As the title says, i've spent some considerable amount of time chasing what I thought to be a parasitic draw. After measuring with a multimeter and seeing in the neighborhood of 30mA draw key-off with a known good battery (newish too), it has to be the alternator. It puts out good voltage with the car running, but I'm told a bad diode internally can cause power to flow out of the battery while the key is off, as well as reduced output from the alternator.
If it's the diode, would it show as a "draw"? If not, can an in-car test reveal if the alt. is in fact at fault? It's getting freakin cold here and I'm tired of disconnecting my battery every time I park my car and having to go put the negative cable back on every morning...
92 240SX: would a bad alt. diode show as a parasitic drain?
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Q45tech
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Re: 92 240SX: would a bad alt. diode show as a parasitic drain? (Hype)
A decent sized quality battery will have >50 ampere hours [until voltage drops to 10.5 v] of capacity [Personally I use an 80 amp/hr unit].
30 ma x 24 hours is not even an ampere hour per day so a fully charged battery [less than 3 year old] should last 50 days.
Precision alternator testers will show the faults.
excessive reverse flow thru alternator is somewhat common as they get old.
Warning most after market rebuilds may not have totally new electronics.Only Nissan factory units are really like brand new.
30 ma x 24 hours is not even an ampere hour per day so a fully charged battery [less than 3 year old] should last 50 days.
Precision alternator testers will show the faults.
excessive reverse flow thru alternator is somewhat common as they get old.
Warning most after market rebuilds may not have totally new electronics.Only Nissan factory units are really like brand new.
Re: 92 240SX: would a bad alt. diode show as a parasitic drain? (Q45tech)
Thanks for your reply.
I'm aware that the ~30mA I see with my multimeter is normal, which is why I believe it to be the alternator at fault while the key is off. My question is, will my multimeter be able to tell me that the alternator is drawing current? There's no other circuits it could be, and with the neg. terminal off the battery overnight, the battery is just fine when I reconnect it in the morning.
Hopefully I'll find the time to make it to an autoparts place and have the alt. checked by them. I hope they can determine if the diode is bad from an in car test.
I'm aware that the ~30mA I see with my multimeter is normal, which is why I believe it to be the alternator at fault while the key is off. My question is, will my multimeter be able to tell me that the alternator is drawing current? There's no other circuits it could be, and with the neg. terminal off the battery overnight, the battery is just fine when I reconnect it in the morning.
Hopefully I'll find the time to make it to an autoparts place and have the alt. checked by them. I hope they can determine if the diode is bad from an in car test.
Re: 92 240SX: would a bad alt. diode show as a parasitic drain? (Hype)
woohoo replying to my own post, go me......
I had it checked at two different auto stores today and they both ran the pretty much the same test. Start the car, hold it at 2000rpm or so, turn on some accessories... all the while they have their tool measuring. It had a small thing telling me both times "diodes good", as well as the charging system working well (which i knew!). Maybe I need to have it checked out on a more accurate bench test?
I had it checked at two different auto stores today and they both ran the pretty much the same test. Start the car, hold it at 2000rpm or so, turn on some accessories... all the while they have their tool measuring. It had a small thing telling me both times "diodes good", as well as the charging system working well (which i knew!). Maybe I need to have it checked out on a more accurate bench test?
Re: 92 240SX: would a bad alt. diode show as a parasitic drain? (Hype)
Diodes are there to change from AC to DC--try pulling 1 fuse at a time till the draw is identified.