We just had a new PCM put into this car in March 2013. We then had a transmission issue (started dropping out of gear and speedometer not working properly). Turns out the PCM they put in had a cracked circuit board. Aamco said that the bad PCM caused damage to the overdrive in the transmission. Had to rebuild the transmission. But when we got it back, we still had some of those same issues (dropping out of gear and speedometer, etc.). Dealer replaced the PCM again. As soon as we got it back, the "check engine" light would come on, the check gauges light and the airbag light would randomly turn on and off, and several of the gauges (voltage, fuel & oil pressure) would peg at the extremes. Even had the car turn off all by itself, and the idling was rough. The dealership said that by a "massive coincidence", the BCM has now failed once they replaced the PCM! We say they damaged the BCM when they put in the new PCM. Dealer says that is not possible since they do not have anything to do with each other.
Lots of misinformation in your story. First of all, you are wrong about the mechanic damaging the Body Computer. While it is possible to intentionally do that, it would take a lot of effort to purposely do something like remove wires from the connectors and reattach them in the wrong order.
Second, it's highly doubtful anyone would get an Engine Computer with a "cracked board". They are sealed in a protective jelly inside the housing and are real hard to get out. I've taken some out to purposely damage one tiny circuit to create problems for my students to diagnose, and it's not an easy job. The boards are really tough and almost impossible to break. If one would be cracked in a previous crash it would have been rejected by the rebuilder and it certainly would not have passed their performance checks. If this was a used computer from a salvage yard and it came from a vehicle that was in a crash there would certainly be evidence of that on the housing.
Third, computer and other electrical problems will not cause physical damage to the transmission. When an electrical problem occurs the system will put the transmission into second gear and it will stay there until you turn the ignition switch off and restart the engine. A physical problem will not result in the speedometer quitting. That points to an electrical problem.
You have a bunch of electrical problems in all different circuits so I would start by looking for things they all have in common. Since the Air Bag light turned on, check that computer for diagnostic fault codes. Those may give a hint as to the cause. The Body Computer talks back and forth with the instrument cluster, which is also a computer module, and either one could be responsible for the warning lights in multiple circuits turning on.
It's interesting too that the transmission problem you described was supposedly fixed once by rebuilding it, then fixed for the same thing by replacing the Engine Computer. Instead, start with some more common things like a defective alternator. Have it load-tested to see what it will deliver for maximum current and "ripple" voltage. If it will only deliver exactly one third of its design rating for current and ripple is high, it has a bad diode. 35 amps from the common 100 amp alternator is not enough to meet the demands of the electrical system under all conditions. Computers hate low system voltage and excessive ripple, (variations in voltage), and they'll do weird things, like turn on warning lights, and turn things on and off.
Also look at the smaller battery wires where they attach to the under-hood fuse box and body. Those often become loose on any brand of vehicle and cause intermittent no-start conditions to flashing dash lights while driving. Those are a couple of examples of things that are in common with all the systems you're having trouble with.
June, 18, 2013 AT 1:12 PM
Hmmm. Not sure what to think.
First of all, the DEALER is the one that said the 2nd PCM had a cracked circuit board, so I can only take their word for it and assume that info is correct.
Also, it's the DEALER that said their diagnostics indicated a bad BCM after installing the 3rd PCM. But not previously? That seems extremely unlikely. They did a diagnostic check after they installed the 2nd PCM, so why wouldn't the "bad" BCM have shown up then? And the dealer says they use NEW OEM parts, not re-built units (although I wonder).
As far as the transmission repair, the mechanic who did the transmission work (a 30-year veteran) said that the bad/cracked PCM (he didn't know it was cracked, by the way) could have caused the damage to the tranny overdrive since it sends control commands to it. And they were able to determine that by just tapping the side of the PCM, they could get the vehicle to do strange things! That's how they figured out the PCM was bad and not the re-built transmission. It was at that point that I had to take it back to the dealer.
I see that there could be other issues going on. Could bad voltage cause the diagnostics to show false readings, like indicating a bad BCM, even though it may not really be malfunctioning? The strange behavior of the indicators was NOT something that was occurring before they did the last replacement of the PCM, but showed up immediately after I got it back from the dealer when they installed the 3rd PCM. Could a faulty electrical system have caused this whole mess? And if so, wouldn't the dealer's diagnostics show that?
I'm so confused. Seems like a lot of conflicting information! : (
June, 19, 2013 AT 1:20 AM
You've added a few details that made me rethink my response. If tapping on a computer makes it change its behavior, that is indeed a potential sign of a bad solder connection inside but first I'd rule out loose or corroded connector pins. Those terminals are pretty tough and they're well-sealed so the few times I have actually found a problem it was after someone was working in the area or had that connector apart.
Most of the time new computers are very expensive so it's customary to get rebuilt units, even through the dealer. I worked for a very nice Chrysler dealership all through the '90s and the only times we got brand new computers was when the vehicle was under warranty and Chrysler was paying the bill. When customers were paying the bill we always installed rebuilt, (remanufactured) computers to save money, and I don't recall ever having a problem.
There's a lot of computers on your vehicle and they all talk back and forth to each other over two wires called the "data buss". You can't damage one computer by having another one send out harmful information on that data buss. That's why I'm looking for a different cause for the multiple problems, and the charging system is the one that is responsible most often. GM owners know more about that because they have a huge problem with their generators developing voltage spikes that cause damage and interfere with computers and their sensor signals. ("Generator" is the new industry standardized term. The common term is "alternator" but that was actually used first by Chrysler in 1960 and they copyrighted that term).
There really isn't anything simple a mechanic can do to accidentally damage a computer. All mechanics live in constant fear of parts failing while we're working on the vehicle, then having to explain that to the owners, but it happens quite often. I run into that with some of the radios I'm asked to fix and there is one specific problem that DOES occur when the mechanic has to disconnect the battery. When he reconnects it later is when the radio is dead, so of course the owner "knows" the mechanic caused the problem, but I have the service bulletin that explains how the problem actually developed months or years earlier. It just won't show up until power to it is disconnected for a while.
That's one example of what I mean by living in fear of something breaking while we have your car. Most such problems are more varied and random. I would be suspicious too, but when that happens to my vehicles it's me who is working on it so I don't have to worry about who gets the blame.
I don't have a good answer about how they did the diagnostic tests and what would show up. We rely pretty heavily on diagnostic fault codes that can be set and stored in every computer. A lot of codes point specifically to an internal failure inside a computer but most codes relate to external sensors and actuators or the wiring to them. In the absence of any codes, about all you can do is condemn a computer if it isn't performing the proper functions. My concern is there is an underlying cause in a different circuit that got overlooked and is causing the incorrect behavior. We never like throwing random parts at a problem, but in the case of a computer, if the problem goes away when you substitute a new one, that pretty much proves there is no other defect.
June, 20, 2013 AT 9:08 AM
Thanks, doc.I'm having a 3rd party auto repair facility check into the possibility of some kind of voltage/electrical problem as the primary cause. Hopefully they can find the real culprit.