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Basic alternator question (wandering voltage)

dyeager535

dyeager535

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So with my truck battery going dead lately, after having sat at most 4 weeks with no load (other than the ECM and radio presets/clock) typically in under 2 weeks, I started to diagnose the problem.

The amp draw just sitting is very small. Within what is considered average. Pulled fuses to make sure, didn't make appreciable difference in draw, so I figure all is good there. No stereo amp or anything else that might be drawing I can think of.

I then started watching the volt gauge as the vehicle was running at idle, and noted that the voltage would run just over 14V at times, then be under 13V at other times. Seemingly random, and not an indication of load. I never really caught it dropping in voltage, just happened to notice that the volt readings weren't staying consistent. Multimeter verifies the volt gauge is correct in it's readings.

I "rebuilt" it a few years ago, one of the parts store rebuild kits and bearings. Diode trio, voltage regulator, brushes being the main components of the kit. Battery is new within the year.

It's a 78amp alternator off of a 1985 (or '86 can't recall) Monte Carlo SS, with the partial serpentine belt. I believe this makes it a 12SI (plastic fan). I'm quite certain the belt isn't slipping, but I haven't tried tightening it up...I've assumed if the belt was slipping I'd get some noise or more noticeable/faster occurring fluctuations in voltage.

Kind of curious what exactly would cause the alternator to act this way? Failure of the regulator typically seems to cause either a no charge or overcharge condition, but this one isn't "dead". I can throw another kit in it, I'm sure cheap parts could be an issue, just curious where I should suspect the problem to lie, perhaps spending more money for a GM part this time around. :)
 
First thing that comes to mind is to *always* use the multimeter, as the dash voltage gauges are about as accurate as an idiot light IME.

That said, in your case, maybe a ground strap somewhere is flaky? Hard to check, but the idea is you want a stable current path from the alternator to the battery, both on the positive and negative sides. Any resistance on either end would cause voltage drop and potentially make the regulator mis-adjust its output.

-- A
 
I know the stock grounds are all good, I've been into this thing enough in the last couple of years to know that all are clean and inspected. :) At least I can bank on that. Braided ground to body, and cable from battery to alternator bracket. (along with the battery to core support and frame) I'm not seeing any other anomalies that would indicate a ground problem elsewhere. All my injection stuff is grounded to the engine, so it would be pretty noticeable if the engine wasn't grounded well.

I use the multimeter, but the gauge IME (at least the voltmeter) has been dead on. But like you said, I use it solely to let me know if there is a problem, if I see something or suspect something, I break out the voltmeter. I did verify that the different voltage readings were accurate.

Now that I think about it, the single post terminal on the back of the alternator...the rubber/plastic/whatever it is grommet was kind of crummy. Problem was I couldn't find a replacement for it, so I had to leave it. Wonder if I need to check that out. I would expect if that were bad though, and the stud was grounding to the case, I'd fry the alternator in short order?
 
dyeager535 said:
Now that I think about it, the single post terminal on the back of the alternator...the rubber/plastic/whatever it is grommet was kind of crummy. Problem was I couldn't find a replacement for it, so I had to leave it. Wonder if I need to check that out. I would expect if that were bad though, and the stud was grounding to the case, I'd fry the alternator in short order?
Click to expand...

Yeah, if the insulator were bad enough, it would zap out in short order (as it were!) But, if it's just cracked, it might intermittently short out briefly, then heat up and move away (or there may be corrosion or other crud in there that heats up and breaks the connection.)

I had a battery terminal once that LOOKED perfect and SEEMED tight, but starting the truck pulled enough juice that the connection was broken, and wouldn't reconnect for an hour or two until it cooled off. Drove me up the wall until I went through and scrubbed every terminal in the system.

And I'm talkin' out of my butt here, but another possibility is a flaky internal connection in the alternator? They ARE subject to heat and vibration... :dunno: If it were me, I'd swap the alternator out with a spare, just to see if the behavior changes. That way you could at least narrow it down to the alternator, or the rest of the truck :haha:

-- A
 
Dorian, don't discount that braided ground strap as being good. I had a truck once that started giving me some problems and after a long time trying to figure it out i replaced that braided ground strap (even though it was clean and looked perfect) and my problem went away. I couldn't believe that was the problem since it appeared to be good so i swapped the old strap back in just to have the problem come back again.
 
If I had to guess based on the readings, it is most likely the regulator or diode trio. The trio has a tendancy to loosen up a little, but that is very uncommon. The regulator can actually give warning, like my old alternator on my 80 did. I would see voltage fluctuation like you are describing, so I tore it down at my buddy's shop. Sure enough, the regulator tested bad, as did the trio. Replaced them, cleaned the comm, and put new brushes just because I was there, and the problem has never resurfaced.
 
Yeah i was thinking regulator /Diodes also. If a diode goes bad it can leak to ground causing your dead battery problem. Might be time to buy a new alternator.
 
Buy new? Are you kidding? lol :)

They probably are pretty cheap comparatively, but call me crazy, I like "rebuilding" the alternators. Not much is that easy to do, and the rebuild kits are dirt cheap....maybe that's the problem. :)

Anyone recommend any aftermarket alternator components as being better than others? I should probably go Delco parts, but going that route, for the cost I'd be better off buying a brand new (not parts store) one.

I will not discount the ground issue, but with the ECM/injectors grounding through the block, I would *think* any issues bad enough to affect the alternator would screw up the way it runs, and idle doesn't change based on the voltage I'm seeing.
 
R & R the alternator. If one diode is shorted or open your 78 amp alternator is putting out 10 amps. That is bairly enough to keep the engine ruuuing. The voltage fluctuation is telling you that the alternator can't keep up with the changing load conditions.
 
Oh, I've got a few alternators sitting around, but the others are ridiculously small, like 37 amps, 10SI's. Probably should recycle them or something, worth more that way I bet. :)

I've been sticking to the 78 amp because up until now, it's been working fine, even with the electric fan(s, although only one is ever used). Anything smaller and I suspect I'd be disappointed in it's performance when the fan came on, with the heater on high.
 
If you want to save some money, tear down the bad 78 amp alt and the good 37 amp alt. Put the diodes and regulator from the 37A and install them in the 78A alt. Compare and use the biggest brushes in the 78A alt, and VOILA a new fully functioning 78A alternator at no cost.
 
I was looking at some parts kits, it didn't appear that there were any differences in the regulators and diode trio's between the same series (12SI) alternators, so I guess that's a good choice. I thought maybe there would be differences based on amp rating, but it appears not.

I was kind of hoping my rebuild kit was the wrong one and the parts burned out due to that, but it doesn't appear that is the case. Unless someone else knows differently?
 
dremu said:
REAL CK5'ers have two spares sitting on the shelf. (And the serious ones know that said spares *work* :haha: ... I am a real CK5'er, but not a serious one =))

-- A
Click to expand...
haha.i got a stock one sittin on the shelf next to my box o' carbs and random brackets.lol.as far as i know it still works too.been sittin for a few years though.
def sounds like it is in need of anether rebuild.one from the parts store is only 50 bucks.how much is the rebuild kit?
 
Rebuild kits are like $13. Even cheaper now than a few years back, which I guess should tell us something. But whether you get a rebuilt alternator or a parts kit, both are just cheap parts IMO. With rebuilding an alternator (basic rebuild, not cleaning up the commutator) taking less time than it takes to drive to the parts store, rebuilding makes sense IMO.

If I was smart, and I've never made that claim, I would at least buy AC Delco replacement regulator and diode trio, so I have a reasonably good idea that they will work for awhile.
 
I'll tell you what, with all the time we talked about it here you could have rebuilt or replaced the thing already. :deal: :D
 
Look up Romaine Electric on Google, and see if there are any local distributors. That is what my buddy used for all his rebuilds, and they last a good long time. I have never had one fail, but that may be because they only had a hundred thousand or so miles after the rebuild.
 
4X4HIGH said:
I'll tell you what, with all the time we talked about it here you could have rebuilt or replaced the thing already. :deal: :D
Click to expand...

Where's the fun in actually DOING things? The fun is in armchair mechanics, jeez. :)

Working sucks, that's all I have to say. Money to buy things, no time to use it. But I do get to spend it on gas getting to work at least.
 
Good or bad, I'm going to say it's not the diode trio or regulator. Bought the kit ($17) pulled the alternator, and tore into it. Nothing out of the ordinary. Visually inspected the "hard" parts, and didn't see anything wrong.

Put the new parts in, put it on truck, fire up, 14V with all accessories running. Great! Problem solved. Then the truck warmed up. Once the electric fan kicked on, the voltage dropped to below 12V, and would stay there unless I increased engine RPM (idle speed doesn't change based on load due to the injection) above curb idle.

Once the accessories and fan were off, the voltage readings went up a bit, but not to 14 unless I hit the throttle.

I'm a bit mystified here. I checked the grounds, cleaned all the connectors, inspected all the wiring, but nothing.

I was mistaken, I've got another 78 amp alternator in the garage, I am probably going to throw it together with it's hard parts and the new replacement parts, and see what happens. I can only guess that it's the rectifier that is screwing up when the components heat up.
 
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