March 13, 2005  -  I spent several hours in the shop today following wires for the factory ammeter, and posted on this page are my findings. However, before I begin, let me catch you up to date, especially important for those of you who aren't quite up to speed.

1967-1972 Ford trucks could either have a full-instrumentation instrument panel (hereafter referred to as FIP) or standard-instrumentation panel (SIP). The FIP panel includes a ammeter for monitoring the charging system, instead of the idiot light setup of the SIP. There have many people asking whether their SIP-equipped trucks to be upgraded to FIP. I've been one of the people asking this question, and it's been long reported to me that the FIP trucks had a wiring harness with a 'shunt', which powered the ammeter. I've always understood this statement as being that a shunt was basically a resistor (or resistor wire/cable) to reduce the voltage to the ammeter. However, my findings today show differently.

As Dirty Harry once said, "A man's got to know his limitations"....and I know mine. I know almost nothing about electrical wiring, so for this exercise I had to enlist the help of my father-in-law Earl, who just recently retired after many years with the local electrical company, and has probably forgotten more  about electrical theory and implementation then I'll ever know. After I explained the basics on Ford's setup, we set out to find out all we could. (And I should mention, he was very patient with me today, trying to help me understand a lot of the basics...thanks Earl!) I'm going to to my best to try to explain what I learned today here, but since I'm still up up to the 'apprentice automotive electrician' level yet, some of what I transcribe here might take a little tweaking in the future for better clarity.

Anyway, here goes...

First of all, I'm not entirely sure whether I was simply misunderstanding other's descriptions of a shunt or whether it was a general misconception, but if I WAS understand correctly, then we're going to need to 'un-learn' what we thought we knew about the ammeter in Ford pickups. Some of us (or just me?) have been thinking of a shunt as a noun...that is, something that we could see and hold in our hands, like a resistor, a magical piece of something that needed to be wired into the circuit. However, a shunt is not an object, per se...it's a method of connection. To "shunt" something means to divert. The Ford ammeter is not a true ammeter...it's more of simply a different connection method for a voltmeter. There is no in-line resistance wire/cable and no inline resistor to the ammeter. All the Ford factory ammeter does is basically to measure the difference between the output of the battery vs. the output of the alternator and reports that difference. In other words, it measures the direction of current flow.

We spread an entire '67 F100 wiring harness out on the floor and hooked up the FIP and a battery and started taking readings. First of all we traced the wires coming from the ammeter back to their source and verified their continuity.

The red wire (A) coming from the discharge side of the ammeter goes through the wiring harness and hooks up to the hot side of the starter solenoid. The yellow wire (B) coming from the charge side of the ammeter goes through the wiring harness and ties into the 10 AWG cable which goes between the hot side of the starter solenoid and the alternator. The points at which the two wires connect to the solenoid-to-alternator cable are about 23-1/8" apart. However, this cable does not "meter" power to the ammeter...it has virtually no resistance. Using a sensitive ohmmeter, we were only able to detect about .10Ω resistance between the point where it begins at the starter solenoid and where the shunt, or power diversion, is connected. Therefore, it appears that even though the yellow wire is connected in the middle of the solenoid-to-alternator cable, where the factory spliced it in, it would be just as effective to hook it up at the alternator itself at the BAT connection where the cable itself is attached.

Fig. 1 - Click this thumbnail to open the full-size image in a new browser window, so you can pop back and forth between this page and the picture.

After hooking up a battery to the wiring harness to check for power at the ammeter itself, we found we had the EXACT SAME READINGS at both ammeter terminals! If there was a resistor inline anywhere in the harness whatsoever, it would have shown up at the ammeter...and it didn't.

For the purposes of this investigation, I had three 1967 wiring harnesses laying out on the shop floor for comparison...two with FIP setups and one SIP. The two FIP setups did not have any fuses, but I've been informed that '68-up harnesses should have two 4-amp fuses in the circuit...the fuses on the  harnesses are inline immediately after the starter solenoid connection and after the splice on the wire to the amp gauge.

Also, I've been told that there are five wires coming from the factory splice in the solenoid-to-alternator cable on '68-up wiring harnesses. The five wires at the splice go to the 1) alternator, to the 2) voltage regulator, the 3) switch, the 4) amp gauge, and to the 5) starter solenoid. However, there are only three coming from the '67 splice. Two yellow wires (one of which goes to the ammeter and the other to the voltage regulator) and one large black wire with a yellow tracer. I ran out of time this evening before I could trace the large black wire to it's source. I will do so soon and report back.

DASH MOUNTED “AMMETER”

This gauge is a charging indicator. It shows whether the battery is being charged by the alternator or is being discharged by an electrical load (lights, engine, radio, etc.) It measures current flow and strength between the battery and the alternator. The gauge consists of a permanent magnet – a pointer – and an armature. The pointer is held in the upright center position by the magnet when there is no current flow. When the alternator is providing power to the battery, the “ammeter” pointer deflects to the + charge side and the opposite occurs when the battery drain is more than the alternator provides. The current is reversed and the pointer moves to – discharge. If the “ammeter” were installed in the main circuit between the alternator and battery, it would work the same, but it would have to be to handle all of the current that is exchanged in this circuit. To overcome this drawback, a smaller wire is tapped into the circuit near the alternator, and then routed to the “ammeter”. This circuit is continued from the other terminal post on the “ammeter” and routed to the main circuit near the battery. This is technically called a “shunt” or “shunted-circuit”. It simply creates another path for the electricity to travel without exposing the gauge to the high currents of the main circuit.

The “idiot light” compares the battery voltage to the alternator voltage. When the battery voltage is below the alternator output – the light stays off, and when the alternator voltage falls below the battery – the light comes on.

I have included sections from period MOTOR’S and CHILTON manuals on the subject.

Earl

• Attachment 1 - Ammeters
• Attachment 2 - Ammeters
• Attachment 3 - Alternator Indicator Light