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1995-2002 Range Rover P38A
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Discussion Starter #1
Hi

Just wondering if there would be nay problem putting a 2001 HVAC controller into my 1996 car?

I have the chanjce to pick up a later unit and I believe they are a little better than the earlier ones

Thanks
Steve
 

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1995-2002 Range Rover P38A
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You'll need to add a relay for the AC compressor. Early HEVAC powered it directly, the later one powers a relay to switch it on.

Early.jpg

Early one

Late.jpg

Later one
 

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Discussion Starter #3
Thanks for that info, mine has had the mod done (i cant remember the TBS offhand) which adds a relay into the circuit I believe. I'll double check !
 

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As noted by Steve early P38s under a TSB to add the relay in late 98...ish. The relay and changed control head became standard about the time of the Bosch change over.
 

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1995-2002 Range Rover P38A
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The later ones will still drive the earlier models without the relay mod (though it is better to have it done as the compressor clutch then gets a proper feed straight from the battery...

I took a look at what in the HEVAC unit actually drove the compressor clutch, and it's just a MOSFET, like the BECM uses for the lights. The difference between early/late HEVACs must only be in the software as the latest ones use the same MOSFET to drive the output, but I am guessing the status pin has been taken out of the coding so it doesn't throw an error when driving the relay directly.

The relay mod is good for driving the compressor direct with battery voltage, but if the later HEVAC tries to drive a compressor clutch directly it won't kill it either, as the MOSFET is pretty heavy duty and has a solid heat sink on it!
 

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Discussion Starter #6
Thanks all, it looks like I will be getting that later unit for mine :)
 

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Someone correct me if I'm wrong but doesn't the later HEVAC use different exterior temp sensor which confuses the operation ?
 

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I think it's the same sensor, it's just located in a different place. On an early car it's in the intake trunking but on a later one it's down behind the front bumper.
 

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Yes, same sensor, just mounted down in front of the radiator on later models. Again, only real difference in HEVAC units is that there was a software change in the later ones which mean they won't update the ambient temperature until the vehicle is moving above 5mph, so it doesn't provide erroneous readings from the latent heat around the radiators.
 

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Discussion Starter #10
I had read somewhere there were quite a few software tweaks which mean the system operates a little better, less cold air blown on startup when it's cold outside for instance
 

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1995-2002 Range Rover P38A
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Is the info on www.stockholmviews.com correct for diesel versions too?
I'm having a diesel and got a newer HEVAC unit and wonder everything will be okay with a rerlay at no 3 position.
At present relay 3 is blank.
 

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if you read the wright up y
u know as much as I do . l would Imagine that the hevac system would be the same for diesel or petrol , If not hopefully someone will say so
 

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As noted by Steve early P38s under a TSB to add the relay in late 98...ish. The relay and changed control head became standard about the time of the Bosch change over.
Can someone give me the part no for relay no.3 please,as it seems smaller than a standard yellow one.
 

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Discussion Starter #15
In my update they use one of the spare standard size relays, are you sure you are looking at the correct relay?
 

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It should be a smaller yellow one - same as the one used for the horn I believe.

Mind you I don't know if you can just drop the relay into the fusebox and expect it to do anything - I know the space is there for it, but I can't see why the wiring would be routed through it, especially if it was empty from the factory.
 

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Yes, it should be smaller than the standard yellow one.
May be someone can confirm whether the wiring is there. So that I need only the relay n fuse with the HEVAC unit
 

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I don't know if it helps, but the later HEVAC units will still drive the AC compressor clutch directly if needs be.

The relay was added to give a decent full battery voltage to the compressor, which is obviously better all around, but you won't kill the HEVAC controller if it is driven direct from it.

The compressor clutch is driven by a MOSFET in the HEVAC controller, and the only main difference between early controllers and later ones is that the error reporting on the MOSFET is ignored, so it doesn't throw a book symbol when it's only driving a relay, rather than the compressor clutch.

If you have to run it without the relay to begin with then it should be fine.
 

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Thanks Marty nz. Let me try that. waiting for the long weekend to start.
 

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I went through RAVE electrical section and it says, for older vehicles, F27 is a 10A fuse and it connects to RLY2- ABS power rly. pls see the attachment:oops:. I guess that with the late HEVAC we need the late model fuse box too.


Sorry, can't upload XPS files:?:. pls refer Y2 page 12 in RAVE electrical section both before and after 1999 model year.
 
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