My 1994 XJ12 recently had a Mirelli ignition failure. I had the plugs, plug wires, cap and rotor all replaced along with a catalytic converter that overheated during the incident. I also had the original air suspension swapped to a traditional setup while the car was down, so all in all it was at the shop for about six weeks. All of that work seems to have been done well and the car runs and drives great but unfortunately, since getting it back I have noticed the HVAC only blows ambient temp air. I am pretty sure both the A/C and the heat worked fine previously, but I hadn't really tested them lately and I have always had good experiences with this shop so I don't want to accuse them of anything. I am not sure how the work they did could affect the system anyway.

The blower and fan controls work, it blows air and it gets stronger if you turn it up, there is just no heat or A/C. The compressor also engages and cycles with the a/c on as I would expect. Pressure at the low pressure service valve coming off the compressor also reads as ok (using the gauge you get with a cheap a/c recharge kit).

I would appreciate any thoughts what this could be? A fuse or relay maybe?

Thanks!

 

Speaking of the AJ6 only; 40 oz. of R134 at low pressure of 40 PSI and my AC is nice in Sunny and HOT Florida.

 

If the compressor is cycling, you should get some cooling. A running compressor also tends to eliminate that you're low on refrigerant as low charge generally means compressor doesn't run.
At the moment is sounds like the blend door motors in the climate control unit may be stuck closed or disconnected?

Larry

 

My A/C at 40 oz of R134 at 40 PSI works fine' I have one anomaly; as I accelerate air flow through the vents stops, when I stop accelerating and level off speed air flow through the vents resumes.
Any ideas?

 

I like Larry's thought about the blend doors, because that seems like it could affect both A/C and heat. But I'm trying to think what position which door would have to be stuck in to prevent you from having no air conditioning and no heat. My recollection is that the air from the blower fans always passes through the A/C evaporator, but the blend door varies air flow through the heater core to regulate outlet air temperature. I'll have to see if I can find the blend door diagrams.

Another possibility is that even though the clutch is engaging, the compressor has failed internally and is not pressurizing the system. You would need both high- and low-side gauges to determine whether the compressor is actually working. On the other hand, the compressor on your V12 is right on the top front of the engine, if I recall correctly, so it is possible something was disturbed during the ignition service.

The problem with your heat could be due to the heater water control valve having been disconnected during the service. On the V12s, I think the valve is mounted near the firewall in the V of the engine. Check to see if its vacuum hose or electrical connector has been disconnected. Also check your coolant level, since low coolant can cause a no-heat condition.

Cheers,

Don

 

Hi Roger,

It is typical for the compressor clutch to disengage during wide-open throttle, but it is not normal for the blower fans to stop working. I'll have to ponder what could cause that odd behavior. I wonder if one of the sensors has an intermittent connection (solar sensor, ambient air temperature sensor, etc.). Are both of your blower fans working?

If your A/C were just getting warmer during acceleration, I would suspect your system is overcharged with refrigerant, and as the system pressure increases at higher engine rpm, the high-pressure switch is disabling the compressor clutch.

Cheers,

Don

 

Thanks Don, in my problem the temp. doesn't change, airflow stops. During my research a "common" cause for this is a failed vacuum check valve.
So...I'm going to start by replacing the large, 30 year old CAC4188 check valve on the vacuum tank with a new smaller check valve. This may turn out to be a "adapt, innovate and overcome" solution.
Will advise as I get involved.

I'm fairly certain that the CAC4188 is the problem. I swapped the one in my car out with a SMALL vacuum check valve sold in the chain auto parts stores. Air flow was greatly reduced but CONSTANT.
I ordered one that MAY be the appropriate size and will fit from the local NAPA store for pickup tomorrow 5/12/2021 will update.