75ZT Community

Hi folks. V6 180 Sport Auto.
When switching to A/C from economy mode or switching straight to auto mode I can no longer hear the compressor cut in.
Car has been laid up for 5 months as I was housebound due to a medical condition. Prior to this, had it re-gassed last spring, A/C worked as expected until I stopped using the car early September.
Can anybody get me started troubleshooting this problem please? I have no idea where to start.

Thanks very much.

Hi Mick,
I Googled your problem and this came up:
Based on common Rover 75 (and MG ZT) issues, the failure of the air conditioning compressor to cut in when moving from Economy to Auto mode is frequently caused by a loss of refrigerant pressure, a faulty ambient air sensor, or electrical issues with the compressor clutch coil. The AC system requires a minimum pressure to engage the compressor clutch as a safety measure.

Here is a breakdown of likely causes and troubleshooting steps specifically for the Rover 75:
1. Most Likely Causes
Low Refrigerant/Pressure Switch Failure: The system has likely leaked, causing the pressure to drop below the threshold required for the compressor clutch to engage.
Ambient Temperature Sensor Reading Low: If the external temperature sensor on the front bumper is faulty or disconnected (sometimes occurring after bumper removal), it may report a temperature below freezing (e.g., -29°C), preventing the air conditioning from turning on.
Failed Compressor Clutch Coil: The electromagnet coil on the compressor itself may have failed, meaning the compressor cannot engage even if the signal is sent.
Electrical/Relay Fault: A faulty yellow relay in the under-bonnet fuse box can cause the compressor to stop working.

2. Troubleshooting Steps
Perform a Diagnostic Self-Test: You can check for HVAC fault codes on the Rover 75. Press and hold the AUTO switch and the Air Distribution switch simultaneously while turning the ignition key from position 0 to II. The display will show "FC" (Fault Codes) on the left and any active codes on the right.
Check the Ambient Temperature Reading: Look at the exterior temperature display on your dashboard. If it shows an impossibly low temperature (e.g., -30°C), your ambient air sensor is faulty, which disables the A/C.
Check the Compressor Clutch: With the engine running and AC switched to AUTO, check if the centre of the AC compressor pulley is spinning. If not, the clutch is not engaging.
Check Fuses and Relays: Inspect the under-bonnet fuse box for blown fuses and check the specific yellow relay for the AC.
Inspect for Leaks: The condenser is a common failure point on the Rover 75, often leading to a loss of gas.
Summary of Actions
If the system has no pressure, it will not engage. The most common fix is a recharge of the AC system after checking for leaks, or replacing the external temperature sensor if the dashboard shows a freezing temperature.

Thanks Mike.
Looks like first port of call is a re-gas and check for leaks. My local garage that does all my bits and pieces including re-gas should be able to troubleshoot it that far.
Temp. sensor is showing correct outside temp.
I can check the integrity of the yellow relay myself.
I have a condenser, been in my stock cupboard for many years (bargain price at the time).
I can get a replacement compressor and belt from DMGRS if necessary.
What I can't readily find is a receiver / drier. This one on ebay ... Click Me... says it is compatible but, I'd need to double check with someone that knows first.
Hard to fathom what could have failed when not in use. Probably electrical if not gas.

Anyway step one will be taken in the next few weeks. preferably before (Road Tax) cleans out my pension for a month.

All my AC problems have always been down to minor gas leaks that don't show up under the normal quick checks. Most machines check for leaks with a vacuum test, and if the leak is small it doesn't pick it up. It is possible to check the pressure using the trinary switch, but couldn't tell you which pins to check right now. The times I have done it, all it proved was there's not enough gas. My diesel passed leak tests for 3 - 4 years but didn't last a year between refills. The last place to look at it wanted to spend £1k on new pipes, one of which is NLA. I think I found the leak (that turns out to be common on the CDTs) but struggling with the car dues to other problems with it.
So, first check, is the gas OK? If not most likely is the condensor. There is no separate receiver drier AFAIK, there's a drier pack inside the condensor. A replacement condensor should come with that.

Thanks Duncan. I will get a re-gas asap. I was not aware the drier was part of the condenser. That will save searching it as stated on dmgrs and various eBay sellers offerings. I have a condenser still in original unopened packaging, bought from Jules in North Wales donkeys years ago. I’ll presume it has the drier until proved differently. if it has to be fitted. If it is a matter of a re-gas every year or so until either I or the car are no longer serviceable then so be it.

Mick the condenser will come with the drier, we have fitted quite a few condensers at the meets so if you wanted to it could be done there, at the same time have the cooling fan checked, most the time you can shine a torch through the front grill and see if your condenser is in a decent condition or not.

Thanks Steve,
August 2022 you changed out the gold resistor for me last time I came to a Nano Meet. The Condenser was in good shape then. Doesn't mean it is now. I'll take you up on your kind offer once the re-gas is done, I'll do that soon as so it will all have leaked out by the next Nano. The cooling fan seems to operate correctly but, doesn't go into second speed when A/C turned on as it should if A/C was working. Definitely worth checking while the front end is off. I'll see what I can see through the grill as well.

Best wishes

Mick