Strange issue with HEVAC temperature control

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Chewster

Chewster

Active Member
Posts
148
Location
Lancaster
Hi All - first post so be gentle eh (!)

I did a search on the forum and couldn't find anything that similar to my problem so thought i'd start a new thread about the HEVAC in my P38 diesel

Airco works fine - I get cold air and it cools nicely to what I set it to, the issue is with cold weather

Symptoms: When I get in the car and it's not been running all seems fine, if it's cold then heater blows lovely hot/warm air and the cabin temp gets up to about what I set it to - all good so far. The problem is that after 30 minutes the heater just blows cold air, the only way I can get the heater to chuck warm air out is to set the temp to HIGH, and then it blows hot air only, I can't keep it on high all the time as it gets too warm

As soon as I set the temp to anything but high I just get cold air again. If I stop and restart the car it doesn't make any difference, the only time (in cold weather) that the heater works is for the first half hour! It's always about the same time as well before it craps out

Does anyone know if there is a timer for the heating regulator? The only thing I can think of is that:

1) For some reason the heater just switches off after a set time
2) For some reason the temperature sensor in the cabin is not working correctly and informs the HEVAC that the cabin is up to temp when it isn't
3) Heater blend doors are stuck (but why work when the car is cold?)

If anyone has some ideas I would be grateful !! It's fine now in summer but the pessimist in me says winter is coming

As usual I have a problem with EAS but i'm waiting for a cable and then i'll have a go at that myself before looking here.....plenty of good stuff for EAS on this site so I hope to fix it myself

.....oh and Hello all by the way!
 
Chewster said:
Hi All - first post so be gentle eh (!)

I did a search on the forum and couldn't find anything that similar to my problem so thought i'd start a new thread about the HEVAC in my P38 diesel

Airco works fine - I get cold air and it cools nicely to what I set it to, the issue is with cold weather

Symptoms: When I get in the car and it's not been running all seems fine, if it's cold then heater blows lovely hot/warm air and the cabin temp gets up to about what I set it to - all good so far. The problem is that after 30 minutes the heater just blows cold air, the only way I can get the heater to chuck warm air out is to set the temp to HIGH, and then it blows hot air only, I can't keep it on high all the time as it gets too warm

As soon as I set the temp to anything but high I just get cold air again. If I stop and restart the car it doesn't make any difference, the only time (in cold weather) that the heater works is for the first half hour! It's always about the same time as well before it craps out

Does anyone know if there is a timer for the heating regulator? The only thing I can think of is that:

1) For some reason the heater just switches off after a set time
2) For some reason the temperature sensor in the cabin is not working correctly and informs the HEVAC that the cabin is up to temp when it isn't
3) Heater blend doors are stuck (but why work when the car is cold?)

If anyone has some ideas I would be grateful !! It's fine now in summer but the pessimist in me says winter is coming

As usual I have a problem with EAS but i'm waiting for a cable and then i'll have a go at that myself before looking here.....plenty of good stuff for EAS on this site so I hope to fix it myself

.....oh and Hello all by the way!
Click to expand...

Sounds like one of the sensors has failed, there is a sensor by the clock with a little fan inside that monitors cabin temperature, they fill with fluff. The is also a sensor on the heater matrix pipes plus a solar gain sensor centre of dash at the edge of the screen.
I assume you are using it in auto mode? Try running it in manual mode and see if you can control the temperature.
 
Thanks for the tips, I will try the manual mode

Is there any way for me to check the sensors without diagnostic kit? I guess not......I live near a very good LR indy so if it needs hooking up for diags I can get it done very reasonably - I just like fixing things myself! (or breaking them........which to be honest is the more usual outcome!)
 
Sounds like the problem I had. It turned out to be the sensor by the clock. The fan has failed so I think the temp on the sensor increased behind the dash and so the cold air would blow! I changed for a second hand unit and all is now well!

You could try shining a torch through and see if the fan starts to turn to see if that is OK or not.
 
there is a sensor by the clock with a little fan inside that monitors cabin temperature
Click to expand...

Porsche 928 has the same. one of the ways to check is to blow some cigarette smoke at it and see if it sucks in.. of course in more modern times, cigarette smokers are not so common.
 
Well I am currently in Holland for work (so no issues with finding a smoker....for testing purposes only of course!) but when in the UK I live just outside Lancaster

I will check the sensor by the clock first as it sounds the most likely culprit

Thanks for all the replies guys
 
GENTLY and slowly insert an opened up paper clip through the sensor grill in the dash. If the fan is working your hear and feel it. If you have a can of compressed air, shoot that into the grill. The fan isn’t totally enclosed and any crap should be blown out.
 
Will give that a go - as it happens I can 'appropriate' a can of compressed air from work, purely for work purposes of course, wouldn't dream of using it for anything apart from the original intended use of cleaning the crap out of my PC.......
 
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