Mechanics

WHY DOES MY BCM ONLY WORK WHEN THE HEADLIGHTS ARE ON?

2002 Saturn L300 • 96,000 miles

A few months ago the service engine soon light, the service light with the wrench icon & the security light all came on & remained on to this day. Along with that, the speedometer needle, gas gage needle & temperature gage needle were jumping all over the place & made fast clicking sounds. The odometer was tacking on 20 miles for every block I drove & soon the mileage was up in the 600 thousands. The shifter would not engage into reverse or drive when in park, but I lifted up the boot piece & manually lifted a lever that allowed me to shift it into drive or reverse. The AC went from blowing ice cold to blowing at room temperature.

After researching the problem it sounds like the BCM but at night I noticed the car did not behave this way & everything worked perfectly fine, the gages, AC & shifter, although both the service engine lights remained on. I wondered what could be different in how the car operated at night as opposed to daytime that could make the car run differently & I figured out that the one thing different was that the headlights were on at night, so when I'd start the car in the day I'd immediately turn on the headlights & sure enough, the cars functioning would be normal & my odometer went all the way down to just 2,000 miles. If I turned the headlights off while the car was running it would go right back to functioning crazy!

I figured turning on the headlights was somehow engaging the BCM & figured I had found some kind of bypass & it worked fine this way for about three months then I noticed the Airbag light come on & the ABS light (in addition to the service engine lights) but still, the gauges weren't jumping around, the AC blew cold & it shifted into reverse or drive as long as I turned on the headlights when starting the car. Then about a week ago, everything just stopped working, but now including the electric locks, the gas gauge, the speedometer and temperature gauge & all the warning lights remained on & I was back to manually engaging the shifter.

After reading tons about the BCM's in L300's, I decided to disconnect the battery cable for a couple of hours & when I reconnected it, everything worked fine again as long as I turned on the headlights, but all the warning lights remain. The car always starts & has never broke down or died but once the electric locks, speedometer & AC stopped working I want to get it fixed. Now just yesterday, it wouldnt start at all, it would crank for a bit, then start to start but then go back to just cranking. I disconnected the battery for a couple hours, reconnected it now everything works again as long as I turn the headlights on when I start the car.

I hear BCM's are costly and. Can only be programmed by the GM dealership now that Saturn is no more. My question is, Is the BCM really bad because it still works as long as the headlights are on. Could it be a fuse or just that the software needs to be upgraded as stated in the recall on the BCM's for this car (bulletin 02-C-02 & 02-C-07 why TWO recalls?)

Thank you for your help, I appreciate it : )
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Avatar
Gettup
October 9, 2012.



Have you ever have the trouble codes read?


KHLow2008
Oct 9, 2012.
You need a new bcm I have replaced a ton of bcms for all different reasons on those carsOnce the mileage starts rapidly changing bad bcm I have replaced a ton of those bcms just for that reason alone. As far as only working partially with the head lights on I would say ithas to do with the part of it that failed. As far as to why it does that cant say I dont build themso cant give a good reason.I have worked on these cars for a very long time at the saturn dealer so I have seen a lot of these bcms go bad.


Saturntech9
Oct 9, 2012.
Thank you both for your replies. Answer KHLow2008, yes, I have the print out from the GM Dealer this is what it says verbatim: A C/S Cluster Inop at time, found updated bulletin online
86001 Multiple failures BCM B1000 ECU Malfunction.
Security Light On, Air Bag Light On, Service Vehicle Soon &
Check Engine Light On. P0171, P0174, P0421, P1632.
Can you decipher them for me please?

Sauturntech9,
They didn't really have an answer as to why the car functions correctly
with the lights on and said it may be fine like that for
a very long time or it can stop working tomorrow.

For some reason, it drives excellent and hate to spend money on a new BCM if
it can be fixed another way or if it stays running ok, but since it wouldnt start yesterday until I disconected the battery, I don't want to be stranded somewhere. Can you recommend a place to get a new BCM other than the dealer that is reasonably priced?

Thank you both for your help, I appreciate it : )


Tiny
Gettup
Oct 9, 2012.
The BCM is a funny component. It is sensitvie to current draw etc and headlights do affect the current draw which can cause some circuits to go haywire, if that answers your question.

If you want to spend time dwiddling on it, you can try checking for poor ground circuits, leaking capacitors inside the BCM or even bad soldered joints of the circuit board.


KHLow2008
Oct 9, 2012.
The B1000 is a commication code saying the bcm cant talk to the ecm from your symptoms bad bcm.As far as the codes P0171 and P0174 is a vacuum leak or a bad mass air flow sensor.Most of the time its a bad mass airflow sensor though.There is a bulletin for those codes.The P0421 is a egr code cant think of it off the top of my head.The P1632 sounds very familar to me also but iam no where near a computer right now maybe khlow2008 can post the definitions for us.As far as buying the bcm somewhere other then the dealer check http://www.parts.com and http://www.rockauto.com you will have to compare the prices of both places with shipping etc.Also cross the part number over to amazon from rock auto also.I have a 5 percent discount code for rock auto if you get it from there.But you will have to have the dealer put it in and program it or find a shop that can gm sps programing.The bcm is shorting out internally and they can do crazy things when that happens because they control so much.Anyone that trys to give you a reason why its doing whats it doing that is just guessing.


Saturntech9
Oct 9, 2012.
Thank you KHLow, I unscrewed the glove box to get to the BCM and when I took
it out I noticed it was marked with blue dots with a felt tip marker, doesn't
that signify it's from a junk yard? & If that's the case then it's not the original bcm
and this one came from another saturn. Is it then possible that maybe all it needs is to
have the updated software installed like the recall on the bcm's state because
it incorrectly registers the mileage? (Bulletin 02-C-02 and 02-C-07 both issued in Aug 2004)

My VIN# is listed in the recall but Saturn says the car was serviced under that recall already butprobably when the original bcm was in it, now that there is another one in it could it also need the software upgrade? Because the recall states that cars that are serviced with a replacement bcm will exhibit the same behavior and will also need the software update.

Another thing I find odd is Saturn said the car was already serviced under the recall back in 2003, but the recalls didn't come out until August 2004, so how can that be?


Tiny
Gettup
Oct 9, 2012.
Thank you saturntech9
can I get that discount code from you please? That would be awesome, thank you : )

I found the P1632 code, it says theft deterent fuel disabled. My car is getting fuel so why the code?

I know someone who works at a GM dealer (thats how I got the diagnostics done but they really didnt explain much and i'm a girl so I just try to learn as much as I can on my own, hence, my reason for posting here (thank you guys for helping me) the dealer will install & program, upgrade software, etc for $100. As for the rest of the problems the codes signify having, is it worth it to fix the car? Is it gonna be expensive?


Tiny
Gettup
Oct 9, 2012.
Can you take a pic of the marking on the bcm and post it for me to see?I have seen blue dots etc on the factory never replaced bcms. You have way more going on then incorrect mileage here the reason for the P1635 is when the security system went into tamper mode when it wont start it will set that code. From the bcm failing.


Saturntech9
Oct 9, 2012.
Hi saturntech9,

sorry it took so long to get back to you. I got injured and was in the hospital for quite some time. My saturn ended up being towed and I just got it back. I am still at the point where I left off, needing a BCM. I uploaded images of my BCM and I believe you are right, that the blue markings could be from the factory, never replaced BCM because if you'll notice, besides the blue paint (after closer observation, it IS paint and not a marker) on the BCM itself, it is also on the interior framework of the vehicle behind and on the side of the glove box indicating it was marked this way while IN the vehicle. If the BCM were from a junkyard, those markings wouldn't be in MY car correct? So is it safe to say that this is the factory installed, original BCM that came with the car?

Thanks again for all your help and expertise, it is greatly appreciated!
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Tiny
Gettup
Feb 13, 2013.
I have seen those marks before looks like the factory marks.


Saturntech9
Feb 13, 2013.
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