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MG MGB Technical - Alternator Help

I have a 1977 MGB 1798 cc engine. The owner before me used red for ground and black for hot on the battery. Not paying attention I needed a jump and hooked up cables wrong. Well that was a no no. I think my alternator is gone. I rewired jumpper cables ad jumped the car off. It run but 9 volts when running on test. Won't hold a charge. How do I go about testing my alternator. Any help is appericated.

Thanks
Tommy
Tommy Ducote

A lot of chain auto parts stores test alternators for free.
Look in your Bentley under section N.
If you don't have a shop manual, try http://octarine.mywowbb.com/forum28/70.html
Section N.25 or N.26
Kimberly

Connecting the battery backwards has probably blown the diodes, hence the low output voltage. Personally I'd replace it wholesale and not attempt to fix just the diodes, other components may have been stressed and cause problems later.
Paul Hunt 2

I agree with Paul about replacing the alternator if the diodes are blown. In the U.S., higher ouput alternators with lifetime warranties are available for about $60- and require very little if any modification.
The Bosch alternator for a '80 Ford Fiesta may require you to change one spade connector to hook it up. Less work than soldering a diode pack. With the Bosch alternator, you may need to purchase an alternator pulley and a different sized belt.

Here's link for installing a Saturn alternator.
It's more work than the Bosch alternator.
http://www.onefastmgb.com/alternatorupgrade.html
Kimberly

Do you guys think I burned up my voltage regulator also?

Thanks
Tommy
Tommy Ducote

Tommy. The voltage regulator is internal to the alternator. Replace the alternator and you have replaced the voltage regulator also.

The fact that your system is only showing 9V indicates the battery is shorted out internally. It should be showing about 12.5V when charged. If the alternator is bad, and the engine is running on the battery, the best indication is that the system voltage does not go up above the battery voltage when the engine is running. Consider one of the gel type batteries as a replacement. Never have to fill it with water and they seem to hold up better than the lead-acid types.

It is not unusual to find that, over the years, cables have been switched out and may not demonstrate the proper color codes. You always need to check everything throughly when learning about a new car. Too many people have done too many things to them over the years to be fully trustful about anything. As you have found, not doing so can be an expensive lesson.

Les
Les Bengtson

My body shop guy did exactly the same thing to mine last year. I replaced the diode pack only and have had no problems in the ensuing months.
Dan

This thread was discussed between 22/07/2007 and 24/07/2007

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