P38A HEVAC problems

Having problems with my HEVAC .It started blowing hot on the passengers side so i renewed qnd the nanacom showed fault as a short on right hand blend motor and potentiometer fault on left hand blend motor so I got 3 second hand motors and fitted them.

So now it blowing hot on drivers side so checked the blend motors out using a ohm meter and fitted a different right hand motor and calibrated again and all was ok blowing cold on both side but when i put the heater on both side s it would not go back to cold passengers side and left hand blend motor fault showed up but when I switched the ignition off and started up again the blend motor went back to cold but when the L/H blend motor would not go back to low until the ignition was switched off .Fault showing left hand blend potentiometer fault so changed that moter with one that checked out with ohm meter and 9 volt battery, still the same .

Use the nanacom to force the motors and the left hand blend can be forced to 90% and return but when forced to 100 % it will not return until ignition switched off .

Anyone had this problem before and any ideas what is wrong Nanacom only shows potentiometer fault. And its not that it has had 3 motors all showing good ohms.

the I had similar problems with mine and checked everything a few times and changed the blend motor. it turned out to be a chafed wire trapped by the radio,repaired and the fault went.

It can also take a couple of times to calibrate I've found. I had the issue on mine and the motor I replaced actually turned out to be OK, but for whatever reason the flap has stuck slightly (the HEVAC drives it to stalling point when calibrating and reads the stall point as it's end value).

The way the HEVAC works is that if it detects the motor has stalled, it tries to back it off. If it doesn't see any change in the motor value in the opposite direction, then it shuts off the motor and logs the fault. It will not try and move that motor again until the ignition is cycled. So sometimes you get it where it's stalled at hot because the flap has stuck slightly. You've then turned the temp down, and it hasn't moved it, so it's then thrown an error and shut the motor off. Then on next power up it sees it should be at cold, so starts driving the motor and it then un-sticks and moves to cold, and happy days - until you go back to hot.

I had the issue where it would stick whilst calibrating (as mentioned above the HEVAC stalls it on purpose to read the value) - which if the stall point is where the flap sticks, not end of travel, then it will calibrate it wrong and then always stick when it moves the motor back.

I think on the second attempt of calibrating mine didn't stick and then has been fine since. The feedback error doesn't always meant the potentiometer has failed - just that the potentiometer didn't move when the HEVAC controller expected it to! So might be worth setting the temp at a mid-point, clearing the faults and then trying to get it to calibrate again and see how you get on.

Hope that helps.
Marty

I removed the motor checked the left hand flap all was ok then did a test on the motor again and it was very weak so replaced the motor and tested with battery set the gears right then tried with the engine running and all the motors seemed ok without calibrating so im seeing how it goes now

Am I right in thinking they recalibrate themselves every 500 ignition cycles?, sure I read it somewhere

tomcat59alan said:

Am I right in thinking they recalibrate themselves every 500 ignition cycles?, sure I read it somewhere

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That is correct Alan. Calibration is all about flaps stalling as they reach the seals in both directions. As the seals flatten they need to go a little further before stalling. After calibration the stalled position in one direction becomes 1 % and in the other 100%. If you force the flaps with Nanocom you may see a value under 1% or over 100% this should be correctable by calibrating the flaps. It just means the flaps have travelled just a little further before stalling. And recalibration is needed.

^^ What Wammers said. If you've replaced the motors then it's always a good idea to recalibrate with diagnostics as different motors will almost certainly report slightly different values back to the HEVAC controller.

martyuk said:

^^ What Wammers said. If you've replaced the motors then it's always a good idea to recalibrate with diagnostics as different motors will almost certainly report slightly different values back to the HEVAC controller.

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Absolutely correct. The stalled positions of the old motor for 1% and 100% that the Hevac has stored maybe be different to the stalled position of the new motor.