P38A Air Con Woes,nanacom

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rover75lover

rover75lover

Active Member
Posts
210
Location
newport.wales.uk
1999 diesel manual Rangerover p 38
Struggling a bit with the aircon as follows
New Condensor,expansion valve and new filter unit,
Took it in and had the car regassed but the compressor wont cut in
The hevac unit puts the chequebook sign up so ive read the fault from a nanocom unit which is "compressor clutch control circuit fault ,current.
I direct jump relay three and the air compressor cuts in and runs fine,so I from reading rave that the trinary switch etc is all good.
I replaced the hevac unit with a known good one off my other rangerover and i get the same response.
Reading rave I checked the voltage at the hevac unit and i get 12 v for 0ne minute so the clutch engages ,drives but then the voltage dissapears .
I changed the hevac unit for a later one and installed the relay update ,well my own with a 4 ohm power resistor in line.
Still the same fault.
Going to buy a dc ammeter tomorrow to see what amps the unit draws,if anyone has any idea on the amps that would be great.
Also can i have a fault on the water temp sensor,evaporator ,solar sensor,evaporator temp etc as they seem to me ,probably in my ignorance ,that the current control circuit needs a response from these to operate? would my nanacom record these as different faults or just "clutch control circuit faults
If anyone knows the substitution values of the various components that would be great.
Alternatively any other help gratefully received.
Have thought about just fitting a bypass "air con " switch,live feed to relay three but concerned that the evaporator temp sensor,water temp sensor,solar sensor ,ambient temperature safety's would be bypassed
Many thanks
 
it is regassed with engine not running wammers.
did they check for leaks before refilling and can you confirm the system is still upto the correct pressure? Any loss of pressure to an extent will immobilize your compressor. :eek:
 
Mark Piercy said:
it is regassed with engine not running wammers.
did they check for leaks before refilling and can you confirm the system is still upto the correct pressure? Any loss of pressure to an extent will immobilize your compressor. :eek:
Click to expand...

I beg to differ, aircon cannot be fully regassed unless engine is running and compressor engaging. You can certainly put gas in with engine not running but as soon as engine is started with aircon switched on and compressor starts pressure will drop. More gas is then applied to bring pressure up as compressor circulates it through the system. If pressure is low compressor will not run.
 
that does make sense. I recall when I had my old omega regassed he did start the car In the final stage while plugged into the aircon machine.. The pressure in the op's system must still be low? Is there any other thing he can check?
 
I can give a blow by blow account of how to regas a system. But you are better going to Utube and watching one of the many videos of how it is done. If the system has been opened you need special equipment to regas the system. You can't just stick a bottle of gas from Halfrauds in it. Usual problem with OPs fault is that there is not enough pressure in the system to close the compressor circuit. Or the switch that enables the compressor circuit is sticking open circuit.
 
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no leaks that im aware of,they vacced the system out as so many new parts were fitted then held it at a vac pressure for 25 minutes with no change. Filled her up weight wise then i bridged the r3 relay out so they could finish it off so to speak so i imagine the pressure is good. Ive checked the trinary switch and thats completing the circuit so i doubt its a gas deficiency problem.
 
If i bridge the r 3 relay and the compressor runs for 20-30 minutes am i correct in thinking that as the current flows through the trinary switch ,after r3 relay, the gas pressure must be good as the a/c compressor doesnt cut out
 
rover75lover said:
no leaks that im aware of,they vacced the system out as so many new parts were fitted then held it at a vac pressure for 25 minutes with no change. Filled her up weight wise then i bridged the r3 relay out so they could finish it off so to speak so i imagine the pressure is good. Ive checked the trinary switch and thats completing the circuit so i doubt its a gas deficiency problem.
Click to expand...

Think your home made circuit maybe contributing. Mine does not have the relay 3 circuit and the aircon works fine fitted with the later HEVAC unit. If yours has relay 3 all the later circuitry should already be in place. It should then run off any HEVAC unit.
 
Power to pull relay 3 comes from C0792 pin 7, white plug centre of HEVAC unit. Then goes through C0229/C0067 to C0571 in base of fuse box. See RAVE for plug/socket locations, wire colours and pin allocations. On earlier cars pin seven plug C244 (C0792 on yours) went direct to compressor clutch via dual pressure switch.
 
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Check the part number of the HEVAC unit had the same problem the later cars need to get a feedback from the pump clutch within that minute or it will close down ,I tried two hevac units until I got the correct one there is a post on here some where about wiring a bulb in line to create a current draw and it does work .
 
boycie said:
Check the part number of the HEVAC unit had the same problem the later cars need to get a feedback from the pump clutch within that minute or it will close down ,I tried two hevac units until I got the correct one there is a post on here some where about wiring a bulb in line to create a current draw and it does work .
Click to expand...

There is no feedback from the compressor clutch. All the later units do is pull a relay via the dual pressure switch, the relay then powers the compressor, rather than powering the compressor clutch direct from the HEVAC via the dual pressure switch.
 
I have a becm in my workshop, with the faults the compressor cuts out after one second.

We'll investigate today & if the this problem is caused by the becm, I'll let you know.
 
Rick-the-Pick said:
I have a becm in my workshop, with the faults the compressor cuts out after one second.

We'll investigate today & if the this problem is caused by the becm, I'll let you know.
Click to expand...

Yes the power to the HEVAC to pull the compressor relay 3 on later models or power the compressor earlier models comes from relay 7 fuse 8 30 AMP in the BECM. ;)
 
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 .
 
I wouldn't have thought the BECM would have anything to do with the AC compressor running. The engine ECU, yes, but BECM?
I know the HEVAC has power from the BECM - 2 feeds if I remember correctly - one a permanent feed, and another ignition switched feed, which is also shared with the radio and possible something else that I can't remember off the top of my head.

I would expect that if the BECM was dropping power on either of the power feeds then it would globally affect the whole HEVAC operation, rather than just the AC compressor.

I know the HEVAC 'requests' the use of the AC from the engine ECU - and if it determines that the engine can handle the added load of the compressor, then it sends a 'grant' back to the HEVAC, which then turns the compressor on.

All the HEVAC units use a MOSFET to drive the compressor clutch - a bit like what the BECM uses to drive all the lamps etc. and the earlier units used the 'status' line of this to determine if there was a load (compressor) present on the output.

I know on GEMS vehicles at least, there was a shim washer in the compressor clutch that could be removed as the clutch wore, to ensure a solid engaging of the compressor, but don't know about diesel or thor petrol models.

The wiring upgrade was mostly for earlier GEMS vehicles where the compressor clutch would wear and slip and no longer engage to have the HEVAC control a relay and this then gave a direct +ve feed to the compressor (being able to supply more current than the HEVAC MOSFET) - however, this wiring had the added power resistor in it to simulate the load of the compressor, to make the HEVAC think that the compressor was operating.

From looking in later model HEVAC units, for vehicles which had the relay installed as standard, from the factory, they still use a MOSFET to drive the output, but I am guessing that the feedback/status part of the programming in the firmware has been written out, so it doesn't flag the low current of the relay as a 'fault' like it did on the earlier ones.

I could be wrong about the BECM power feeds, but I don't think that there was a dedicated feed to just run the compressor - I was under the impression it was a general switched feed for the whole HEVAC unit.

If you have a Nanocom, then I would have a look at the live data when the AC is running and when it cuts out... see if the A/C request/grant to/from the engine ECU is still working as it should, and that for some reason the engine ECU isn't suddenly telling the HEVAC that it can't run the compressor.

Another thought I just had, would also to be to check the HEVAC live values for the evap temp sensor reading. The HEVAC uses this to cycle the A/C compressor off when the evaporator gets cold, so it doesn't freeze over - it could be possible that the sensor is faulty, that when it gets cold, it drops the connection, and the HEVAC thinks it's gone frozen and shuts the compressor off... I only suggest looking at that, as the default value for the sensors is -40? (from memory) so if it sees that reading on the evaporator, then it would also shut off the AC compressor.

If that all comes back clear, then I'd try and get the pressures read when the system is running... if they are in spec, then maybe the dual pressure switch has a fault that after it's up to pressure for awhile, it drops out, which would also stop the compressor from running.
 
martyuk said:
I wouldn't have thought the BECM would have anything to do with the AC compressor running. The engine ECU, yes, but BECM?
I know the HEVAC has power from the BECM - 2 feeds if I remember correctly - one a permanent feed, and another ignition switched feed, which is also shared with the radio and possible something else that I can't remember off the top of my head.

I would expect that if the BECM was dropping power on either of the power feeds then it would globally affect the whole HEVAC operation, rather than just the AC compressor.

I know the HEVAC 'requests' the use of the AC from the engine ECU - and if it determines that the engine can handle the added load of the compressor, then it sends a 'grant' back to the HEVAC, which then turns the compressor on.

All the HEVAC units use a MOSFET to drive the compressor clutch - a bit like what the BECM uses to drive all the lamps etc. and the earlier units used the 'status' line of this to determine if there was a load (compressor) present on the output.

I know on GEMS vehicles at least, there was a shim washer in the compressor clutch that could be removed as the clutch wore, to ensure a solid engaging of the compressor, but don't know about diesel or thor petrol models.

The wiring upgrade was mostly for earlier GEMS vehicles where the compressor clutch would wear and slip and no longer engage to have the HEVAC control a relay and this then gave a direct +ve feed to the compressor (being able to supply more current than the HEVAC MOSFET) - however, this wiring had the added power resistor in it to simulate the load of the compressor, to make the HEVAC think that the compressor was operating.

From looking in later model HEVAC units, for vehicles which had the relay installed as standard, from the factory, they still use a MOSFET to drive the output, but I am guessing that the feedback/status part of the programming in the firmware has been written out, so it doesn't flag the low current of the relay as a 'fault' like it did on the earlier ones.

I could be wrong about the BECM power feeds, but I don't think that there was a dedicated feed to just run the compressor - I was under the impression it was a general switched feed for the whole HEVAC unit.

If you have a Nanocom, then I would have a look at the live data when the AC is running and when it cuts out... see if the A/C request/grant to/from the engine ECU is still working as it should, and that for some reason the engine ECU isn't suddenly telling the HEVAC that it can't run the compressor.

Another thought I just had, would also to be to check the HEVAC live values for the evap temp sensor reading. The HEVAC uses this to cycle the A/C compressor off when the evaporator gets cold, so it doesn't freeze over - it could be possible that the sensor is faulty, that when it gets cold, it drops the connection, and the HEVAC thinks it's gone frozen and shuts the compressor off... I only suggest looking at that, as the default value for the sensors is -40? (from memory) so if it sees that reading on the evaporator, then it would also shut off the AC compressor.

If that all comes back clear, then I'd try and get the pressures read when the system is running... if they are in spec, then maybe the dual pressure switch has a fault that after it's up to pressure for awhile, it drops out, which would also stop the compressor from running.
Click to expand...

There is a 30 AMP feed from fuse 8 in the BECM via relay 7 to the white plug in the middle of the HEVAC unit. C244 early and C0792 late. The Hevac unit switches this supply to pin 7 same connector to either power compressor direct via dual pressure switch, early models. Or pull relay 3 later models. Power for compressor clutch on later models then comes from fuse 27 30 AMP engine bay fuse box to power clutch via dual pressure switch. If the electronics in the BECM fail and relay 7 in BECM is not being pulled or has failed, the compressor clutch cannot engage under either early or late scenario as no initial power is being provided from BECM to either power clutch early or pull relay 3 late.
 
Marty, You'll be surprised how much control the becm has.
The becm supplies power to the Hevac & radio, but also the blower fans etc. The alternator also returns signals back to the Hevac, then back through the becm.
These becm's are very complex the way they work. We're always updating the schematics.
 
Yes, I know there is... it's one of the ignition switched supplies

If you actually look at the RAVE diagrams though you see that RL7 also controls the radio and the same splice is shown as going off to the navigation system. My point here is not that the BECM doesn't supply power to the HEVAC - I know it does. But it's shared with at least the radio, so if this was where the problem was occurring - then surely the radio would drop out aswell?

I didn't know the alternator returned power directly to the HEVAC (I know it gets a switched position II/III feed from the engine bay fuse box), but knew the BECM got a feed from the alternator, to know when it was putting out charging voltage, so it would switch on high-current devices - like seat heaters, window demisters etc. The blower fans are only controlled by the BECM, not powered directly from it - it provides a ground to the relays in position I/II, so they will shut off whilst cranking, but the actual power for them is from the engine bay fuse box.

I don't doubt there is more to the BECM than I am aware - but I have also spent a lot of time looking through the power board especially. Just in this case, if the BECM were the culprit, I personally think you would have more symptoms - like the radio going off at the same time as the A/C - which the OP hasn't mentioned anything about.
 
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