Chrysler Repair: 1996 LHS heater/ac fan runs constantly, serial data bus, silicone grease

Question
Hi Roland,

I have a 96 LHS that has a heater/ac fan that will not shut off.  I did the self diagnostic test and the (ATC) display read '34'.  The engine temp gauge works, the heater/ac temp control works but the fan can not be adjusted or shut off.  The fan does run slightly slower in 'auto' mode.

Is this some engine temp sensor problem ?  I was told by one garage that Chrysler said it was the 'air temp sensor' in the dash and it costs $150 for the part.

Thanks,
Mike

Answer
Hi Mike,
I disagree that this is the air temp sensor in the dash because that one is going to produce a code 26, not a code 34. The 34 refers to the engine's coolant temp sensor signal, and that would be delivered by serial data bus line which also presents it to the dash gauge. So I am thinking that because you do have a dash gauge reading this is a false code. Usually when the fan runs high speed all the time with the ATC (or any time it is activated with the conventional unit) it is problem with blower power module  (or the blower resistor block in the case of a conventional unit) because these units are the ones that should provide for variable blower speeds. The fix is to replace that module. In your case it is a solid state module located in the air duct to left of the blower motor under the dash. You have to remove the lower right under-dash pad to see and remove it from the duct. It has a five wire plug: two are connected to the blower motor (another way to find the module), one is a ground (black), one is a dark green bringing power from the fuse block under the dash, and one is a control wire from the body computer (red/light green, goes to pin 4 of the black plug on the body computer located behind the trim panel cloth next to the passenger's right shin). I would be inclined to remove the module (2 screws) from the duct and examine to see if either of the transistors show signs of melted silicone grease on their bases which would be a sign of a bad transistor, or any electrolytic capacitors that look wet (a sign of failure). If the bad component(s) were not identified it would be difficult to get the correct replacement, but if they were then you could try replacing any component that looked faulty by soldering in a replacement from an electronics store. Otherwise, I would try a wrecking yard for a car of a similar age (early 90's-on) that had an ATC unit with an identical plug and buy that one used for very little cost and try it out. Otherwise it means taking a greater chance and buying a new one from the dealer. This unit is supposed to put out 14 different blower speed voltages and the fact that it doesn't implicates the "blower power module" to the best of my knowledge. Please let me know what fixes it for you. I would not look at anyother parts first because I suspect they are more expensive and less likely to be the cause. I don't believe the "coolant temp sensor signal not being received", if that is true, will cause the blower to run at high speed all the time.
Roland