alternator not charging

[DaveA]

Last week the alternator in my '99 Response LX went out. I replaced the stock 51 amp alt. with a marine Prestolite rated at 90 amps. I'm also using a Hellroaring battery isolator with dual batteries. The new alternator is self exciting; however, it's my understanding this won't work due to the battery isolator. The alternator came with a wire labeled ignition. I connected this to the purple excitation wire and connected the orange positive and black negative wires to the alternator. Initially, everything seemed fine, the dash guage read ~ 14 V with the engine running. Part way through the next outing the alternator stopped charging. I get 0 volts between the positive and negative posts at the alternator with the engine running. There is also no voltage at the purple excitation wire, even with the key on. I thought this should have 12 v with the ignition on, is this correct?

[68Slalom]

I would pull it out and have it bench tested then maybe if it's no good return or exchange it

Edited June 5, 2009 by 68Slalom

[DaveA]

The problem was a faulty ground connection. Troubleshooting went as follows, hopefully this will help someone else. Hook up flush pro, start engine, measure output at (+) & (-) on alternator -> 0 volts. Measure output at (-) on alternator & excitation wire -> 0 volts. Reset circuit breakers, search for blown fuse - no joy. If alternator is bad that still doesn't explain 0 volts on excitation wire with ignition on. Check connections, seem to be ok. Hook up ohm meter, mesure resistance between (-) connector on alternator & motor mount - open circuit! Come to think of it, I did have to replace that connector since the new alt. had larger posts. Removed connector, ensured continuity between bare wire & motor mount. Replaced connector. 12 V between (-) on alternator & excitation wire with ignition on. Start engine, 14.4 V output from (+) & (-) on alternator. Charge battery, line up 3rds for the weekend .

[68Slalom]

The problem was a faulty ground connection. Troubleshooting went as follows, hopefully this will help someone else. Hook up flush pro, start engine, measure output at (+) & (-) on alternator -> 0 volts. Measure output at (-) on alternator & excitation wire -> 0 volts. Reset circuit breakers, search for blown fuse - no joy. If alternator is bad that still doesn't explain 0 volts on excitation wire with ignition on. Check connections, seem to be ok. Hook up ohm meter, mesure resistance between (-) connector on alternator & motor mount - open circuit! Come to think of it, I did have to replace that connector since the new alt. had larger posts. Removed connector, ensured continuity between bare wire & motor mount. Replaced connector. 12 V between (-) on alternator & excitation wire with ignition on. Start engine, 14.4 V output from (+) & (-) on alternator. Charge battery, line up 3rds for the weekend .

I'm not sure if I understand, isn't the alt. already grounded when it's bolted up to the bracket that bolts to the block? I know you need a ground but the block should have a main ground directly to the battery, I could be wrong. Maybe marine stuff has paint/coating ect. that doesn't allow that as a factor

Good thing you figured it out

[DaveA]

Marine alternators differ from car alternators, which typically have just a (+) charging lead. Marine alternators are treated to resist corrosion and have an enclosed brush assembly to shield any arcing, which could cause accumulated gas vapors in the bilge to ignite. They also have separate (+) and (-) connections, and usually a a connection for an excitation wire. This needs 12 V when the ignition is on. Many alternators are self exciting; however, if a battery isolator is used the separate excitation wire is needed.