P38A Heated seats anomaly

4.0 HSE Thor

Previously, when you hit the heated seats button(s) the lights would come on and nothing happened - heated pads were knackered. Have both seats out at the moment for other stuff and figured I'd get the elements sorted, so that's been done. Uprated thermostat too however, when plugging them back in, when you hit the heated seat button, the light only comes on briefly then goes out. Any idea what would cause that? Can hear the relay in the BECM go off - voltages at the plug under the seat are correct.

Possible linked issue: have now noticed the same behaviour with either of the heated screens (front and back) or using the prog button. Momentarily comes on then goes off and returns to normal operated. All faults cleared for hevac with Nanocom, no issues. When hitting prog, you can hear the blend motors etc all turn immediately then all go back to normal. Temp adjustment, flaps and fan speed all work off the buttons correctly.

Using Nanocom on the outstation memory, I can move the seats around happily enough but again, turning the heated seats on, you can hear the relay click but nothing.

Heated seats that work

There’s an anomaly...

What he said, mine don’t work either

Is usually the heater pads like Dom says. Replacements Are cheap enough online,

presuming that’s what is up with yours? My model shouldn’t have them fitted buy you never know with these

Pads are fine, they were replaced today and checked.

They are timed as is the windscreen.
No idea how that helps tho cos timer is in minutes rather than seconds
Are they drawing too much current so switch off to protect battery or overheating?

oakey said:

They are timed as is the windscreen.
No idea how that helps tho cos timer is in minutes rather than seconds
Are they drawing too much current so switch off to protect battery or overheating?

Click to expand...

Possibly.. But all of them? Seats, windscreen, rear window, prog? Is there a common denominator here?

Yes but sorry no idea what it is
Suggest in HEVAC panel somewhere

RangeRoller dt said:

Heated seats that work

There’s an anomaly...

Click to expand...

My original ones did. Never, ever switched them on! Replaced my driver's one for a knackered one but better leather. I guess the passenger one still works but it was only ever switched on at the test drive!

I’ve got the HS buttons but not sure my DT should have my model hevac?? Wiring is unoriginal and most I’ve seen have slidey handles like a fender

There's an override function detailed in RAVE to avoid current overload if too many systems are in operation.

Another possibility is dirty Prog or Auto switch contacts. Mine used to randomly deactivate Auto mode until I pressed the button a few times.

CIRCUIT OPERATION
Driver and passenger seat heaters are PTC
regulated. Heated seat activation requests and
timing functions are generated by the Hevac ECU
while the Ignition Switch (X274) is in position II. The
BeCM (Z238) provides power switching only for this
function. In the case of both rear windows and the
sunroof being operated at the same time or an
uninitialised rear window or sunroof function being
operated, the seat heaters will be inhibited to
maintain current demand within a safe operating
area.
The signals are sent from the HEVAC ECU to the
BeCM (Z238) and are active low. Therefore the lines
are normally held at 12V.

Is the HEVAC ecu part of the hevac unit in the dash, do you know? From what I recall, it was only the switch gear and display in that. Was quite a light thin unit.

HEVAC controls the relays which if I remember are in fuse box. Check RAVE.

Pete.

pwood999 said:

HEVAC controls the relays which if I remember are in fuse box. Check RAVE.

Pete.

Click to expand...

Yep. When selecting any of the affected buttons on the hevac unit, you can hear the relays clicking in the BECM. Just click off again almost immediately though

Best to check using a voltmeter. The signals from HEVAC to BECM are active low, i.e. normally at 12V when off, and go to 0V when you switch them on. Check the voltage at both HEVAC & BECM connectors.

You could also simulate the HEVAC operation by first disconnecting C246 and then grounding C255 pins at the BECM. This should activate the heaters.





Nanocom can also show the signal from HEVAC. BBS document has typo because it references pin-15 for both seats, but it should still show the BECM inputs.

Okay had a few moments of progress sort of.

1) reseated the top row of connectors on the BECM and, with the engine off, ignition position 2, the lights come on and stay on. Turn the engine on and they go off, will not go back on.

2) tightened up the battery connectors as was getting an alternator fault, that's no longer an issue.

3) engine running, I can switch the heated seat on but similar to before, after a few seconds it turns off again. Same applies with prog or heated screens.

4) checking voltages at the various plugs returns the correct values depending on what's selected.

5) using Nanocom, outstation memory, it apparently let's me turn the heated seats on as you can hear the relay click - it doesn't click off until I switch it off in Nanocom. The seats don't warm up though (presume because the engine isn't running)

6) hevac does have 2 recurring errors but don't think they're relevant:
(36) condenser fan control circuit short to positive 12v (logged)
(40) air conditioning request line fault (current)
Both reappear after being cleared.

Update. Carpet is bone dry.

Lifted it up and found the sponge underneath is absolutely soaking wet. So, not going to diagnose anything further until that's all dry. Just need to figure out how to dry it thoroughly.