User manual

read a higher than normal voltage until the electrolyte reaches equilibrium. The same is

true for a battery, which has been discharging at a heavy rate, when your remove the

heavy load, the voltage will be “lower” than normal and return to a value based on its

state of charge after a period of time.

Don’t jump to conclusions with limited information.

The warning light usually indicates that the voltage at the battery is too low and the

battery is not charging. The voltage needs to be least 13 volts to charge the battery. It

can be low for a number of reasons:

1) Alternator is defective

2) Slipping belt

3) Alternator wiring problem, regulator connection and warning light circuit.

4) The special “starting circuit” for the alternator has failed. (Isolator equipped

vehicle.)

5) The connection between the alternator output and the isolator is open.

6) Bad isolator; an open internal diode between the vehicle battery terminal and

the alternator terminal or an open diode between the “E” terminal and the

alternator output terminal. (Note that this diode connection is such that current

can flow from the E terminal to the alternator terminal, but not in reverse.)

7) The connection between the isolator battery terminal and the battery is open.

8) Alternator is “current limited” due to excessive loads or because it’s damaged.

If the house batteries are discharged, all of the loads may be more than the

alternator can deliver.

If the dash battery warning light comes on, I would first measure the voltage at the vehicle

battery with the engine running to verify that the voltage is in fact low. If it is low, I would

then shut off the engine and then attach a meter wire lead to the alternator output. In an

“isolator equipped” vehicle, this is connected to “A” terminal on the isolator. There is an

annotated picture of an isolator on page 2 of this document. If you have a battery

separator, then the alternator output connects directly to the battery just like a normal

vehicle. There may be an open fusible link in the wiring harness between the alternator

output and the battery.

Connect the other meter wire lead to a bare metal part on the frame. This can be a

challenge; you can probably find a wire attached to the chassis or connect it to the

negative terminal of the battery. Use alligator clips so you don’t have to hold them on.

Crawling under a running vehicle to make a measurement is dangerous and not

necessary. Start the engine and make the measurement. This alternator output and the

battery should be connected by a diode in the isolator and if the alternator is working will

be about 1 volt greater than the battery voltage when the engine is running. So if the

battery is reading about 13.5 volts I would expect the alternator to read around 14.5 volts.

Again the actual voltage can vary, but the difference should be about 1 volt. If the diode