Brand new alternator overcharging

[91Pioneer]

My 89 MJ had low voltage and it was dying so I kept it on the charger when I wasn't driving it.

 

Today I put in a brand new AC Delco 335-1008 alternator that I got from RockAuto, it says it fits my truck. https://www.rockauto.com/en/moreinfo.php?pk=3468069&cc=1181674&pt=2412&jsn=1

 

Anywho I fired it up and the charge gauge went to the middle right away then climbed up to the red zone overcharging. I shut it down within like 15-20 seconds as I'm worried.

 

What should I do now? The voltage regulator is built in in '89 but it's brand freaking new. Should I run it longer so it "learns" the voltage or what? I think I'll go back and let it idle for a minute or two and see if that helps calm it down.

 

Grounds are good, battery connections are tight. No obvious any other issues. I didn't touch anything other than the battery terminals, the BAT terminal on the alternator and the wiring plug on it.

[Ωhm]

Alternator needs counter voltage supplied by the battery. If battery is in poor condition or very poor condition, no counter voltage could put the alternator in a run-away situation. Put a voltmeter across the battery and check what your numbers are. Hopefully it’s the battery and not the alternator.

[91Pioneer]

I ran it for about 3-4 mins and did a few voltage readings. Currently now after that with it off its 12.5V at the battery.

 

When I started it for 2 mins at idle it was putting out 17.7 -  17.8V, with the high beams on it was 17.1V, with high beams and vent blower on high it was 15.1V. With DRLs on and vent on high was 16.9 - 17.0V.

 

I shut it down then because the fuel pressure regulator was leaking pretty good. Maybe the fuel pump is going crazy and pushing way too much psi.

 

I will put the battery charger on it overnight and try more tomorrow. I have things to do tonight.

[acfortier]

Might be worth getting the battery tested at an auto parts store just to rule that out?

[91Pioneer]

I have another battery I could try tomorrow out of my 91. My battery did take a beating this past week driving and charging when not driving but I did not drive very far.

[91Pioneer]

OK well I did a couple of things, I set the battery charger on my 91 battery, not on my 89. SO... I swapped in the 91 battery which tested good, AND I put on a brand new positive battery cable because the terminal looked a little sketchy but was tight. And, it's still overcharging. My old battery was sitting at 3.8V so I don't know why that is, maybe it's bad or maybe the alternator damaged it?

 

UPDATE: I called AC Delco tech hotline. They said the voltage regulator is bad and I need to warranty it. UGH he explained that's why my battery drained overnight. OK unfortunate but it is what it is.

[fiatslug87]

Send back the alternator and get a replacement.

[91Pioneer]

7 minutes ago, fiatslug87 said:

Send back the alternator and get a replacement.

I edited my post. I contacted their tech support and I submitted for a replacement to RockAuto. The tech said if it's new and overcharging the 80% most likely cause is a failed internal voltage regulator.