Has my alternator gone bad?

[round_midnight]

I was just driving my Cobra and noticed that the electrical system voltage shown on the voltmeter was varying substantially with engine rpm. At idle, the voltage was 12-14 V, and at 3000 rpm the voltage was 18 V. Is that an indication that the voltage regulator in the alternator has failed? It is a single wire type alternator. Is it something that can be repaired or do I have to replace the entire alternator? Thanks.

[Tim Brewer]

It sounds like there's nothing wrong at all, that's usually normal. Start your car and just check the voltage comming directly out of the alternator, or just look at your headlights to see if they're dimming. Alternators don't have an exact constant voltage at all times.

[strictlypersonl]

The internal voltage regulator in the alternator is probably bad. You should NEVER see more than about 15.5V. Also check that the alternator is properly grounded.
You are cooking the battery to death.

[csx wnab]

Originally Posted by strictlypersonl

The internal voltage regulator in the alternator is probably bad. You should NEVER see more than about 15.5V. Also check that the alternator is properly grounded.
You are cooking the battery to death.

i have to second that

[Tim Brewer]

But first, check the voltage with a meter straight from the alternator, don't trust your gauge. Then if that doesn't check well, swap out your alternator.

[csx wnab]

Originally Posted by Tim Brewer

But first, check the voltage with a meter straight from the alternator, don't trust your gauge. Then if that doesn't check well, swap out your alternator.

i second that too.

[undy]

If you have a specialty auto alternator/starter/electrical repair shop nearby they can repair or rebuild your old one too. That should save you some $$ over a new one.

[Ron61]

I agree with the above posts. First check your voltage with a volt/ohm meter by placing the leads across the battery terminals and then comparing them to what your gage is reading. My top reading used to be about 15 volts.

Ron

[RICK LAKE]

around_midnight You didn't say if the system in the car is 12.0 volts or 16.0 volts? Are you running a 16.0 volt battery? A 12 volt system with a fully charged battery should show a charging rate of 13.7-14.3 volts at 2,000 rpms and a steady reading not bouncing. You are way out of the gate. You need to take the battery and Alt for a test. The battery could be internal shorted causing the high voltage output from the ALT. A load test will confirm this. A good 12.0 volt battery should be in the 12.3 to 12.7 voltage with a volt meter. Are you sure that the cables from the battery are tight to the soleniod, starter, and ground mounts?? Also make sure the the battery terminals and cables are CLEAN and tight at the posts. Do you have a side mount battery?? They can have problems withthe nuts that are bonded into the lead when they are built and cause crazy problems. The next thing to check, The belt and pulleys for slipping on the pulleys and glazing of the belt. Make sure it's tight but not or tight to cause a wearing out of the water pump bearing. Check for a coolant leak from the bottom of the water pump. This leak will cause the charging system to jump all over the place and yet when you look under the hood may see nothing. Is the car running undersize pulleys, like March??? They have caused some charging problems with 1 wire ALTs. If everything checks out OK, take the batery and ALT to 2 different places and have it check. Sears, Pepboys, Autozone, and R&S all have free checks but have testers are NOT CALIBRATED and try to sell you things you don't need like Batterys, Starters, Cables, and the Biggie ALTS. They are all paid on commisions and upsales. Just be careful. Rick L. Double check the ground and power wires for tight connection first.

[round_midnight]

Kind of a puzzling end to this story. I ordered a new alternator from Summit. I got ready to install it and decided to do a couple of the things suggested. I wiggled the battery cables and found them to be tight. I hooked up a voltmeter to the alternator output and started the car. Voltage was rock solid at 14.5 V! Have started it several times since, and no further problems. So, I guess I just need to keep an eye on it to see if the problem returns, and see if Summit will take the new alternator back. Thanks to all posters for your help and advice.

[undy]

Originally Posted by round_midnight

Kind of a puzzling end to this story. I ordered a new alternator from Summit. I got ready to install it and decided to do a couple of the things suggested. I wiggled the battery cables and found them to be tight. I hooked up a voltmeter to the alternator output and started the car. Voltage was rock solid at 14.5 V! Have started it several times since, and no further problems. So, I guess I just need to keep an eye on it to see if the problem returns, and see if Summit will take the new alternator back. Thanks to all posters for your help and advice.

It sounds like an internal regulator problem. Chances are that it'll return. I'd swap out the alternators and never look back.

Dave

[RICK LAKE]

round_midnight Like Undy said I would keep the new Alt in the car and the tools to change it on the side of the road. We will see when you take the car for a long drive and see how it holds up. I am guessing that your forum name will come true with a breakdown around midnight on the side of the road. Side note have a good flashlight and jumper cables too. Once something is going bad it doesn't not fix itself. Rick L.

[patrickt]

Originally Posted by undy

Chances are that it'll return.

The chances are 100% it'll return. I have yet to have a problem on the Cobra magically disappear and never return. It just doesn't happen. I've had them go away for a while, but they always return... and generally at the least opportune time.