The Snakes Alive CMP Wiring Project

[Jacques Reed]

Some aspects of the following may have been covered before on MLU but it may still be of interest to restorers.

I have noticed most wiring diagrams in Ford CMP Maintenance Manuals and Handbooks do not show the wiring for an ammeter equipped instrument panel. A discussion about wiring with a fellow MLU Member prompted me to investigate it a bit further. The only reference I could find was a 1945 Australian “Drivers Handbook for Ford” which showed an “Ammeter” in the dashboard layout diagram. All other publications show it as a “Battery Indicator” There are also some wartime Ford commercial truck wiring diagrams showing an ammeter and not a battery indicator.

Thanks to Mariano Paz’s 1945 Ford CMP wiring diagram data plate, my original harness, and the link below I have a clearer picture. Also going back to Electricity 101- Ammeters vs Voltmeters helps to make it clearer. 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.

1939-40 Fords, Mercs, and Lincolns, and early war CMP’s used a gauge with “BATT” displayed on the face which is really a voltmeter. Ammeters display “DIS-CHG” on the face. 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.

On indicators with “BATT” displayed the current flows from the starter solenoid first to the voltage regulator battery terminal, then to the ignition switch, and then to one terminal on the Battery Indicator. The other terminal on the indicator goes to earth. This is shown on all the early wiring diagrams and is correct. I must admit I thought in the past that was wrong but I was looking at my ammeter’s wiring and not that of a voltmeter.

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. If an early BATT gauge harness is used on an ammeter equipped truck or an ammeter harness used with a battery indicator equipped truck it could cause serious electrical problems.

Things to keep in mind if repairing, making or buying a wiring harness, or changing gauges.

And yes, the battery indicator and oil pressure gauges are on the wrong side in the photo and “BATT” should be on the bottom. That’s how I bought it.
I have been told the gauges are the same and can be used if you change the faces, but you can only swap a BATT, not an ammeter, with the fuel gauge, and the temp with the oil pressure gauges due to the mounting hole positions.

Hope this is of some help and interest.

The following link may help to explain it all.

https://fifthaveinternetgarage.blogs...gauge-and.html