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) .

range rover conversion HVAC wiring campervan and off roader
1995 HVAC to engine ECU wiring, 199 on is basicaly the same but connector C246B is now called C0793

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 CU turns on the compressor and sends a signal back to the HVAC that it is on, you cannsee how more interdependant the later systems are on each other.

here is a list of wthe 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

simply connect the BS and BG wire on pins 13 & 14 on the HVAC, either C246B or C0793 depending on which year you have. you will have to use your brain to turn it off if the car is overheating.

It is ossible 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 🙁

for the compressor: I sed the M57 compressor because of the multi V Aux belt, a made two adapters for the pies 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.

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One thought on “HVAC on a P38 with a M57 BMW engine

  1. P38 Autobox comms with the M57 engine – BeaDy

    […] Making the HVAC work on a P38 with a M57 conversion […]

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