Expert: Roland Finston - 8/8/2011

Question
1998 Chrysler town and country limited 3.8L
My problem is between the dash gauge cluster and the climate control. The gauges do not work unless I unplug the climate control. When the climate control is plugged in the a/c heat and fans work fine but I have no tach speedo etc. The dash just goes dead. I have pulled the cluster and reflowed solder on the circuit board as I have read to do. There was no change in the problem. Can you help with this please I can't figure out what is wrong. I did do the test on the dash and said code 999 and also reset climate control still did not help.
   Thanks

Answer
Hi Curtis,
There are only 3 wires that the units 'share'. First is the panel lamps driver, an orange wire from pin 1 of the black 36-pin plug from the juncion block under the dash, which actually comes from the body computer to which the block is attached. The other 2 wures are the twisted pair of wires for the digital data bus which come from pins 3 and 34 of the 36-pin gray plug that is diagonal to the 36-pin black plug on the junction block, which also actually come from the body computer. The twisted pair goes to pins 10 and 9 of the cluster and also to pins 14 and 16 of the HVAC module blue plug. They are spliced together near the 36-pin plug under the dash to the right of the steering column where wires from a large number of units that are on the 'bus' meet.
I am not certain how these digital data bus wires can seemingly cancel eachothers' signals when both units are involved, but seemingly work when one of the other is out. Clearly the bus carries the info for the gauges. What is their specific exact role in the HVAC is not clear. But the purpose seems to be to communicate to the body controller when rear defogger, the recirculation mode, and the request for AC have been made.
You might want to try a fault code readout to see if you can get an sign from the body computer as to what might be seen as going wrong with the data bus communication. Think about anything that is unusual about the functioning of the HVAC which might give a clue.
This would appear to be a data bus problem, but the cause is 'opaque' at the moment.
Roland