AGM and Alternators

YJJPWrangler

YJJPWrangler

Well-Known Member
Joined
Jul 19, 2005
Location
Charlotte
Im having issues with my jeep not being charged by the alternator. I’m running a yellow top optima. Battery is brand new as of today. Alternator was also tested today and performs as it should. Motor is out of a 91 S10 blazer with a 100amp alternator. I have a 8ga wire running from the alternator directly to the positive post of the battery(bypassing the disconnect).

On cold starts it cranks right up just fine. After it’s been running for a while it will not crank and acts like the battery is dead. I threw a voltmeter on it while it was running and watched as the volts slowly ticked down.

Do I need to get a higher output alternator? Is it an ECU setting? For what it’s worth, I do not have my voltmeter gauge hooked up but I don’t think that has anything to do with it. I’ve read a bit how older style alternators can’t charge AGM batteries(news to me).

Any ideas?
 
Measure the volts while running. You did see something over 12v at idle. If you tach it up you should see 14.4. The alt doesn't know/care about the battery type


What type of alt are you running?
 
@Black Bear its a standard 100-105 amp alt for the 4.3. After talking with a friend, I suspect that one of the diodes is failing when it gets hot. When I had it running at idle, the volts read 14.04 and started ticking down and ended up around 13.90 and falling. Even driving it around the neighborhood I was averaging 1500-2500rpms(idle is 7-800) so it should have been charging correctly. I’m running a 2 wire alternator and would like to step up to a 145-160amp alt
 
Monkeynono said:
Single wire alt or does it need a load to charge?
Click to expand...

It’s a 2 wire
ADC791A2-F3E4-43F1-8BE8-CA5D267CEAD1.jpeg
 
I’ve work on an old scout that had been convert to Chevy 350 and turbo 350 and the alternator wouldn’t charge without the gauge in the dash hooked up. Just something I’ve dealt with, sounds like yours starts out charge then falls off.
 
I do have a light that is hooked up to the alternator and it shows red when charging. I just don’t have the voltmeter hooked up
 
YJJPWrangler said:
Im having issues with my jeep not being charged by the alternator. I’m running a yellow top optima. Battery is brand new as of today. Alternator was also tested today and performs as it should. Motor is out of a 91 S10 blazer with a 100amp alternator. I have a 8ga wire running from the alternator directly to the positive post of the battery(bypassing the disconnect).

On cold starts it cranks right up just fine. After it’s been running for a while it will not crank and acts like the battery is dead. I threw a voltmeter on it while it was running and watched as the volts slowly ticked down.

Do I need to get a higher output alternator? Is it an ECU setting? For what it’s worth, I do not have my voltmeter gauge hooked up but I don’t think that has anything to do with it. I’ve read a bit how older style alternators can’t charge AGM batteries(news to me).

Any ideas?
Click to expand...
Motor I sold you was out of a '91 Caprice Taxi, with parts from an Astro van and a 4.3 Chevy WT. I ran/run a 130 amp alternator as best as I can remember. But that shouldn't matter at the point where you're at. So many funky issues with your build. Has this computer ever been replaced?
 
@R Q. No kidding. All kinds of issues. I have not replaced the computer. I’m thinking it’s a wiring issue for the alternator excite wire. I need to spend some time under the dash and trace some wires. Other than the charging issue, everything else works :laughing:

Or it’s just a bad diode. Easiest solution would be to throw in a NEW not remanned alternator
 
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YJJPWrangler said:
On cold starts it cranks right up just fine. After it’s been running for a while it will not crank and acts like the battery is dead.
Click to expand...

YJJPWrangler said:
When I had it running at idle, the volts read 14.04 and started ticking down and ended up around 13.90 and falling.
Click to expand...
That's correct... the alt will vary the voltage based on load/battery, so it decreasing 0.15VDC isn't smoking gun...
What was the voltage when attempting to crank hot? Depending on runtime, a charged yellow top will run the engine a long time (barring e-fans, winch use, etc.)...
What did you do to get it to crank... jump/jump box or just let it set to cool?
YJJPWrangler said:
I do have a light that is hooked up to the alternator and it shows red when charging. I just don’t have the voltmeter hooked up
Click to expand...
If OE, I believe that's backwards... "amp" light should come on when NOT charging?

If it spins over nicely when cold and not when hot *AND* the battery voltage is good, this smells of a starter issue to me vs. "charging"
 
Just for fun, have you tested the starter? I've had failing starters present with those same symptoms.
 
@grapehead I have not tested the starter.

@Caver Dave the voltage at the battery when it was hot read around 13.90. I hooked my battery charger up to it and put it on the 30amp quick charge setting and it fired right up. Even with it running you could see voltage drop on the battery

Could the starter/solenoid be getting hot and causing it not to crank? If thats the case, then why would it crank easily when the charger is hooked up?

I need to pull the amp light and check my connections there for sure.
 
YJJPWrangler said:
@grapehead I have not tested the starter.

@Caver Dave the voltage at the battery when it was hot read around 13.90. I hooked my battery charger up to it and put it on the 30amp quick charge setting and it fired right up. Even with it running you could see voltage drop on the battery

Could the starter/solenoid be getting hot and causing it not to crank? If thats the case, then why would it crank easily when the charger is hooked up?

I need to pull the amp light and check my connections there for sure.
Click to expand...

Adding the charger could be supplying enough amps to push through whatever the resistance is, vs without. If the battery voltage does change with hot starting, you can try measuring the voltage at the starter/solenoid when cranking. If all looks good there, my bet is the starter/hot start issue.

Here is GM 3wire diagram that might help:
GM_10SI-3.jpg


If my memory serves me right, you can loop the wires together on the 3wire, and it will work, but you have to rev the engine up briefly to get it to excite.
 
Check all your grounds, and the starter for sure. I had a 77 Ford that would kill a battery and not charge right. I chased it for months only to be the starter in the end.

If you're saying the battery has 13.9 at hot idle, that should be sufficient for starting, which makes me think it's a starter issue. Anything above 11 should start.
 
So if it is indeed the starter, at hot idle, the starter is still drawing/draining power but not enough to activate it?

Hypothetically, would it be drawing enough to lower the battery's voltage during idle? If that's the case, then I should definitely look into a higher output alt as I have a winch and planning on adding rock lights/radio etc in the future.

I would assume that testing the starter would be moot since it only happens when its hot. I'm going to retrace my alternator light wiring to confirm

@paradisePWoffrd I don't have an ground coming off my alternator. Everything else looks the same as mine. Could this be an issue? Or does it ground itself to the motor?
 
Did some more reading and looked at my wiring. My alternator excite wire goes to a 2 post bulb. The other post goes to the ground. I have read on other forums that the excite wire should go to lamp as well as + ignition source. Any comments on this?
 
I don't know if this is helpful, but on my samurai, I did a GM alt swap and had similar problems. Wouldn't charge consistently & the battery would discharge and couldn't figure out what the heck the problem was. Mine has with the 2 wire pigtail/plug going into the circled area below:

7273112-BAC__ra_p.jpg
It turned out that I had the wires reversed - the wire colors on the pigtail were not consistent with the directions I had. Once I had that changed around, it was charging correctly and has been working fine ever since.
 
YJJPWrangler said:
@Caver Dave the voltage at the battery when it was hot read around 13.90. I hooked my battery charger up to it and put it on the 30amp quick charge setting and it fired right up. Even with it running you could see voltage drop on the battery

Could the starter/solenoid be getting hot and causing it not to crank? If thats the case, then why would it crank easily when the charger is hooked up?
Click to expand...
IMHO, 13.9VDC directly from the battery is a bit high (for diagnosis later)
How long between no crank, battery charger, and it spinning over? Assume you likely had the hood up as well (even though your battery isn't there)?

With the close proximity of everything in your engine bay, the starter could very well be getting heat soaked...
 
shawn said:
What was the underlying failure?
Click to expand...

Something was causing the starter to draw on the battery constantly. That’s been 15+ years ago so my memory is fuzzy. I put a new starter on it and it was never an issue again.
 
@Caver Dave i drove it over to @getstucksome hkuse so maybe a .8 mile trip. I had it running for 10 minutes prior. Pulled in, shut it off and talked for 10 mins. Went to go crank it back up and it had issues. Waited another couple minutes and it fired right up. Didn’t hook a jump box to it or anything. I’m leaning heavily towards heat soak on the starter.
 
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YJJPWrangler said:
@Caver Dave i drove it over to @getstucksome hkuse so maybe a .8 mile trip. I had it running for 10 minutes prior. Pulled in, shut it off and talked for 10 mins. Went to go crank it back up and it had issues. Waited another couple minutes and it fired right up. Didn’t hook a jump box to it or anything. I’m leaning heavily towards heat soak on the starter.
Click to expand...

I had a zj that would randomly do this. Sometimes it would require a jump, then it would start fine for 2 weeks. I ended up replacing the battery and never had that problem again.
 
So I did some wiring this morning. Apparently there are two ways to wire this alternator up based on the exciter wire. I had originally wired it to a dummy light. I took it out and ran the exciter wire to a switched 12v positive bus bar and redid some grounds. I now have a charging alternator. Ran it up to temp and at hot idle, it will still crank and run but I noticed that the battery was slower to charge than previous. Any good rule of thumb for charging? It would slowly tick up in voltage. Cut the motor off and the volts would drop quickly. I'm still thinking that the starter is robbing some power. I've got a new starter ordered as well as a heatshield for it, I should pick it up later today.
 
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