[Dbl_D718]

I'm usually decent at figuring out electrical problems, but these have me stumped. First up is what seems like an alternator issue, but replacing the alternator didn't help. Sometimes at idle the alternator quits charging. It only happens with the car in gear and I'm coasting or braking to a stop, or if I'm stopped like in parking lot turing the wheel. The RPMs drop to around 6-700 and that's when the alternator quits. I got a replacement alternator and still have the same problem, so I'm ruling that out. I've read that there's a minimum charging RPM for the alternator and I'm wondering if maybe that's what I'm seeing, because anything above the 6-700 RPM and the alternator charges fine. What is that minimum RPM, and why would I be having problems with it? The problem seems to be a lot more common if I've been driving kinda agressively, especially if I've gone WOT.

The other issue is relatively new and I'm not sure if it's related to the first. When the car is running (and alternator charging normally) the battery light on the dash will come on, followed by the check engine light for an instant and the whole car shuts down electrically for a split second, then comes back. The gauges all drop to 0 and the car shutters, then it starts back up and goes back to normal. Not sure if it's a symptom, but the torque converter is always unlocked when everything comes back. Also, if the car is sitting still instead of driving down the road, it'll die and need restarted, so I'm assuming that going down the road is the only thing that keeps it running otherwise. Whatever code it is that sets the check engine light isn't tripped long enough for my Aeroforce to read it. The symptoms make it seem like there's a loose connection somewhere, but I've triple checked them all. My next guess is the power or ground wire going to the PCM, but I don't know which wires those are. Any ideas?

[blazerman3]

Your engine is idling too low...you are right that the alt can`t charge the circuit because it`s running too slow. What has made the engine idle slower is the key. When did it start and have you had work on it recently? I would start there before doing anything else.

[Dbl_D718]

It only gets that low if it's in gear, like while I'm coasting through my apartment parking lot or braking coming to a stop sign. It idles at around 800 I think if I'm sitting in park, or even after I've come to a complete stop in drive at a stop sign. Just when I'm slowing down does the idle drop to the 6-700 RPM.

The tune was done by PRJ, and I don't remember this happening while I was there getting tuned, and nothing's been changed since then.

[me6894]

6-700 is normal. IIRC stock setting is like 670ish.

How do you know the alt isn't charging at idle? Are you monitoring volts?

[Dbl_D718]

The voltage gauge on my dash drops quite a bit from normal, all lights dim, and the voltage readout on my headunit drops to around 12.5 volts.

[me6894]

Where did you get the alt from? Either way I would get it tested on the car. At 1500 rpms it should put out 14.5ish volts and uder a load should put out 80 amps or so.

[Dbl_D718]

It was the stock alternator until today, and I replaced it with a stock one from a 55K mile GP that I got from Morad and have the exact same problems. When everything's running right, it does charge at ~14.5 volts.

[cak387]

Dbl_D718, on 06 March 2010 - 03:59 PM, said:

The voltage gauge on my dash drops quite a bit from normal, all lights dim, and the voltage readout on my headunit drops to around 12.5 volts.

I have these same problems, except I don't have a volts gauge. My radar reads right at 12 when I'm stopped and I notice all my dash lights dim and the heater slows down by itself. I always thought it was just the alternator about to go out, but that doesn't seem to be your case Dbl and it may not be mine either.

[me6894]

Its perfectly normal for that to happen. That is for the heater to slow down a little and the lights to dim a bit at idle. My car has done it for 2 years and even with a volt booster. The alt just doesn't charge much at idle.

As for the engine light and battery light coming on, that's not normal and sounds like it could be a bad connection somewhere. Try cleaning the battery terminals. Also, have you had the battery tested lately?

[Dbl_D718]

Yeah I know that minor voltage drops at idle is normal, but dropping all the way to ~12.5 isn't. My car never used to do that until a couple months ago, and it's only intermittent at this point. I've always just assumed the alternator needed replaced because my stereo tends to wear them out prematurely and I've just been waiting for warmer weather to do it. (Note: this problem occurs with the stereo off too)

The battery terminals are nice aftermarket ones and since I had them both off today while doing the alternator, I know they're clean and all. Other connections all seem solid and clean as well. I've also got some redundancy with aftermarket alternator and ground wires on top of factory ones, so those shouldn't be problem areas either. I haven't actually had the battery tested, but it's an Optima Red Top that's about a year old so I'm hoping that it's not the problem. Couldn't hurt to get it tested though I guess, I'll do that Monday.

My plan is to get a video of both problems posted, but I couldn't get either to happen on the way home from work tonight.

[me6894]

Its not uncommon for mine to drop to 12.0 volts with the radio on and blower blowing on high.

[blazerman3]

But should`nt the alt pick up the draw and recharge the batt? All 3 of my GM`s have reman alts fron NAPA and I haven`t had a problem with `em. I`m thinking the voltage regulator should command the charge around 12.8 or so.

[Dbl_D718]

Ok so my car did the second problem really freakin bad tonight. I got it on video this time in hopes that someone will have an idea on what to do. The car hasn't done this even once for a week and a half, then on the drive home tonight it was horrible. My phone is the camera, so pretty bad vid quality, sorry.

Skip to about 20 seconds on this one to see it.
Video 1

On this one, I'm sitting at a red light (with the car in neutral so it doesn't die in traffic) and all of the revving and everything happens on it's own. My foot doesn't touch the gas until the camera comes off the dash for a couple seconds as I shift to drive when the light turns green.
Video 2

WTF?!?!?!

[nitehawkjcb]

What the f*ck... That is VERY weird... It especially sucks since your Aeroforce won't store the codes. My car did something similar when my battery died due to a bad alternator, but the car was not driveable once it happened. Check all the wires going to/from your battery and alternator again. Sorry I can't help more.

[Dbl_D718]

Bump for more ideas. I'm looking for a replacement PCM (but this one is only a couple months old) and it was suggested that I check and clean the ground on the transmission bell housing, so I'll be doing that Friday. It hasn't happened again since I took those videos, but something is obviously wrong and I gotta figure it out so I'm not constantly worried about my car getting me home.

Any wire harnesses, grounds, or specific wires I should look at? I can't think of anything I did to the car right before this started happening, so I'm really just lost.

[ChibiBlackSheep]

Think it's possible to get your BCM tested? A lot of times when that starts to go bad weird electrical problems that you can't figure out just start happening.

[Dbl_D718]

I'd be willing to try it. Not sure what's involved with testing it though. Something I'd have to have a shop do I assume?

A guy I was talking to today turned out to be a mechanic at a local shop and after a brief desciption of what happens, he said crank position sensor could be it. Does that make sense?

Also, it happened about 3 times on my way to work today. A new symptom I got to observe today is that it'll turn off cruise control when this happens. Dunno if that helps narrow down the possibilities at all. Where could I find a main electrical connection that would power the PCM, BCM, cruise control, etc; maybe it's loose/corroded?

Am I correct in assuming all of the lights in the "message center" on the dash are BCM controlled?

[me6894]

There is no way to test the BCM.

I take that back, start the car and find the bcm, with the car running shake the bcm and hit it. If it acts up its probably a loose connection inside it.

You really need to be looking at a scanner while this happens or get a snapshot to really see what's going on. Ask the dealer if they're willing to let you use their Vehicle Data Racorder. If the do not know what this is, find another dealer.

The VDR is just a big button that hooks to your OBD2 port and you keep it in your car then whenever you can get it to act up (assuming the dealer can't) you press the button and it records a snapshot that a tech can look at and hopefully see something that's off.

And no, I would not suspect the crank sensor.

This post has been edited by me6894: 23 March 2010 - 11:28 PM

[Dbl_D718]

Where does the PCM get power and ground? Seems like that is a likely source of the problem.

[Dyonysis]

BCM ground is a stud to to one side of the gas pedal bracket. Stuff your head down in the wheel well and you should see it fairly easily.

[Dbl_D718]

Well it got to the point today where the car isn't driveable anymore. I tried limping it to the dealership but it didn't make it there, gonna have to have it towed tomorrow probably. I'm almost positive it's a loose connection now because driving down the road doesn't restart it and then when I turn the key nothing happens except that the check engine light flickers. All the while, the radio/windows/interior lights/everything not under the hood still works fine, so that should rule out a battery or battery terminal problem. The positive posts on the under-hood fuse boxes are solid, so where the heck are any other main connections I can check?

This sucks. I was gonna buy a motorcycle tomorrow, but now I gotta spend that money to fix the car.

[Dbl_D718]

Well I got the car into the shop today and fotunately they reproduced the problem. The guy confirmed that the PCM was loosing power intermittently like I suspected, and he determined it to be the PCM connection. He said he tested the power and ground on the harness and saw no problems, plugged it back in and couldn't get the car to act up again. It's wierd though because I've tried unplugging the PCM an plugging it back in before and the problem came back, so I'm kinda expecting it to happen again.