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Alright i think that this is my problem. I have a 2000 gt with bullitt intake, with the PA converson harness for the alternator. Everything has worked fine for a long time. Now i am getting the dreaded P0300 MULTIPLE MISFIRE. After checking everything out I am getting 29-30 volts AC from the alternator its charging fine but its getting a constant missfire under load. Anybody got any thoughts on this. How much AC voltage does a bad diode let thru. I tested my new work van and it puts out .5 volts or below so i know what its supposed to be. Does a bad one put out 29-30 volts like I'm reading.
 

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My new alt puts out something like 50VAC right now. I was freaked out by it also but haven't noticed any ill effects. I read that the proper voltage was in the milliamp range. Like I said, haven't had any problems. My lights don't blink, etc.. This is in a 97 Tbird 4.6, though.. (here cuz I am buying a 'Stang SOON and reading everything I can find until then.. )
 

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It's not normal for the AC voltage to be so high. IMO it's likely one or more alternator diodes are shorted.

If this were my car, I would not continue to drive with such high AC ripple voltage.
 

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There should be NO AC on the line.
If you have a question on your results look at the line with an oscilliscope for AC.

If you have a known good alternator and the meter reads AC, try waving the probes around in the air above the engine, you may be seeing secondary ignition.
 
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