svenology · 2025-09-04T11:05:30+0100

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.

svenology · 2025-09-04T11:17:35+0100

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.

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?

pwood999 · 2025-09-04T11:19:55+0100

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.

pwood999 · 2025-09-04T11:24:08+0100

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?

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 ??

Datatek · 2025-09-04T11:26:26+0100

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.

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.

svenology · 2025-09-04T11:28:08+0100

from @wammers on https://www.landyzone.co.uk/land-rover/p38-hevac-unit-versions.228929/post-2624184

"It is the dual pressure switch that controls the compressor."

So maybe I do have a bad switch or bad relay. My HEVAC controller is the newer version with heated seats buttons (is that truly the tell tale?)...

Datatek · 2025-09-04T11:41:20+0100

svenology said:
from @wammers on https://www.landyzone.co.uk/land-rover/p38-hevac-unit-versions.228929/post-2624184

"It is the dual pressure switch that controls the compressor."

So maybe I do have a bad switch or bad relay. My HEVAC controller is the newer version with heated seats buttons (is that truly the tell tale?)...

The dual pressure switch controls the compressor but only if the HEVAC provides the power.
Heated seats were I think available on the early version.

pwood999 · 2025-09-04T11:55:04+0100

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.

svenology · 2025-09-04T11:55:41+0100

Datatek said:
The dual pressure switch controls the compressor but only if the HEVAC provides the power.
Heated seats were I think available on the early version.

and on the new setup with the relay, the HEVAC would only measure the resistance/load to the relay, no longer to the compressor clutch as it used to... correct?

RangeRoller dt · 2025-09-04T12:42:34+0100

svenology said:
is that truly the tell tale?

No

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