P38A 2.5 D aircon issue -> no code in EAS unlock, but get the service book

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this is the switch I am thinking...
I do not ever hear the fans going on when the AC is allowed to go... but presumably just like my other cars, that wouldn't happen until the system is active for a while.

I do get the fans to go on for engine temp every now and then when it's hot... (driving up 25% slope for 15 minutes straight at 3000 rpm) which I presume is a Diesel issue... having to do with a stuffed catalytic converter.... as I have also seen a message "Catalyst overheat see manual".... yet in my Italian manual there is nothing on this.... but that's for another day.
 

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pwood999 said:
Also has the HEVAC control unit been replaced with an early version ?? If it has, then it needs an extra resistor so the output drive transistor thinks it's powering the clutch direct.
Click to expand...
I never replaced the unit... BUT now that this drive transistor comes into play, I wonder if that is the issue...
That would in my mind make the most sense currently for the book only showing when HEVAC is trying to use the AC. And the AC clutch only being engaged so briefly.

I did find it "suspicious" that the HEVAC module has a perfect screen and backlight.... which my California 2001 RR didn't have and I had to fix that.

Where would this resistor get put in? And what Ohm rating? Can it be done at the fuse box?
 
Fans are a separate issue to the compressor only engaging once.

- Pressure Switch 2 is to turn on the fans either if pressure >>247psi, but the engine ECU will override if engine temperature gets too high.
- Dual Pressure switch only turns them into fast mode is pressure >>305psi.

Switch-2 has nothing to do with your compressor issue unless it never engages.


1756981006512.png
 
svenology said:
I never replaced the unit... BUT now that this drive transistor comes into play, I wonder if that is the issue...
That would in my mind make the most sense currently for the book only showing when HEVAC is trying to use the AC. And the AC clutch only being engaged so briefly.

I did find it "suspicious" that the HEVAC module has a perfect screen and backlight.... which my California 2001 RR didn't have and I had to fix that.

Where would this resistor get put in? And what Ohm rating? Can it be done at the fuse box?
Click to expand...
Before you go that route, take the HEVAC out & check the part number to verify if that could be the issue.

The tech bulletin for early P38 & HEVAC is attached & shows the resistor. You already have the relay installed, so do what Keith said & bridge it. That will prove if the dual pressure switch is working as well as the clutch. Did you check the gap ??
 

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svenology said:
this is the switch I am thinking...
I do not ever hear the fans going on when the AC is allowed to go... but presumably just like my other cars, that wouldn't happen until the system is active for a while.

I do get the fans to go on for engine temp every now and then when it's hot... (driving up 25% slope for 15 minutes straight at 3000 rpm) which I presume is a Diesel issue... having to do with a stuffed catalytic converter.... as I have also seen a message "Catalyst overheat see manual".... yet in my Italian manual there is nothing on this.... but that's for another day.
Click to expand...
If your engine is getting hot enough to trigger the fans, I suspect you have a faulty viscous fan. Stand by for a cracked head.
The aircon fans only come on when needed, the viscous fan is enough a lot of the time if it's working properly.
 
I'm fairly sure one of my HEVAC is the early version, mainly because I had clutch gap issues causing intermittent operation. Both of them in my P38's and my spare all have heated seat buttons.

Only real way to tell is to take the unit out & check the label.
 
Datatek said:
View attachment 349132
Remove relay #3 and jumper pins 30 & 5 on the drawing. The relay pin numbers will not actually be the same but a DVM will tell you which pins are which.
If the aircon runs with the jumper, you are left with 3 main possibilities, faulty relay, evap sensor or the HEVAC drive transistor, assuming the connectors are OK.
Click to expand...
Followed your advice...
when I jumper the pins 30 & 5 I have a working A/C circuit.
Clutch engages. Fans run every now and then... I get cold air. AC clutch also seems to be cycling... i.e. turn off for a few seconds every now and then.
Climate control works perfectly... I even can do hot on one side and blow cold on the other... so blending seems to be working as well. As yesterday, I can't detect any issues with the blend motors or any of the others for that matter.

I still get the book symbol it seems. Which again clears immediately after an ignition cycle...
(I also did a full from the beginning cold start and watched the AC clutch... it takes about 1 minute before the AC clutch engages for maybe 1-2 seconds... no book symbol... then some time passes, probably almost a minute... AC clutch engages again for a few seconds and the book symbol comes up.

I did replace the evap sensor... (that nail/pin like thing you get to from the glove compartment, right?) The old one seemed fine too... no difference in resistance.

So that leaves us at a bad HEVAC drive transistor? Or any other possibilities?

And keep in mind the HEVAC book symbol, ONLY when wanting AC with no detectable HEVAC code.... no book symbol when the AC is in off mode....

Argh... P38s....
:(

But I think you saved me from pulling the bumper...
 
It does sound like early HEVAC behaviour, because when you bypass the relay it sees the higher clutch current draw ?

Could also be a bad relay, so swap that for a new one first ??

Depending on ambient temperature, the compressor should be on for more than 1-2 seconds though, at least until the Evap Sensor sees the cold air where it needs to be to cool the interior.
 
pwood999 said:
It does sound like early HEVAC behaviour, because when you bypass the relay it sees the higher clutch current draw ?

Could also be a bad relay, so swap that for a new one first ??

Depending on ambient temperature, the compressor should be on for more than 1-2 seconds though, at least until the Evap Sensor sees the cold air where it needs to be to cool the interior.
Click to expand...
the relay I swapped already... so that seems to be just fine.
But explain to me this... how does the HEVAC "see" the current the AC clutch draws? Isn't the clutch engaged by the relay and the relay's solenoid current drawn is the only thing that the HEVAC sees.
Meaning the hypothesis is, that if it's an OLD unit in a newer model car, the HEVAC throws an error, because it thinks the clutch is NOT working correctly because there is NOT ENOUGH power drawn. As the relay only needs little vs. the clutch a lot more...

Or am I seeing this wrong?
 
yes that's correct. You said you bypassed the relay (pins 30 & 5), after which the HEVAC engages the clutch properly ?? If yes, then it could be the relay is not pulling enough current.

However if the compressor still only engages for a few seconds each time, you need to check the connections on the dual pressure switch. Also measure the voltage at the clutch connector when the clutch engages. (back-probe the connector for this because it needs to be connected)

Best to get the HEVAC out & check the part number. There are several revisions, and unless you had the car from new, you cannot be sure it's original.

It's easy to do:
- Undo the window switch panel, and then the gearshift surround & ashtray.
- Take the two side panels off (2 screws each)
- Unscrew the HEVAC.
- Check part number (usually on the bottom or back).

The alternative is to add the resistor in parallel with the relay coil. (12ohm 50W) It needs to be screwed down according to the LR Bulletin above. BUT do NOT fit this without checking the HEVAC version.
 
in https://www.landyzone.co.uk/land-rover/air-con-woes-nanacom.312452/

@boycie says https://www.landyzone.co.uk/land-rover/air-con-woes-nanacom.312452/post-3951061

I am sure you are correct Wammers you normally are it was over 2 years ago I was trying to sort mine and I am getting old ,the problem was I had an early model Hevac unit in a 1999 model car once I fitted the correct part it worked fine .
Click to expand...
That seems to confirm suspicions now of a wrong/older HEVAC front end in a new car
 
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