A/C problems

[EppNation]

2008 Mercury Mountaineer 4.6L V8
Automatic climate control, dual zone

Been dealing with A/C issue since shortly after I bought the car Aug 2021, I have solved some of the issues with new parts but parts of the system still are not working as they should.

First I'll disclose the modification that the previous owner made. This model DOES have the rear air option, but the PO completely removed the refrigerant lines that go to the rear evaporator. They used compression fittings to seal off the tubes, which can only be seen when removing the front passenger inner wheel well.
I'll also say, leaking/low freon is not my issue here. I have a leak detector, and a proper a/c manifold. No leaks anywhere and freon levels are filled to 30-40psi low and 200-250psi high (depending on the ambient temperature at the time).

Symtpom: A/C compressor not staying engaged when demanded.
A/C compressor was replaced Sept/Oct 2021
Seemed to have solved the issue for a while.

Symptom of the compressor not staying engaged returns, but only after vehicle is hot. It works fine for first 10min of driving but then compressor is disengaged and I can occasionally get it to turn back on by turning the entire A/C system off for a few seconds and back on again.
I ended up putting a jumper in the compressor relay when needed, but when I do this I must keep the fan speed on high (7/7) or else the evap will freeze over.
Also, along with the A/C compressor not engaging, the air often times does not come out of appropriate vents. System would often switch to floor/defrost vents on it's own even when forward facing vents are selected. I could occasionally get it to work properly again by turning the entire A/C system off for a few seconds and back on again.
I had some luck too by turning the blend door setting from max cold to max heat, and back to max cold again. Doing this (assuming the compressor was working or if I had relay jumper in) would get the air to come back out of forward facing vents.

I have since replaced the A/C control panel, both blend door actuators and ran a switch into the cab so I can "jump" the compressor relay on demand. It was when I was replacing the blend door actuators that I realized that the mode door actuators are powered by vacuum.

The problem continues. The system randomly changes from forward-facing air to floor/defrost vents. Sometimes it will return to forward-facing vents after a couple of seconds, but other times it won't until I reset the system.

I don't want to keep throwing parts at this, so I'm looking for some confirmation that a failing vacuum pump COULD be the culprit of the mode doors intermittently not working.

I never have passengers, the back is always filled with tools/gear. So restoring the 'factory setup' of replacing the tubes to the rear evaporator has never been a priority. In my head, this is likely the reason the compressor is not working like it's supposed to (not engaging). Now that I have a switch in the cab, I can control the compressor manually on the go. I'd eventually like to have the compressor issue solved too, but sourcing the tubes and installing them is...not a priority lol.

So, is it worth my time/effort in replacing vacuum pump or is this possibly a ECM problem?

 

[EppNation]

I am assuming there is an electronic vacuum pump on this car, but now that I'm looking one up I'm not finding one...

 

[FordFlintrock]

Vacuum pump would typically be on a engine with turbo not the 4.6L. So the vent discharge problem is separate being a vacuum check valve/reserve tank/line leak problem.
What are the gauge pressures when compressor stops working? You also have ambient and cabin temp sensors which could be faulty and cause you problems. Scan tool needed to read those when problem surfaces. The A/C relay could also get funky when hot.
How much of a charge was put into system? My source has 44oz. with rear A/C and 27oz. without, which would be your charge weight. Block off kits for rear A/C are common enough and should have no affect.

 

[EppNation]

I read through some other threads here and I think my problem with the vents is the vacuum check valve. I ordered one and will report back.

My gauge pressures when the compressor stops working go from "normal" when working and then they just gradually fall. The compressor works fine when I jump the relay, and yes, I have tried swapping the relay with several new ones and the problem persists.

When filling system, I didn't go by weight because of the block-off kit, I went by the chart based on outside ambient temp on what to fill the gauges with...WITHOUT jumper installed (system works fine for first 10min of cold start). I have periodically checked pressures with and without jumper and they remain consistent.

Also forgot to mention that I recently replaced the condenser, thinking the compressor issue may have been a restriction in the condenser...problem persists.

On the control panel the display for outside ambient temp I would say is accurate. I just ordered cabin temp sensor. As much as I don't like throwing parts at a problem, a $20 part is money better spent than paying a shop more for an advanced diag...in my opinion.

The issue is definitely 'behind the relay'. Something is preventing the signal from closing the relay when appropriate. I imagine it is supposed to cycle periodically to prevent the evaporator from freezing over, which I have bypassed by jumping the relay, but it's the only way I can have cold air. I'm in Tampa, and the A/C is used 360 days a year.

Faulty cabin air sensor makes sense, thanks for the suggestion!

 

[EppNation]

Also, I have code P0443, not sure if it's at all related.

I replaced the purge valve and noticed the bigger hose was really loose on there to I put a hose clamp on it with the new valve.

Code keeps coming back though. Before I realized this car does not have a dedicated vacuum pump I thought maybe this issue was caused by a lack of vacuum somewhere in the system. Now I'm not sure.

I don't mean to sidetrack the A/C thread, but didn't want to leave this detail out in case it's related.

 

[EppNation]

I will follow up and report back on this issue.

I understand the importance of sharing solutions in these communities, especially when asking for and receiving help, but a much more pressing issue has come up which I am making a new thread for now.

 

[EppNation]

Having to buy a new PCM due to an overcharging alternator, the new PCM has solved all my A/C control issues.