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MG MGB Technical - Alternator/ Drained Battery?

I have a 74 1/2 MGB GT with a slight problem. I cant decide for myself if I have a bad battery or alternator, or both. For the past 5 days or so, I have been driving around like a fool w/ my ignition light gleaming at me. It didnt go off when I rev the engine this time. So today I crank on the car and turn on my lights. Things seem ok till I put on my turn signal and brakes... radio goes off temporarily. I drive around a little more and then I cant get any turn signal when I am applying brakes. Then no turn signal at all. No radio. Luckily though headlights and brakelights stayed on, and I was able to get home safely. After I killed the engine I tried to start again, with no luck.. nothing but a click,click,click. I still have headlights though and fuel pump turns on. Any help is appreciated.
mack taff

Mack, You might have a dead short somewhere in your wiring. We call it "chasing the snake", and can be a real nightmare to find. One way to narrow things down is to first loosen up your positive post connection on your battery. See if you have a real small spark when you tap the cable to the battery. If you do, then connect the cable back up and start removing one fuse at a time until the spark goes away. Once the fuse has been identified your short is in that circuit. Get a wiring schematic and start tracing out wiring until you find your break. It helps in getting to the problem a little faster but will still take some time. If you have no spark at all in the beginning of all this, then pull your alternator and have your voltage regulator bench tested and repaired as necessary. Hope this helps a bit.
Victor Gardino

Mack. It sounds to me like you may have the classic case of--bad alternator. The ignition light coming on will normally indicate either a bad battery or a bad alternator. Most frequently, an alternator which is failing to charge properly. In this case, the car is running on the battery, until the battery charge drops low enough that the car will no longer run. The first thing to do is to remove the cable clamps from the battery and hook the battery up to a charger. Charge until the battery is fully charged. Reconnect the battery terminal clamps and try to start the engine. It should start. Then, have a friend run the engine at about 1,500 rpm and put a DC volt meter across the battery clamps. You should see about 13.5-14.5 volts, if not, you are not charging at the correct voltage and you have a charging system problem. Remove the alternator and take it to Checker, et al where they should test it for free, in hopes of selling you a rebuilt unit. If you are showing a good charging voltage at the clamps, you could have one of two possible problems. First, the battery terminals and/or the inside of the clamps could have crud on them, making for a poor connection. Get one of the metal (not plastic) battery terminal/clamp cleaners and clean everything properly, then re-assemble. The other possible problem is that your alternator is putting out sufficient volts, not insufficient amps to properly run the car and recharge the battery. (It should be capable of doing both at the same time.) However, the ignition warning light most frequently indicates a bad alternator. Victor's information may be of some use, but this technique is most frequently used when you do not have the ignition warning light and the battery (after having been checked and proven good) keeps draining overnight. Les
Les Bengtson

This thread was discussed on 16/09/2001

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