P38 Blend motor problem

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T

TigeTikker

New Member
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7
Location
Estonia
Hello, I hope someone can help me with this problem.

Car: Range Rover P38 2.5 diesel 1998. Automatic with AC.
So the drivers side(left hand drive) blend motor is not working and i have no idea why.
What have I done/tested:
1) I have taken out the servomotor (so it is not connected to the flap), problem still exists. Flap itself moves freely.
2) Checked all the wiring and connectors (for blend motor and HVAC overall). Everything is OK.
3) Taken apart servomotor (gears fine, potentsiometer 10kohm, motor moves fine). Aligned also the arrows to default position.
4) Tested other 3 used blendmotors (gears fine, potentsiometer 10kohm, motors moves fine) - Did not help. Still same problem.
5) Tested another HVAC unit - Did not help. Still same problem.
6) When applying voltage to the blend motor it moves both ways.


Can there be some sensor fault that would cause only one servomotor to not move?
Everything else works (except outside temperature sensor that seems to be faulty).
Can the blend motor still be faulty somehow?
Does it have to be calibrated with a computer?
 
You need to measure the resistance of the potentiometer across its full range, ideally with an analogue meter, make sure it's nice and smooth from 0 to 10, no gaps, spikes or otherwise erratic reading.
This is often what fails
 
Datatek said:
You need to measure the variable resistance as the shaft is turned.
Click to expand...
Yeah, it goes from 0ohm to 10.3Kohms. There are no gaps also.
kermit_rr said:
Also check the solder joints on the under side of the board in the blend motor itself
Click to expand...
They are fine by visual. Also measuring confirms it.

I have tested 4 (including the original) blend motors and it cant be that all have same mysterious problem.
Doesn't those have to be calibrated using computer? Or i´ll just connect them and they should work?

I actually got one moving the other day, but it only moved when starting the car. After the car had started and the servo moved to some location(seemed to be random) and then the book marking came on and it did not respond anymore.

Do you guys think that i should spend the cost for brand new blendmotor? I have already wasted money trying to get this working and as it seems that the used blendmotors are fine whats the chance that the new one will get this problem solved..
There cant be anything else wrong in the servomotor if the gears are fine, motor moves both ways and potentsiometer shows values from 0ohm to 10.3Kohms. Or am i wrong?
 
If you unplug the hevac power for 10s then plug back in and turn on the ignition, it should do a calibration. I forget which plug it is.
If the motor is going so far then stopping, that sounds like a problem with the pot
It will only work in situ, it needs to 'feel' where the limit of flap travel is in both directions
 
I wish I understood any of what you are all saying. My heating works in as much as it heats the cab, but fan seems a bit random, temperature control I have no confidence in and the air con is disconnected. I’d like to have a go at fixing it all one day, but I don’t even know where to start.
 
You need to clear the service book symbol ( or ID the reasons for it being on), the Hevac may well be protecting another malfunction and your LH blend motor is perfectly fine. The fact that the hevac moves the blend door at start up strongly suggests it's fine.

The motors can be recalibrated, but as far as I know the start up sequence ( if it succesfully moves the relevant doors through the full range) supercedes it anyway.
 
Last edited:
Blend motors DO NOT calibrate every time the ignition is switched on. They move to their stored max and minima then return to their selected position. Calibration is totally different.
 
Datatek said:
According to RAVE a " self tuning" calibration cycle occurs every 500th ignition on cycle
Click to expand...
Yes it does but that is not everytime the ignition is switched on as some think. Calibration stalls the motor at each end and the ECU stores the new max travel in each direction info.
 
I had strange HEVAC behaviour on my P38 where the 16-pin connector appeared to be intermittent. Turned out to be similar to the EAS driver pack connector issues, where the pins & sockets inside the 16-way housing were not making good contact. I suspect over time all these connector types suffer expansion of the socket side, and hence cause random connection problems.

Also not 100% sure on default modes with book-symbol, but if any of the HEVAC temp sensor inputs are invalid, it might try to compensate by moving the blend flaps to end stops, hence initial movement followed by non-operation ?

I have definitely had excessive heat when my heater pipe sensor was giving wrong readings. HEVAC decided the water pipes were too cold, so increased the cabin heat to max to compensate !!

Pete
 
+1 re: connectors. I had exactly the POs issue a few weeks ago. Blend motor apparently not moving after the start up sequence... in the end I just replaced one connector pin, and the entire connector block. No new blend motors, no recal
 
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