The Snakes Alive CMP Wiring Project

[Tony Smith]

Originally Posted by Jacques Reed

Ammeters- measure current flow in a circuit and placed in series. Voltmeters- measure electrical potential difference between two points in a circuit and placed in parallel.

Later in the war, the Ford commercial instruments, used in the CMP’s, were changed from battery indicators to Ammeters. This was prior to the military round gauge instrument panel being adopted. As of the May 1943 F15-A Spare Parts List. however, only “Battery Indicators” are listed, and not ammeters.

When the Ford commercial ammeters replaced the battery indicators on CMP’s the wiring could be the same for both the Ford gauges and the military round gauges. The current flows from the starter solenoid to the ammeter first, and then to the voltage regulator battery terminal.

Hope this is of some help and interest.

Don't know why I didn't pick up on this at the time.

The Ford Commercial Amp Gauge IS NOT wired in series. The gauge uses an inductive coil to measure the current flow (and direction). Look at the back of the gauge. There are no terminals, but there is a steel "bridge" device. The wire from the "Batt" terminal on the voltage regulator (itself in turn connected to the neg terminal on the battery) to the Starter solenoid is an 8ga wire and PASSES THROUGH this bridge. The current flowing through the wire creates an electromagnetic field, the strength of which is read in the inductive coil, and deflects the needle on the gauge.

The later round "Military" Amp gauge does have terminals on the back of the gauge and IS wired in series, and the wire from the Voltage regulator is in two lengths with terminal ends that connect to each terminal on the back of the gauge.

So the wiring harnesses for "Ammeter+Commercial instruments" and "Ammeter+Military Instruments" are different. If you have the early "Inductive" style harness, it is a simple matter of cutting the VR to Starter wire and adding terminal ends to suit the Mil gauge. If you have the later "Series" harness, you need to replace the VR to Starter wire with a single length of 8ga wire.

The Inductive Amp gauge was probably superceded for reasons other than just the availability of round military gauges. The Inductive coil gives a good "Indication" of charge/discharge, but is not terribly accurate for the actual current amperage value compared to a gauge being wired in series. In addition, the unsecured wire passing through the bridge is prone to chafing and shorting out, particularly on a military vehicle which might be expected to see some rough roads and a lot of shaking. Worst case is a vehicle fire, best case scenario is a shorting out of the electrical system and a "Fail to Proceed".