We've been concentrating on the Main Relay not turning on. That is turned on by the Engine Computer, but where the problem usually comes in is it's the Body Computer that is turned on when you turn on the ignition switch, then, if the programmed-in VIN matches, it is what tells all the other computers to turn on. The assumption has been the Body Computer hasn't been sending messages onto the data buss to the other computers. From what I've been reading, this appears to be a common problem that is solved by replacing the Body Computer. Some people have reported it only took disconnecting a battery cable to let the computer initialize and reset when powered back up, but you tried that already.
It's possible there's a poor connection between a pair of mating terminals in the connector, or even a broken wire right next to a terminal. Don't ask me how a wire that never gets flexed can break, but I've seen that more than once.
If the problem acts up again, the only course of action left is to connect a scanner so you can see what the computers are doing. You'll be able to see, for example, the state of the ignition switch as seen by the two computers in question, and whether that agrees with the position of that switch. You can watch if commands are being sent and received between the computers.
Also be aware that data buss runs between all of the computers, and a short anywhere will kill it and stop the messages from being sent. At one of the Chrysler schools I attended, the instructor used a paper clip at the load-leveling computer at the rear of a Jeep to short the two data buss wires together. That resulted in a crank / no-start. Her contention was that any computer on the vehicle could short internally and cause that. There would also be no communication with the scanner at that point, so the only solution would be to unplug every individual computer until the engine would start.
Later she qualified that to say she did that to make the point that the data buss could be shorted anywhere, but as for the computers, there was too much circuitry inside them that isolated any internal short from the data buss. To say that a different way, a computer could fail but it wouldn't take down the entire data buss. Because the wires are so well-protected inside the bundle they're in, there is a better chance of finding corrosion on the connector terminals. The data signals on older Chrysler products were only 0.2 volt digital signals, so they weren't affected too much by corrosion, but it is the type of thing we have to look for.
Related to that corrosion, it seems computers under the hood are designed pretty well to be immune from the effects of water and heat. It's the computers under dashes and seats that get less attention even though there's a lot of moisture in those areas. Ford has had a real big problem with their "Generic Electronic Modules", (GEM) doing weird things. That's their version of the Body Computer. A lot of problems have been caused by leaking windshields and water dripping onto the connectors, but corrosion forms there even when there is no leak. Anything that sits close to the driver's feet is prone to moisture problems. Watch out for connectors with wires that go straight in and you can see the back side of the terminals. Being exposed like that is a perfect place to find corrosion. Wires that run through rubber weather-pack seals are less troublesome. Those are what you'll find under the hood.
Friday, January 24th, 2020 AT 7:28 PM
