P38A Making sense of HVAC error codes

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Montyjohn

Montyjohn

Well-Known Member
Posts
220
Location
UK
I get the book symbol often on my HVAC.
If I leave both sides in LO, then the book doesn't appear.
I can change the distribution buttons with no issue.
When the book symbol comes on, it disappears after a restart, so I believe the motors are all winding back to their start position correctly.

It used to be that if I touched the left temperate control the book would appear, but lately, it only does it on the right side. I'm sure I've got that right but memory is an odd thing.

Anyway plugged in my RSW diagnostics the other day and got the following:

Right blend door motor fault S/C to 0v)
Right blend door motor circuit fault (O/C)
Right blend door motor circuit fault (feedback)
Left blend door motor fault S/C to 0v)
Left blend door motor circuit fault (feedback)
Distribution door motor circuit fault (feedback) door motor circuit fault (feedback)
HEATER CORE TEMPERATURE SENSOR CIRCUIT SHORT TO GROUND

So everything is broken it seems.

Cleared the codes, and played with all the heating settings a few time to force the book to come on a handful of times.

Now the only code I get is

Right blend door motor circuit fault (feedback)

Do I therefore ignore all the other messages and assume they are just noise?
I assume it needs a new blend motor for the right hand temp control, or at least a good clean out.

I can see in the history it had a blend kit fitted (STC3259) back in 2015, so shame it's gone again.

Since it appears to reset ok after each use, even after the book symbol comes on, I suspect it may be more to do with the flap rather than the motor itself.

Any experience or thoughts to start me off in the right direction?
 
Could just be the potentiometer inside the motor, or bad connections on the contacts in C261 connector.

Mine works about 25% of the time after cleaning the contacts. I have a replacement connector ready to fit, but not had time to do it yet !!
 
The variable resistors/pots are likely the issues especially as running them a few times clears the fault.
The hvac does a fault check every time it powers up and only the 3 blend motors cause a book symbol as far as I'm aware
 
Montyjohn said:
I get the book symbol often on my HVAC.
If I leave both sides in LO, then the book doesn't appear.
I can change the distribution buttons with no issue.
When the book symbol comes on, it disappears after a restart, so I believe the motors are all winding back to their start position correctly.

It used to be that if I touched the left temperate control the book would appear, but lately, it only does it on the right side. I'm sure I've got that right but memory is an odd thing.

Anyway plugged in my RSW diagnostics the other day and got the following:

Right blend door motor fault S/C to 0v)
Right blend door motor circuit fault (O/C)
Right blend door motor circuit fault (feedback)
Left blend door motor fault S/C to 0v)
Left blend door motor circuit fault (feedback)
Distribution door motor circuit fault (feedback) door motor circuit fault (feedback)
HEATER CORE TEMPERATURE SENSOR CIRCUIT SHORT TO GROUND

So everything is broken it seems.

Cleared the codes, and played with all the heating settings a few time to force the book to come on a handful of times.

Now the only code I get is

Right blend door motor circuit fault (feedback)

Do I therefore ignore all the other messages and assume they are just noise?
I assume it needs a new blend motor for the right hand temp control, or at least a good clean out.

I can see in the history it had a blend kit fitted (STC3259) back in 2015, so shame it's gone again.

Since it appears to reset ok after each use, even after the book symbol comes on, I suspect it may be more to do with the flap rather than the motor itself.

Any experience or thoughts to start me off in the right direction?
Click to expand...
Odds on it's the blend motor and not the flap. The distribution flap is the one that usually sticks not the temperature blend flap.
As others have said, it's likely to be the potentiometer at fault.
Ignore the historic codes that cleared.
If you use the HEVAC on Auto the blend motors will fail more often as they will constantly be on the move. I use ours on manual and we rarely change the temperature setting hence no wear. ;)
 
I also find it best to keep both temp settings equal, or it will start hunting to get the right temperature on both sides. Passenger side can't be more than 2 different from the driver's side anyways
 
Might buy a handful of these potentiometers just have handy when I start pulling things apart.

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I noticed here: http://www.stockholmviews.com/p38/index.html#hevac it mentions that there are two versions. Once that good for 500 cycles, and another for 10,000 cycles but I can't find this specified anywhere in the technical data.
 
Montyjohn said:
Might buy a handful of these potentiometers just have handy when I start pulling things apart.

Buy Piher PT15NV02-103A2020-S PT15NV02-103A2020-S Trimmer linear 0.25 W 10 kΩ 270 ° 1 pc(s) | Conrad Electronic

Top %brandid% Selection | Order easily now: Piher PT15NV02-103A2020-S PT15NV02-103A2020-S Trimmer linear 0.25 W 10 kΩ 270 ° 1 pc(s) | Fast delivery | Reasonable prices
www.conrad.com www.conrad.com

I noticed here: http://www.stockholmviews.com/p38/index.html#hevac it mentions that there are two versions. Once that good for 500 cycles, and another for 10,000 cycles but I can't find this specified anywhere in the technical data.
Click to expand...
The potentiometer is an industry standard part, look in Farnell or CPC, the dimensions and value of the pot is in your link.
 
Datatek said:
The potentiometer is an industry standard part, look in Farnell or CPC, the dimensions and value of the pot is in your link.
Click to expand...
COuldn't find it in CPC, but Farnell seem to have them.

Notes so I don't forget
I think V02 is the correct mount
E option is the long life option
It's an N rotor as G is on 11 weeks back order
No idea what the PM 50% wiper position is or if it's important
 
Montyjohn said:
COuldn't find it in CPC, but Farnell seem to have them.

Notes so I don't forget
I think V02 is the correct mount
E option is the long life option
It's an N rotor as G is on 11 weeks back order
No idea what the PM 50% wiper position is or if it's important
Click to expand...
Not sure that's the right one, it must be the linear version designated by the letter A.
PM 50% just means the device is delivered with the wiper half way round the track, not important
 
Don't know part numbers but there are 2 types, the one i got was 90° out. It doesn't matter but it means the arrows on the gears don't like up and you have to work it out
 
Datatek said:
Never found that on my 2 P38's at least not if it's on manual.
Click to expand...
Definitely does that on both the ones i've had... as soon as I push one side higher or lower temp (though I think more than 2 degrees, more like 5) the other blend motor gets dragged in the same direction. I can watch the flaps do that all day with the nano, so it's presumably programmed behaviour
 
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