No charge, but tach works just fine?

Got Smoke? said:
Carried starter to AZ and had it tested today...failed big surprise.

Carried it back to rebuilder....found a bad Bridge Rectifier replaced...charged me...said was fine.
(I don't believe the guy is taking me for a ride before anyone jumps to that conclusion, he's and old man in a wheel chair with a VERY good reputation around here)

Went directly to shop...installed...no charge :bang :bang :bang
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Now try the jumper wire trick. If no charge then, you got a bad pcm I believe brother.
 
You mean if it DOES charge with the jumper, he has a bad PCM? The jumper wire basically bypasses the PCM control.
 
Need some more help. He had the alt tested again yesterday and they told him it was OK. I had him test connections after he put it back on and we came up with a couple of puzzling results.

I checked my truck, which is working correctly, and here's what I had on the small studs:

Key OFF - +12V to both small studs
Key ON - Weak test light illumination on bottom stud (didn't check with voltmeter), VERY low and fluctuating test light illumination on top stud (almost like a "sizzle")
Engine running - Strong illumination on lower stud, strong "pulsing" illumination on upper stud.

Here's what he's getting on his truck:

Key OFF - No test light illumination on either stud. He's getting an arc when he jumps the bottom small stud to ground but it would not illuminate the test light.
Key ON - Nothing on either stud.
Engine running - when he jumps the top small stud to +12V to force charging, his voltmeter goes DOWN.

Any ideas? Sounds to me like he has something backwards in the alternator. Is that possible?
 
I had a similar problem with mine recently, first thing make sure you disconnect the small connectors from the small bolts and then hook one to ground and one to hot, doesn't matter which one. If that doesn't force it to charge then either the belt /pulley is slipping, or the alternator is junk. Also make sure your battery and grounds are clean and tight.
 
Completely misread your puzzling results post sorry about that.

What is he using as a ground compared to you? have him check all ground connections. There may be an internal short inside the alternator. What did they do to test it when they found the bad diode bridge? Also if the diodes don't check themselves it can allow feedback into the circuit which is no good, they act like one way check valves
 
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I'm using the negative battery post on the passenger side battery. Since he doesn't have a passenger side battery, I'm not sure where he's grounding the test light.

I'll also suggest he disconnect the "terminal block" on the small studs on the alternator and jump it as "rickiefulton" suggested.
 
BigPapa said:
I'm using the negative battery post on the passenger side battery. Since he doesn't have a passenger side battery, I'm not sure where he's grounding the test light.

I'll also suggest he disconnect the "terminal block" on the small studs on the alternator and jump it as "rickiefulton" suggested.
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Well that is very interesting indeed... So does he only have one point of ground going to the frame/engine block? That could very well be the issue, though I still don't trust a rebuilt alternator unless I was the one to do it.

Has either of you done a voltage drop test on your wires or resistance?
 
duck_04 said:
Can someone turn off :bang for Mark please LOL
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Sorry Matt....that's just what I feel like I'm doing is banging my head up against a wall. Truck is running fine except for not charging, and a few fluid leaks which ALSO have me beating my head up against a wall.

BigPapa said:
I'm using the negative battery post on the passenger side battery. Since he doesn't have a passenger side battery, I'm not sure where he's grounding the test light.

I'll also suggest he disconnect the "terminal block" on the small studs on the alternator and jump it as "rickiefulton" suggested.
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My test light will reach all the way from the D-side battery to the alt, so all grounding with it is done on my driver side battery. At this point I'm willing to try anything, so I will disconnect the "terminal block" tomorrow and try jumping it then.

ChuMaDD said:
Well that is very interesting indeed... So does he only have one point of ground going to the frame/engine block? That could very well be the issue, though I still don't trust a rebuilt alternator unless I was the one to do it.

Has either of you done a voltage drop test on your wires or resistance?
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I have the large negative cable for the D-side battery going to the front of the block. It was put on bare metal and coated with battery protectant. The OEM grounds at the front of the d-side fender are also attached to the battery terminal. I also spoke with another mechanic friend of mine last night, and he went out in his driveway and looked under the hood of his 97 to check for sure but his neg batt cable on the P-side only went to the block.

Alt tested good 3 times back to back at Auto Zone Thursday afternoon.

I have not done a voltage drop yet.

Interesting tid bit not sure if Scott mentioned above....jumpering the bottom small stud to the air box stud on the pass inner fender when the key was on produced an arc?
 
Got Smoke? said:
Interesting tid bit not sure if Scott mentioned above....jumpering the bottom small stud to the air box stud on the pass inner fender when the key was on produced an arc?
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Yes, I mentioned it in post #25 but I thought it was key OFF.
 
Hmmmm... interesting..... IF you dont have it figured out by sunday ill find the known good alt off the puller and bring it down as well as a few more things that need to come that way.... I WILL see this thing drive lol we have all pulled our hair out on this thing ha ha. Ill also bring my good meter from work.
 
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BigPapa said:
Yes, I mentioned it in post #25 but I thought it was key OFF.
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sorry didn't read it word for word. Honestly...I don't remember :doh: I will figure that out tomorrow.

Fireman450 said:
Hmmmm... interesting..... IF you dont have it figured out by sunday ill find the known good alt off the puller and bring it down as well as a few more things that need to come that way.... I WILL see this thing drive lol we have all pulled our hair out on this thing ha ha. Ill also bring my good meter from work.
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alternator is good and charging Lucky was confirmed with 3 back to back test's at Auto Zone.

Something is funky with the truck wiring, no driving until I get a new compressor wheel and housing somewhere.
 
I'd suggest disconnecting the block that's connected to the small studs like rickiefulton suggested. That will allow you to "force jump" the alternator without interference from the factory wiring and you can test voltage to the tabs on the block with them disconnected so you can see what the PCM's calling for.

I just can't ever think of the simple stuff like that. :bang

Oh, sorry about the :bang

:hehe: :hehe: :hehe:
 
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Ok now I'm on the computer and not my small dumb phone. Well my 97 when it was sorta working my voltage kept bouncing around, so I tried grounding one stud and running 12v to the other small stud without unhooking them and I fried the 12 volt input from the pcm, meaning I fried the pcm, but only the 12v output. I pulled the alt and opened and found 2 diodes fried which was my problem to start. I rebuilt it reinstalled and of coarse it dont work, due to not getting 12v power the small stud, but my pcm was still sending out the ground field which is good. So I ran 12v keyed power to one small stud, and hooked the other one (ground field from pcm) back up and charging perfect now. I will check what color the ground field wire is and let you know.. Good Luck!!!!!
 
He found that the alternator is good. After he disconnected the leads to the small studs and jumped it properly it is charging. The truck's a puller, not a daily driver. For what he does with it he can use the jumpers. I told him he had the choice of going deeper to find out why it won't charge on it's own or run an external regulator.
 
BigPapa said:
He found that the alternator is good. After he disconnected the leads to the small studs and jumped it properly it is charging. The truck's a retired puller, not a daily driver. For what he does with it he can use the jumpers. I told him he had the choice of going deeper to find out why it won't charge on it's own or run an external regulator.
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Fixed it for ya Scott. ;) The sport got out of my price range so I'm putting my truck back on the street, but after sitting in a garage for over 2 years being tore apart its pesky things like these that are driving me nuts.

I wasn't jumping ground and 12V hot when I tried bypassing with the OEM wiring intact. When I disconnected the junction block and tried jumping both Neg & 12V hot the alternator charges. However I cannot leave it like this as it goes past 15.5V's in less than 10 minutes and throws the GEN light on the dash. I'm just going to put a $15 external regulator on it for now, I may try and track down the wiring problem later but I'm VERY close to having my truck road worthy again and this is a small speed bump I'd love to get over.

:thankyou2: to everyone for helping me figure this out, especially you Scott I appreciate it very much my friend!
 
Sorry man. I thought you said you were building for pulling with the solid diffs and all. If your putting her back on the road I would suggest tracking down what the real problem is.
 
BigPapa said:
Sorry man. I thought you said you were building for pulling with the solid diffs and all. If your putting her back on the road I would suggest tracking down what the real problem is.
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No it still has the 4.88's & locked diffs from pulling. That's my last big hurdle before she's ready for the street again. :st:
 
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