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Discussion Starter #1
AC/HEVAC/Compressor Clutch confusion..... A solution? Maybe.....

Afternoon all.
Been trying everything I can think of to narrow down “issues” with my ac.
Will put details here in case it helps anyone else! Thought I would put a new post rather than updating my other ac posts. Hope that’s ok.
To summarise I had been having various issues, replaced condenser, receiver dryer, had a regas, then replaced trinary switch. The compressor would kick in, and then out again.
Assumption was this might still be because HEVAC was not getting a correct read from the trinary switch.
I removed the HEVAC unit yesterday to inspect, and it had a sticker on suggesting it had been replaced in 2008 and the unit had come from keymancars.
I took a picture of this as it had a part number (or so I thought) but this didn’t correspond to anything I could find. (370125029809). 2013-08-03 17.15.29 edit.jpg
Put everything back together, and did some more internet searching.
Made a list of a few things to try, and as weather is good today, gave them a go.
First question, was did I have Relay 10?
Yes I did. I think this means I have the extra wiring harness, possibly confirmed by presence of Relay 3?
I had tried jumping trinary switch previously, but wasn’t sure if this was when handbook symbol was already showing or not.
Today I tried jumping the trinary with engine off, then starting up.
Same issue. Compressor kicked in, and then out.
I tried jumping the other pressure switch.
Same as above. Compressor kicks in, then out.
I found this link last night.
http://www.stockholmviews.com/p38/index.html#hevac
This raised the question about using pre/post 98 HEVAC units with opposite age cars.
As I have a 2000 4.6, I wondered whether the unit replaced in 2008 was perhaps not the right one?
The link suggests jumping the Relay 3 (85/87 terminals I think) as a solution to the HEVAC unit not registering the right current draw, and thus cutting the compressor clutch. I’ve attached photo of my temporary jump, but there is also one on the above link. 2013-08-04 11.31.42 edit.jpg
Bingo! Compressor clutch ran fine, still stopping and starting, but I believe this is normal, and when it was running, it would run for longer, and cold air would be felt from the vents, and most important…. No handbook symbol!

Following this, I removed HEVAC unit again, to find out what I should have noticed yesterday…..
As you can see from the attached photo, the HEVAC unit shows part AWR1012. 2013-08-04 11.40.59 edit.jpg
I’m not sure whether this means it’s older or not. There are ebay items listing this as ’97 vehicle, but also ebay items listing AWR5051 as being from older cars.

The question for the experts is….. Am I likely to cause any damage to anything by using this Relay 3 hack? The fact that the compressor clutch still cuts in and out, suggests to me that pressure/temperature readings are having an effect, so there shouldn’t be a risk of overloading the system. I could be wrong of course….

Clearly, if I have the “wrong” HEVAC unit, then a better solution would be to get the right one, but these aren’t cheap, and buying from ebay etc, has no guarantee of a proper solution if the unit doesn’t work properly.

On a side note, with HEVAC unit removed, I also found what appears to be a blend motor harness which has been cut. Photo also attached. 2013-08-03 17.14.21 edit.jpg
It looks like a normal blend motor harness is plugged into HEVAC, and I think that this cable has likely been left as it is very tight in the behind dash lattice of supports, so perhaps has been cut and left. There was certainly no play in it. As my blend motors appear to be working, this will be on the back burner, but if they start to play up, I guess it will need more investigation!

Any thoughts gratefully received!
Cheers
Matt
 

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Re: AC/HEVAC/Compressor Clutch confusion..... A solution? Maybe.....

Ok.. here goes (again... this may be a bit of a long one!!)

It's interesting that you mention this and have a look at that... In short (explanation to follow), you do have the wrong HEVAC ECU for the vehicle. But, the RL3 Jumper method (Maybe make a shorter one) will fix your issue, and problem will be solved... There's just been a pretty long discussion on one of the P38 groups on Facebook, about this exact issue..

The explanation/reasoning...

The earlier models of P38 had the A/C compressor clutch driven directly from the HEVAC ECU. This obviously had a MOSFET or other power transistor in it with current sensing (like the ones in the BECM that tell you if a lamp has blown) to engage the compressor clutch directly from the HEVAC ECU. The coil (at least on my 2001 - but I presume it's pretty standard for electro-magnetic A/C clutched) is about 4ohms. Ohms law says that current (I) = voltage (V) / resistance (R). So 12v system, with a 4 ohm coil will draw 3A. This is a fair bit of current to be drawing, and I'm presuming that over time the power circuitry in the HEVAC gets weaker, and also as the clutch wears - the HEVAC ECU struggles to pull the clutch in and operate the AC compressor.

The idea behind this I guess was so that the HEVAC could tell if the compressor clutch was 'actually' engaged but monitoring the current drawn through the output of the power transistor - and I presume that if it detected that there wasn't a high enough current draw, then it would throw up the chequebook and log an error of something to do with the A/C compressor, or the trinary switch).

An interim update to this was to add an extra wiring harness between the HEVAC ECU and the A/C compressor - which incorporated a relay, and a power resistor to mimic the load of the A/C compressor to fool the HEVAC into thinking it was controlling the A/C compressor clutch directly. Instead, it was now wired to control a relay, which had an added directly fused feed from either the battery, or fusebox (I'm not 100% sure which - as I've never seen on being fitted) to allow a better voltage, and higher current to be drawn to make sure the compressor clutch definitely engaged.

The revamp (obviously in '99 when the GEMS/THOR thing happened... I think from VIN XA417600 is what I got from a parts website) removed the high power driving circuitry from the HEVAC controller, instead just running a low current 'switching' supply to the fuse box, where RL3 was added in to drive the A/C compressor directly from a fused feed (basically incorporating the 'updated' wiring harness from the later GEMS models into the vehicle wiring and removing the power resistor to mimic A/C compressor clutch, and removing the high power driver/current checking from the ECU).

So... That means that if you put an OLDER HEVAC ECU into a newer vehicle (Post '99 with RL3 that drives the A/C compressor) then you need to remove that relay and jumper it as you have done - so that the HEVAC is directly driving the A/C clutch. OR you can add a relay in, but will need to put a power resistor on the input to the relay to 'fool' the HEVAC ECU into thinking it's got a load of the A/C Compressor.

If you use a NEWER HEVAC ECU in an older vehicle (pre '99) then you NEED to add in a relay/fused battery feed into the wiring at the A/C compressor, or you will kill the driver circuitry in the HEVAC ECU as the newer systems are NOT designed to be able to drive the compressor directly, as the older GEMS systems did.

The easiest way to avoid any issues is to replace the HEVAC ECU with the correct part number for your model year.

I've compiled a table with what the HEVAC Units are, and what they've been superceded by... However, I don't know at what point the changeover in wiring occurred to do with the A/C compressor - as it isn't clear. My guess would be from XA411505, as I think that was the first vehicle with the Bosch system.

Part NumberWith A/CWith Front ScreenWith Heated SeatsVIN RangeSuperceded?
AWR1009NONONOALLManual Unit
AWR1012YesYesYesTo: TA344172AWR5051Earlier Models Need External Temp Sensor Moved
AWR1011YesYesNOTo: TA344172AWR5050Earlier Models Need External Temp Sensor Moved
AWR5051YesYesYesFrom: TA344173 To: WA381430JTN100000Earlier Models Need External Temp Sensor Moved
AWR1010NoYesYesALLManual Unit
AWR5050YesYesNOFrom: TA344173 To: WA381430JTN100010Earlier Models Need External Temp Sensor Moved
JFC101900YesNONOFrom: WA381431 To: XA411504JFC102410
JFC102400YesYesYesFrom: XA411505 To: XA417599JFC102550
JFC102410YesNONOFrom: XA411505 To: XA417599JFC102540
JFC102550YesYesYesFrom: XA417600LatestEarlier Models Need External Temp Sensor Moved
JFC101890YesYesYesFrom: WA381431 To: XA411504JFC102400
JFC102540YesNONOFrom: XA417600Latest
JTN100000YesYesYesTo: WA381430JFC102550Earlier Models Need External Temp Sensor Moved
JTN100010YesYesNOTo: WA381430LatestEarlier Models Need External Temp Sensor Moved






For models needing the Ambient Temp Sensor moved to down by the radiator, the parts are listed below:

JTF100230Ambient Temp Sensor
JTU100030Bracket
YMQ103420Ambient Sensor Link Harness





In Fact, I've just dug this up, with the TSB about the link harness... it says up to VIN XA411503:
http://www.rangerovers.net/forum/7-range-rover-mark-ii-p38/23210-c-clutch-harness-recall.html

And the Link Harness Part Number is: YMQ104590AA

Told you it would be a long post... but that should put together as much information as I can collate from across the web, and from google image searches as to what functions each part number has...

Marty

**EDIT The information regarding what part number supercedes what was found on allbrit.de The column relating to what models have heated front screen was purely from google image searches of the relevant part numbers - if anyone has any corrections, then let me know!
 

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Discussion Starter #3
Re: AC/HEVAC/Compressor Clutch confusion..... A solution? Maybe.....

Hi Marty,
That’s brilliant, thanks so much for detailed response.
The link you posted was one of the many I read through last night!
Perhaps I don’t have the harness then, as the updated loom in my car (Vin YA….) replaced that.
Judging by your table, I should have JFC 102550, which on fleabay seems to be anywhere from £100.
The main concern was whether it would be likely to cause any damage jumpering the relay, so if that’s ok, then hopefully that’s another one crossed off the list!
The jumper wire I was using for all sorts of tests, so needed to be longer, but now I know it’s ok I will make one that fits better!
I will pop that jumper in now, and see how she runs. Fingers crossed the ac keeps working and I don’t have a leak…..! Will keep an eye out for reasonably priced HEVAC units, but if this work around is good, then that won’t be an urgent requirement.
It goes to show though that there is no telling what has been done by previous owners…. To be fair, looking at ebay the HEVAC units look identical (heated seats/screens etc aside), so I guess you could say its an “easy” mistake to make. But then part numbers are put there for a reason!
Thanks again for all your help on this anyway. And for the offer to read codes etc this week.
Hopefully this may help others having ac issues as something relatively easy to check.
Cheers
Matt
 

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Re: AC/HEVAC/Compressor Clutch confusion..... A solution? Maybe.....

Hi Matt,

You won't have (or need) the harness - as the later models are wired so there's a direct connection from a battery power source, instead of the HEVAC controller doing all the work.

You shouldn't have any issues running with the jumper in there, as it's basically getting the system to run like the earlier GEMS models and having the HEVAC ECU do the controlling.

A JFC102400 unit should also work in your P38 as I think it's the earlier version of the JFC102550 - it's still after the changeover to the later method of controlling the HEVAC ECU.

You might also find on eBay, that if you search for something like P38 HEVAC and then weed through the listings for post 1999 controllers, you might find more than just by searching the part number.

Cheers,
Marty
 

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Discussion Starter #5
Re: AC/HEVAC/Compressor Clutch confusion..... A solution? Maybe.....

Thanks Marty,
I’ll keep an eye on ebay and see what comes up.
Unfortunately, the celebratory beer is back on ice…..
Nice cold air coming from the drivers side. But….
(We know it’s one step forward, two steps back with RR right?!)
Yes, you’ve guessed it, nice hot air coming from the passenger side.
The distribution servo seems to be working ok, air can be directed up, down etc, so I guess both temp servos are stuck in opposite positions.
I think this is an intermittent fault as I have had “nice” warm/hot air blowing drivers side as well recently, and not just ambient air.
I’m not going to rush into sorting this out just yet, but it looks like I might as well start looking for all 3 blend motors as well……
Keep you posted!
Cheers
Matt
 

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Re: AC/HEVAC/Compressor Clutch confusion..... A solution? Maybe.....

Hi Matt,

What's the temperature set to?

If air is coming out where it should, then distribution servo is working.
If you ramp the temperature on the drivers side and it goes from cool to warm etc, then that blend motor is working ok...

If you have temp on both sides and drivers is cold, passengers is hot - it says to me that the LH blend motor is failing (of the flap is sticking). But your auto elec's diagnostics should be able to pull the code(s) from the HEVAC. There will probably be a bunch of codes stored, so clear them, and then re-read them to see if any come back. If nothing there, then move the servos (change temp on each side, change distribution, recirc etc and see what codes pop up when the servos try to work.

Cheers,
Marty
 

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Discussion Starter #7
Re: AC/HEVAC/Compressor Clutch confusion..... A solution? Maybe.....

Hi Marty
Seems to me to be temperamental. Initially drivers stuck on cold, passenger on hot. Regardless of movement on temp.
That said, drove to shop earlier, handbook on. Coming home, no handbook.
Not long enough to see temp change really tho.
Will see how it goes. Going to get codes read anyway just in case!
Cheers
Matt
 

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Re: AC/HEVAC/Compressor Clutch confusion..... A solution? Maybe.....

Marty- I dunno about the OP... but THANKS YET AGAIN FROM ME!!!!

I been trying to find the answer to this one for a while and here it is, finally!
 

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Discussion Starter #9
Re: AC/HEVAC/Compressor Clutch confusion..... A solution? Maybe.....

Hi darthdude,
What was your problem, compressor or blend motors?
 

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Re: AC/HEVAC/Compressor Clutch confusion..... A solution? Maybe.....

Hi Matt,

It could be a number of things - the blend flaps sometimes stick inside the housing, so the motor will stall and then not operate properly, or it could be a stripped tooth on a gear inside the blend motor, or a faulty feedback potentiometer sending a bad signal back to the HEVAC unit. See what the auto elec finds this week and then go from there..

Either way, you'll have to pull them out at some point - I'd leave that (depends on how much it bugs you) until you have a replacement set of motors to go in!!

Darthdude... No worries - It only took an hour or so to collate all that info, I was curious to see what the differences were over the years - whether there were just a couple of part numbers... but 14 part numbers across 7 or 8 years of production!

Cheers,
Marty
 

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Discussion Starter #11
Re: AC/HEVAC/Compressor Clutch confusion..... A solution? Maybe.....

Cheers Marty, I had read up on the blend motor stuff, and was quite happy that it seemed to have gone away..... Still, be nice to have everything working soon! Will let you know how I get on. Cheers Matt
 

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Discussion Starter #12
Re: AC/HEVAC/Compressor Clutch confusion..... A solution? Maybe.....

Afternoon all,
Update on the HEVAC handbook symbol…
Had the auto-electrician round today. I think he used an Autodata diagnostics machine which read the fault codes.
Initially, just about everything was apparently “at fault”, but I guess the ECU hasn’t been cleared in a while!
After clearing codes, the Left hand blend motor was diagnosed as at fault.
Cleared that, and it came back again, so clearly might as well plan for doing all 3 blend motors soon…. As the drivers side works, it isn’t going to be an urgent job. The passenger side is intermittent, so with patience it will change temperature eventually.
A second code then came up which was interesting. The Heater Core temp sensor was either open/closed circuit. This gave a Heater core temp reading of 6,534 degrees celcius…. Rather warm! Haha
Might as well change that too when I decide to take the dash apart.
Cheers
Matt
 

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The heater core temp sensor can be changed with the removal of the drivers knee panel, and possibly the foot well panel. Then its just a simple unclip/reclip of the sensor and one connector.

The blend motors however, require a bit more work to get to!

Marty

Sent from my S4
 

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Discussion Starter #14
Re: AC/HEVAC/Compressor Clutch confusion..... A solution? Maybe.....

Cheers Marty, I thought that sensor looked relatively easy to change. The Audodata printoff suggested it was upper left side of dash, but I looked on RAVE and it suggested the location is as you said. I guess the danger in not changing this sooner rather than later is risk of icing up the system. Is that right? Maybe i'll do the sensor and spend a bit more time psyching myself up for the blend motors! Cheers. Matt
 

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Discussion Starter #15
Re: AC/HEVAC/Compressor Clutch confusion..... A solution? Maybe.....

Evening all,
Just thought I would post an update to this thread.
Parts (blend motors and heater core temperature sensor) turned up Thursday, and finally found some time today to get started (along with good weather forecast next couple of days…)
Removed centre console panels, stereo, upper facia, dash display, glove box.
Left steering wheel on, didn’t touch airbags etc.
I found it was possible to lever the rear vent ducts apart where they join near the gear shifter, allowing some movement in the upper ducts that pass the blend motors.
Using this method on the driver side it was possible to get a screwdriver on the distribution motor screws, and one of the temperature motor screws. There was then just enough space to get a hand in holding a screwdriver bit only to release the second screw. Luckily they didn’t seem to be done up too tight!
On the passenger side, flexing the duct up and down allowed a screwdriver to get to both screws.
Due to the way the blend motor harness was routed through the dash there was no way to remove the whole lot as one, so I pushed the pins out of the plug end of the harness, and then was able to pull the wiring harness back through the dash to get the blend motors out. As I was only having a fault on the left motor, the other two can be “spares” although I hope I won’t need any for a while!
With the motors removed, I checked the flap arms for any signs of resistance, but all seemed to move freely.
At this point I changed the heater core temperature sensor.
For refitting the new blend motors, I have routed them slightly different in the dash. There was space to push them lower through the dash (HEVAC opening), and then up to their locations without splitting the harness. I am sure the original was routed behind everything for a reason…..But there is space behind the dash and the wires are out of the way of the stereo etc, so I am happy enough!
Then it was “simply” a case of putting everything back together…. Never quite as quick as taking it apart! The main struggle was connecting the ducting back up behind the dash display.
I’ve put the dash top switches, stereo and dash display back together, and then wired in the HEVAC unit to test before final reassembly. Interestingly, my dashboard has always had a faded section at the left of the message box, but on reconnecting it, it seems to be back to normal…..
Initially I was getting the handbook display back up. Not encouraging! I switched the engine on to allow things to warm up/settle down/circulate etc, and get some temperature to allow testing.
As the dash isn’t fully back together, I could also reach the blend motors to feel for movement vibrations.
Left side was moving ok and changing temperature. Distribution flaps were all good. Driver side was stuck on cold though!
I tried the old IT support favourite saying “off and on again”, and checked HEVAC plugs, and it seemed to then come on with no handbook symbol.
By this point I was losing light though, so will get back to it tomorrow. Fingers crossed the handbook symbol stays banished!
Assuming it all tests out ok, then I will finish putting the panels back on.
All in all so far it has taken about 4.5 hours just plodding through it methodically. Assuming all seems ok in the morning, I reckon another hour to put the final panels back on.
Not such a bad job (assuming it all works as it should tomorrow!)
 

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Discussion Starter #16
Re: AC/HEVAC/Compressor Clutch confusion..... A solution? Maybe.....

Finally a finish to this thread…. I hope!
Further testing this morning showed that the right temperature motor wasn’t working.
Maybe I shouldn’t have tried to save money with the Britpart set….
I ended up splicing that motor out of the harness (pushing pins out of the plug) and re-fitting the original working motor back into the plug.
All tested fine, arms all seemed to move as they should, so I put everything back together.
No handbook symbol so far!
The hardest part of this job in my opinion, is the original location of the wiring loom for the blend motor harness. That said, it isn’t so hard to identify which wire goes to which motor, so actually, splicing one out of the harness and installing a new motor into the loom isn’t so hard.
Once the old motor is out of the loom, the wires can be removed easy enough, and you can then lay the new wiring in a more accessible way.
I had put this job off as it seemed quite a daunting job, but it isn’t. Anyone else putting this job off, don’t be put off, it isn’t so hard.
Cheers
Matt
 

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Re: AC/HEVAC/Compressor Clutch confusion..... A solution? Maybe.....

The change of a servo or temp sensor needs Testbook etc. ideally to perform a calibration, this also happens every 500th (power cycle) so that wear in the servo's can be accounted for, perhaps that servo is ok just its too far from what the HEVAC ecu had for the old/removed servo, just a thought.
 

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Discussion Starter #18
Re: AC/HEVAC/Compressor Clutch confusion..... A solution? Maybe.....

Thanks Larry, that’s an interesting thought.
I did test it prior to completely removing it. I took the back off to see what was happening.
With ignition on, it would turn one way and then stop. Handbook symbol would come on. No matter what adjustments were made to the temp switch the servo would not work. Each ignition on, the motor would spin the same way.
Needing to calibrate it would make sense.
Perhaps I won’t dispose of it just yet! I had considered asking for a partial refund, but that is often more effort than it’s worth!
If I get any more issues I will look into calibration.
Cheers
Matt
 

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Re: AC/HEVAC/Compressor Clutch confusion..... A solution? Maybe.....

Dear All,
Sorry to revive this but i have a question. My car has VIN non 315750 so should have the older direct drive HEVAC. However i have a relay in my fuse box but no evidence of the upgraded wiring harness. I am pretty certain i relaced my fuse biox a while ago and would not have been aware of the extra fuse.

I am assuming that if you have an older direct driving HEVAC unit you cannot harm anything by installing the AC wiring harness and relay upgrade? All you will be doing is reducing the load on the HEVAC output, instead of driving the AC clutch direct it will simply be switching a relay.

Regadrs
Paul C

PS
Also i need a new clutch on my AC compressor. Over here in the UK you can only buy whole compressor units but i notice that in the USA you can buy new replacement or remanufactured clutches alone. But the USA firms will not sell to me direct. Is there anyone in the USA who would be prepared to buy one for me and then post it to me - all your costs covered etc?

Thanks in anticipation.
 

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Discussion Starter #20
Re: AC/HEVAC/Compressor Clutch confusion..... A solution? Maybe.....

I'm not sure about the Vin, but when was your car registered? Is it pre or post '98? That was the major change in the wiring harness. As for the upgrade, it wasn't applied to everyone from what I understand. I'm no electrical expert, but I guess the TSA upgrade was intended for the older HEVAC/system, so should be fine to install (if needed). However, if you need a new clutch on the compressor, that would be the thing to fix first I think.
 
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