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Old    Chris (maxx_wake)      Join Date: Sep 2003       06-12-2006, 9:45 AM Reply   
I need some help solving a charging problem on my 83 Supra. A little while ago I went to launch the boat and the battery was dead. I wanted to get out on the water so I bought a new battery and swapped them out and boat ran fine but started to crank a little slower towards the end of the day. Next time I cranked it in the drive way before going to the lake and the battery was once again low.

My next step was to take the alternator off and take it to a shop to have it tested. The guy at the shop tested it and said it was bad so I told them to rebuild it.

To make a long story short the alternator shop screwed me around for almost a month and a half before finally getting my alternator back to me last week. Ran it last week with a fully charged battery and it still wasnt charging!

My question is what leads to touch directly on the alternator to test the voltage output of the alternator? I put the voltmeter on the battery terminals and it didnt change from 12.6 wether the boat was running or not. I believe my alternator has an external voltage regulator so I would like to eliminate the alternator as a problem source before I move on to something else.

I will post a picture of the alternator connections when I get home today but it has three smaller wires and on larger wire connected to it.

Also, my boat has what looks to be an in dash amp meter. It reads from -50 to +50 amps. What should this gauge be reading. Right now it pretty much stays parked at 0.
Old    Robb Davis (boarder_x)      Join Date: Mar 2006       06-12-2006, 10:16 AM Reply   
There should be a red and black wire on the alternator. Hook the voltmeter to those wires. I can't remember how high is should be (14.3 I think) but don't quote me on that. However your starter wants 12V just to engage.
Old    Rod McInnis (rodmcinnis)      Join Date: Sep 2002       06-12-2006, 12:03 PM Reply   
The alternator should have pretty much a direct connection to the battery, so if you measure the voltage at the battery and it isn't at least 13.0 volts when the engine is running at a fast idle then the alternator isn't working.

If you have an external voltage regulator (very unusual these days) then the problem could be the regulator. The more common alternator these days has only one connection (well, two if you count the frame of the alternator, which provides ground). External regulator or internal the main power is transfered through a large wire bolted to the alternator.

While it is possible, and even common these days for there to be only one wire to the alternator it is possible that there could be additional wires. Alternators need a little power to generate power. It used to be common that this initial power was supplied to the alternator from the ignitions circuit via the "idiot light", the light on the dash that was typically labled "Alt". If this light burned out then the alternator wouldn't get its initial excitation voltage and might not work.

If the alternator has an external regulator then there will certainly be additional wire(s) on the alternator, and several wires connected to the regulator. The regulator could have failed, or one of the wires into/out of the regulator could have broken.

The small wire of importance on the alternator would be the "field" wire, which would typically be labeled "F". With a typical external regulator I would expect this terminal to be 0 volts when the ignition was off, ~12 volts when the ignition was on but engine not running, and some lower voltage when the engine was running and full output of the alternator was not needed.

If you can identify this connection measure the voltage on it. If you don't see any change between ignition on or off then I would replace the regulator. If you see it go from zero to 12 volts then I would suspect the alternator.

You can force the issue by using a jumper wire to connect 12 volts to the field terminal. With the engine running ~ 1200 RPM you should see the battery voltage step up dramatically (from 12 volts to 13 or so) when you touch 12 volts to the field terminal.

Likewise, if your alternator has an "idiot light" setup that connects directly to the alternator then there should be a small wire that has 0 volts on it with the ignition off and some small voltage on it when the ignition is on.

Old    Chris (maxx_wake)      Join Date: Sep 2003       06-12-2006, 4:45 PM Reply   
Ok here is a picture of the alternator before it was rebuilt.


Robb, can I just touch the post with the thicker red wire on the right side of the picture and the post with the thinner black wire at the top of the picture with my voltmeter leads to see if the alternator is putting out voltage?
Old    Robb Davis (boarder_x)      Join Date: Mar 2006       06-12-2006, 5:06 PM Reply   
Yup. Those are the two you need to test. I'm sure they cleaned up the post on the alternator when they rebuilt it. You should clean the connections on your wires as well. Make sure you have the ability for current to move. ... You'll know if your alternator is putting out good voltage. After you test those two posts, then test + and - of your battery. Make sure the reading is the same. If they aren't, you may have bad connections.
Old    Chris (maxx_wake)      Join Date: Sep 2003       06-13-2006, 7:43 AM Reply   
Ok I think I found the problem. The connector on the bottom post in the picture was so corroded that I had to cut it just to get it off and the new crimp on connector I put on there was loose. I fixed this and it appears to be working now. Thanks for all the responses. I have been reading on this forum for a long time but I just recently got a boat and so far posting on here has helped me learn a lot about how to maintain/work on it.


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