P38A HEVAC strange problem

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pwood999

pwood999

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Near Heathrow.
Suddenly getting weird behaviour from my HEVAC. Was fine driving from Heathrow to Bournemouth yesterday afternoon, but then misbehaved randomly on the way back in the evening.
  • Air is very hot at all settings from 16-26 deg, but goes cold at "LO" setting ?
  • Distribution & Blend flaps seem to be working ok (can hear them cycle before starting car).
  • No book symbol.
  • A/C on-off working correctly.
  • Exterior temp reading working fine.
  • Same issue in Auto or Manual modes.
After quick pause for fuel, restarted car, and it started working correctly for about 20mins, and then back to random heat for the rest of the journey.

Will get the Nanocom on it later, but anyone had same issue ? Could it simply be the mini-fan behind the vent sticking ?

Pete
 
pwood999 said:
Suddenly getting weird behaviour from my HEVAC. Was fine driving from Heathrow to Bournemouth yesterday afternoon, but then misbehaved randomly on the way back in the evening.
  • Air is very hot at all settings from 16-26 deg, but goes cold at "LO" setting ?
  • Distribution & Blend flaps seem to be working ok (can hear them cycle before starting car).
  • No book symbol.
  • A/C on-off working correctly.
  • Exterior temp reading working fine.
  • Same issue in Auto or Manual modes.
After quick pause for fuel, restarted car, and it started working correctly for about 20mins, and then back to random heat for the rest of the journey.

Will get the Nanocom on it later, but anyone had same issue ? Could it simply be the mini-fan behind the vent sticking ?

Pete
Click to expand...

What do you expect, selecting low or high disengages automatic operation. Selecting LO will just give you ambient fresh air. Selecting HI will turn it into an oven.
 
Setting 16deg makes the air fully hot as if on HI. It was like the blend motors were going from Ambient to full hot for 1deg setting change.

Anyway tried again this afternoon. 7 mile drive and all is working correctly. No faults on Nanocom so far.

Must be loose connection somewhere.
 
pwood999 said:
Suddenly getting weird behaviour from my HEVAC. Was fine driving from Heathrow to Bournemouth yesterday afternoon, but then misbehaved randomly on the way back in the evening.
  • Air is very hot at all settings from 16-26 deg, but goes cold at "LO" setting ?
  • Distribution & Blend flaps seem to be working ok (can hear them cycle before starting car).
  • No book symbol.
  • A/C on-off working correctly.
  • Exterior temp reading working fine.
  • Same issue in Auto or Manual modes.
After quick pause for fuel, restarted car, and it started working correctly for about 20mins, and then back to random heat for the rest of the journey.

Will get the Nanocom on it later, but anyone had same issue ? Could it simply be the mini-fan behind the vent sticking ?

Pete
Click to expand...

Yeah, had that a few years ago. Fixed itself like yours. Has done it since but only once in a whle and cures itself each time. Generally goes to red hot on the hottest day of th year when I'm sat in traffic. I'd just about decided it was a safety feature to dump heat if the engine felt it was getting too hot.
 
Either a loose connection, or maybe a dodgy temperature sensor which is throwing the logic off, and causing it to think it needs to drive either full hot or full cold to achieve the desired temperature.

Might be the one in the dash, giving a bad readout as a target back to the HEVAC - or maybe the heater core temp sensor is intermittent and causing it to do strange things...
 
martyuk said:
Either a loose connection, or maybe a dodgy temperature sensor which is throwing the logic off, and causing it to think it needs to drive either full hot or full cold to achieve the desired temperature.

Might be the one in the dash, giving a bad readout as a target back to the HEVAC - or maybe the heater core temp sensor is intermittent and causing it to do strange things...
Click to expand...

IIRC mine threw up a short-circuit on the right blend-motor but it fixed itself. I assume that had something to do with it.
 
Could have been that it stalled itself and the HEVAC threw a hissy fit. If it detects a problem with the motor at all then it logs a fault and then won't try to operate that motor again until the next time it's powered up. I've known of a couple where the driver IC's for the blend motors have failed and caused a short circuit error - but it's one that won't go away!
 
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