Intermittant instrument cluster
The body controller (BCM) must send a message to the engine controller (PCM) to tell the PCM that it's OK to start and run when a theft system is involved. As long as the OK-to-run signal can make it to the PCM, all will be well.
But during the times when the gauges are flat, the bus communication system is probably crashed and there's no way to deliver the message. The PCM has no recourse but to disallow fuel at that point, killing the engine. The only thing that doesn't fit is that the starter isn't taken away at some point, but it may be something that doesn't happen in every model year.
The solution is with the communication bus, the CCD. It operates on two circuits, 2.5 volts each and connects to every module on the van that needs to share information.
The instrument cluster printed circuit board is actually much more than just a board, it's a processor in its own rite. It controls gauges, telltale bulb illumination and supports the CCD bus both through voltage regulation and supplies the point of circuit termination (ground). The termination seems to be the key.
Without ground, the bus voltage is pushed far above its normal 2.3 to 2.5 volt range, into the 7-volt area. Bus communication depends upon a voltage matrix very close to 2.5 volts, so this effectively crashes the network. Everything becomes a zero.
One very good diagnostic tool for when the system crashes is to apply a firm, open-palm slap to the dash just above the instrument cluster. Works almost every time.
While the problem can be temperature related (parked in sun), a good jolt usually brings it back online and allows the van to start and run. It seems ham fisted but it's almost impossible to diagnose by disassembly. The act of touching the area around the cluster normally brings it back and you're faced with the question of whether it was coincidence or not. I prefer the slap.
Testing the data link connector (DLC), the trapezoid-shaped connector under the left dash area, will tell you if the bus is crashed or not. The third terminal from the left on each row (8 apiece) is CCD bus. Remember that 2.3 to 2.5 volts is normal while the key is on and the bus is active. You should see the voltage dancing, varying rapidly in response to data transmission if tested with a digital voltmeter. Anything above this is likely to be FAR above, indicating a slap is at hand... you know what I mean.
The board isn't cheap... close to $450 and it requires a calibration before the tach or speedometer will operate. This may involve a trip to the dealer; I'm not sure if aftermarket scan tools can accomplish this task. Labor to install a board can also vary according to your repair facility.
Nov 29, 2008 |
1997 Dodge Caravan