Ford GP (series 1) slow build

[Tony Smith]

Originally Posted by Mike Cecil

Took it to the auto elec wizard to be told the field coils are cactus and it will need an expensive rebuild!! Now what could have caused that? The generator was wired into the circuit as per the wiring diagram from Vintage Wiring of Maine, but never worked, so it seems it was cactus from day one. My biggest concern is that I may have done something wrong and caused the problem, but I don't know what, and don't want to repeat the process.

Oh well ...

Mike

If the Generator has been out of a vehicle for some time before your rebuild, it may have lost some of it's magnetism and needed to be "flashed" to restore it's ability to generate a charge. Even if the Generator has retained it's magnetism, it might be a good practice to do this to ensure that the polarity of the Gen is correct when it comes back from the Auto Electrician.

Residual magnetism in the generator Field Coils is necessary for the Generator to build up any voltage during start-up. Full output is not necessary if the battery has sufficient charge to crank the engine. Once the engine is running, voltage flowing through the Genny will reform the full magnetic field.

To restore the small amount of residual magnetism necessary to begin voltage buildup, connect either a 6 or 12 volt battery (not dependant on the voltage of the Generator, we are just using DC to create an electromagnet) to the Field Coils while the generator is at rest, as follows:

If the Generator is still in the vehicle, remove Field wires F+ and F- from the Generator. Sometimes, one of these will be marked "Ground" instead (depending on polarity. In a Jeep, it will be Neg Ground. Some Fords, like CMPs, were Pos Ground).
CAUTION: If still in the vehicle, failure to isolate the Field leads from the regulator during flashing procedures can damage the regulator.

Measure the Field Coil resistance from the F+ to the F- terminal (or "Field" and "Ground"). You should be able to read some resistance as you are measuring a continuous winding, but we are looking for a continuous circuit. An infinite resistance reading would indicate damaged Field Coils, needing replacement. Also check to be sure there is no short to Earth from either terminal to the body of the Generator.

Connect F- (or "Ground") to the Negative terminal of the battery.
Using an insulated cable like a jumper lead or spare battery cable connected to the Battery Positive terminal, hold it to the Field+ (or "Field") post on the Generator for about 5 to 10 seconds, then remove. (NOTE: for Positive Earth vehicles, reverse these connections!). This will reform a small magnetic field in the Field Coils. Small cheat, if the Generator is still in the vehicle, instead of connecting to terminals on the battery, you can use the "Batt" post on the Generator and a good Earth on the engine block to do this, but be familiar with the polarity of the vehicle's electrical system.

Reconnect F+ and F- to the regulator. Start the engine and check if the Generator is producing any output charge.