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Billy
Billy, Chevrolet tecnician
Category: Chevy
Satisfied Customers: 510
Experience:  Over 25 years experience,GM grand master for 8 years,certified smog and government safety inspector
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The pcm stays energized... and the prndl stays on..anti theft..airbag

Customer Question

i have a 1999 malibu... something is draining the battery, and i do know that the pcm stays energized... and the prndl stays on. Also the anti theft and airbag lights sta on too, but only when the car is off. It seeme to run okay when it starts, but when you turn it off, those three stay on.   When I disconnect the pcm bat fuse, it goes off. there is something also clicking under the glove box, when I plug the battery in. Have you ever had anything like this.
Submitted: 5 years ago.
Category: Chevy
Expert:  Billy replied 5 years ago.
hello and welcome to just answer.

I suspect that you will have to replaced the ignition switch. not the ignition lock cylinder but the ignition switch (electrical)
I think some of the electrical circuits are on because the switch is shorted out internally.

Best regards
Billy
Customer: replied 5 years ago.
Sorry for taking so long to get back... small emergency (if there is such a thing).

I changed the switch already, and it is doing the same thing. I have that BS schematic from Haynes. The problem is they only show the wires coming out of the cpu. I tested all the fuses that are supposed to be "hot at all times" and those that are "hot at on or run" And all are the way they are supposed to read EXCEPT the one that is PCM ACC. That one says hot. When I un plug the fuse, the lights on the PRNDL cut off... and the ANTI THEFT,and Air Bag.

Now I do know that when I connect the battery, the fuel pump relay, the fan #2 relay, and the fan mode relay energize( eng comp 14, 15), but the fans don't come on. If I take the main fuse from the pcm None of them click anymore. But that box under the dash still clicks L/H BEC BAT #2 (fuse #3 eng comp). The car will sart and run, but its draining the battery, and who knows what else it might do... There was a mechanic that did a valve job on the car, because it overheated, a short while back. He is the one who mentioned the other problem.

I am willing to give a little more for some helpful knowledge... also, is it possible the other mechanic shorted the pcm? Have you ever seen this sort of problem?

Troy
Customer: replied 5 years ago.
Oh yeah, and now I can run the obdii scan without even having to turn the car on... comehow the pcm is already on.
Expert:  Billy replied 5 years ago.
hello again,

I will check the wiring diagram for you and will get back to you on the weekend or monday if that is ok with you.

Billy
Customer: replied 5 years ago.
That will be fine... Its a crazy set up, I know but If you could see where the pcm acc fuse is getting power from, we might just find out the other stuff. I do know that if I unplug the ignition, and connect the battery then all the lights come on. If I unplug that little box under the glove, all the light (warning that is) come on. Is it possible that the box under the dash(the one that clicks) is sending the power to everything and bypassing the ignition?

I know this is a headache... but its' definitely something new and interesting. I am willing to pay a little more for your insight, and we can work that out with progress.

Good luck on your search.

Troy
Expert:  Billy replied 5 years ago.
hello again,

quick question, is your vehicle has a 2.4 or a 3.1 liter engine?
also which fuse are you looking for again? first you said pcm bat but then you said it was the pcm acc fuse that is on all the time.
please let me know so I can provide some info for you.

Billy
Customer: replied 5 years ago.
<p>hEY,</p><p> </p><p>1. It is a 3.1L</p><p>2. The pcm bat says hot at all times</p><p>3. The pcm acc says hot in run or start (book), but it is hot always.</p><p>4. The fuel pump relay clicks, and the fan relay clicks when I hook up the battery, but the fans dont come on. </p><p>5. The PRNDL, air bag, and anti theft lights stay on... go off when I take out the pcm bat or the pcm acc fuse.</p><p>6. There is also a 4in. black box with 3 plugs, under the glove, it clicks when I hook up the battery too. If I unplug it all the warning lights come on.</p><p>7. The car starts and runs, but not everything is cutting off with the key.</p><p>8. If you hook up the little obdii, it works even without the key, other cars need to be turned on.</p><p> </p><p>I wanted to give you as much of a picture as I can, so if it seemed wordy, then I apologize... but all this is actually going on.</p>
Expert:  Billy replied 5 years ago.
hello again,

I need you to go to the left side IP fuse block and with a tes lamp check the PCM ACC and the IPC/BFC ACC fuses wit the ignition key OFF. only these two fuses at the moment.
please tell me the results.
Customer: replied 5 years ago.
The both have power (current) going through (the light lit). I went a little further, and unplugged the #44 eng comp fuse (pcm bat)... and they both register no current.

If you like you can call me XXX-XXX-XXXX... it might make things easier. I may not be at the computer when you are.

Troy
Expert:  Billy replied 5 years ago.
can you reconnect everything and pull the PCM ACC fuse only with the key OFF,
and then test the IPC/BFC ACC fuse again. tell me what you have found.

sorry, unfortunatelly can't call. it is against company policy.

Billy
Customer: replied 5 years ago.
the left side of the pcm acc fuse block is hot... and the IPC/BFC ACC no longer shows current
Expert:  Billy replied 5 years ago.
ok...now with the PCM ACC fuse still out turn the ignition on and retest both teminals where the fuse used to be, also just to make sure at the same time retest the IPC/BFC ACC fuse as well.
we are getting closer.

Object Number: 264785  Size: FS
Customer: replied 5 years ago.
Both sides of the pcm acc block have current (no fuse). The ipc/bfc is still lit, but only the right side if I take out the fuse.
Expert:  Billy replied 5 years ago.
ok then...so now we now for sure that the pcm is backfeeding through the circuit 1500 to the underhood fuse block (PCM ACC fuse)
next step is to find out what is causing it.
it could be the pcm it self or some other module is shorted out. this is going to be fun : (
I will do some more research and get back to you.

Billy.
Customer: replied 5 years ago.
Have fun...
Opportunity for enlightenment can come at anytime, and what more than an experience like this to keep ou on your toes and stay the grandest.
Don't worry, I am going to add more money, before this is over. I just need to know how much the actual cost for the repair is.

God Speed
Expert:  Billy replied 5 years ago.
I guess, I feel very enlightened now... : )

The black box under the dash is the BCM, you could try to disconnect and reconnect one connector at the time with the ignition off, and test the PCM ACC fuse for power each time you disconnect a connector at the BCM.
I have a feeling that the problem is somewhere else though and not with the BCM.

There is two other things you will have to inspect as well. check both left and right side instrument panel fuse boxes for water intrusion. pull fuses and relays if you have to and check for signs of corrosion or rust. If any found then you may have to replace the affected fuse box.
The second thing is a little more involved. You may have to remove the passenger seat for this one, but I need you to get to the AIR BAG module under the passenger seat and
check for water damage there as well. Unplug the connector if you have to and check to connections at the module and module connector.
if the carpet feels wet around the module then we have a problem.

please let me know the results.
Customer: replied 5 years ago.
all the fuses checked out okay. There did not appear to be any water or moisture damage. I did a little experimenting though and came up with the following......
1.If I disconnect the BCM BAT fuse. the anti theft, air bag, check eng, PRNDL all light up. If I disconnect the plug (pink) to the bcm they light up too.

2. if I take out the #44 eng comp (pcm bat) fuse, the current goes away @ the pcm acc, and ipc. Then when I turn on the key all the light come on like normal. I did not try to start it without the pcm bat fuse, though.

Is ther a muliple connection from the BCM to the PCM? If I take out the pink plug, it seems to have an effect on the dash... and if I leave the pink plug in and remove the smaller white plug, it does the same thing. the middle plug only seems to affect the inside lights. When I plug the connectors to the BCM the lights go off,leaving only the PRNDL, air bag, and anti-theft.

One note: my friend had a mechanic do a valve job. the mechanic is the one who informed him and then me of the issue. Is it possible that he (the mechanic) shorted the BCM or the PCM? I just got the car to check it out the other day. It is a bit beyond me, because I don't have enough diagrams to try and trace. So I have brought you into the mix, and I don't know whether I should apologize. I just knew that you or one of your peers have come across something like this, making it "easy money". With that said, I believe there is some relation to the BCM PCM and dash... what I don't know, do you? The car still runs, it will just kill the battery when you cut it off.
Expert:  Billy replied 5 years ago.
hello again,

don't worry, there is no need to apologize...I was the fool who answered your post ; p
Just kidding, all we can do is to try to fix this thing. Truth to be told though, the only problem at this time is the distance. It is not easy to try to guide someone through my thinking process and vice versa it is difficult for you to explain to me the process on your side.
However, if you need schematics tell me which and I will try to find it for you.
Open the hood and visually inspect the wiring around the engine to make sure nothing got pinched or shorted out from the head job.
At this point I would be tempted to plug in an ECM if nothing more then just to try.
It seams that the ECM is backfeeding through circuit 1500 to the underhood fuse block and by doing so powers up the BCM and IPC through fuse IPC/BCF ACC (normally ignition powered fuse)
I have replaced some ECM's for various malfunctions in the past but never for a internal
short to power like this, but there is a first time for everything.
You may want to try a used ECM or if you can just grab one from an other malibu and just plug it in for testing.
And please if you need diagrams tell me which one and I will try to find it.
Expert:  Billy replied 5 years ago.
there is one more option for you to try. disconnect the pcm connectors and check every single input to the ECM as well as every output from the ECM. if all well then replace the ECM.

Billy
Customer: replied 5 years ago.
I replaced the pcm... the lights went out, but now every time I tried to start the car the theft light blinks, and the car now won't start. I do know that the ignition switch is damaged (you can pull the key out evern in "on or ACC". So I don't know if the used computer even works, becasue now it won't start, whereas before it started but the lights stayed on, with the key off.

Thats alright, I found a site to help me with the Passlock. I tok a couple of ideas and formulated my own solution. Thanks for all your help! I believe the passlock was the original problem and someone altered the car so it would start. That is probably why the cpu stayed on, and when I plugged the new cpu in... well I bought a BCU too, and let it learn my key. The car starts up fine now.

Thanks for your time.
Expert:  Billy replied 5 years ago.
Hello again,

The reason the engine didn't start after you have installed the used/replacement ECM is very simple. Every time you replace an ECM, first it must be calibrated to the vehicle and then the vehicle theft deterrent system must be re calibrated to the new ECM.
This is a normal process and NO NEW GM vehicle will ever start without relearning the passlock system after an ECM replacement.

The passlock system was not the original problem with the battery draining and because of the way the system is designed (not enough space here to describe the operation) it is ABSOLUTELY IMPOSSIBLE for the VTD to cause the malfunction.

It was the ECM that had an internal short to power (for unknown reasons, although I would check with the guy who did the head job) that caused electricity to back feed through the system and it seams to me that by replacing it we solved the problem.


Furthermore, I assume that the "new" ECM had come out a similar vehicle and that is why it allowed the engine to operate/run after the installation, but keep in mind that it controls everything including the transmission on your vehicle.
Without actually calibrating it to your vehicle it could cause all sorts of troubles for you on the long run.
I highly recommend that you take it in for programing.
Billy, Chevrolet tecnician
Category: Chevy
Satisfied Customers: 510
Experience: Over 25 years experience,GM grand master for 8 years,certified smog and government safety inspector
Billy and 4 other Chevy Specialists are ready to help you
Customer: replied 5 years ago.
I understand all that, and I will have it taken in. However, someone bypassed the passlock system, and in doing so also installed one of those anti theft plug ins... the little black multi-wire under the dash. Whatever they did to bypass, possibly caused the short. See the shorted cpu would start the car no matter what bcu was in... it was with the new cpu the I had to let the car learn the settings. I do not know the whole story on the car, and it has been fun sharing this experience with you. I am sending the money, and hope your next experience brings you better joy.

Take care, thanks... and God Bless!

Troy
Expert:  Billy replied 5 years ago.
I'm glad your problem is solved, thank you for the accept and Happy Holidays to you and your family.

Billy
Customer: replied 5 years ago.
Happy Holidays to you and yours too. I hope to get you again sometime... not for another malibu though, something a bit more interesting...

Thanks again
Expert:  Billy replied 5 years ago.
thank you,

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