Intermittent alternator function 99 blazer 2wd
 

I have a 99 blazer with 183k miles. I had an issue where the alternator was not charging so I replaced with a new one. I didn't have the unit tested before replacing. The vehicle has quite a few miles so I figured this was pretty cut & dried. My understanding is that the entire 'charging circuit' is contained in the alternator so it made sense that maybe something had gone bad. The battery is about 1 1/2 tears old but has been drained to dead several times now because of the charging issue that follows.

Now - sometimes - when I start the vehicle the alternator will not charge. The meter on the dash will show ~9v. Sometimes if I rev the engine it will immediately jump up to 14v and act fine. Sometimes it doesn't.

I went through the battery cables and ground cables and disconnected, cleaned contact points, and reattached. No change to the behavior.

I took the vehicle to my local mechanic and it never acted up for him.

This issue tends to happen the first thing in the morning. If I leave the vehicle out overnight it almost always acts up. If I put it in the garage it still may happen but much less likely.

Could it be a bad alternator? It's new - not rebuilt - but stuff happens. Any thoughts or troubleshooting paths to chase down would be appreciated.

My Tahoe started doing this a few months ago. It stopped charging one night. I remove the alternator & bench tested it and it tested fine. Same results at the auto store. Reinstalled it after checking & cleaning all of the connections and it worked fine for a week. Then just stopped charging altogether. The first alternator I picked up to replace it was a reman from Autozone. It was bad out of the box with huge swings in output plus some bearing noise... Returned it and then bought a new alternator from NAPA and haven't had a problem since.

What makes things more complicated with these trucks is that the PCM controls the output of the alternator by driving the field using a pulse width modulated signal. You can rewire the alternator to make it a voltage reference control (covered in a tech article if I remember correctly) to see if it is a problem with the signal from the PCM, but I doubt that is the cause of your problems.

I've been burned by poor quality components in the past. This was a new alternator from NAPA - I tend to trust their parts, but still...

I have replaced the PCM in the past year. It was a Delphi unit from NAPA also, so I think it is probably OK.

Thanks for the feedback. I'll talk to my NAPA guy and see if he will be helpful.

Well the guys at NAPA let me exchange the alternator for a new one. Installed it yesterday and I still have the same issue. I started the vehicle this morning and no charge.

This appears to now happen every morning now that it has gotten colder. If I garage it overnight it is fine, so it appears to be temperature related somehow.

I'm at a loss as to what to even try next.

https://blazerforum.com/forum/lighti...g-nutty-53055/

That was damn hard to find no matter what search terms I used! Kyle can you change that thread title?

The relevant post in that thread:
How To: Convert from PWM to Battery Reference

Be aware that's a clipping from zr2usa and it seems that info no longer exists on that site, at least I couldnt find it.
Plus read the rest of the above thread.. didnt solve the whole problem for the OP.

Note there are a couple things you may have missed. The wiring back to the PCM and the Fuselink itself in the big wire to the battery

That post is also a Tech Article. I have that post open in a tab so I'll see about changing it's title when I get a chance later.

Thanks for the input guys. I think I'm going to try the following:

1) I'll check the fuse link in the circuit.

2) make sure there is not some sort of an 'open' condition in the one wire that goes to the alternator. I may try running a new wire to the alternator from the PCM assuming that there is an issue somewhere in the wiring harness - assuming I can figure out which wire coming out of the PCM is the correct one. My Chilton's manual seems less than clear based on my level of circuit diagram understanding.

3) if that doesn't work I'll convert over to the voltage reference control.

I'll post back next week (the xmas traveling starts today for me) with results.

Thanks again.

OK, even though I haven't posted in a while I have been pursuing this.

I rewired the alternator per the tech article. My particular vehicle only had one wire going into the B pin of the connector. There was no change once I rewired to include the wire from the alternator output to the D pin. The vehicle still will only charge sometimes - usually once it is warm. Now that it is even colder I sometimes have issues where it won't charge even in the garage - temp usually 45 or less. If 50 or above it seems fine.

My battery charger has a function such that it will display the relative strength of the battery as a percentage of full. Last Monday before I messed with the vehicle I hooked up the charger and it listed the strength as 48%. The vehicle would start but not charge. I shut it down and hooked the charger up in its 'start assist' mode. I then started the vehicle and the voltmeter showed the alternator started to charge.

I thought it likely that the battery had some sort of issue even though it is only 1 1/2 yrs old. Since the vehicle was working I took it to a local Advance store and they ran some tests on it for me. The battery checked fine but showed that it needed a charge. I was surprised that it checked OK but not that it needed a charge. The Starter Test indicated that the starter pulled 224 amps - the guy said it should have been more like 170 amps. The charging system test showed no problems except that there was more of a ripple (45mV) on the output than he thought there should be. His input was that he felt there was a slight short somewhere in the system that was draining the battery over time and possibly causing the larger current pull during starting.

I took my multimeter and checked all the circuits in the main fuse block with the vehicle off. I didn't find any circuits that showed any current except the main ignition circuit. Once I disconnected the underhood lamp that circuit showed no current also. If there is a drain my multimeter isn't sensitve enough to register it or I'm not testing the right circuit.

That was 5 days ago. The vehicle has been parked ever since (My daughter normally drives it and has been sick). I did the same check with my charger - garage is at 45 deg and battery was at 50%. I disconnected the charger and started the vehicle. Vehicle started but no charge. Turned it off, hooked up the charger in 'start assist' and started the vehicle. Instantly starts to charge. I turned the vehicle off and have the charger hooked up to recharge the battery.

It seems like a real possibility to me that there is a drain on the battery somewhere. I don't think a battery should lose half its charge in 5 days when the average temp in the garage is around 45 deg - but I don't know that for sure.

Any ideas?

Ever figure this out? I am running into an almost identical issue! After having the battery and alternator test as 'good' at a few places, I put everything back in and it started working. I figured it was a loose connection and I was just stupid for missing it...

Thanks for any follow-up!