Another HVAC Problem

Shipfaced · Post by **Shipfaced** » 20 Nov 2018, 06:56

'94 850 w/ECCU - Last summer the AC worked intermittently, I pulled the ECCU, cleaned all the contacts and it worked for a week or so (in both auto and manual modes) then went back to intermittent. We now need heat and it does nothing (no lights, no fan, no heat trickling out at highway speeds) but occasionally the fan will blow on high for a few seconds and then back to nothing. I know this subject has been beat to death but I couldn't find and posts with exactly the same symptoms. My gut says to replace the mosfet first as it is giving bad info to ECCU but I am concerned that there is no heat getting pushed through the system. Thoughts? Suggestions? Thanks.

Post by **jreed** » 21 Nov 2018, 05:19

It's possible that a failing fan motor could have caused the MOSFET to fail. You can test the fan to see if it is drawing excessive current, which could burn out the MOSFET.
It might also be helpful to open up the ECC and inspect for damage to the components on the boards.

Shipfaced · Post by **Shipfaced** » 21 Nov 2018, 05:31

Thanks for the reply. When I had the ECCU out and cleaned the connections I also inspected the boards and all looked good. The fan seems to only run when it is dead-cold on start-up. Runs for 15-20 seconds then nothing. While it is running I have no control over it. It sounds fine but I intend to pull it out and inspect this weekend. Would a failed mosfet cause the other symptoms?

Post by **jreed** » 22 Nov 2018, 06:24

I haven't experienced a failed MOSFET but others have reported them on this site. My memory is that the symptoms of the failures were either no current to the fan or else full current to the fan.
I don't recall anybody reporting that the fan had full current for a short time and then stopped, but it can't be ruled out.
I imagine that you have already checked the connectors, pins and wires in the MOSFET and fan circuits for signs of overheating or intermittent contact. You could try swapping in a known good MOSFET to see if it fixes the problem. If so, it may be helpful to check the current draw on the fan to make sure the fan isn't drawing too much current and causing damage to the MOSFET.
Regarding the observation that you're not getting heat... it could be because the fan is only running at start up when the engine is cold and there is not yet enough heat coming from the engine.

Shipfaced · Post by **Shipfaced** » 22 Nov 2018, 06:52

Thanks for the info. I intend to tear into it this weekend and try to gain more knowledge. Absolutely will clean and exercise connections to fan and MOSFET and test fan.

Post by **jreed** » 23 Nov 2018, 06:24

On the ECC unit, when you turn the AC and REC switches, do the orange indicator lights come on?

Shipfaced · Post by **Shipfaced** » 23 Nov 2018, 10:01

Only when the fan runs (on full), which is maybe 20% of the time. Otherwise no lights, no nothing. When the fan does run it seems to be only in “on” position (not “acc”) and goes off when I go to “start”. After starting it stays off. I have jiggled key to half-a** test for bad keyswitch.

Post by **scot850** » 23 Nov 2018, 16:36

Check the motor itself. Chances are the bearings are worn out and locking up or the brushes are worn out. Loads in junkyards and easy to replace.

Neil.

Post by **wizechatmgr** » 23 Nov 2018, 17:07

Mine did this when the blower motor went - however mine usually would end up blowing the fuse after a period of time when it did run.

Shipfaced · Post by **Shipfaced** » 23 Nov 2018, 17:13

Sounds like I have a course of action for tomorrow! Will pull the fan and thoroughly test it. Thanks for all of the responses.