Strange Charging Issue

[Bob L]

I just made a parts run with my 98 1500 4.3. It was about 60 miles each way and just before I got to the JY I noticed the voltmeter at about 12 volts. As I continued on it was slowly dropping like the alt wasn't charging. I came to a red light and when I stopped it jumped back to 14V. When driving off it dropped back down to about 12 and continued to fall. I got to the Junk Yard and left it idleing and charging close to 14v. I was there about half an hr and it was reading between 10 and 12 when I got back to leave I was down to almost 10 v. When driving away I noticed it would go back up when I took my foot off the throttle. I ended up making it all the way home by taking my foot off the gas and shifting to nuetral downhill and stopped at a rest area and let it idle for about 10 min.
Alt. is only 2 months old and was replaced due to bad bearings in the old on so I will replace it again Mon. on warrantee. I have never seen one that only charged at idle but not at higher rpm's

 

[df2x4]

That is very strange. I would suspect the alternator as well, though. Seems like bad parts straight out of the box are becoming more common these days.

Might be worth double checking your battery terminals if you're still using the factory side terminals. I once had an issue where the negative terminal actually broke loose inside the battery and would make intermittent contact while I was driving around.

 

[Bob L]

I replaced the bat about 6 months ago, I took the factory bolts out and replaced with some 3/8 x 1 1/2 with washers so I can tighten and have an inch of bolt left for jump starting. I did check the cables and alt connections and all looks good I will check engine grounds when I get back out later.

 

[Bob L]

I have replaced the alt suspecting a problem with internal reg. same problem still I replaced the belt tensioner as it was weak and could be moved by hand and replaced the two ground straps from the frame to rear of rt. head and frame to firewall both were very corroded and dissintigrated when pulled on. Still having this issue. Charges fine at idle even with lights ,radio and blower on. Charging cuts in and out when driving down the rd. mostly under exceleration. What next fuseable link looks good may run a jumper wire to see if it helps.

I wasn't able to get at the bolt for the ground on the rear of the head so bolted that ground to the head where the dipstick bolts on. I can't see where that should matter. Main battery ground to engine looks good.

 

[90halfton]

Have you verified these readings with an actual meter?