MTPZ06

My neighbor has a 2000 A4 Vert, and he's having a strange starting issue. The car is throwing no codes at all, but the only way he can get it to start is to disconnect the battery? Once he reconnects it, the issue was obviously cleared. He can start the car, drive it just fine, and then shuts it off...comes back a few minutes later and it won't start. Disconnect the battery again, and it starts just fine? Doesn't matter if the car is cold or hot, it wont start until whatever electrical issue is cleared through a battery disonnect. Again, no codes what-so-ever...

Anyone hear of such a thing??

Bill Curlee

YES

This is a BCM issue. It was discussed here on the forum a while back. The person even narrowed it down to a componet. See if you can search for it.

The BCM actually completes the circuit to allow the Theft Deterrent Relay (TDR) to close and supply 12 VDC 60 AMPs to the starter solenoid.

Heres the circuit:



BC

BC

Bill Curlee

To test out this theory,,,,, when the problem happens, ground the YELLOW/Black wire on the TDR and if the car starts,,,the BCM is the issue.

BC

MTPZ06

Thanks Bill! This should help him out.

Bill Curlee

I should clear up what I ment. ground the yellow/black wire and then do all the stuff that you do NORMALY to start the car. You know Clutch in and turn the key to start.......

BC

ersatz928

When the car no-starts, try this.
1) Turn key off and remove from ignition lock.
2) Pull fuse #25 (in the passenger footwell fuse/relay box), wait 5 secs, and re-insert fuse.
3) Try to start the car : IF it now starts OK, you have a BCM problem : it does not send the PCM the "Fuel-Enable" signal.

It sounds like the same exact BCM problem I had with my 01 6spd. I did a lot of troubleshooting and this is the "story". And no, my BCM never got wet. I also checked all the BCM grounds and power feed circuits, and my door wires.

When everything works properly : the BCM is the "Power Master" of all the computer boxes hanging on the serial data bus. When you shut the car off, then remove the key, and then open the door, this starts a power-down timer in the BCM. This timer is about 20 or 30 minutes. If the BCM sees no activity (key put in ignition, a door open...) during this 20 minute period, it sends a "Power Sleep" mode signal to all the other modules on the bus, and they go into a low-power sleep mode (so the battery doesnt get drained). The BCM goes into a sleep-mode too, but it still has enough functionality to recognize that a door has been opened, or a key was inserted into the ignition switch, which terminates this power-down timer and wakes-up all the modules. This sleep functionality of the BCM is also there for the anti-theft horn alarm. In sleep-mode, the BCM can also recognize a hatch-release request from the RF remote control (key fob), and then the BCM activates the hatch release relay. (The hatch release signal comes from the remote radio receiver and is transmitted to the BCM on the serial data bus.)

If you have this no-start problem, the issue is that the BCM does not come out of its sleep mode : it does not respond properly to the door-open switch inputs (or the key-in switch input, or the hatch release request). If you try to start your car before this timer runs-out (as evidenced that the interior lights come-on when you open the door), the car will start. If this timer expires, and all the modules (including the BCM) go into sleep-mode, your car won't start. The problem is internal to the BCM : It does not come out of its sleep-mode when you open the door, or stick the key in the ignition (the "key-in/out" switch). If you pull passenger footwell fuse #25 (the one shared by the Instrument cluster and the BCM), and then reinsert it, this forces a power-on-reset of the BCM logic, which forces it to come-up in the waked-up state, and now the car will start. Fuse #25 is the power feed to the BCM logic/CPU circuit. So that I could still drive the car, I rigged-up normally closed pushbutton switch in-series with fuse # 25, so I could reset the BCM before I started my car. And yes, I verified that all the correct power-feeds, grounds, and switch inputs were present at the BCM connectors. I even bought a spare BCM module, and started tracing out its internal schematic, in an attempt to find the defect of the wake-up circuit.

Now for the weird part. After using my pushbutton BCM reset switch for many months, I noticed my BCM began to behave normally. I noticed that the interior lights once again came on when the door was opened. I still have my pushbutton wired into the fuse-box, but have not had to use it anymore.



You have 3 options to fix this :
1) Kludge-fix it with a pushbutton switch
2) Replace the BCM with a new one (get the correct pn, there are 2 or 3 BCM part numbers used, depending on the model-year), but you must get the replacement BCM programmed by a GM dealer for your cars RPO options. Because the car won't run without this programming, you will have to have a GM dealer do the BCM replacement and reprogramming.
3) Find a used BCM, of the correct pn on EBay (or wherever), whcih has the same options as your car does, and therfore already has the correct RPO programming.


My pushbutton kludge fix : Take a mini-fuse, cut apart the plastic and save the metal pins (cut the fuse element that is between the pins. Then solder on a length of insulated wire, from one pin to an automotive blade-type fuse-holder. The other side of the fuseholder pigtail to some more wire, to the Normally Closed (NC) contact of a momentary bushbutton switch. Connect the other terminal of the switch (C) to another length of insulated wire, the other end is soldered to the 2nd fuse pin. Install shrink tubing to insulate all the connections. Install a blade fuse (same Ampere rating as fuse #25) into the added fuse holder, then plug the mini-fuse pins into the fuse-box, where fuse #25 was. To start your car, push and release the bushbutton, then rotate the key and start. Since I don't now need to use the bushbutton, I just coiled the wire up in the pasenger footwell, tucked out of sight in case I need it again.

Note : The antitheft horn alarm will not be functional if your BCM has this wake-up problem, nor can you pop the hatch from the keyfob or the dash switch unless you hit the pushbutton first.

Bill Curlee

THATS THE POST IS WAS THINKING ABOUT!!!!

You really did your home work!! WELL DONE!

Bill C

Bill Curlee

MTPZ06

What did you find out about your problem??

MTPZ06

It was for my neighbor, and I gave him the write up and diagram, but prior to that I also mentioned he take it to WCC since he didn't want to go the dealer. I haven't touched base with him since I gave him the info, but I've been curious as to which direction he went. I'll let you know when I see him next.

M3MOTORSPORTS

Going through this with my 99 A4...What did your neighbor find out?

01Torch C5

I recently started having a no start problem with my '01. I turn the key and hear a relay engage but nothing else happens. The first time it happened I figured it must be a dirty battery terminal. I pulled the battery cables off and they were still shiny as new. I tested the battery with a load tester and it was plenty hot. I reconnected the battery cables and the car started. The next time it happened, I disconnected then reconnected a battery cable and it started. The third time I just turned the ignition off then tried to start it again and it worked. I have done that ever since. I never know when it will start the first time or not. I connected a voltmeter to the battery and watched the battery voltage during a no start and there is no voltage dip nor big current draw. I don't get any codes or any other symptoms when this problem occurs. I haven't had the time to do any further diagnosis.

dgrant3830

Your's sounds like either the iginition switch needs to be worked on or replaced and/or possibly the starter solenoid, all depending on if you hear the TDR relay or not in the passenger footwell when it doesn't turn over.

VetterGetIt

i had all these symptoms and mine was the starter. the actual starter motor and not a solenoid or an electrical component FWIW

good luck to him

11ellswray

i know this is an old post but at least I am using the search function right?
So if I am reading this right, and the BCM is at fault, then when I open the doors the interior lights will stay off? I am having similar issues with me 01 Z06 but my interior lights come on every time I open the door. I did ground out the yellow/black wire and the car started.