Testing an Alternator
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Mountain Pass
Joined: Feb 2003
Posts: 147
Likes: 3
From: UP of Michigan
Testing an Alternator
My '88 F150 plow truck is not charging batteries. In fact, I have to jump it or put the battery on a charger every time I need to start it now. A real pain to say the least.
How do I check if the alt. is bad, or are there any other suggestions for things to check? The battery has been replaced already with the one that was working fine in my '94 (marine battery) when I got it a couple months ago.
Thanks!
How do I check if the alt. is bad, or are there any other suggestions for things to check? The battery has been replaced already with the one that was working fine in my '94 (marine battery) when I got it a couple months ago.
Thanks!
Junior User
Joined: Jun 2006
Posts: 88
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From: Victoria B.C
Originally Posted by flareside_thunder
I got one easier...start er up and pull the Positive lead off the battery post....if she shuts off your alt is bad.......Luck
JR
JR
These alternators are internally regulated.
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Mountain Pass
Joined: Feb 2003
Posts: 147
Likes: 3
From: UP of Michigan
This truck is not roadworthy, nor do we have any auto-parts chains around here. I'd have to take it to a garage to have it tested, so I was hoping to test it myself.
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Lead Driver
Joined: Aug 2006
Posts: 5,670
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From: Sand Lake, MI
I've done that for years also, while it is not a sure fire test when removing a cable while its running it will show a alternator that has no output at all. If it stalls with the battery disconnected the alternator or regulator is not putting out any juice and the truck/car is running strictly from the juice the battery provides.
The truck/car could still remain running and still have a bad alternator, putting out just enough juice to keep running.
And no I've never seen it damage a alternator or regulator doing so, not that it couldn't happen, just that in the hundreds of times I've done it over the years I've never had a problem.
I replace them as a unit, alternator/regulator both at the same time. If one's dead the other is close to it anyway.
The truck/car could still remain running and still have a bad alternator, putting out just enough juice to keep running.
And no I've never seen it damage a alternator or regulator doing so, not that it couldn't happen, just that in the hundreds of times I've done it over the years I've never had a problem.
I replace them as a unit, alternator/regulator both at the same time. If one's dead the other is close to it anyway.
Postmaster
Joined: Jun 2007
Posts: 2,621
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From: Sweetwater Texas
Originally Posted by Bandit-1
You should never run the vehicle with the battery disconected as the voltage regulater sense a very low voltage, then it puts out bascially full output with nowhere for that much power to go to.
These alternators are internally regulated.
These alternators are internally regulated.
I drove mine with no battery for 30 miles once with no side ffects.
"Mifflin Clay"
Joined: Jul 2005
Posts: 3,177
Likes: 4
From: Mifflin, PA
Originally Posted by Dean88
What can this hurt again?
I drove mine with no battery for 30 miles once with no side ffects.
I drove mine with no battery for 30 miles once with no side ffects.
I use an ammeter that fits over the battery cable or alternator output wire to indicate a charging current. If it's present - say 10-30 amps or so, the alternator is working.
Looks like this:
Last edited by alchymist; Dec 1, 2007 at 11:35 AM.
Junior User
Joined: Sep 2007
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Just get yourself digital volt ohm meter from anywhere( radio shack, parts store, Sears), start the truck put the meter to volts connect to battery posts. If the voltage is above 13.5 volts Alternator is working, next turn everything electrical, including the cig. lighter and see what the volt reading is, if still around 13.5v alternator is fine. You either have a battery being pulled down (draw problem) or the battery is bad. You mentioned a marine battery, they are not built to quick loads and rapid recharging. Your old battery could have been bad and now that you have a marine battery(deep cell) and now is bad. Let us know what you find.
P.S. a cheap meter is only about $15 at parts store.
P.S. a cheap meter is only about $15 at parts store.
Thread Starter
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Mountain Pass
Joined: Feb 2003
Posts: 147
Likes: 3
From: UP of Michigan
I do have a digital multi-meter, but I'm pretty ignorant with almost everything electrical. Thanks for the info. on how to test it...I'll check it out ASAP. It's been snowing for most of the week here.
My '88 truck came with a marine battery when I bought it several years ago and last year I started plowing with it. That battery was very low or dead on several occasions this fall and was not taking a charge from a charger either.
SO-this '94 150 I just bought a couple months ago had an odd-ball sized marine battery in it (previous owner plowed a little with it last year), which I wanted to replace with a proper battery anyways. Apparently, marine batteries are common around these parts for plow trucks.(?) Prior to relocating it to the '88 the marine battery in my '94 worked just fine. Since it's been in the '88, it is also not charging while I run the truck to plow. I hope that makes sense.
Thanks all for the help!
My '88 truck came with a marine battery when I bought it several years ago and last year I started plowing with it. That battery was very low or dead on several occasions this fall and was not taking a charge from a charger either.
SO-this '94 150 I just bought a couple months ago had an odd-ball sized marine battery in it (previous owner plowed a little with it last year), which I wanted to replace with a proper battery anyways. Apparently, marine batteries are common around these parts for plow trucks.(?) Prior to relocating it to the '88 the marine battery in my '94 worked just fine. Since it's been in the '88, it is also not charging while I run the truck to plow. I hope that makes sense.
Thanks all for the help!
Freshman User
Joined: Oct 2007
Posts: 39
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just because its got the voltage doesnt mean its working right. i replaced mine just about a month ago it was showing 14.13 volts but when i had it tested under a load it wasnt doing its job. replaced and no more slow starts.