Well I've tested the coil and got 15ohms so I'm guessing that's gone but I think I knew kinda knew that but great way of testing. I've bridged the low pressure sensor then cooling fan runs at high speed still NO 12-14 volts at the compressor only 9.45 still. I'm guessing there is sensor issue but don't know where so really stuck😶

While ripping out an old hands free kit from the console I've done some testing on my working A/C (fingers crossed I haven't just jinxed it) so hopefully you have a reference of 'good' figures to work to.

These were all taken from the back of the A/C console 'ECU'.

Compressor Clutch.
With the compressor running and clutch engaged (A/C on, blower fan running and temperature set low) measuring on the back of connector C1630 (black 3 pin connector in the centre of the ECU) I measured 13.3V between pin 2 (Black/Grey (V8 ) or Red (TD6) - Compressor clutch) and pin 3 (Brown - Earth).
With the clutch disengaged (A/C off) I measured a few millivolts across the same pins.
Disconnecting the same connector from the ECU and measuring the resistance between the same pins on the incoming lead I get 2.5Ohms (coil resistance of the clutch).
NOTE you need to disconnect the connector to measure it otherwise you are measuring the internal resistance of the ECU combined with the coil which gives an false high reading (I was getting 15.9Ohms).

Refrigerant Pressure Sensor.
With everything off and left to settle for a few minutes I disconnected connector C1629 (blue 26 pin connector). I de-shell the outer case of the connector and checked the resistance between pin 10 (Black/Grey - Pressure sensor signal) and pin 20 (Brown - Pressure sensor ground). I measured 11.54kOhm which equates to roughly 11.5bar according to IID live data (on my vehicle the pressure settles to that figure when the system is off).
If the measured pressure drops to 1.9bar or increases to 33bar the A/C disengages (clutch off) and won't engage again until 2.3bar or 23.5bar is reached respectively. According to the live data on my vehicle the pressure of the system sits around 19-20bar with the A/C running.
With the de-shelled connector plugged back in you could also CAREFULLY check for the voltage between pin 21 (Green/Yellow - Pressure sensor supply) and pin 20 (Brown - Pressure sensor ground) which should be around 5V...DO NOT SHORT THE PROBES THOUGH, you could blow the reference supply, I didn't check this as mine was working OK and it's very fiddly to do (probably easier to pierce the relevant cables).

Evaporator Temperature Sensor.
Again with everything off and left to settle for a few minutes I disconnected connector C0923 (black 18 pin connector). I de-shelled the outer case and checked the resistance between pin 6 (Yellow/Black - Evaporator Temperature sensor) and pin 9 (Blue - Sensor Ground). I got 4.24kOhm which equates to roughly 19C according to IID live data.
The sensor is a NTC thermistor (Negative Temperature Co-efficient) so the resistance will increase the lower the temperature. Also according to live data the temperature of my evaporator was sitting around 4C when running, if it drops to 2C then the A/C disengages (clutch off) to prevent icing of the evaporator and won't re-engage until 3C is reached.
It's also worth noting that if ECU determines the temperature sensor signal voltage is out of range (<0>4.784V) then it defaults the temperature to 0C...which means the A/C will never engage.


It's worth noting the below on what conditions are needed before the Compressor Clutch is commanded to engage:

Compressor Control
The compressor is engaged by pressing either the automatic mode switch, defrost switch, A/C switch or maximum A/
C switch. To prevent a dip in engine speed when the engine is at idle, a time delay of approximately 0.5 second is
built into the compressor engagement process. The time delay allows the ECM to increase throttle angle and fuelling
in anticipation of the additional load on the engine when the compressor engages.
When it receives an input to engage the compressor, the ATC ECU sends a message to the ECM, via the K bus,
instrument pack and CAN bus, to advise that it wants to engage the compressor. Provided there are no engine
management problems, the ECM responds by increasing throttle angle and fuelling and sending a message granting
the request to the ATC ECU over the CAN bus, instrument pack and K bus. When it receives the grant message, the
ATC ECU energises the compressor clutch provided the following conditions exist:
l Engine speed is more than 400 rev/min
l Evaporator temperature is more than 3 °C (37 °F)
l The refrigerant pressure is within limits
l Battery voltage is less than 16 V
l The blower is running
l There are no faults detected by the ATC ECU.

Disclaimer!
I will not be held responsible for any damage caused by disconnecting and reconnecting connectors while the system is live!

Last edited by Weejock on 17th May 2015 6:42pm. Edited 1 time in total

William97841 wrote:

Well I've tested the coil and got 15ohms so I'm guessing that's gone but I think I knew kinda knew that but great way of testing. I've bridged the low pressure sensor then cooling fan runs at high speed still NO 12-14 volts at the compressor only 9.45 still. I'm guessing there is sensor issue but don't know where so really stuck😶

This is really interesting, thanks. when it's not hammering down outside I will go out with my multimeter and do the same measurements. Is it Easy to take the shell off the blue and black connectors?

Yes, the outer shells are held on by a single tab on one side, you just need to prise the outer shell over it and the connector slides out. If you look at the blue connector first it's easier to see as the shell is blue and the connector white.

Picture worth a thousand words:



Prise the blue plastic case up over the small white tab you can see in the centre left of the case then slide the connector out like this:

Click image to enlarge

De-shelling the multi plugs is really easy isnt it! thanks for telling me how. So i have been out with my multimeter, here are the results next to yours (yours in italics, my results in red):

Compressor Clutch.
With the compressor running and clutch engaged (A/C on, blower fan running and temperature set low) measuring on the back of connector C1630 (black 3 pin connector in the centre of the ECU) I measured 13.3V between pin 2 (Black/Grey (V8 ) or Red (TD6) - Compressor clutch) and pin 3 (Brown - Earth).
With the clutch disengaged (A/C off) I measured a few millivolts across the same pins. I get 9.3 volts under all situations (ignition off, ignition on with AC button lit, ignition on with AC button not lit), so this is a problem (the problem). I know the compressor clutch is good, its new and if fed with 12v it engages.


Refrigerant Pressure Sensor.
Connector C1629 (blue 26 pin connector) resistance between pin 10 (Black/Grey - Pressure sensor signal) and pin 20 (Brown - Pressure sensor ground). I measured 11.54kOhm I got 12.09kOhm, slightly lower pressure in my system which would relate to the fact it hasnt been recharged since I replaced the compressor. But still within working margins I think, so no problem here.


Voltage between pin 21 (Green/Yellow - Pressure sensor supply) and pin 20 (Brown - Pressure sensor ground) which should be around 5V. I got 5v, so all good here.

Evaporator Temperature Sensor.
Connector C0923 (black 18 pin connector) resistance between pin 6 (Yellow/Black - Evaporator Temperature sensor) and pin 9 (Blue - Sensor Ground). I got 4.24kOhm which equates to roughly 19C according to IID live data. I got 4.68kOhm which reflects the slightly cooler temperature today, so all good here.

So in conclusion I think it is still likely to be a duff HVAC ECU unit, what do you think?

Those results look OK.

Where were you feeding the 12V to the clutch to get it to work? At the compressor connection or from the HEVAC unit end to the clutch?

I'm just thinking there might be an open circuit somewhere between the HEVAC and the clutch. With no load on the circuit (i.e. open circuit) it could possibly float at 9.5V all the time.
I would confirm the resistance of the clutch coil too at the HEVAC ECU connector cable, it's not just confirming the clutch coil but the whole circuit right through to the earth (although doesn't disprove a short to ground bypassing the coil, a short to ground though shouldn't leave a floating 9.5V).

After that then it does look like the HEVAC ECU and is probably what I would investigate next. I would be stripping it down and looking for a blown output transistor.

I should add that it's probably worth getting the system checked with a diagnostic tool before condemning anything. I have an IID tool which gives live data of the A/C system. If you are getting sensible figures on the live data and it's commanding the A/C clutch on but it's not engaging then it can only be the HEVAC ECU output to the clutch or the circuit from it (if the clutch is checked as OK).

Hi so I tried a few of the tests but I still was stuck so I took it to s air con speacialist they tested it and now sucked it and refilled it but now I get 14.445 with engine running for a few seconds then back to 9.45 for a few seconds then back up to 14.45. So now I'm gonna get a nissens compressor and get it refilled and see what happens. I'll keep u up date soon. If I can't do it then it's time for windows open lol

So does that point to some sort of dodgy pressure sensor?

Taking it a Land Rover specialist in air con see what they say. But still think it's the compressor

Let me know how you get on and if they find a solution!

Is the compressor clutch engaging when you are getting 14.4V?

I suspect the clutch coil circuit is open circuit, either a wiring issue or the coil itself. I was only getting 13.3V under load and commanded on, higher than this when commanded on suggests there is no load. Likewise 9.3V when commanded off also points to a no load situation.
You could try shorting the circuit to ground with a 12V test light (non LED, bulb type) and measuring the voltage.

If I could get to my compressor clutch connection easily I could try and disconnect mine and compare measurements, unfortunately it's awkward to get to.

Let us know how you get on.

Any update on this?

Hi guys just a quick update sorry for the delay I changed my compressor first and also changed my condenser and now everything works including the cooling fan. Which does come on when the air con is active. So all good now. Thanks guys for all your help

I've solved mine too, things I have learnt:

- The compressor clutch supply cable will read 9.45v at all times when disconnected (unless the hvac ecu has the clutch on when it will be 12-14v). When connected to the clutch the resistance means the voltage sits at 0v or 12-14v when the circuit is live. This seems to be earth leakage of some sort, which the clutch resistance normally prevents and appears normal.

- The pressure sensor lowest readout appear to be 5 bar when read with IID tool. This confused me for a while as I thought that meant the system had some pressure. I took off the pressure sensor and it read 5 bar when in free air. Rigging up an airline to it for testing showed it worked fine and when pressurised above approx 11bar turned the compressor clutch on.

So I got my local aircon man to regas the system and all working. I had thought the compressor clutch burning out was as a result of some other electrical problem but I have now put that down to old age.