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2001 Suburban Not Charging

What say guys…first post to this forum.
Wife’s 2001 Suburban 1500 4WD…Battery not Charging. Dash Voltmeter shows only 12 (battery voltage-no charge).
DVM confirms only actual battery voltage when running.
Took the battery and alternator into O’Reilly’s last Thursday and both checked out ok. Battery less than 2 years old. Alternator is original.
I put new battery bolts on.
Cleaned the battery bolt holes;
Cleaned alternator connections;
Cleaned remote positive connector;
Confirmed continuity between all connections between alternator and battery;
Confirmed continuity between small positive cable going to the electrical center;
Checked all grounds including back of block; blew out dust from PCM connectors;
After I did all that, started up and I got 14 volts but it randomly fluctuated between 14 and 12 every now and then.
Son took it to work Friday and said it stayed up at 14.
Wife took it to work Saturday and said it just stays at 12.

I decided to check the brown and gray wires on the alternator connector. Not sure what they’re for.
I think the gray is the “field sense” and brown is to the “PCM” ?
Key off, no voltage across either of them(to ground);
Key on, not running: Battery voltage =11.6v; Gray = 0; Brown = 9.98v
Engine running: Batt voltage = 11.4v; Gray = 0; Brown = 9.96 volts;

Any advice is appreciated. Can’t do with out the 'burb for very long and a new alternator is $150 !!

Shade Tree Steve

The charging system in your vehicle uses the PCM to regulate the voltage to the battery. If the voltage is erratic on voltage gauge, then one would have to conclude that there’s a problem with the PCM regulating the voltage to the battery.


Those brown and gray wires go to the power train control module, according to the schematic I have.
They appear to be inputs to that module, the brown being the “Generator Field Duty Cycle Signal” and the gray being the “Charge Indicator Control”.
Edit: Tester above^ suggests that those signals are outputs from the power train controller.
Could be, I guess. Usually regulators are present inside alternators but they may have done it differently on your truck. If so, you may be having more serious problems related to the computer. Let’s hope that’s not the case.

I’d check the ground from the alternator to the battery. If you don’t see it charge, see what happens when you use a booster cable - the BLACK WIRE ONLY - and bridge the connection from a beefy piece of metal on the engine to the NEGATIVE post on the battery. Then start the car and see if it still doesn’t charge.
DO NOT USE THE POSITIVE PART OF THE CABLE - just let that one dangle and don’t hook it up to anything.
If you find that it charges when you put the negative booster cable on, you likely have a bad ground connection from the battery to the frame or from the frame to the engine.

There’s also this thing called a “Battery Cable Junction Block” that ties the positive of the battery to the alternator and the rest of the car. Follow the fat cable from the alternator to where ever it goes - that would be that junction block.
It probably has some bolted up connectors. I’d check that to make sure it is clean and tight.

Worst case, take the alternator out and see if there are any motor repair shops in your area. Perhaps it has an intermittent problem of sorts. There’s a shop in my area that can test and repair them while you wait, if they are not busy. It may set you back $30 or so but it will be good as new.

Thanks for the ideas Tester and RemcoW. I’ll head out to do more checking now.

Good luck. Let’s hope it is something simple as a bad connection and not the power train controller.

ok, problem solved.
My nephew was kind enough to drive 70 miles and bring his laptop, computer diagnostic program, Alldatadiy subscription and the alternator from his own '03 Suburban.
We didn’t connect his alternator right away but rather went through the troubeshooting process.
We were immediately stumped when he attached the OBD2 connector but the program didn’t recognize a vehicle attached.
After searching the web, he found a post that said to check the cigarette lighter fuse because they’re on the same circuit. Sure enough, my fuse was blown. We replaced it and thankfully the program noticed the vehicle.
When the analysis was done, it showed codes P1637 and U1041. The U1041 is the ubiquitous ABS/Brake warning light. I’ll clean that ground later.

In the end, P1637 error code troubleshooting chart lead us to a bad alternator.
We installed his alternator to verify and sure enough, it worked.

So after a week of cussing, scraping knuckles and puncturing skin…it’s what we originally thought…the alternator. What led me astray were the, not one but two positive bench tests at O’Reilly’s of the alternator and battery. If I had purchased an alternator and it didn’t fix the problem, O’Reilly’s wouldn’t have taken it back so that’s why I didn’t do that earlier.

I actually upgraded to a 145amp for an addtional $10.

Anyway, thanks for everyone’s help.

Great - glad you were able to fix it yourself and hope it didn’t dip into the beer and fishing fund too much.

Wow that is a story worth remembering. Thanks for the update!

Off topic? Where?

I agree. Very nice to hear the outcome and an issue got fixed for once.

Ran into a somewhat similar alternator problem on my 91 Dodge D150 pickup. The voltage regulator went out and is integrated into the PCM. Instead of replacing the whole computer, I got an external regulator off on older Chrysler at the u-pull, mounted it and routed some wiring…Good to go.

Glad you finally resolved the problem, sorry it was such a convoluted, time-wasting, and tedious process. It’s a good example of why bench-testing parts isn’t a reliable way to come to a conclusion of what is at fault in a system.

Some people balk when they come in with a complaint of no charging and hear that it costs $50 for me to do a complete electrical system test, but we charge and load test battery, test starter draw and alternator output with both a carbon-pile tester and a labscope, scan for and troubleshoot any relevant trouble codes, perform voltage drops on all battery and ground cables, test regulator or control system when possible, and give a definite guaranteed diagnosis.

I agree that GM’s tying the power for the data link to the lighter fuse is a bad idea, since that lighter fuse gets blown a lot.

I agree that GM's tying the power for the data link to the lighter fuse is a bad idea, since that lighter fuse gets blown a lot.

Yeah, that sounds like they’ve put technology in a car for the sake of technology.
I’m all for technology when it makes something better, stronger, cheaper and more manageable but things like that are just stupid.
Like using the PCM to determine the charge level when there’s perfectly cheap known technology out there that just works. They really do that, then?

Asemaster is doing an electrical test the way it should be done even if it does involve a fee for that service.

That process eliminates the parts throwing and customer returns.

Whenever someone says “that was a stupid thing to do” it usually indicates they don’t know the whole story-

I’ve caught myself thinking like that, usually when I’m wrestling with something frustrating. But found out too many times I probably didn’t have enough information to understand how/why it is the way it is…

I invent better mouse traps for a living. This ain’t one of them.
It is just there to obfuscate the system, making it more difficult to diagnose the system, unless you have special tools - ie if you’re a dealer.
There was nothing wrong with using a regulator and ELD, like we’ve been doing for decades. Easy to diagnose. Unless you decided to hang a 400W amp in the trunk, we were all happy.

Maybe if you’d actually read the article you’d understand why a simple regulator won’t work anymore. Seemed pretty well explained.

If you have better ideas why not dazzle us with YOUR solution, keeping in mind the issues described in the referenced article.

Oh, I’ve read it. We can argue about this forever but it just seems to be an overcomplication of part of a system that works fine on most other cars. They don’t do it this way, after all.
We clearly disagree on this but we’re not here to argue (I hope) so let’s just move on.

Just my humble opinion, but I’ve never had a starting or an idle correction problem on my cars or family members cars due to increased electrical load. I understand the theory behind why it’s done but I can flick on everything at once on my Lincoln (even the headlamps are automatic) and there’s no hint of an idle stumble.

@RemcoW: “it just seems to be an overcomplication of part of a system”

Had a late model Chrysler product in, no left brake light. Shop helper put a new bulb in, still no taillight. Out comes the DVM, good ground but no voltage. Hmm, car’s awful new for wiring problems. Then I remembered reading about this somewhere. Hooked up scan tool, accessed body control/lighting, found and cleared fault code for open/short circuit left brake lamp, and brake lamps now functional.

Had an Explorer in for no washer fluid operation. Diagnosed that without opening the hood, the car had a fault code for open washer pump relay.

I look at some of these systems and think “Really? We need this?”

It would appear that the alternator on the 2001 Suburban is wired through the ECM/BCM so that when accelerating, the alternator is shut off to allow more power to the wheels. Such a set up often causes charging problems that are difficult to diagnose and when bench testing the circuit to the ECM is jumped. Often these systems don’t immediately pick up a charge at start up but when the throttle is pressed the voltage jumps up to normal where all is well until accelerating. If the throttle is depressed far enough to drop the MAP to some predetermined threshold the alternator is shut off, as is the AC and the blower. Wthe throttle is released back to cruise the charging system should immediately kick on but often doesn’t and finding the cause can be difficult. The voltage regulator has frequently been the cause.