Alternator question??

[ericboutin]

Yesterday I figured I had better try and figure out why my batt light is on again...last time it did this I had to replace both batts, the time before that it was the voltage regulator. Anyway I'm thinking regulator again cuz the batts are still new. The guy does the load test with his machine (Advance Auto) and it shows the alt is not putting anything out.

My question is this...how should he be checking it? He put the clamps on the hot and neg side of pass. side battery and then the little coil checker (tech term) around the neg cable?? He didn't seem to sure of what he was doing and I think the ole diesel kinda set him back as he wasn't sure what to do with the the other probe.

Continuing on...we put a new regulator on just in case and sure enough batt light is still on. I go in and say give me another alt. I pull off the old in the pking lot and he swaps my pulley for me and I put the new one on......ugh crank her up and the batt light is still on??? Anyway I go and get him and he says the new one must be bad and he doesn't have anymore will be today before he does sooo I take off with the "new" one in the truck and I'll swap out today.
Second question - on the drive home the batt light finally goes out and the gauge starts to finally move out of the red area like it's charging, the needle is not moving quite as far as it did before but appears to be charging. I was thinking I would have noticed immediatley with a new alt that it would be charging and the light would have gone out immediately? Would a new alt take a min or two to "turn out the batt light"?
Anyway sorry for the long post - but now I'm curious as to whether I should keep this alt or still go and exchange it for the new one he has on order?

Thanks

 

[krawlr]

The current clamp should have gone around the big wire on the alt.

I've seen Bosch alts that needed a little engine rev before they kicked in. Haven't seen it on a Ford before but most of my experience is in imports.

A good tech knows how to bypass the reg and full feild the alt to tell if it's a reg or alt problem.

Now that it seems to be charging I would do voltage drop tests and clean the poor connections you will probably find.

 

[geonc]

Eric, that new alt prolly just needed to be "tickled" get some rpms to "exite" the field

Many larger AC Delcos and some of the Leese Nevilles are the same way.

 

[The Warden]

A good tech knows how to bypass the reg and full feild the alt to tell if it's a reg or alt problem.

This may not be the case on an '87-'91 truck, but you can do this on an '80-'86 truck in your driveway. You short two of the plug connections together or short one plug connection to ground...I'm not certain, but when I get home, I can check my shop manual and give the details on this if anyone's interested.

It's amazing how much a full-fielded 100 amp alternator will load down an IDI I used the hand-throttle to bring the RPM's up to 2000 or so, and full-fielded the alternator (thought the alternator was going bad; turned out to be a bad connection at the VR)...and I don't have a tach, but I estimate that the engine bogged down at least 300 RPM's, and possibly more like 500.

 

[argve]

Eric,

Me thoughts are that your alt was bad and the batts were low... Now a alt will not produce rated amp output at idle - they do this typically much higher on the rpm scale so while you were sitting there puttering she was charging but not much, so once you got her up to steam the alt was then kicking out the amps and it just took some time to bring your batts back up to *****.

Now that the light is out go out and stab the batts on the post not the terminals and see what your voltage is both running and not running. Then test the terminals as well (right there on the batt) and compare (both running and not running) I bet that you might find that you have some dirty connections between the batt and the cables - which is typical.

 

[Cowboy_Customs]

I just installed a nice rebuilt 100 amp Alt, and 100 amp regulator the other day, like said before, it doesnt put out lots at idle. Once I get it up around 8 or 9 hundered RPM, the needle really shows the juice flowin! Also, I too went to Advance Auto and had the old one tested, the kid said it was bad, so I go get me a rebuilt, install it, n fire er up, well, that batt. light was still on, So I go n get me a Regulator and find out that it had a broken wire! Im tempted to pull off the new one and have that same kid check it. To bad I cant get a reimbursement from them for a not so bad alternator I got rid of cause of their..... employees!
I too would agree to go through and clean up all you terminals, and I cleaned up the terminal connections on the wiring on the alt. My guage showed immediatly that it was charging, and the light was off. But my batteries never dropped below 12.00 volts either.

 

[ericboutin]

Update!

Well I just got back from Advance a little bit ago...thanks for all the replies they were all right on the money. When I got home from work I went out and fired up the Dewey and this time the needle jumped right into charging mode so that was cool. I figured I'd go ahead and ride over and have the guy re-check everything just to be sure. He must have done a little checking on where to put the 3rd probe too cuz now it was reading the RPM's but only up to about 940 ...anyways this time all showed good - had like 13.7 volts from the regulator and 189 amps from the alternator, I think I have that right. Anyway last night the old one was showing no alternator found. So anyway now she's got a nice new "rebuilt" 100 amp alt and a nice new shiny regulator too. Thanks again.