SOLVED! 07 Trailblazer not charging.

[Stugar]

I just picked up an 07 Trailblazer 4.2L which had been sitting since October. When I got it it needed transcooler lines, power steering lines and a R&P, and brakes and ball joints, all is done except ball joints. I didn't exactly notice for sure whether it was or wasn't charging on the way home from the sellers house, he'd had to charge it to get it running but I drove it home. I did take out the cooling pack, battery, and battery tray to facilitate changing the line sets, but it went back together just fine. Also, I had to mess with all the bundles of of wires near the PCM to get the clamp for the PS lines off, I tried to avoid moving them too much though, and I have no other issues but this, it starts up and runs just fine, but no charging.

So, what I know.

• There's no voltage coming from the alternator; battery voltage while running is the same as once it's stopped, plus or minus a few tenths of a volt
• Oreilly's says the alternator is good
• There are 0 ohms between Battery + Terminal and Alternator lug, I have not tested this at load though
• Generator Turn On Signal sits at 0V engine off, key on
• Generator Turn On Signal (GTOS) sits at 2V engine on (it is supposed to be at least 3.7V)
• The Generator Battery Control Module (GBCM) has power and ground
• There is continuity between the GBCM and the Alternator on the GTOS wire
• I am not sure if it's throwing a DTC as none of my code readers will pull up anything, but they're cheapo generic code readers.
• There is continuity between Pin J of the GBMC and Pin 1 of the PCM
• There's 9k ohms between the GTOS and the Generator Field Duty Cycle Signal
• No fuses are blown.

Per the Service Manuals I tried replacing the GMBC since it's supposed to control the Generator Turn On Signal PWM. Just to test it out I didn't go through the trouble of pulling the entire battery ground, I just plugged it in in place of the old one, started the engine, and still no charging, still 2V on GTOS while running. About the only thing I haven't tried is manually supplying voltage 3-5v to GTOS.

I'm unfortunately leaning toward a PCM issue now. Damn this thing is getting expensive to fix. Any thoughts? Anyone run into a similar issue? I know the GBCM GMT360's aren't the most common trucks, being only the last three years of production, but hoping somebody else has run into this, if not on the 360 then maybe on one of the full size platforms that got GBCM a few years earlier.

This wiring diagram I was able to find matches the one on page 154 of the 2007 Wiring Diagrams PDF for reference, while being a bit more convenient to link to.

 

[gmcman]

Did the alternator stop charging the moment you picked it up or after you had worked on it? Did you have to jump start the engine and if so did this happen after the jump start?

I had a similar issue once, with the engine running, try wiggling the signal harness near the alternator. When you checked the voltage, did you verify voltage after the connection or at the connection?

Mine was the fact the 2-wire connector was loose and had to spline a new connector.

 

[Mooseman]

Have you tried just unplugging the GBCM (aka: SARVC)? This would make the alternator work as a normal alternator without the controls from the GBCM.

Found this article that has very useful information about the SARVC system:

Editor's Report | Automotive Resources | MOTOR Magazine

RVC Redux The July Trouble Shooter column on charging system problems on a 2007 Chevy Silverado generated some interest among Motor readers, and prompted the following insights from Mohammad Samii, owner of Sammy’s Auto Electric Service, Inc., in Champaign, IL. Samii has written the “Auto...

www.motor.com

This part also has an interesting part about it when the SARVC fails:

The RVC generator’s default mode has another feature. If an anomaly is detected in the system, the generator will disregard the L terminal input and will turn itself on in a default mode with a 13.8V setting on the regulator. This feature is designed to ensure the continued operation of the generator and charging system if there is a failure in any of the associated modules that participate and control the RVC charging system. Of course, the dashboard warning light would be on if the generator is running in default mode, to indicate a problem that needs to be addressed as soon as possible.

So if it fails, the alternator is supposed to work regardless of it.

I have also attached the system description from GM-SI. In there, it says that the turn on signal is a 5v PWM signal so a normal meter would not display this properly, which is probably why you're seeing just 2v.

A better scanner would be required to scan codes on this system (U and B codes). A cheap Elm327 Bluetooth adapter and the Car Gauge Pro app on an Android device would be able to do this.

I'm also leaning towards a wiring issue or that the alternator is defective despite what the tester said.

 

[budwich]

lots of good testing so far. A couple other checks might help. First you checked the resistance between the GTOS and field cycle... but you also need to check the resistance between the GTOS and a KNOWN ground since you are indicating that the GTOS seems to be low which might indicate a short to ground or equivalent. Having said this, what were you actually measuring... ie was this with the controller connected or disconnected... basically you need to do this test in "both directions" with the plug disconnected.

Another test that is suggested in the manual (I have never tried this one)... is disconnect the alternator. Turn the key on and measure the voltage on the "L connector" (load). Should be zero. Then start the engine and do the same measurement. Should be ~3v. Go from the results.

Ultimately, as suggested by mooseman, disconnect the controller will result in the alternator going to "normal on" operation putting out about 13.6 volts.

 

[Stugar]

Did the alternator stop charging the moment you picked it up or after you had worked on it? Did you have to jump start the engine and if so did this happen after the jump start?

I didn't notice, I was paying attention to other stuff like transmission since I was told it had a "rather profuse leak" from the lines, the PS pump, since I know that thing can puke out a quart in a hurry. I didn't notice till after I rolled it out and took it for a test drive. I do know that at the time of doing all the repair work the battery needed charged again, so it probably wasn't charging on the 15 minute drive home now that I think about it.

I had a similar issue once, with the engine running, try wiggling the signal harness near the alternator. When you checked the voltage, did you verify voltage after the connection or at the connection?

Mine was the fact the 2-wire connector was loose and had to spline a new connector.

I checked it by side probing the connector at the very end of the connector. The connector seems to be tight in the alternator.

Have you tried just unplugging the GBCM (aka: SARVC)? This would make the alternator work as a normal alternator without the controls from the GBCM.

I am definitely going to try this as soon as the battery is charged and it's down on the ground, currently have the LCA's off to pull Ball joints. I am always hesitant to unplug modules that are on a databus because I've assisted on a car (really old, can't remember which) where we damaged some stuff by trying to run it without a module plugged in.)

Found this article that has very useful information about the SARVC system:

Editor's Report | Automotive Resources | MOTOR Magazine

RVC Redux The July Trouble Shooter column on charging system problems on a 2007 Chevy Silverado generated some interest among Motor readers, and prompted the following insights from Mohammad Samii, owner of Sammy’s Auto Electric Service, Inc., in Champaign, IL. Samii has written the “Auto...

www.motor.com

This part also has an interesting part about it when the SARVC fails:



So if it fails, the alternator is supposed to work regardless of it.

Makes sense I suppose, typical GM redundancy. I have encountered plenty of alternators that test good but don't charge (see the "too many Mercury Cougars" in my sig)

I have also attached the system description from GM-SI. In there, it says that the turn on signal is a 5v PWM signal so a normal meter would not display this properly, which is probably why you're seeing just 2v.

I thought any decent multimeter, which mine isn't a Fluke, but it's not terrible, should be able to read the PWM signal, it's just rapidly turning the circuit on and off to get an average voltage which just about any voltage sensing should pick up as a fixed DC voltage. But I suppose I could be wrong here

A better scanner would be required to scan codes on this system (U and B codes). A cheap Elm327 Bluetooth adapter and the Car Gauge Pro app on an Android device would be able to do this.

A better scanner has been in the plans for a while, probably looking at something from Autel, but with the coof affecting the business I work for I'm on a 20% pay cut currently, really stretching the budget just fixing this car.

So yeah, thanks for the help all. I'll give the simplest stuff a shot first, then start chasing more possible causes if that fails.

 

[Stugar]

So, after buttoning up the suspension bits I hooked up the freshly charged battery, unplugged the GBCM, started it up, and it charged. great... I cycled it a few times, still charging. Then I plugged in the GBCM and it quit charging again, but the weird part is that unplugging it again it's still not charging.

While I had it apart I checked for a short to ground or power while I was at it, unplugged the alternator, the GBCM, and PCM, and got nothing. I probably should have checked by plugging in each of those and checking, but I was, at the time In a bit of a hurry to wrap things up since I have to be to work in the morning.

I've got a theory that the alternator will stop working if it's much above room temperature, which is why it tested fine at Oreilly's on the bench, but not when put back in a warm engine bay after reassembling things. Anyone have experience with the Pure Energy brand reman alternators? $29 plus core seems like too good of a deal honestly. I'm more inclined to go with the Remy since they're the OEM for our charging system, but it is twice the price...

In other news, backing it out of my buddies garage (he has a lift, no way in hell I'm doing it on the floor when he never uses that thing) it had no power steering. I get it to it's oily parking spot, check the reservoir, nothing. Check the path from and too, nothing. Neither tie rod boot is full of fluid. I start poking other things, reach up under the car, poke the steering shaft boot, and a quart of PS fluids pours just to the side of my hand and comes rushing at me! The brand new (well, remanufactured) rack I JUST put in is leaking profusely from the steering shaft. It had drained the reservoir in the couple of minutes that I was testing the alternator, I'd literally just topped off the reservoir prior to starting it up.

 

[Mooseman]

I've had better luck with new alternators than remans. I've had an ACDelco rebuilt crap out on me on warranty, replaced it with a new one by some unknown name, lasted to the truck's grave. I also had a Remy crap out on me, pretty sure it was also a reman.

Sucks about the rack. Was it bought or rebuilt locally? Not much choice for those.

 

[Stugar]

It was an Oreilly rebuilt unit, not a local one, so at least it has a lifetime warranty. I'd ordered a new Detroit Axle unit, but it came smashed up because they don't package their stuff at all, it was just thrown in a box. So I went and got the reman unit rather than wait another 2 weeks for the new replacement, which would've probably show up damaged just like the first.

 

[Mooseman]

Good to know about Detroit Axle. Most here have had good experiences in the past except lately, they've been having some quality problems, and now, packaging and shipping problems.

At least it shouldn't take too long to exchange it at the store, the one advantage of brick and mortar stores. Still sucks you have to do it again.

 

[Stugar]

TBF a lot of that is on UPS throwing the box around, but proper packaging would have prevented it.

I'm lucking I can go to their store to pick up some stuff at least, that's what I did for the suspension bits, the 18 piece kit is the same cost as 4 ball joints from a chain store.

 

[Stugar]

Well, replaced the alternator, and steering rack this weekend. I'm down to an hour and a half each way in and out on the steering rack. Warranty rack of course had one 16mm threaded inner tie rod and one of the smaller (14mm?) tie rods installed, luckily that 18 piece kit from Detroit Axle came with inner tie rods so I just swapped that over to the new rack.

Got the alternator in this morning, got everything buttoned up, plugged in, started it and it was charging. Kept charging the whole time I ran it and I ran it till it got up to temp, so I'm pretty sure it was the alternator. Proof that these computer controlled alternators just can't be left to sit for a few months and then be expected to come back to life problem free, this isn't the first car I've had such an issue with.

Thanks for the advice everyone. Now I just need to get it in for an alignment, and get the GMOS4 interface I ordered from Crutchfield last night installed, and the tires swapped over from the envoy, and I should be good to go for a while!

 

[mrrsm]

FWIW... The VOP (Video Original Poster) goes through a similar Diagnostic Approach using a DVOM and the Wiring Diagram on a 2006 GMC Envoy Not Charging.

He eventually winds up calling the problem as requiring the R&R of the Alternator. But... He ALSO decided to Replace the Serpentine Belt AND the Belt Tensioner at the same time as being symptomatic with the Alternator Failure:

 

[Stugar]

Yeah, Belt, Tensioner, and Idler pulley are on the menu soon.

And finally I got the conclusion to the Detroit Axle issue, they issued a refund so I won't have a spare rack sitting around and I get my $200 back.

BTW, does the option to edit a post, like to say "solved" instead of the "Need Help" tag disappear after a certain amount of time, or is it that you can only edit your latest post? Seems odd that you can't go back to add relevant information and a conclusion to your original post.

 

[Mooseman]

BTW, does the option to edit a post, like to say "solved" instead of the "Need Help" tag disappear after a certain amount of time, or is it that you can only edit your latest post? Seems odd that you can't go back to add relevant information and a conclusion to your original post.

Done. After a certain number of days, a post can't be edited. You can ask to have it done by a mod by hitting the Report button or asking in the post.

 

[Stuntmanmike1977]

Hello all. Maybe a similar issue here but just a few days ago I was just driving to the store and the battery light and battery not charging message came up on the dash and the voltmeter went to between 9-12. Only happens for about 30 seconds up to a few minutes then goes back to normal! Happened about 10 times. Only thing I've done was clean up the ground straps to body and block with no change. Am very ignorant when it comes to electrical issues. Don't know what the GBCM or field cycle is, no scanner or meter. Don't know where to go from here and can't afford a garage diagnostic job. Any simpler advice would be greatly appreciated or do I just replace the alternator which would be a little while. 03 LTZ 4.2. Thanks in advance.

 

[Mooseman]

I think you can take it to a parts store to have it tested however if it's an intermittent issue, it might not show itself as bad at that moment. I personally would just replace it given the age and mileage.