Intermittent alternator issue

MM3846

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Feb 18, 2014
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161
LI, NY
Riddle me this batman.

I installed a voltmeter in my cab so I could monitor that like any other gauge. Last week while driving I noticed it was at 12V, shrugged it off and hoped it would fix itself. It did... for a little while. Over the past week every now and then after a bit of driving it stops charging. Went to work yesterday morning, 13.6V. Drove to the store after work, 13.6V. Store to home, 12V. Came home, let the truck sit half an hour, started up, 12V. This morning, 13.6V, sitting at a light idling with headlights, hvac blower and rear defrost on, 12V. Rev up the motor and it goes back to 13.4ish. Start driving and it goes to 13.6V (I've always seen this truck dip down to low voltage at idle with a couple things on). Also, the thing emits this high pitched, barely audible CRT TV like hum even with the engine is off.

I checked all the wiring yesterday after work and everything seemed fine. Everything is pointing to the alternator to me- its some US Industries brand parts store special, replaced another identical one under warranty a little over a year ago bc that one shorted out on me, literally zapped me and died. That one came with the truck installed by the PO and he gave me the receipt showing it was only a few weeks old. Just wanted to make sure I'm not missing some mystical Lucas part in the middle.
 

discostew

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Sep 14, 2010
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Northern Illinois
I had an alternator on my own truck that would drop the tach signal when it was cold out. If it was below say 30 degrees.

I would make sure your ground strap is clean and tight. Did you look around for a good used one? The alternators have a low failure rate so a used one wouldn't scare me at all. Cruise the vendor section and ask one of those guys what they can do for you. I think I would rather have a used genuine alternator than a new aftermarket.
 

MM3846

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Feb 18, 2014
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That came up when I searched, my tach moves up and down a couple hundred RPMs when cold, but idk if thats the motor or the alt. I'll check all the grounds again after work today. I have a PM out will Will Tillery with the one he has up in the for sale section.
 

p m

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One of the diodes in the rectifiers may have burned out. The alternator will still work, including tachometer, but it will operate at roughly 2/3 of its capacity (and average output voltage will be lower).
This is something most auto parts stores should be able to diagnose.

Edit: checking the ground is a good suggestion.
 

MM3846

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Feb 18, 2014
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Replaced the alt with a Bosch reman last night. The fit and finish of the Bosch was worlds above the one I pulled out. Everything seems to be working better now, and this alt has a 2 year warranty.
 

MM3846

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Feb 18, 2014
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Welp, another update. Didn't drive the truck all day saturday, started it up that night and it sat at 11.5 volts with the battery light lit for about a minute and then out of nowhere up to 13.8. No issues after, only drove it 10 minutes. Sat all night until this afternoon, same thing. Took about 15 seconds until the alt "kicked in" so to speak. Drove for a few hours doing christmas shopping and no issues. I'm gonna check the wiring again tomorrow after work.

Am I missing anything?
 

MM3846

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Feb 18, 2014
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It looked fine, I'll check it again tomorrow.. It was dark when I got home from work. Of course today I had zero issues.
 

discostew

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Sep 14, 2010
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Northern Illinois
I would check the voltage drop from the alternator case to the battery neg. post while the trucks running with lights and blower and wipers on. Then the voltage drop from the field terminal on the back of the alternator to the battery pos. post. I wonder if you dont have some corosion between the bracket and the block, or the bracket and the alternator. Voltage drop would find that. I would think .3 of a volt would be on the high side of what I would expect to see.
 

MM3846

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Feb 18, 2014
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161
LI, NY
Right on, I'll look for that. I think I have some 2 gauge with lying around too, I can fashion up a new lead.
 

MM3846

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Feb 18, 2014
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161
LI, NY
I haven't had a chance to check for a voltage drop, but yesterday and today the same thing happened. Start the truck, not charging. 1/2 mile of driving later it kicks in. This morning its 0 degrees out and the battery was definitely weak even though after work I was driving around running errands with the truck charging the whole time. It should be a normal temperature outside tomorrow so I'll recheck everything after work. Could it be an issue with the AGM battery? It isn't overcharging, which to my knowledge is really the only thing that kills an AGM.

The issue seems to come up when its either cold out or the truck sits for more than a day or two without being started.
 

discostew

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Sep 14, 2010
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Northern Illinois
Check the smaller terminals in the connector that's not a cable. I wonder if any of those are fagged out and not making good contact when it's cold.
 

MM3846

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Feb 18, 2014
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Will do. I'll run a new cable from the alt to the fuse box (or wherever it goes) and replace all the ends on everything the alt touches. Thanks.
 

robertf

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Jan 22, 2006
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I dont know why its reading 12 at times, but the 13.6 has been normal in my experience with the bosch and denso replacements. I never measured the original magnetti marelli output, but the replacement stuff seems not so good
 

MM3846

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Feb 18, 2014
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161
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I dont know why its reading 12 at times, but the 13.6 has been normal in my experience with the bosch and denso replacements. I never measured the original magnetti marelli output, but the replacement stuff seems not so good

13.6 seems fine to me. The truck came with a cheap parts store replacement when I bought it, and that read high, 14.5-6V. It shorted out on me one day when I was working on the truck and I replaced it under warranty (PO gave me the receipt) and output of that one read normal, but idling with accessories (headlights, wipers, blower) it would drop to 12V. Then I replaced the battery, and then that alt started not charging a few weeks after. That one would start up fine, then after a few miles of driving would drop to 11.5V. After replacing that one with this Bosch reman, I've been not charging at startup intermittently. When it's working, I have 13.6-14V ~2000RPM and 12.5-13V idle w/ accessories.

Sound like the truck is draining the battery if it is not in use for more than a few days.

That I can deal with, since its my DD. However, the truck had over an hour of running time (with stops and starts at different stores) yesterday afternoon and the battery was definitely fully charged when I parked it yesterday evening. My gauge dropped to a little over 10V when starting this morning. I've never had an issue with parasitic loss before this alt/battery combo.

EDIT: forgot to include, when I'm not charging now, I have no tach when the motor is below 1000RPM. Once the truck starts charging, tach works normally.
 

Ed Cheung

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Aug 23, 2006
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Hong Kong
Maybe unplug the battery if you are leaving it overnight and plug it back in the morning and see if there is any voltage drop.
 

MM3846

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Feb 18, 2014
1,221
161
LI, NY
Maybe unplug the battery if you are leaving it overnight and plug it back in the morning and see if there is any voltage drop.

Thats a good idea, I'll try that tonight. The only thing that could be killing the battery (I would think) would be the factory alarm.