Air Conditioning Gremlins

[Smokey Bones]

Gentlemen, good evening. This is my very first post, so be gentle.

I recently purchased an 04 MM, SB with 129k on the clock. From the get go, I have been having strange a/c issues. First, on several occasions, when driving in excess of the posted speed limit, the a/c would shut off. Several times it would not come back on. (Fan was blowing, but no dice on ice)

Took it to the garage, and checked the pressures, and they were, ehh... So I evac'd the system, changed the orifice tube, and recharged according to factory specs.

Shortly after doing this, the same situation occurred again...

Today, I fired up ole girl, and after a short drive, I decided to turn off the air. The eatc turned off completely (like normal), but the cold air began to blow onto the floor. The blower will not turn off when commanded to.

I performed the eatc test and got 888's across the board, so I know I'm good there.

Any thoughts or suggestions as to where I should start?

[RF Overlord]

If the blower has a mind of its own, it could be the BMSC (Blower Motor Speed Control) module. They are a known issue. Try tapping on it with a screwdriver handle and see if the blower speed changes. It's in the engine bay on the firewall, behind the passenger cam cover.

Note: this car does NOT have a blower "resistor"...it's an electronic module that uses PWM (Pulse Width Modulation) to control the blower speed.

[Smokey Bones]

Overlord, thanks for the fast response. I'll try that in the a.m. Hopefully that is the problem. For what it's worth, the unit that is in there now looks relatively new, within 6 months or so, based on a lack of dust and dirt. Thanks.

[Zack]

The AC shuts off at a WOT and delays About 10 seconds before coming back on

[EMAS]

In addition to shutting off the compressor at WOT to give you all the available power the computer can shut off the compressor if the computer thinks the engine is running hotter than it should. That is done to avoid adding heat in front of the radiator.

[Smokey Bones]

Zack, I'm familiar with this on a p71 car, but the thing is I drove the car about another 30 miles at 55-60, at night and no other traffic. It's almost as if the compressor never came back on. But when I got home I turned the car off and it sat for about 5 minutes, I fired back up and clutch immediately engaged, and a/c worked.

EMAS, the car has never run hot since I have owned it. The temp normally stays just below the half way point.

So, what would make the blower motor continue blowing on my feet, even when I command the eatc off? I'm afraid that maybe the gremlins have multiplied, one in the compressor/clutch and one in hvac on the firewall? Or am I making this harder than it has to be?

[EMAS]

EMAS, the car has never run hot since I have owned it. The temp normally stays just below the half way point.



It is not what the gauge indicates that matters, nor what the actual engine temp is, it is what the computer thinks that matters. On our cars there is a sender for the gauge and a separate sensor for the computer. A bad coolant temp sensor could be telling the car it is running hotter than it actually is and the computer will react accordingly. Looking at the coolant temp reading and whether the compressor is commanded on with a scan tool when it actually is occurring is the best place to start diagnosing this problem.

I haven't seen the problem in the 00 era Fords but on the late 80's and 90's cars a clutch coil that shorts out when it is hot is not uncommon. With those the AC can seem to come and go as it pleases though it usually acts up when you are doing a lot of idling and the underhood temps rise.

I do think you have multiple gremlins that just by chance are happening at the same time.

[Smokey Bones]

Well, I think I have discovered the issue. Yesterday, while sitting in traffic, the air stopped blowing cold (as if the clutch turned off), and began to blow hot. So I immediately went to turn it down, and the fan wouldn't respond. High was the only option. I then went to turn it off, and the screen went black, but the fan continued to blow on high.

Since I already had the part, I decided it was time to tackle the problem. So I replaced the bcm in about an hour, and it wasn't too difficult. I just pushed the coolant hoses out of the way (still very tight), and dropped the new unit in. I used a magnetic 8mm to keep the screws in place until they were started, and a swivel with extension really helps. Overall, not a bad job.

Once the new unit was in, everything functions like new, all speeds and settings, including "off" and the clutch seems to be cycling properly now.

Thanks for everyone's help.