P38A HEVAC Temperatures Wrong

My HEVAC has recently started behaving weirdly. All works ok in the morning as the engine warms up, and remains so for about 30-60 mins. Heating & Air-Con works properly.

Then after being driven for a while the Aspirator temp rises to about 30-32deg (but cabin is still around 20deg & aspirator fan is working ok), and the Heater Core reading drops to 30-36deg (engine temp still at 95-98deg). Consequently the heater output is all over the place & flaps moving around quite a bit !!

• Anybody else had similar issues?
• Could this be HEVAC Ground related, or simply bad connectors with resistive contacts?
  • neither sensor is grounded directly, but have both sides into the HEVAC, hence why I don't believe it's an earthing problem.

I guess it's time to open up the dash again. Will also re-check the fan, and check the heater feed & return with separate temp meter.

Pete

Have you put the Nanocom on it when it's cold? I have seen before the heater core temp sensor read weird values - if they have ever had coolant drip into them from a leaky O-ring then they can corrode over time, or go open circuit at random times... more commonly they fail where they show something like -19 degrees until they warm up though.

The other thing could be air leakage in dash from badly sealed joints. A mate's P38 that we pulled the dash out on the weekend had an issue where in some distribution motor settings it blew the air into the dash cavity which messed with the aspirator temperature. His drive to me was about 1 1/2 hours, and you could put your hand on the side of the centre console (with the outer trim panels off) and it was noticeably warmer than it should be..

Sensor wise, I am not sure if all of the sensors join up to a common ground inside the HEVAC (haven't probed it out yet) but the Heater Core, Evaporator, and Solar sensor all share a common feed, and then have separate returns back to the HEVAC. So unless all of these 3 sensors values change at the same time, then it probably isn't an issue with the HEVAC. If all the sensors drastically change value at the same time, then it could be something common - either in the wiring or the controller.

Not sure if that's any help...

Marty

Interesting. In the past mine randomly used to put the heaters on hot. Usually on a very hot day. I used to manually put the temperature on LOW and then after a few minutes put it on HIGH for a minute and then move it back to the desired temperature and it would behave again. Not done it for a year or so though. Yours do anything like that?

Not tried the Nanocom cold yet, but good idea. Yes tried going through from full hot to cold & back, but still playing up.

I'm am suspecting hot air leak behind the aspirator, although the outer panels don't get that warm. Will def check the heater temp sensor contacts.

pwood999 said:

Not tried the Nanocom cold yet, but good idea. Yes tried going through from full hot to cold & back, but still playing up.

I'm am suspecting hot air leak behind the aspirator, although the outer panels don't get that warm. Will def check the heater temp sensor contacts.

Click to expand...

If you have a leaking seal dash to top of heater unit that may give you the problem.

Definitely looks like air leak. This morning aspiratior temp is 2-3 dog high after 10-15mins.

Heater core still resolutely at 35deg so prob contact issue there.

pwood999 said:

Definitely looks like air leak. This morning aspiratior temp is 2-3 dog high after 10-15mins.

Heater core still resolutely at 35deg so prob contact issue there.

Click to expand...

The seal on the top of the heater to bottom of dash degrades. I could not believe how bad mine was when i took the dash out to do the heater. It was just dust.

Had another look at the HEVAC sensors today. The reason for heater core sensor being wrong was bad corrosion on one of the connector sockets & the corresponding pin on the sensor. No idea why only one was bad, but cleaned both (Vinegar & Salt method) connector & sensor. Now the aspirator, evaporator & heater core readings are within 1-2 degrees of each other (engine off because water pump still removed). Moved the sensors so I could blow warm air over all three together, and all the readings go up together

Hopefully once the water pump is fixed the HEVAC will now work properly.