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Alternator bench tests good but does not charge

H

hueyrescue

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My '94 Suburban stopped charging a few days ago. I've had some battery cable issues so I focused there. However, I noted that my alternator produced only 1V when disconnected from the battery with engine running. I removed the alternator and it was bench tested by a highly-skilled tech and passed. In the meantime, I took reasonable steps to fix the battery cable issues including removing all side-terminal boots and grinding down/cleaning visible corrosion and replacing all positive studs. I replaced the alt cable with a 4ga wire and put everything back together. Still not charging. The tech suggested I check the input to the alt volt reg. and the wire to terminal L shows 12V, which, I believe, should be enough to trigger the volt reg. I've checked all of the fuses in the dash box but did not see anything specific to the charging system and none were blown. Any insight will be appreciated.
 
The field terminal requires voltage to turn on the regulator. Make sure that terminal has voltage with the key on and the connector removed from the alternator.

If the alternator bench tested good, that is all they do. Attach the output to a battery, and apply voltage to the field terminal and spin it.

Make sure your fusable like between the alternator and the battery is ok. If it opened, it won't charge either.

Good luck
 
Internal regulator.not much to it.. Should be 3 wires. The heavy one under the nut goes to the batt. Then on the 2 wire pug one is for the tach and the other should be the exciter. You tested for voltage right at the stud on the alt while it was running ? Check for voltage going in on the plug. Who tested it ? Most autoparts stores have no clue how to test an alt unless you brought it to an actual rebuilder.
 
jmiller said:
The field terminal requires voltage to turn on the regulator. Make sure that terminal has voltage with the key on and the connector removed from the alternator.

If the alternator bench tested good, that is all they do. Attach the output to a battery, and apply voltage to the field terminal and spin it.

Make sure your fusable like between the alternator and the battery is ok. If it opened, it won't charge either.

Good luck
Click to expand...

Thanks! The F terminal shows no voltage with the ignition turned on, L shows 12v. Any idea which is used as the field terminal on the 6.5? Seems that it can be either but maybe I've misunderstood. No fusable link on the alternator-to-battery cable, just 4ga Cu. When I first put things back together and started, the alt light went out after I revved the engine and stayed out for about a minute before coming back on again with the usual symptoms (tach jumpy, batt meter dropping, etc). I am guessing that the alternator is not coming on for some reason but I don't know the system well enough to diagnose.
 
Here you go...pic worth 1,000 words. I can't figure out how to save them not upside down so if a Mod can, go for it. I turned them the right way but when I save them they are still upside down.
 

Attachments

  • 93 alt pg1.pdf
    1.6 MB · Views: 66
  • 93 alt pg3.pdf
    1.7 MB · Views: 37
  • 93 alt pg2.pdf
    1.4 MB · Views: 34
  • 93 alt pg4.pdf
    1.4 MB · Views: 32
BTW, you are a slacker....:D
According to the book, it should take you .5 hrs to diagnose and replace the alt...:rof:
Too Funny...
 

Attachments

  • alt book time.pdf
    1 MB · Views: 11
Acesneights1 said:
BTW, you are a slacker....:D
According to the book, it should take you .5 hrs to diagnose and replace the alt...:rof:
Too Funny...
Click to expand...

Ha! Thank you for taking the time to look this up and scan it in. Very helpful. It is strange, though, that the alt bench tests okay (done by a well-respected rebuilder, by-the-way) but the book says replace. I'll have to give some more thought to what might be going on and try a few things the book mentions.
 
Something that can happen and is not easy to test for on the bench, damaged diodes. The alt can still put out 12V, but will not be strong enough to keep the battery charged up.

Don
 
hueyrescue said:
My '94 Suburban stopped charging a few days ago... my alternator produced only 1V when disconnected from the battery with engine running. I removed the alternator...
Click to expand...

If it wasn't dead before, the shock unload of removing the battery/electrical loads from the output terminal probabally did it in. Shock unloading generally frys the avalanche diodes in the alternator.

Likely the regulator has a crack in it that fails with heat or your brushes are shot/sticking. You have verified 12V to it. Double check with a pin back probing the connector to make sure it has a steady 12v and not cutting out from a bad wire or loose fuse. If it has 12v your alternator is toast. The test bench isn't a vibrating diesel...

The only other oddball thing I can think of is a major short that is bouncing the gauge and stalling the alternator via shock unloading and loading. battery plates shorting or bad battery is also possible.

The pulley isn't slipping on the shaft?: alternator or crankshaft? I had one loose 200 RPM on the tach as the alternator fan started hitting the case from a slipping pulley - defective from the factory.
 
I had a problem with the alternator on the Suburban several years ago. Batteries kept going down, so removed the alternator and took it to the shop. This is a shop I really trust, he has done a lot of work for me. Anyway, he put it on the machine and it put out over 100 amps. He proclaimed it fine and handed it back to me. I checked all the wires, everything looked good, charged the batteries and put it back together. Couple days later, batteries are almost dead. Took both out, both under warranty so got two new ones. Couple days later, both batteries almost dead. Took the alternator back out and took it to two different places. Watched as they tested it and its putting out fine. OK, I'm getting pretty perplexed by now. Charge the batteries, check the wires drive it and couple days later, the batteries are almost dead. It sure acts like a bad alternator, but why? So, I keep my old alternator (its one I had specially built years ago) go buy a reman alternator, install it and everything starts working great. OK, now I know its the alternator. Take it back to my alternator guy and tell him what happened. He takes it apart and finds the wire that connects the 12V post on the back is broken. The testers all use alligator clips to connect, but when I tighten the wire on the post with the nut, it turns the post just slightly and only makes intermittent contact. Had him repair it and all is good. Kept the reman as a spare. I have found usually once you have a spare on the shelf, the parts on the truck seem to keep working. :hihi:
 
Acesneights1 said:
I can't figure out how to save them not upside down so if a Mod can, go for it.
Click to expand...

Good info contained in these, thanks for sharing. I'm not a mod and didn't stay near a holiday inn express but...
To solve the inverted issue I copied them as images into Word07, flipped the upside down ones and saved them all as single PDF with five pages.

Hope it helps.
View attachment 1993 Charging System Diagnostics.pdf
 
bobbiemartin said:
Kept the reman as a spare. I have found usually once you have a spare on the shelf, the parts on the truck seem to keep working. :hihi:
Click to expand...

:skep: Send some of your luck my way! Bolt in spare out in the middle of nowhere and... it was defective. Bolt in spare alt after the rear bearing blew up and... It lasted 6 miles before also giving up. Spare CS130 alternators are just dead weight... :mad2:
 
WarWagon said:
:skep: Send some of your luck my way! Bolt in spare out in the middle of nowhere and... it was defective. Bolt in spare alt after the rear bearing blew up and... It lasted 6 miles before also giving up. Spare CS130 alternators are just dead weight... :mad2:
Click to expand...

I have 3 of these boat anchors with bad bearings. I haven't taken the time to attempt to fix them. At $120 ea for a new one, is it worth the time?

One of the reasons I put the second unit in. Hopefully if one quits the other one get me around.
 
One thing I found is there are several different size bearings (front & rear) for the CS 130. On a reman, no telling what you get, plus the bearings probably come from China. You have to go to a good shop and impress on them that you don't want cheap, you want good. As in put in the best you can get and I'll pay whatever it cost. I'm also using an Iceberg rear cover. You have to modify the bracket, but it seems to help. I did the iceberg mod probably 10 years or so ago. Not saying it fixes everything, but it helps. And CS130's need all the help they can get.
 
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