P38A New hvac fault, bugger!

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Flossie

Flossie

Well-Known Member
Posts
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Location
Shropshire
You all may recall I had the book up and nano saying pot fault right hand blend motor? Today, I have tackled the job and soldered in a new pot that I got off kermit_rr, bit of faffing about as the pot drive is 90 deg out but simple once I sussed it.

20240504_135827.jpg

New pot fitted , note the arrows 90 deg out . Refitted it and that fault has gone according to nano.
20240504_143841.jpg


Now, my new fault which won't clear, on the distribution motor.
20240504_163137.jpg

I've calibrated it twice but the fault won't clear and cycled the ignition 'just incase ', still there and the book also.
I did take the distribution motor off to check the flap moves easily and also had a peek at the gears inside to check for wear then buttoned it all back up.
When I force the distribution motor with the nano I can see the big white gears moving and the airflow changes as expected so I'm at a bit of a loss now.
 
Flossie said:
You all may recall I had the book up and nano saying pot fault right hand blend motor? Today, I have tackled the job and soldered in a new pot that I got off kermit_rr, bit of faffing about as the pot drive is 90 deg out but simple once I sussed it.

View attachment 316369
New pot fitted , note the arrows 90 deg out . Refitted it and that fault has gone according to nano.View attachment 316370

Now, my new fault which won't clear, on the distribution motor.
View attachment 316371
I've calibrated it twice but the fault won't clear and cycled the ignition 'just incase ', still there and the book also.
I did take the distribution motor off to check the flap moves easily and also had a peek at the gears inside to check for wear then buttoned it all back up.
When I force the distribution motor with the nano I can see the big white gears moving and the airflow changes as expected so I'm at a bit of a loss now.
Click to expand...
Usually, that would be the distribution flap jamming at one end of it's travel. Are you sure you put the blend motor back on with the flap in the right position?
 
Datatek said:
Usually, that would be the distribution flap jamming at one end of it's travel. Are you sure you put the blend motor back on with the flap in the right position?
Click to expand...
It will only go on one way? Got me thinking now! I'll take it off again tomorrow and double check everything.
Thinking about it again, the motor wasn't moved and I had to turn the flap maybe 10 deg clockwise to get the flat on the splined motor drive to line up before it would push back into place , the screw holes lined up easily.
I guess it could be wrong if the spline on the flap rotates more than one turn ie 2 or more turns, otherwise I can't see how to get it wrong.
I can't 100% remember but when I worked the white gears I seem to recall the operating arc wasn't much, maybe 180degs?
I've put my back out being stuck in awkward positions and can't face going out to it again until tomorrow.
 
The flap on the distribution only moves around 120 degrees before it hits mechanical stops , observed after removing the distribution motor again and manually moving the white gears. Took the drive unit apart, enligned the gears after checking for bad joints and reassembled back on to the heater box taking care to mate up with the flats on the spindle.
Fault still there, won't clear, book symbol up. :vb-groan2:
I did notice however, that if I turn the ignition off then back on, the book disappears for maybe 15 secs (self test I think?) Then reappears as soon as the self test finishes.
Squeezing my hand down I can feel and hear the BLEND motor moving during this self test and when it stops moving is when the book comes back.
All flaps move easily btw.
I'm stumped as what to do next.
 
Are you 100% sure the pot is not reaching it's end stop ? The pots I bought recently also need the gear in a non-standard position. Also check the distribution gears are correctly aligned - there are little marks on them as well.

How free are the flap gears ? Even stickyness at one poiunt can mess with the calibration stall points. They need to move with single finger pressure.

The distribution flaps on all the heater boxes I have seen, have rubber edges & cannot stick. It's the plastic end spindles that become tight in the heater housing. Unfortunately it's a dash out job.
 
Last edited:
Distribution gears? The white(and black) ones on the box itself?
I haven't removed any of them.
What I did with the replacement pot was, I noted how many degrees end to end was on the old pot and where the arrow on it was pointing when it was enligned with the other larger white gear wheel, turns out that it's 1/2 way through its sweep. I then, without moving the position of the white gear wheel, set the new pot exactly 1/2 way through its sweep and fitted the black pot gear which turns out to be 90 degrees clockwise as in the pic above.
No faults showing on the nano for the blend motor now, just this distribution fault.
 
If I haven't removed any of the gears on the box just moved them by hand to check for smooth movement, do I still need to enlign them? Can't see why unless they have been removed but happy, more than happy, to be told they do!
 
Flossie said:
Distribution gears? The white(and black) ones on the box itself?
I haven't removed any of them.
What I did with the replacement pot was, I noted how many degrees end to end was on the old pot and where the arrow on it was pointing when it was enligned with the other larger white gear wheel, turns out that it's 1/2 way through its sweep. I then, without moving the position of the white gear wheel, set the new pot exactly 1/2 way through its sweep and fitted the black pot gear which turns out to be 90 degrees clockwise as in the pic above.
No faults showing on the nano for the blend motor now, just this distribution fault.
Click to expand...
In diagnostics, you should be able to see the percentage feedback from the potentiometer, you should also be able to see that percentage change if you command movement from diagnostics.
 
Datatek said:
In diagnostics, you should be able to see the percentage feedback from the potentiometer, you should also be able to see that percentage change if you command movement from diagnostics.
Click to expand...
Yeah...I sort of did that yesterday, I took screenshots as I tried to put a video on here before and it wouldn't let me. Screenshots aren't much help on a moving screen really.
I'll put some up anyway.
 

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Darn, posted accidentally before I'd finished. The right blend motor above stayed on 65 even though I'm getting no faults on nano for that. The left blend motor..that number changed?
More pics to follow.
 
That last number changes as in the pic above but not on the rh blend.
Nano reporting no faults on the rh blend though.
 
Flossie said:
That last number changes as in the pic above but not on the rh blend.
Nano reporting no faults on the rh blend though.
Click to expand...
What does the green band represent? I don't have a Nanocom but the percentages look right, 0% in 0% feedback, 105% in 100% feed back. With Faultmate, I can actually watch the numbers change in real time. The RH blend motor should do the same.
 
The Nanocom numbers also change in real time, but not in photos !! RH blend pot or motor definitely not correct.

Best way to see is to unplug from HEVAC, and put a ohmmeter across the pot wires. Then move them with a 9V battery & see if the ohms reading changes correctly. Short pieces of copper wire poked into the connector is fine if you don't have thin probes.

In case it helps, here's where the distribution gears align. Move the grey gear all the way to one endstop, and make sure the little square marks on the other two are aligned.

IMG_0835.jpg
 
To diagnose and examine the blend motor assy it is easiest to remove it from the car
and take it apart, when this is done just remove the two white gears and lift out the
circuit board that holds the motor and potentiometer.

Use a multimeter (or even better an old fashion meter with needle and scale) to check the
potentiometer for condition by measuring the resistance while slowly moving the
potentiometer wiper through the whole register the value should smoothly change its
value without any large jumps.

Use a 12v DC source to check the DC motor and make sure it spins fine.
Connect and remove the power several times to make sure all windings in the
motor anchor is OK also change polarity to reverse direction a few times "just in case".

If no fault is found reassemble the blend motor and perform troubleshooting on wiring
between motor and HEVAC control panel.

heater blend motor potentiometer replacement Range Rover P38
10K linear potentiometer


In case of faulty potentiometer, unsolder the potentiometer and replace it.
It is a 10K linear trim potentiometer PHIER PT 15NV is the type.
Conrad P/N 432024 - 89. Wiper position preferably like in the G version
but not critical . This potentiometer comes in two different versions, one
that are rated for 500 cycles and one that is suppose to be good for
10000 cycles. (Not hard to guess witch one Land Rover uses, huuu). Get the Long Life version

Is this any help
 
Missed this bit ooops

Heater Blend Motor repair Range Rover P38


When you reassemble the blend motor, with its original parts make sure to get the
markings on the big white gear to meet the marking on the potentiometer gear.

worm gear of the HEVAC servo
Potput gear


If you replaced the potentiometer and didn't get the one with correct wiper/gear
position just make sure the white gear is centered at it's widest part against the
other white gear, then tune the potentiometer to middle position and thereafter
install the potentiometer feedback gear without taking care about markings
 
Fixed it I hope!
Found this post on a goggle search last night.
Went out and plugged the nano in and 'distribution motor stall fault' still there and won't clear. Then I did this
Screenshot_20240506_120932_Gallery.jpg


And the book symbol disappeared.
I then ran through all the values for all 3 motors and the rh blend was now reading the same as the lh blend which it wasn't doing yesterday.
Cycled the ignition a few times and still no book symbol and look at me nano now! Yay! :banana:
20240506_110851.jpg
 
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