positive post together with one of the thin wires from the dashboard ammeter. The other thin wire, connect to the sharp knife or the steel pin.
Connect the positive side of the analog ammeter to the other side of the 4mm wire.
Connect the negative side of the analog ammeter to the positive cable of the battery tester the negative cable of the battery tester connects to the negative battery post.
You must now decide as to the amps readings you want your ammeter to read.
Mine I decided as follows the alternator is a 50A so the max the ammeter must read is 50A the ammeter has 0 in the center to the right 1st line 2nd is max reading charging to the left 1st line 2nd line max discharge.
So I decided the 1st line must read 25A.
Now turn up the battery tester until the analog ammeter reads 25A. Start by cutting into the insulation or prodding through the insulation approximately 750mm from the battery positive along the 4mm wire note the reading on the dashboard ammeter move the knife or prodder closer or away from the battery positive post until you get the desired reading that matches the analog ammeter. This has to be done fairly quickly as the battery discharges so you have to adjust the battery tester to keep the right reading. It is better to do it with a friend to help keep adjusting the battery tester.
I hope this will help as my ammeter works perfectly when I start it reads 25A as it charges it goes down to 3-5A when I switch on the headlights it actually reads higher telling me the alternator is working.