No Batt Charge At Cruise

[fred75]

  Ladies and Gentlemen of the Forum,  Have a interesting one here; Truck Battery voltage drops while at cruise. Will climb slowly back up to 12.9 at idle. While at highway speeds it will slowly drop to approx- 10v, let it idle at a light, it will climb back up. If the heater, lights and wipers are on it will fall into the red zone (maybe 8 volts) on the gauge, and will not climb back up at idle until all are off.

  Cleaned leads at starter, no burnt fuseable links, new alt and battery. Cleaned junction leads at firewall, replaced two terminal lead at alt and cleaned alt main lead out. Checked batt, frame and engine grounds, all good. Cleaned batt connections. Starts fine if I let the voltage build back up at idle before shutting off.

  Stumped.........

[topp]

Belt slipping?

[Da67goatman]

Alternator brushes might be shot, which would cause low charging.

[beastie_3]

Test your alternator, even though it is new.

[alaskaskiff]

Definitely check the alternator.  I put in a new 'reman' one two weeks ago.  It lasted 18 hours before it died.  Well within the 1 yr waranty.  Put in another and so far so good

[slammed79]

Belt slipping?

You may not hear it, but if its stretched out it wont charge when the rpm's are up, then you're running of the battery.

[1979C20]

Alt belt either too tight or too lose.

[fred75]

  Thanks for all the replies, will get the alternator bench tested. Liked the tension and condition of the belt, but will take another look at those two components.

[Irish_Alley]

Do your lights dim, and other problems, have you check the volts with a volt meter

[fred75]

  Well, to all that mentioned loose or slipping belt, you were right on. Did a bench test the Alt, Alt is good.
This is once again a huge reminder to check my work and refrain from the extreme over confidence I sometimes suffer when it comes to simple tasks. How dare anyone suggest a loose belt, don't they know I installed the Alt???

 I would have lost big money betting the belt was tight enough after I installed the Alt.

Good Job All.........

[thirsty]

Glad you got it fixed Fred. Cheap too! Tension is free at our garage lol

[topp]

Glad it worked out for you!

I always start at the easiest fix first, and then move to more expensive....

[slammed79]

Over a year at Checker Auto parts gave me that hunch. Too Many times was a customer just as confident as yourself. Glad its handled.

[Lt.Del]

i recall driving my 79 pick up down to North Carolina to watch the last North Wilkesboro NASCAR race many years ago. As i was approaching Yadkinville Valley, home of Daniel Boone, my alt guage voltage was decreasing.  It was showing close to the red line.  I found a auto parts store and turned the truck off. I tried to turn it back on, dead.  Looked at the alt and belt and sure enough the V belt didn't have much V left in it. Walked inside the store and paid $13 for a new belt and was on my way again in no time.

[fred75]

  Roger on all the comments, prolly one too many serpentine belt with the auto tensioners. Focusing on idler pulleys and tensioners, one forgets the basics, from where it all came.  When working on some of this older stuff, I prolly need to take a step back, and appreciate the Era!!!