Alternator Diodes?
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Member
Joined: Feb 2009
Posts: 31
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Alternator Diodes?
This problem isn't with my truck, but its "cousin".
We inherited a pristine 1991 Grand Marquis from an aunt about 6 months ago. It sat a few years before I rescued it from under a bunch of pine trees.
Anyways, it has a 5 liter engine and the past few days as the car sits it drains the battery.
I know it is not the battery itself. It has the same battery as my truck, and I swapped fully charged batteries between them a few times.
I undid the negative battery terminal and put a test light between the negative terminal and negative post. One of those testers with a probe and alligator clip that has a small bulb in it.
It faintly glows, so I pulled all the fuses and relays, to no avail. I also checked all switches and accessories.
Finally, I unplugged the alternator and the light went out. The alternator has two plugs in it, one going to a small black plastic box on the side of it. Is this an external voltage regulator?
If I leave the alternator unplugged at night, the battery does not drain.
Now, I asked a few auto parts stores (not AutoZone) if when they bench tested the alternator if it would show if a diode was bad. I got yes-it should, no, and maybe as an answer. One place even said the only way to tell for sure was to hook the car up to some kind of diagnostic scope, for $100.
I put a multimeter on the car and it is charging fine, 13.8 volts I think.
Could bad diodes be the reason the battery is being drained, or am I just breaking the circuit of the cause when I unplug the alternator?
Can a bench test tell if a diode is bad? If no, any other way a novice could a test, with just a multimeter?
Sorry 'bout the long post.
Thanks in advance, guys.
We inherited a pristine 1991 Grand Marquis from an aunt about 6 months ago. It sat a few years before I rescued it from under a bunch of pine trees.
Anyways, it has a 5 liter engine and the past few days as the car sits it drains the battery.
I know it is not the battery itself. It has the same battery as my truck, and I swapped fully charged batteries between them a few times.
I undid the negative battery terminal and put a test light between the negative terminal and negative post. One of those testers with a probe and alligator clip that has a small bulb in it.
It faintly glows, so I pulled all the fuses and relays, to no avail. I also checked all switches and accessories.
Finally, I unplugged the alternator and the light went out. The alternator has two plugs in it, one going to a small black plastic box on the side of it. Is this an external voltage regulator?
If I leave the alternator unplugged at night, the battery does not drain.
Now, I asked a few auto parts stores (not AutoZone) if when they bench tested the alternator if it would show if a diode was bad. I got yes-it should, no, and maybe as an answer. One place even said the only way to tell for sure was to hook the car up to some kind of diagnostic scope, for $100.
I put a multimeter on the car and it is charging fine, 13.8 volts I think.
Could bad diodes be the reason the battery is being drained, or am I just breaking the circuit of the cause when I unplug the alternator?
Can a bench test tell if a diode is bad? If no, any other way a novice could a test, with just a multimeter?
Sorry 'bout the long post.
Thanks in advance, guys.
Senior Member
Joined: Oct 2009
Posts: 1,526
Likes: 4
I have never figured out how to test the internal regulators....Fords are usually hit n miss...one minutes they could be working just fine and another they could be totally shot. have the alternator rebuilt or buy a used one from a boneyard....