Hello. The heat and A/C in my 850 has recently stopped working. It all started with a plastic/electric burning smell coming from the vents on my to work last week. Once the burning smell started I immediately turned off the a/c and luckily I was very close to work so I was able to turn the car off with in about a minute of it all happening as well….So not really knowing what happened I crossed my fingers that things just got hot and that all would be good once I restarted my car after work…Well that was not the case, now every time I start the car the blower just blows full blast for either the entire time the car is on, or it stops after a few minutes and the rec light and auto a/c light just flash.

I have had some time to do some trouble shooting on the unit and have found the following: When trying to get codes from the ECC computer, I was having trouble even getting the computer to connect to the ECC. The light would just not flash or show any signs it was talking with the ECC unit. I was however able to get it to work sometimes by starting the car first, then turning it off, then moving the key into computer testing position. When I was able to get it to connect I found about 15 codes and was able to clear all of them except code 419. I tried clearing codes multiple times but could not get code 419 to clear. So at that point I start looking into some of the hardware. I ended up pulling out the head unit that has the temp ***** and toggle buttons on it and took it apart. I found a row of capacitors that had black burnt spots all around them and it smelled exactly like the burning smell I encountered a few days before. At that point I tried cleaning the burnt area away with a que tip and electronic cleaner and reconnecting it, but it made no difference. So at that point I know I would need to replace the head unit.

Well I ended up buying one off of ebay this week (off of a 96 850) and tried hooking it up to see if all was good…well it was not…the fan just blows as it did with the old unit when the car is either running or the key is in the “on” position…so I carefully took the used one I bought on ebay apart to see if it was burnt or maybe a junk unit, and it looked just fine, but I did notice that the computer board was slightly different then the one on my 94 850…so here I am stuck…in Florida with no heat or a/c and scratching my head. If anyone can chime in with some trouble shooting tips for me or let me know if I need to find an exact head unit out of a 94 please let me know. Sorry for the long post and thanks in advance for your help.

 

You have a shorted out power stage (blower resistor). The MOSFET transistor that controls fan speed shorts out you wind up with a fan that runs all the time.

Unfortunately the MOSFET is no longer available through the normal suppliers like Mouser but I have seen some writeups where people have substituted other part numbers but the pinouts were different and the performance was slightly different.

You can buy the assembly from any of the major suppliers like eEuroparts or FCP. They aren't cheap though. Most of them list it as a blower resistor for the electronic climate control.

...Lee

 

Hi Lee. Thanks for getting back to me. I do have an update on this. I went to the local pick in pull here in town and pulled an ECC head unit out of a 93 850 and all is working again....at least for now which is fine by me. The MOSFET transistor you talk about...is that the piece that sits next to the blower motor and has a large heat sink on it? If so, I pulled that unit out as well when this all stopped working. I didn't do any tests on it, but did a visual inspection and hit it with some electric cleaner...even after I did all that the ECC head unit out of the 96 would not work...but as soon as I connected up the ECC out of the 93 all was good. So from what I have found at least the 93-94 ECC units are swappable...