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MG MGB Technical - voltage spikes on bosche alt.

I have a 78 b with a bosche alternator. i also have a voltage gauge installed in place of the clock in the dash. my voltage gets very erratic. jumping from 8 to 15 volts and settles to about 13. now (began to do this today) i get run on when i shut off the key. the battery has been getting charged but now i am beginning to wonder if the alt is going out.


thanks
andy heston

Sir;
Could be the alt but My gurss is a loose connection in the car. Check out all brown wires especially in the area of the starter. You will take out the alternator if this is allowed to go on. Bob Thompson/International Auto
Bob Thompson

Andy, I agree with Bob. Take the wires off the starter, clean them up, then put them back on properly and see if that corrects the problem. I had a similar issue with a 76B and it almost fried everything before I figured out what the cause was.

Robert
Robert Rushing

If the above fixes don't do the job, then it may pay you to take the alternator to a good Auto Electrical shop and have it tested. Many of the rebuilts used to convert to the Bosch Alternator have been "rebuilt" by less than worthy rebuilders, and are a problem in themselves. I have had consistent luck with Bosch alternators rebuilt by Autolite, and I am sure there are others who do a good job.

If you are not happy with that Bosch unit, you can replace it with a Lucas 18ACR unit with no modifications, or you might want to try your luck with that great old piece of 'merican Iron, the AC Delco unit, http://www.mgbexperience.com/service/delco.html , tho it will mean creating your own mounting brackets and rewiring. Good luck!
Bob Muenchausen

i took the alt. to the shop and they said it was fine but that tghe voltage output was a little low (between 12.5V and 13v). so i asked them to put a new regulator and they were apprehensive saying that they did not think it was going to work, but they did it anyway and low and behold 14v output. now it is time to check all those connections.

a side not: i remmebered today that during the last engine pull to replace a clutch i did not put the transmission to transmission crossmember ground starp on as i put a new on on the engine to the chasis. the alternator conversion was done subsequent to that so i don't know if it is contributing to this.... any thoughts?
andy heston

Andy,
if all you did was exchange a tranny crossmember ground strap for an engine ground strap, this should not have led to the problem you had.

Many of the replacement voltage regulators sold individually or used for rebuilding, are made in China these days and tho the Chinese are quite capable making very good stuff when working for foreign companies, they don't always do so well for themselves. It is worth the extra bucks for a "genuine" Bosch-made voltage regulator.
Bob Muenchausen

Switching between 8 and 15 is almost certainly dodgy connections, if the alt was failing it would only drop to battery voltage i.e. about 12v. It should be showing about 14-14.5v when running and I see that is what your alt now gives. If the lights aren't dimming and brightening at the same time then the problem is not at the starter but possibly in the brown closer to the ignition switch, the white or green, depending on where you have your voltmeter connected.
Paul Hunt

This thread was discussed between 06/08/2002 and 07/08/2002

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