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Discussion Starter · #1 ·
I THINK I have a bad BCM. No crank, No interior lights, locks, seats and windows not functioning,etc. Head lights, horn, wipers, flashers and brake lights DO work.
Dealer tells me BCMs are ‘VIN Locked’ so used ones cannot be used. And, Refurbished have to be programmed by a dealer.I’m 50 miles away from the dealer and trying to avoid a big wrecker bill.
Any suggestions? Help will be much appreciated!
 

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2004 Trailblazer LS 4.20 L
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any remote start or security bypass modules installed? is the key fob the original? Are the imterior bulbs any good? ive seen them all burned out before. Do you have a voltometer or continuity tester to check fuses? Any missing fuses or relays?
 

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2004 Trailblazer LS 4.20 L
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any remote start or security bypass modules installed? is the key fob the original? Are the imterior bulbs any good? ive seen them all burned out before. Do you have a voltometer or continuity tester to check fuses? Any missing fuses or relays?
check for broken wires in the door and cleanup ALL the grounds that you can see. start with the battery to the body. bad connection right there will cause no crank in fact its the most common reason for jumpstarts. how old is the battery?
 

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Discussion Starter · #4 ·
Battery is brand new. Body grounds are good, as is ground on side of block. Are BCMs VIN locked? I've tried 2 used ones and they don't change my symptoms.

I've convinced myself (by chasing thru wiring diagrams) that I have a bad BCM. Should I send mine off for repair or buy a refurbished? Will a good one, refurbished or repaired, allow me to start the vehicle even tho I might have to drive it to the dealer for programming? Trying to avoid a 50 mile tow bill.
 

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2004 Trailblazer LS 4.20 L
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Battery is brand new. Body grounds are good, as is ground on doesn't seem like the BCM is the problem since you have tried two other. have you checked actual wires and connections or just looked at diagrams? after market remote starters and security bypasses dont show up on internet schematics. you've got to follow them from the ignotion switch. the ignition switch, btw, is more likely the problem. 90% of no starts is because the the battery ground isn't hooked up right. i just magically "fixed" another car yesterday with that problem. guy sold his van for $100 because it wouldn't start or run consistently. the insulation on the battery terminal looked fine but the wire had dissolved. no negative battery connection. he put a new alternator and MAP sensor in there. it was a three dollar fix there's a lesson there.
 

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Discussion Starter · #6 ·
The 2 BCMs were from junk yards. After the second one didn’t fix it I thought BCMs were VIN locked. Can used BCMs EVER just be plug and play? Grounds are good; battery is new. Ignition switch was changed without helping.
 

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im not sure if any are ever completely plug and play, and i dont have a service manual for the 4.2 L or any Trailblazer. However, how small the chances of pulling two junkers in a row, plus one that you had, directs me towards another issue. when did it stop starting? does it crank? anything get spilled on the fuse block? did you check the harness that exits the bcm? there's a connection on that harness that can get loose or corroded. still sounds like a problem in the dash board with a security bypass or remote starter, to me. BCMs in general dont go bad without some other electrical failures or malfunctions. that's one reason why "replace BCM" is nearly the last thing on the repair procedures.
 

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Discussion Starter · #8 ·
Problem started when battery went to zero over a period of 3 weeks of no activity. Plugs, harnesses, grounds all look good.
NEW INFO:
1. a buddy loaned me his OBDII Autoxray scan tool. Showed O2 sensor bad but no ‘trouble codes’.
2. Did the 34 minute key on- key off procedure on all three BCMS. No help. Battery stayed up throughout the 3 tests. This took a couple of days. Left original BCM connected.
3. Started to determine whether to buy a refurbished, or to send mine out for repair. This took 2 days.
4. When I went to pull my original one out, BATTERY VOLTAGE HAD DROPPED TO 2.4 VOLTS. It had been back in the circuit only 2 days!
I now believe the original BCM was the root cause of my battery going to zero over the 3 week idle period. Repaired BCM (if repairable) should be back to me by about 5/14. I’ll post an update then.
Thanks for your continuing advice. I’m disappointed almost no one else gave me any.
Battery and ignition switch were replaced early.
 

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I can’t really help here, but yes many electrical components are notorious in them for having to be matched to car as originally equipped. Particularly radio and airbag module under carpet rear of handbrake. If you got a junkyard BCM you should get radio and airbag module from same donor vehicle. I learned that after adding keyless entry to mine.
 

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2004 Trailblazer LS 4.20 L
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Well, it is not an easy answer really. I have heard of people temporarily swapping BCMs for one reason or another and being successful. That's iffy imo, but very possible. Looking at grounds tells you very little, you should pull them off and make them shine. Bad grounds can be intermittent and will drain batteries. Same with stuck relays. They get old, weak, and tired. It costs almost nothing to clean the paint off the ground points and sand the terminals. just takes time, not a week, awhile. Also, BCMs aren't known to fail suddenly unless there was a collision or fire or multiple component failures simultaneously. Somethings to consider. its often the simple stuff that gets overlooked . Good luck, let's hope you get it fired right up.
 

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Discussion Starter · #11 ·
BCM was not repairable. My $164 is now a ‘sunk cost’. New deal is $349.99 more for a remanufactured and fully programmed to my vin on my suv. Funny they could pull the program out of the main chip which was totally shot. I told them to ‘do it’. Tow bill would have pushed $200, reman unit would have been $79, and dealer programming would have been $138. That’s about the extra $349.99 I’m paying. Coincidental, or sharp pricing? I’m getting cynical about this. Should get something back late next week. I’ll advise the result.
 

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yeah that funny alright. i can't seem to find out how exactly the BCM gets programed but i found a big thread about a no start trailblazer. its from another site so hopefully the link works.

[/QUOTE]
 

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Discussion Starter · #13 ·
Thank you Mr. Sparky for the info. I followed all the links and learned that I’d fumbled thru most of the steps myself. I had found no ground on the starter relay coil - when there should have been one - and jumpered the contacts. Starter cranked but I pulled the jumper and didn’t try to start it.
Just for the hell of it I’m going to call the folks programming my refurb BCM (Module Experts) and ask that they also return the one I sent. If they decline, I’m gonna get even more paranoid. BBB has a NR rating for them. That may just mean that they don’t have enough information about them to rate. Items in hopeful!
Thanks again for chasing these links for me.
 

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Discussion Starter · #14 ·
Also offering what I heard when I sent my BCM out for repair:
“Diagnostics are completed on the Envoy module, and this unit is beyond repair due to the issue being traced to the main processor (computer chip). The chip is not able to be replaced on this module, as it is made into the circuit board assembly, so you would need to replace the unit to correct the issue. There is nothing that can be done to save this unit. It must be replaced.

We can provide a remanufactured replacement, fully clone programmed to the KEYS, VIN, security, mileage, and vehicle options, plug and play ready to go and with a lifetime warranty, at $499.00 (minus the repair cost), so we would need an additional $349. This would take a couple of business days, and it will come fully clone-programmed plug and play. No dealer required. Just install it.”
 

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so the starter relay has a broken ground somewhere? that's not something that you'd think of right away. the dealer has to give back your old parts i think. they may charge you a recore but im fairly certain that you have the right to all parts that are replaced. they should at least offer. im not sure that ground on a relay could fry a BCM but im guessing it could not. nice work! good luck!
 

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Discussion Starter · #16 ·
The missing ground is furnished by the PCM when directed by the BCM to do so. BCM wasn’t telling it to give the relay the ground. You’re right on the core charge- some are $90. I guess I’ll just trust them. Should be back by Friday.
 

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still sounds like a broken wire but i usually fight replacing original electrical components. until it really burns me lol.
 
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