Chrysler Repair: 93 Conc.Guages, A/C & overhead readout not working, roland roland, bias supply

Question
Hi, the guages , ac controls and overhead console on my 93 concorde went out about 2 wks ago. 3 days ago they came back on and worked until i shut the car off to go into the store. they  are not working again. everything else works fine, car starts and runs great cruise, lights, horn, door locks all work. i have noticed that when i start the car the tach jumps up to about 100 RPM and just stays there but when i turn it off the gas guage jumps and the shifter position lights on dash blink on but are off while key is in ACC or running. any thoughts?

Answer
Hi Dave,
Did you check for the presence of DC voltage on each of the two data bus wires? With the ignition switch on, the violet brown is supposed to have +2.5 and the white/black -2.5V as a "bias" supply. If not then there is something wrong with the data bus. A readout of the bus using the Chrysler diagnostic readout box might give us a clue as to what is wrong. I have the trouble shooting manual for the '94 models which should be applicable, if you can get a readout. Let me know what the voltage check shows.
Roland

Roland, have continuity from data port to bcm @ .000 and to dash from data port @~.05 - .07. lights and only high defrost from ac and no lights at all on overhead console brehaut@adelphia.net


Hi Dave,
Thank you for the very complete description of the problem. I have a couple of uncertainties so let me give you my assumptions and you can tell me otherwise if incorrect. I assume that you have an automatic temperture control sort of HVAC control and I assume that when you say "not working" that there are signs of "life" at these three consoles (such as illumination) but that they are not producing any readings (gauges/overhead) or control over the cabin temp (HVAC control head).
If that is indeed the case, then it would eliminate the possibility of a problem with 12V power supply to the units and that is consistent with what I see of their being no common power supply for them that if out would explain the failure.
So that leaves us with the one common factor which is the digital data bus (CCD) which is required to function for all three to be working. The digital signals for all three of them originate at the body computer on pins 7 and 8 of the 24-pin bone-colored plug of the body computer which is located on the right side cowl/kick panel just in front of the passenger door opening, behind a trim panel.*  Data bus uses two wires that are twisted together (white/black and violet/brown) and both must be connected and biased properly for digital data to be carried successfully, so either one could be faulty and that would shut down the communications. My inclination would be to go to the body computer and check out the pins/sockets of the bone plug for water contamination from say intrusion of rain/wash water dripping down from a leak around the windshield that is located above it. Maybe there were heavy rains that might have leaked water onto the body computer, for example.
The system can be probed electronically by a dealer with the Chrysler diagnostic readout box II (DRB II) and that might give another clue as to where the data bus is failing.
So let me know about my assumptions and theory as to why this is happening.
Roland
*To reach the module:remove the cap on the right end of the dash panel (held on by clips, lever gently off). Remove cowl trim panel and pull carpeting back, exposing dash liner. Cut dash liner along perforated lines and pull back dash liner to expose module just in front of the door opening. remove mounting screw thru oblong hole at right end of knee bolster. Remove push-in fastener at lower corner or module and maneuver module down and out which will allow access to the 3 21-pin plugs.