HVAC on a P38 with a M57 BMW engine
I have been asked about how I got the HVAC working on my P38 Range Rover overland camper ( now officially called Dogger)
Rather just do a reply to the comment I thought I would do a blog post to help you all understand it.
The P38 was at a sweet spot in technology in regards to the amateur converter, why? well, it had very advanced ideas for its time like the uses of a data bus allow remote stations in the doors and keypads but was still not a fully integrated system like what followed in just about every other car.
this means that all the systems can be orphaned with ease, the newer stuff relies on intercommunications between ECU’s that is hard to reverse engineer and/or fool
the main systems on the P38 are
- BECM + out stations , switch panel, display
- Engine ECU
- HVAC
- gearbox ( if auto)
- transfer box
- ABS/TCS
I have listed the BECM with the out stations ,doors and display because they communicate on the data bus so are really all one system.
all the others are separate systems that communicate with the BECM via simple wires that are hi/lo in various configurations, this tells the BECM what’s happening. The exception is the later petrol engines which have CAN-BUS data between the gearbox and the engine ECU. in the early cars this is a single wire with a complex waveform.( I may explain that in another post one day) .
The BG wire coming from the HVAC ECU is the request signal, it is asking the engine ECU to have the A/C compressor on, the engine ECU looks at engine load, temp, and some other things and decides if the extra load of the AC compressor is OK, if it is then it grounds the BS wire, which tells the HVAC unit it is OK to turn on the AC compressor clutch, this wire must be connected on the M57 engine to the original, which should be a thick black/green wire on the M57 Engine.
The M57 ECU does the control of the AC differently, the HVAC requests the AC compressor if everything is ok then the engine ECU turns on the compressor and sends a signal back to the HVAC that it is on, you can see how more interdependent the later systems are on each other.
here is a list of the reasons the later M57 ECU will not allow the AC compressor on.
- Start off status :Road speed < 10km/h AND accelerator pedal > 60% AND engine speed < 1800 rpm
- Acceleration : Accelerator pedal change > 400% / second AND engine speed <3000 rpm
- Coolant temperature > 117 deg C
- Throttle dip : Engine speed < 650 rpm
- while starting
- Fault code stored in DDE ECU 500 : road speed signal or 120,220 : pedal postion sensor
anyhow how to make the p38 work with the M57 DDE4 ECU
simply connect the BS and BG wire on pins 13 & 14 on the HVAC, either C246B or C0793 depending on which year you have. The AC compressor clutch is still controlled by the C273 BK/YW wire as normal, you will have to use your brain to turn it off if the car is overheating and use the BM
It is possible to connect up the M57 ECU to work correctly but you will need to get rid of the error code 500 road speed signal which will involve CAN bus speed signal input to the ECU 🙁
UPDATE 2024: now we have the cruise working that involved getting the speed over CAN into the ECU this error should disappear.
For the compressor: I used the M57 compressor because of the multi V Aux belt, I made two adapters for the pipes by welding universal fitting from Webasto onto the E46 pipes and then it is easy to clamp the pipe with the clips at home.
all other pipes, and wiring stay the same as original.
Update 2023: I changed the pipe that goes from the compressor to the Condesnor on the front of the radiator to a braided type and for the adapter on the compressor I welded a JIC male fitting to it, this was to add reliability to this pipe, it was vulnerable to damage and hard to fit/remove.
Hi Beardy i too have a few questions, I was wondering how to drive the ZFhp24 after changing the gems 4.6 to an OM 606 fully mechanical fuel pump. I can’t thank you enough for the explanation on the hvac. That has been on my mind for some time.
You would need to use a signal generator, but you would need to get throttle position and rpm into it, you could keep the original potentiometer to get the throttle position, the rpm you can get from the signal you have going to the instrument panel.
this post will help you with the signal
https://www.beady.com/blog/p38-autobox-comms-with-the-m57-engine/
bd
Hello.
I have a Range Rover P38 with an M57 engine, and I can’t turn on the cruise control.
Can you help me?
Thanks
You need the speed sending to the ECU over CAN bus.
I have mine working but only in 5thgear which is fine.
Flatcap engineering can sell you a box to make it work
https://www.flatcapengineering.co.uk/
bd
Good Afternoon!
So, we need to do a shunt betwen 13 and 14 on the plug C246B on Hevac and put a Wire betwen 14 and compressor clutch?
Im right?
Tank you very much for your help!
yes connect the 13 & 14 wires together, The AC clutch will still be controlled by the P38 OE BL/YW wire on connector C792 ( different connector number on earlier models)
I have updated a few points on this blog ( and some spelling 🙂 ) if you find any other stuff let me knwo I will add it to the page to help others
bd
Hello .
Do you already have the cruise control working?
Can you help me with this?
I have my p38 with a 184hp M57 engine.
Thanks
Hi! Great stuff and happy to read this. However I have a bit different question; how could I make the p38 hvac work when putting a 4.6 from p38 with stand alone ecu in 1980 Range Rover Classic? Do I just need hvac panel and heater/blower/condensor unit from p38? Or is the control unit seperate? Thanks a lot!
Hi Frans,
that is an interesting question, the simple answer is yes it’s possible
the display panel is all the brains to the HVAC, the A/c is a basic a/c system so if you have an A/C fitted to the classic then it will easily control it.
the HVAC panel will require these inputs from the base P38
The ambient temp sensor, on later p38 is just in front of the radiator
the interior temp sensor, is located by the clock and has a small fan in it to circulate the interior air over the sensor
the sun sensor, located in the centre of the dash by the windscreen, keep the shiled, it is calibrated to use it.
it will need a speed input, the HVAC uses it to know when to measure the ambient temp
outputs are harder, the main two fans left and right have the drivers built in, the classic has one fan, the HVAC will give and error if they arent there.
the 3 flap actuators are air mixers x 2 and recirc, you could use one, but the other two must be connected or errors will occur.
I think that’s all
the RAVE manual is you big help here
bd
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