Faulty Aternator Circuit

[Tim]

I recently installed a used Atomic 4 in my Pearson 10M that I am restoring. I got the engine installed and running. I wired the engire per the wiring diagram Don has posted on forum. Today as a quick test of the alternator I disconnected the battery while the engine was running and the engine immediatley shut off. I assumed that once started the engine should run without the battery connected. I have wired the alterantor output directly to the battery connection on the starter since I don't have an ammeter. Would this indicate an alternator/regulator failure or am I missing something?

[Don Moyer]

Tim,

You're probably correct to assume a problem with your alternator.

I'd reconnect the battery, start the engine, and then check the DC circuit for 13.4 volts if you're using an original Motorola alternator, or 14.2 volts if you're using one of the more contemporary alternators. If you read the same voltage (or less) after starting as before starting, the alternator is not producing.

Regards,

Don

[Tim]

Don,

I installed a new alternator and intially had the same problem (alternator had no output). I only got an output from the alternator after increasing RPM to about 2000, at which point every seemed to function normally. However, each time I start the engine I have to increase to 2000 RPM before I get any output from the alternator. Any ideas? Also, what exactly does the exciter connection do? I did not connect it since the output of the alternator already goes to the coil via the ignition switch. Also, trouble shooting the alternator has stopped since my new (less than 3 hours runtime) electric fuel pump decided to let out all its magic smoke and quit working.

Tim

[Buck]

My suggestion is to connect your exciter lead and see if that solves your problem. Based on your description, the alternator is working as would be expected without an exciter connection.

In a typical 3-wire alternator, the exciter lead provides current to the rotor.

As you've discovered, most alternators will work just fine without an exciter connection, provided you first rev your engine up enough to allow the residual magnetism in the rotor to start inducing enough current to become self-sustaining. Once the alternator turns on, it's stays on and will continue working at lower rpms. If you don't like having to rev your engine to start your alternator, make the exciter hookup. If you decide to run your exciter lead to your coil, put a diode in the line with the arrow pointing toward your alternator to prevent current flowing from alternator to coil with the ignition off.

[Don Moyer]

Tim,

I recommend that you connect your exciting lead to the positive terminal of the coil. The exciting lead is connected to a separate internal circuit from the main output of the alternator, and even in the case of self-exciting alternators, connecting the external exciting lead will usually bring the alternator online at a somewhat lower RPM than if you rely strictly on the internal circuitry.

Don