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The Inductive Amp gauge was probably superceded for reasons other than just the availability of round military gauges.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".

[Jacques Reed]

Tony,

It appears there are two types of Ford ammeters used on the Ford CMPs then.
I took my information from the original harness after tracing out the wires. That round gauge ammeter has two terminals and is wired in series to the voltage regulator from the battery.

Second photo is the back of a NOS Ford Ammeter also with two terminals. The lower left gauge. The face of it is shown in a previous post and has the same face as the one you show.

Added a 1941-42 Truck wiring diagram showing the ammeter in series.

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wiring diagram 41-42 truck V8 only.jpg

[Lynn Eades]

I'm not sure of the significance, but in Australian carriers, If they were fitted with the battery gauge (volt meter), then they usually had a three brush generator. If they had an amp meter, they were usually fitted with the two brush generator. Hopefully I have this the right way around, without looking in a book.
Thanks on the Auction heads up Tony.

[Hanno Spoelstra]

Originally Posted by Jacques Reed

It appears there are two types of Ford ammeters used on the Ford CMPs then.
I took my information from the original harness after tracing out the wires. That round gauge ammeter has two terminals and is wired in series to the voltage regulator from the battery.

Second photo is the back of a NOS Ford Ammeter also with two terminals. The lower left gauge. The face of it is shown in a previous post and has the same face as the one you show.

Added a 1941-42 Truck wiring diagram showing the ammeter in series.

Here’s a photo of my spare instrument cluster. It has an Ampere meter but it is wired separately from the oil pressure meter - there is no bridge.

Now I need to study the various wiring diagrams to see if it can be wired in in my 1943 F15A which has the BATT meter (last photo).

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[Bob Carriere]

Hanno... Jacques...bonjour!!

.....even lowly Chevrolet have a Amp meter.....but really does not tell you much except your system is charging....or an even keel or running on the battery and discharging.......

Neat idea with the solid brass bridge as it must eliminate a whole bunch of wires.

On the Chev, along with the builtin bypass, all my power wiring has to be first connected to the Amp meter which makes for a very cumbersome nest of wires on one bolt connection....... which forever needs tightening!!!....my dash lights are not working now...guess why???

A volt meter, or engine oil temperature gauge, and vacuum gauge would be much more useful......

[Jacques Reed]

Hi Hanno,

Just had a look again at the back of the Ford Cluster. I think the brass bridge between the ammeter and the Oil pressure gauge should not be there. That is used with a battery indicator type gauge to supply power to all the gauges when the ignition switch is thrown.

Will have a look at the wiring again later today and make up a revised Ammeter equipped wiring diagram at the instrument panel using Mariano Paz's great information on the late war wiring plate.

Cheers,

[Jacques Reed]

Hi Hanno,

Here is a revised wiring diagram showing the differences in red between a Battery indicator gauge equipped instrument cluster and an ammeter equipped one.

Not a lot required to change over to an ammeter from what I can see. Mainly the routing of the supply from the solenoid to the Battery terminal on the regulator.

Original Regulator Battery terminal to Ignition switch wire can stay even though Mariano's diagram takes current from the Ammeter to the Ignition switch. Same thing electrically however.

Only one Brass bus strip used, too, from Fuel to Temp gauges. If ammeter to Oil pressure gauges had a bus strip all gauges would be "Hot" all the time. They should be only be energised when Ignition switch is thrown.

Thanks again to Mariano Paz's excellent late war wiring plate photos. Makes it so easy to trace wires compared to the rudimentary Ford Manual schematics.

Fairly happy with it all but always glad for "peer review" as they say. It can be easy to miss things.

Cheers,

CMP wiring 1943 schematic.jpg

[Hanno Spoelstra]

Originally Posted by Jacques Reed

Just had a look again at the back of the Ford Cluster. I think the brass bridge between the ammeter and the Oil pressure gauge should not be there. That is used with a battery indicator type gauge to supply power to all the gauges when the ignition switch is thrown.

Thanks Jacques. I've always thought that my BATT meter did not do much, I now found it has a live feed (bridged to the oil pressure gauge), but no wire to ground - see photo. This makes me wonder why it was moving at all? Will add a ground wire and see how it behaves.

IMG_7907_redlined.jpg

PS: the white terminal block will be removed. I noted this loose yellow & red wire dangling under the dash and it turned out to be live. Using the diagrams on here I found this is the feed to the inspection lamp socket. This socket was removed in the past to make way for a traffic indicator switch.

Will have a look at the wiring again later today and make up a revised Ammeter equipped wiring diagram at the instrument panel using Mariano Paz's great information on the late war wiring plate.

Will look at that in detail later. I am trying to get a better understanding of my F15A's wiring, making it work better and meanwhile draft the specs for the new loom I want to order.

I may switch from the BATT to AMP meter, though I plan to use an electronic regulator. I've just replaced the indicator/ flasher relay by an electronic one, and it is a huge improvement because it uses far less power and it is independent of load. I am becoming a fan of modern electronics. I recently fitted an electronic distributor, quite an improvement so I plan to add more in an effort to improve drivability and reliability.

[Jacques Reed]

Hi Hanno,

Just looking at your picture there should be a wire, 30A, a jump from the oil pressure gauge brass bus to the Temperature gauge brass bus to supply current to the Battery and Oil Pressure gauges when the ignition switch is thrown.
There should be wire, 30, from the ignition switch to the Temperature gauge brass bus to supply current to the Temperature and Fuel gauges and then to the other gauges via 30A.

The wire into the Battery indicator is my concern. If it shows voltage only when the ignition switch is thrown then it is wire 30 and can be connected to the brass bus at any gauge to supply them with current. Diagram shows the Temp gauge but any brass bus is the same electrically.

If for some reason that wire into the Battery indicator shows voltage continuously it could be coming directly from the Battery via the regulator and therefore your wiring is setup for an ammeter. If that is the case it could cause a serious problem to the wiring and gauges if the Battery indicator is sent to earth. A quick check will determine if that wire is "hot" only when the ignition switch is on and therefore set up for a Battery indicator.

I have found with numerous owners over the years and many jury rigged wires on these vehicles it is best to check and double check. My first truck had many disconnected wires and even house wiring used in some places!

Cheers