Reader Follow-Up—

September 1, 2010 | By David Kozar

If Your Ammeter Won’t Respond

WHEN JIM HARP wrote about the minimal needle movement in the ammeters in his Ford cars (June Mechanic On Duty) I just had to respond. I bought a ’75 Ford F-250 pickup new some 35 years ago with the same exact problem. The dealer as well as two other mechanics refused to repair it saying that’s just how those ammeters worked. It was at this time that I learned just exactly how these shunted ammeters do work. Being an apprentice electrician at the time and knowing a little about resistors in parallel, I decided to try and fix the problem myself. As it turned out the fix was relatively easy and worked for the 20-plus years I had the truck. Here is what I did:

1. Turn off all loads in the vehicle.

2. Disconnect the battery.

3. Locate the shunt in your particular vehicle. In my truck, a bulge in the wiring harness directly behind the alternator is where the shunt was located. If you can’t find a bulge, then start at the starter solenoid and cut the outer wrapping of the harness exposing the large conductor (10- gauge, I think). Follow the conductor until you see an 18-gauge wire attached to it. About 12 inches or so along the wire you will find another 18-gauge wire attached to it. These two 18-gauge wires are the wires that go to the ammeter.

In the case of my vehicle the bulge was created when the 12-inch piece of wire was folded during the manufacture of the harness. Unfold this shunted wire and make sure the 18-gauge wires are securely attached. I don’t remember but if they were soldered or welded during their manufacture they are probably OK.

4. Cut the 10-gauge wire midway between where the 18-gauge wires are attached and skin the cut ends with a wire stripper. Splice in a short piece of wire of the same gauge (10-gauge, I think). The length of the wire I ended up splicing in was about 12 inches. This added just enough resistance to the shunted wire to divert more current through the 18-gauge wires and make the ammeter deflect more. You will have to use trial and error to determine the exact length of wire needed for each vehicle.

5. After making sure the temporary splice will not come apart, reconnect the battery and test the operation of the ammeter by turning on a small load like the dome light. Change the length of the wire you spliced in until the deflection of the ammeter is correct. Disconnect the battery between tests. If the splice comes apart during the test then all of the load current will go through the ammeter and probably burn it up.

6. After determining the proper length of wire to use, I recommend that the splices be soldered to ensure that they will not come apart. If using wire nuts, make sure that they are new and tightened very securely. Re-wrap the wiring harness and you are good to go.

Remember: The longer the wire you splice in the more the ammeter will deflect. When testing, start with a small load like the dome light and increase the loads until they are all turned on. Keep an eye on the ammeter and make sure it does not deflect too much. Starting the vehicle should also be part of the test. The charging current can be higher than all of the load currents combined. This is particularly true when an alternator is charging a heavily discharged battery. During these infrequent times the ammeter in my truck would “mildly” peg to the charge side of the ammeter but damage to my ammeter never occurred. The key here is to not use too long of a wire and cause too much deflection.

I’m sure the problem is widespread as I have read many letters over the years regarding this problem. However, I have never read about a repair for the problem. Maybe Auto Restorer will be the first with the answer.

David Kozar Via e-mail