Expert: Brad Sears - 8/11/2009

Question
QUESTION: Hi Brad
I have a 351 cleveland in a 1972 ford f100 and am having problems.
When the engine is running, the alternator charges at 14 volts but is constantly fluctuating to as low as one volt but only for a second or so at a time. Getting the same readings from the positive at the coil.Am also getting a misfire under gentle acceleration. Have just replaced coil,leads and distributor and have put on another alternator that I had spare. If you have any ideas as to whats going on, please enlighten me as It's doing my head in.
Thanks
Nev

ANSWER: This sounds like there is a circuit breaker opening and closing protecting a circuit with a short. That is about the only way that voltage on the live side will drop to near zero. Either that or a bad connection somewhere between the battery and the live side of the circuit. If the alternator stopped charging the battery voltage should not drop below its stored voltage. The miss fire under gentle acceleration could be connected or a completely different problem. Now, will this condition occur with the key on and the engine not running or does the engine need to be running for it to happen? Let me know.
Brad

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QUESTION: Hi thanks you for the quick response.The engine has to be running for the voltage to jump around with the volt meter on the alternator, or coil. if i have the volt meter on the battery itself the voltage doesn't drop. it does appear to have stopped missing now i think that was a vacuum problem.

ANSWER: Are you using a swing needle voltmeter or a digital readout?
Brad

---------- FOLLOW-UP ----------

QUESTION: Hi
I'm using digital readout. I tried it on my other car and it works fine.

Answer
I prefer a swing needle voltmeter for cases like this. The problem with a digital is that it is a time based sampling meter and will not give a true variable reading. I have an idea that the voltage regulator may be acting up and not controlling the charging voltage or that there is a shorted circuit and a circuit breaker opening and closing.
Brad