Auto: erratic voltage while accelerating, big drop at idle while stopped

[junior varsity]

I can't figure out what the issue is with my Tahoe (2001, 215k mi).

While at a complete stop, at idle, voltage drops substantially.  Giving it a bit of gas to raise the rpms gives the voltage needle a bit of rise. Dash lights go from dimmer to brighter, and/or air blows harder (whatever is on responds to increased rpm so I don't think the gauge is playing tricks on my mind). The voltage drop is to a lower than used-to-be-normal position.  While I have not verified if the gauge demarkations are spot on, it usd to be 14ish, drops to around 11-12 V indicated (again, haven't verified with voltmeter yet).

While cruising down the highway, constant speed - level road - constant throttle input and constant rpm, the voltage needle "wiggles" about at the higher (used to be normal) position.

I tried to problem solve this by replacing the battery and the alternator. I out in a "higher output" powermaster alternator.  The problem was mitigated, slightly, for awhile. Now it seems not much different than before.

If the new alternator wasn't "bad" out of the box, what might have led to it's deterioration in less than a year? If it's not the alternator, what else causes these kinds of problems? Bad grounds? Gremlins? Everything else is wonderful about Big Red, especially the memories. It's like a member of the family...with a voltage problem.

Any ideas?

[Ddan]

I did get a bad alternator once.  Is it OEM, or aftermarket?

[mitt]

Could be bad cable.  I would check ground connections. 

[DarkMonster620]

Do you still have OEM battery conectors and cables?

Check not only that they're tight, also check they're clean if they have some sort of rust or chemical rust, the white/greenish stuff, use some club soda or coke to clean it... check all, engine, transmission and battery ground connections for this also check the dashboard ground cables ...

Check the length of the cables moving them with both hands to make sure they're not 'boken' at any point. Get some contact cleaner to clean the alternator plug and altenator itself, just spray some of the contact cleaner into the alternator.

If none of this works, you might either need to replace the cables that go from battery > alternator > main fuse box and it's grounds, all grounds. This happened to a vehicle from a former employer, a Suburban and I did replace all the before mentioned cables, had them made locally from 1/0 gauge and copper terminals.

Hope this helps

[Raux]

what is the belt routing for the alt?
check for bad or loose idler arm.
you could be getting plenty of tension with higher rpms, but a loose belt in the low rpms

[RAT900]

auto erratic asphyxia?

[DarkMonster620]

what is the belt routing for the alt?
check for bad or loose idler arm.
you could be getting plenty of tension with higher rpms, but a loose belt in the low rpms

+1 on this, just enough loosening but not enough to cause 'screeching' ...

[zooom]

I have heard of GM alternators heating up and reversing polarity....

[junior varsity]

I was using the dash gauge as reference, however, it was not the only evidence of something being up: Lights would dim at stop, air wouldn't blow as hard (and/or both), etc.

The replaced alternator is a Powermaster model rather than OEM GM (don't recall if that's a delphi, delco or what), put on new belt and new tensioner assembly earlier this year when the tensioner pulley seized and belt & tensioner pulley bit the dust. I should note that the voltage problem existed prior to such failure.  The replacement alternator seems to have helped, but still the voltage drop / lights-dimming / air-not-blowing-as-hard at idle (potentially worse as the days go on and temp in the concrete of jungle of Dallas continues to hit 100F).

Putting the new serpentine belt was a chore. I would describe the spring-strength of the tensioner as a "sumpregnant dog". Not sure if the new belt would have loosened to such a capacity as to run loose at idle. My brief research prior to parts-purchasing of these items (just quick browse of forums and such online) led me to believe the gator-back belts were less prone to slipping - got the OEM size and the tensioner and alternator pulleys were the same diameter as the OEM parts.  Since the replacement Powermaster alternator is so new, it still looks *very* clean but I will give it a good once-over early tomorrow morning before it gets hot.

I had considered that not only it was a good idea but also quite likely time to replace/upgrade the original motor/frame/electrical-system grounds/connections.  I will certainly replace these ASAP.  Anyone have a convenient way of finding a diagram of where all these are located prior to crawling around in the engine bay (since it is quite hot and I'd like to get in and get out with a quickness).

On connections - when installing the Powermaster Alternator, I put in beefier direct alternator-to-wiring-harness-junction-box wire. That's the positive wire, I should say. The battery's positive cable goes to this lug, and the alternator's positive connects to the same lug.

Other ideas I had brainstormed and tried to solve for troubleshooting purposes was restrictions in air flow at idle, resulting in lower power and inability to crank out some juice.   So I pulled off the stock air intake system, check filter (fine, relatively new), sprayed a silly amount of carb cleaner at that throttle body to dislodge any potential debris or sticking points, etc, and then continued to just let seafoam pour/get-sucked-into the intake.  Then I decided "to hell with the convoluted plastic stock intake setup" and got a simple cold-air intake setup and giant cone-shaped K&N style filter at the end of it. (btw - future work will be much more convenient with gobs o' plastic out of the way it seems)

The only other 'vehicle history' I can think that was adverse to the vehicle's electrical system was driving through a 'flood' in Texarkana to get back to Dallas (from Nashville). About 2 years ago.  Water was more than a few feet deep and cars were getting washed down anything that hinted at being a 'hill'.   "Big Red" had no trouble, though water was high (poor decision to drive through, I am aware, but it arose out of nowhere, and I saw no safe places to simply 'stop' not in the flood's path)

[junior varsity]

I'd like to get it sorted out as I may choose to spruce this beast up in the future (perhaps with full engine swap at this nearly quarter-million mileage).  But before I spend a dollar on switching to electric fans, or replacing the headers with some long tube ceramic coated ones, or even buy a mullet-wig and put a magnetic phoenix in the hood, I want to get it to run 'as it ought to'.


Power delivery wise, it feels pretty good still, even for this mileage.  No, its not as fast as the high revving v8's in BMW's latest M3, but that's not what Big Red is for. Its general proportions and motor is well suited for an urban-assault-vehicle duties (people mover) and towing trailers (small boats, jet skis, and motorcycles).   I had begun thinking of trading it in on a new one or something else, but there's far more sentimental value tied up in it than the pennies a dealership would offer me for it based on the mileage.  And in a phone call to my old man, he reminded me - even a new engine, or transmission, or whatever it might need is far cheaper than a car note since I was still over all very happy with the vehicle.

... though I have comprehensively worn the leather through to the foam on the door-side of the driver's seat by sliding in and out of it for the last 10 or so years.  Perhaps I will bogart the front seats out of a new Tahoe for a 'moderate' upgrade in the future.

[DarkMonster620]

How high did the water hit "Bog Red'?

If my doesn't play trick on me, the ECU is located behind the center consolo at floor level.

When you replaced the altenator, you disconnected negative first and then positive and reconnected inverse order, did you? Just checking

[junior varsity]

How high did the water hit "Big Red'?

If my doesn't play trick on me, the ECU is located behind the center console at floor level.

I think mine is located just next to the large, under-hood fuse box in the engine compartment. Like the fuse area, the computer is also covered in a black plastic shroud that is easily removable (tabs snap in to hold it and you press them in to slide pass the retainer to remove it). It is above axle-height by several inches, potentially above bottom-of-door height (though I'm not certain as I have never thought to look at it for such a reference).

When you replaced the alternator, you disconnected negative first and then positive and reconnected inverse order, did you? Just checking

Yup.

[junior varsity]

how many grounds should I be looking for to inspect/replace/upgrade?   Any 'engine-compartment-suitable' car-audio power/ground wire of 1/0 gauge appropriate for such a task?

[DarkMonster620]

I think mine is located just next to the large, under-hood fuse box in the engine compartment. Like the fuse area, the computer is also covered in a black plastic shroud that is easily removable (tabs snap in to hold it and you press them in to slide pass the retainer to remove it). It is above axle-height by several inches, potentially above bottom-of-door height (though I'm not certain as I have never thought to look at it for such a reference).

Yup.

OK

Check the ECU connectors; if you have never cleaned them, could be that they have moisture and this moisture might have created some rust adding resistance to the electricity and it could be that here lay your issue ...

It happens ...

[DarkMonster620]

how many grounds should I be looking for to inspect/replace/upgrade?   Any 'engine-compartment-suitable' car-audio power/ground wire of 1/0 gauge appropriate for such a task?

Yes, 1/0 is OK, but, try to go for non-car audio type, for the engine/transmission area you need cable that's easy to handle but rigid enough to be able to withstand temperatures and grime and water.