JTheJackal

Hi folks, me again .

I was wondering if a faulty BCM could cause the car to cut out seconds after starting?

Basically, sometimes the engine starts, runs for a few seconds and dies. Other times it starts and drives perfectly.

I know the usual answers to this is check fuel pressure, opti etc. I'm only asking before going down these avenues because I have some telltale signs that the BCM isn't happy (I recognise the symptoms from when I had to replace it last year).

Injuneer

Year of vehicle would be helpful.

If it's a 96/97 here's the BCM schematic, courtesy of Shoebox:

http://shbox.com/1/bcm_1996.jpg

The BCM incorporates the functions of the theft deterrent module (TDM) used in earlier years to send a signal to the PCM that is required by the PCM to activate the fuel pump. See the connection labeled “FUEL ENABLE” in schematic. If that signal doesn't show up, the engine will run for a few seconds after cranking/starting, using up the fuel in the lines from the pump priming. That would appear to be one possibility.

https://www.camaroz28.com/forums/lt1...nature-879835/

JTheJackal

Ah, forgot to leave car details sorry. It's a 1996 Z28.

That makes sense. If I try and start the engine again immediately then it struggles to start up. The interior lights are acting funny and I was getting random "bongs" from around the passenger footwell where the BCM is. I saw these things happen last time I had a defective BCM but not the engine cutting out issue so I was curious.

I have some really dodgy wire splices going to the BCM which I've neglected for too long so I'll sort that first.

Thank you, helpful as always.

Phoenix'97

Just to throw in my own past experience with the BCM, I was installing an LED light in my rear interior housing and was lazy about disconnecting the battery or pulling the fuse for the interior. I had my screwdriver in my hand and when needing to nudge the bulb into the connecting prongs I managed to successfully short out my entire interior lighting system! My BCM was making a strange and very subtle "click" sound as if you are flicking circuit breakers. A trick I was given by my dad's buddy who works on our cars recommended putting in a higher resistance fuse for my interior circuit. Well, doing so restored power almost like resetting it! So, I put the original rated fuse back in and have not had a problem since but man was that scary and I was even convinced I shorted out the BCM and ended up buying a new one from a dealer. So, at least I have one in the event my factory BCM takes a dump some day.

How my story applies to your situation, well, bad wiring could be throwing off your BCM and if it is making that subtle "click" or as you state "bongs", almost like your turn signal making weird noises when the bulb is reversing polarity or something. Also, as I said, perhaps a short term fix would be to throw in a higher resistance fuse in your circuit box and see if you can successfully reset your interior lights if worse comes to worse and for short term diagnosis purposes if the event is recurring. If your wires are spliced you had better look them over and make sure those splices are not breaking. Also, are those splices butt connected with the ends melted to the wires as is proper? Or are those splices someone tying the wires together with household wire connectors or splices or just the absolute WRONG way to splice wiring together? You may need to splice the wires once more and do them right!

GaryDoug

How did you come up with a "higher resistance" fuse? I have never heard of such a thing for a given amperage.

GaryDoug

Yes, fix the "dodgy" splices first.

Is there any chance that the starter interlock anti-theft feature is bypassed? If so, then the BCM may be cutting off the fuel enable signal because of a defective key actuator. I have found that the injectors operate for a few seconds after the anti-theft condition is activated. The fuel pump is not affected. Do you see a lit Security lamp lit for more than a few seconds?

Phoenix'97

If my wording is improper then I apologize! Just use a higher amp fuse was all that I was trying to say.

shoebox

That's a good way to fry everything in that circuit. There is a reason fuses have a given amp rating - it's to protect the wiring.

peterpar

This is something that is so important for people to understand. The wire size of a circuit is determined by calculating the maximum load (amperage draw) it takes to operates the equipment that is connected to it and then selecting a wire gauge capable of handling that calculation. Every gauge of wire has a maximum amperage rating and the fuse protecting this wire should not be higher than this rating. A higher fuse amperage rating would still provide protection for a direct short to ground, but it also allows the circuit to draw an amperage higher than wire is capable of safely carrying which can cause the conductor to overheat because it is too small for the amperage trying to pass through it.

Injuneer

Excellent point!

And.... the fuse amperage rating is also protecting the delicate internals of the solid state body control module. The BCM sees enough problems with bad solder joints, PC boards, etc.. Last thing it needs is a huge surge of current passing through it, on the way to an overload on the other side of the BCM.

Phoenix'97

You do realize it was a TEMPORARY measure to get power back to the circuit as to restart it, right? I could not restore power using the correct amp fuse! Once I put a higher rated fuse in I was able to restore the interior lighting and from there I put the correct amp fuse back in. Explain this one, please.

shoebox

There isn't a rational explanation. There isn't a "restart" scenario I know of. Perhaps the higher current "re-soldered" on it's own a bad connection in the BCM? IDK. Temporary or not - bad advice. It can take only seconds to fry wiring and cause untold heartache in wiring repair.

Injuneer

So you got this expert advice from someone who doesn’t even know the difference between amps and ohms........

Laughable.

Phoenix'97

Explain how a mere fuse can help "re-solder" a bad connection in a BCM?! The situation I was dealing with was that I shorted out the circuit for ALL interior lights, including my factory gauge cluster and console. I disconnected and reconnected the battery and after all diagnosis with a "ticking" BCM which sounded like it was flipping a circuit breaker, I was convinced I fried my BCM but that didn't fix the problem! Only a higher rating fuse powered the circuit back up and BELIEVE ME I was swapping in new fuses of the correct rating with no results! So clearly there is something to this and NO, for very short-term use you won't fry your wiring, especially when the car is not running.

Just because I don't know the difference doesn't mean my father's buddy who is an avid fourth-gen mechanic does not. You want to know why this site is dying, look at your comments...

shoebox

The "re-solder" comment was just a wild guess, since there is no other plausible explanation (did you see my question mark at the end?) . Sending more current through the circuit path might have overcome a marginal connection and the heat from it may have caused the connection to fuse together. Like I said, a wild guess, since no one would ever consider this a means of repairing a problem. You asked for an explanation and I gave it a shot. What is your explanation on why it worked?

Car running or not makes no difference, you had to power up the circuit for it to do whatever it did. When you power up the circuit, current flows. When current is higher than what the wiring or circuit board is rated for, damage can occur quickly. There isn't a service recommendation I've ever seen that says to try a higher amp fuse.