sleepy hollow
Registered
See my intro thread just posted for more info on this car which I just got inspected and registered after 5 years in my FIL's garage.
One remaining project is the AC which does not work. Here is the AC story so far:
A/C is blowing uncooled air. I figured she needed a charge after 5 years sitting so I picked up a can of 134a and was ready to inject using an old AC Pro gauge. To my surprise the system appears to be fully charged. It registered about 40PSI on the low side at 70 degrees ambient temp. So, no need for the refrigerant at this point. Actually that is good news showing the system is tight.
However, the clutch never engages when climate control head is set to max AC.
I can hand turn the compressor (with engine off of course) so it is not siezed.
I did a self-test on the control head and it passed with no fault codes. I also have an OBDII+ bluetooth scanner which I have found to be very useful for its price on my other vehicles. So, I selected all the available PIDs for the Mark VIII related to this problem. Turns out under drive train there are several AC/related PIDs that can be read in real time. So, here are the relevant PIDs and values with engine running and max A/C selected:
A/C pressure switch: <1 volt, usually around 0.25V-0.30V if I recall correctly
A/C clutch: 0
A/C compressor cycling switch: 0
A/C driver request: 0
A/C medium pressure switch: 0
Not a lot to go on, but it seems to me that the request to turn on the system is not being asserted to the compressor from the control head, or is being cut off somewhere in between.
I found a wiring diagram online and it shows a module called the VLCM (Variable Load Control Module) which is the last stop before the clutch will be powered on. Took me quite awhile to locate it as I had no idea what I was looking for. I think I finally found it. Sort of looks like a car stereo amplifier with a set of large heat dissipation fins on the back. It is right of center in front of the radiator/condenser complex. But I cannot see the connector so not sure if it is the correct part. I will have to pull it to see more but that means removing some plastic cowling and who knows what else. Ran out of time yesterday.
I was trying to find a way to energize the clutch directly, but I cannot get to the connector on the compressor. I tried from the top and from underneath, but it is in a very difficult location to remove and test directly without tackling the A/C plumbing that is in the way. Hence my search for the next point upstream. The clutch coil has two wires connected, one of which is grounded nearby. The other is the purple wire connecting to the VLCM.
I also tried to ID the appropriate clutch power fuse or relay in the power distribution box under the hood. The wiring diagram indicates fuse 23 (40A) is the power feed to the VLCM which is activated by the "A/C demand req[uest]" on pin 18. Pin 18 is the purple wire connected directly to the clutch.
So I am thinking that I can apply 12V to the wire harness at the VLCM for the pin 18 wire and maybe get the clutch to energize. I do not see any way to do this further upstream as the VLCM seems to be controlled by the PCM feed on pin 15 (red).
I am expecting the clutch to energize in any event. My hypothesis at this point is that the mechanical system is fine and there is a problem somewhere in the electronic control system. I am thinking after I confirm the mechanical system components are operational, I'll pull the dash module and look inside for a loose connection or PCB issue. I saw a video on a Town Car with a bad solder joint on the climate control head PCB. I would love to find that problem here. But what I wish and what is are often unaligned.
Any thoughts or suggestions would be most appreciated. Given that the car is not worth much, I really hate to have to pay for a repair as I will expect it to be a substantial bill regardless of what is found due to the labor it will take. Of course that pride thing is also at work I confess...
Thank you all for anything you may have to offer.
One remaining project is the AC which does not work. Here is the AC story so far:
A/C is blowing uncooled air. I figured she needed a charge after 5 years sitting so I picked up a can of 134a and was ready to inject using an old AC Pro gauge. To my surprise the system appears to be fully charged. It registered about 40PSI on the low side at 70 degrees ambient temp. So, no need for the refrigerant at this point. Actually that is good news showing the system is tight.
However, the clutch never engages when climate control head is set to max AC.
I can hand turn the compressor (with engine off of course) so it is not siezed.
I did a self-test on the control head and it passed with no fault codes. I also have an OBDII+ bluetooth scanner which I have found to be very useful for its price on my other vehicles. So, I selected all the available PIDs for the Mark VIII related to this problem. Turns out under drive train there are several AC/related PIDs that can be read in real time. So, here are the relevant PIDs and values with engine running and max A/C selected:
A/C pressure switch: <1 volt, usually around 0.25V-0.30V if I recall correctly
A/C clutch: 0
A/C compressor cycling switch: 0
A/C driver request: 0
A/C medium pressure switch: 0
Not a lot to go on, but it seems to me that the request to turn on the system is not being asserted to the compressor from the control head, or is being cut off somewhere in between.
I found a wiring diagram online and it shows a module called the VLCM (Variable Load Control Module) which is the last stop before the clutch will be powered on. Took me quite awhile to locate it as I had no idea what I was looking for. I think I finally found it. Sort of looks like a car stereo amplifier with a set of large heat dissipation fins on the back. It is right of center in front of the radiator/condenser complex. But I cannot see the connector so not sure if it is the correct part. I will have to pull it to see more but that means removing some plastic cowling and who knows what else. Ran out of time yesterday.
I was trying to find a way to energize the clutch directly, but I cannot get to the connector on the compressor. I tried from the top and from underneath, but it is in a very difficult location to remove and test directly without tackling the A/C plumbing that is in the way. Hence my search for the next point upstream. The clutch coil has two wires connected, one of which is grounded nearby. The other is the purple wire connecting to the VLCM.
I also tried to ID the appropriate clutch power fuse or relay in the power distribution box under the hood. The wiring diagram indicates fuse 23 (40A) is the power feed to the VLCM which is activated by the "A/C demand req[uest]" on pin 18. Pin 18 is the purple wire connected directly to the clutch.
So I am thinking that I can apply 12V to the wire harness at the VLCM for the pin 18 wire and maybe get the clutch to energize. I do not see any way to do this further upstream as the VLCM seems to be controlled by the PCM feed on pin 15 (red).
I am expecting the clutch to energize in any event. My hypothesis at this point is that the mechanical system is fine and there is a problem somewhere in the electronic control system. I am thinking after I confirm the mechanical system components are operational, I'll pull the dash module and look inside for a loose connection or PCB issue. I saw a video on a Town Car with a bad solder joint on the climate control head PCB. I would love to find that problem here. But what I wish and what is are often unaligned.
Any thoughts or suggestions would be most appreciated. Given that the car is not worth much, I really hate to have to pay for a repair as I will expect it to be a substantial bill regardless of what is found due to the labor it will take. Of course that pride thing is also at work I confess...
Thank you all for anything you may have to offer.
