Testing alternator

[beerhunter]

Hiya,

Sorry, me again... Gradually working through the list of issues with my 86 2.9.

It has a parasitic drain that means I have to disconnect the battery overnight, which is tiresome. I tried using my (cheap) multimeter to measure the amp draw with key off, to work out which circuit is to blame. However, it doesn't register any draw at all, so is either broken or the ammeter function never worked to begin with

I have a new (better) one on order, but reading suggests a bad alternator diode is the most likely cause, so wanted to test mine with the ohm function (that does work). However, my alternator doesn't have a positive post to use. It has a 3-wire plug at the top, and a 2-wire plug at the bottom.

I disconnected the top plug and tried the positive probe on all 3, with the negative probe on the casing. I got no reading at all. How do I perform the ohm test on my alternator without a positive post, in order to check if there's a bad diode?

Many thanks

N.b. I swapped batteries to rule that out, so I am pretty sure there's a drain somewhere. And the alternator charges when the engine is running.

 

[Lefty]

Hiya,

Sorry, me again... Gradually working through the list of issues with my 86 2.9.

It has a parasitic drain that means I have to disconnect the battery overnight, which is tiresome. I tried using my (cheap) multimeter to measure the amp draw with key off, to work out which circuit is to blame. However, it doesn't register any draw at all, so is either broken or the ammeter function never worked to begin with

I have a new (better) one on order, but reading suggests a bad alternator diode is the most likely cause, so wanted to test mine with the ohm function (that does work). However, my alternator doesn't have a positive post to use. It has a 3-wire plug at the top, and a 2-wire plug at the bottom.

I disconnected the top plug and tried the positive probe on all 3, with the negative probe on the casing. I got no reading at all. How do I perform the ohm test on my alternator without a positive post, in order to check if there's a bad diode?

Many thanks

N.b. I swapped batteries to rule that out, so I am pretty sure there's a drain somewhere. And the alternator charges when the engine is running.

There is a simple trick to test the alternator. It should throw off a good deal of magnetic force while it is running. Hold the blade of a screwdriver against the backside. You should feel a very strong pull that requires a little muscle to work it loose.

 

[lil_Blue_Ford]

So there are a few ways to go about testing things…

With the engine running, you should see somewhere around 14.3-14.6 volts across the battery terminals with a voltmeter. If it’s higher or lower, the alternator is likely bad.

Vehicle off, you should see 12.2-12.5 volts usually across the terminals.

Granted, both of these tests don’t involve amps or load, so it’s more of a general reference. I’ve seen batteries put out that voltage but not enough amps to crank. But generally speaking, these tests give you an idea of if there’s a problem.

With the positive terminal connected and the negative disconnected, hook the multimeter between the negative battery post and negative terminal. Or hook a 12 volt test light, or a lighted probe. If you have 12 volts or the light illuminates, you have a draw. Also, if amps work on the multimeter, any draw will likely be in milliamperes, not amps. So think decimal points.

Disconnect the alternator first if you do have a draw and see if that removes the draw. I’d pull the big wire bolted on first, then unplug it. If you still have a draw, move to pulling fuses in the engine bay to see what circuit is affected, then if needed move to the fuse block in the cab.

 

[beerhunter]

Hi

Thanks, I'll give that a go. But will that test simply show if the alternator is generating while the engine is running? I am already pretty sure that it is, as the battery voltage increases from 12 point something to 13 point something when the engine is running.

What I suspect may be the case is that a faulty diode is allowing the alternator to suck the battery voltage back, after the key is off.

Thanks

EDIT my post crossed with lil Blue Ford. As mentioned above, the alternator does charge, but I'll give the test light a go, and disconnect the alternator, and see what that does. Thanks

 

[lil_Blue_Ford]

Hi

Thanks, I'll give that a go. But will that test simply show if the alternator is generating while the engine is running? I am already pretty sure that it is, as the battery voltage increases from 12 point something to 13 point something when the engine is running.

What I suspect may be the case is that a faulty diode is allowing the alternator to suck the battery voltage back, after the key is off.

Thanks

EDIT my post crossed with lil Blue Ford. As mentioned above, the alternator does charge, but I'll give the test light a go, and disconnect the alternator, and see what that does. Thanks

If you’re only getting 13.something volts running, the alternator is suspect, but I’d go through the rest of the testing. If disconnecting the alternator removes the draw (with a test light the light will go from lit to dark, with a voltmeter volts or milliamperes will drop), then you found the problem for sure.

 

[beerhunter]

OK, so I put a volt meter between the negative post and the battery lead, which showed a 12v draw.
I pulled all the fuses in the fuse box, none made any difference.
I undid the 3 wire plug on the top of the alternator, which also made no difference. It doesn't have a big bolted on wire, just another 2 wire plug at the bottom, and I couldn't see any way to get that out.
Any tips on disconnecting that? It's pretty well hidden.

 

[dirtymike]

It might have already been replaced and is wired in straight. If it is from Autozone, its soldered in. Had to solder in my sons 1989 but my 1987 had a pinch plug.

 

[beerhunter]

Thanks. It really looks like a plug, but I can't see anything to pinch or separate, in order to extract it.

 

[dirtymike]

If you can feel connector block on the alternator with your hands, run up the wires and you will probably find a splice close(6"-10")away from it.

 

[beerhunter]

Thanks, I will check in the morning, it's getting dark now

 

[lil_Blue_Ford]

OK, so I put a volt meter between the negative post and the battery lead, which showed a 12v draw.
I pulled all the fuses in the fuse box, none made any difference.
I undid the 3 wire plug on the top of the alternator, which also made no difference. It doesn't have a big bolted on wire, just another 2 wire plug at the bottom, and I couldn't see any way to get that out.
Any tips on disconnecting that? It's pretty well hidden.

Interesting. Well, the big wire, or wires should be connected to the starter solenoid on the fender, it can also be separated there.

 

[beerhunter]

So I at last managed to wrestle the bottom connector off the alternator ( had to remove it from the engine to get enough clearance - thanks, Ford). It was just a clip-in connector, but I needed to lever it out which wasn't possible whilst in its normal position. And it was actually 3 wires, not 2 as I previously stated.

Anywho, with the alternator completely disconnected from the wiring harness, I still showed a 12v draw from negative pole to the negative clamp. So the alternator isn't the cause of the drain.

What else doesn't run through the under-dash fuse box, that I could test?

Someone had previously replaced the fusible connection to the starter relay with a bladed fuse. I pulled that, and got no change on the voltmeter. Would that be enough to eliminate the starter as the cause? Or could that circuit still be potentially the problem? Apologies if that's a stupid question.

 

[lil_Blue_Ford]

So I at last managed to wrestle the bottom connector off the alternator ( had to remove it from the engine to get enough clearance - thanks, Ford). It was just a clip-in connector, but I needed to lever it out which wasn't possible whilst in its normal position. And it was actually 3 wires, not 2 as I previously stated.

Anywho, with the alternator completely disconnected from the wiring harness, I still showed a 12v draw from negative pole to the negative clamp. So the alternator isn't the cause of the drain.

What else doesn't run through the under-dash fuse box, that I could test?

Someone had previously replaced the fusible connection to the starter relay with a bladed fuse. I pulled that, and got no change on the voltmeter. Would that be enough to eliminate the starter as the cause? Or could that circuit still be potentially the problem? Apologies if that's a stupid question.

There are a few things that don’t run through the under dash fuse block, but I believe all of those that don’t run through the fuse block in the engine compartment. Maybe the relays under the engine compartment fuse block.

The absolute extreme would be to pull everything off the starter solenoid on the fender except the battery wire I guess. If that kills the draw connect things back up one at a time until you get a draw maybe? Sounds like this is a difficult one to locate

 

[dirtymike]

1986 manual should have only two wires, correct?
"I still showed a 12v draw from negative pole to the negative clamp."

I'm confused. You need a clamp on on inline Amp meter to measure Current draw. On one wire at a time! Voltage is always there(12V) but until you close a circuit no current(Amps). Parasitic draw is usually measured in milli-amps, from my experience.

 

[beerhunter]

"pull everything off the starter solenoid on the fender except the battery wire"

Thanks, I think I will need to give this a go. At least this is all very instructive on how the truck goes together.

Before it was imported to the UK, I am told it was used either for towing, or being towed behind an RV. So I wonder if some aftermarket stuff was added in to facilitate these activities, as all standard equipment works, and passes the multimeter test.

Thanks again.