OK, a friend of mine has a 1999 E150 Van with the V8 engine. He was accelerating while towing heavy, and the A/C unit went from vent to the DEFROST setting (without anyone touching the control ****). Later in the trip, while accelerating hard, the A/C blowers both quit working simultaneously.

He has replaced the fuses under the dash, as well as the A/C relay under the hood. Neither the fuse nor the relay have popped since replacing, but the blowers are still not working.

Was wondering if this could be something like his heater/AC blower motor resistor going bad? Just don't understand the issue with the accleration causing these weird problems.

Any words of wisdom to help him out?

 

I have the same problem with my 1997 E-150. When I step on the gas, such as getting on the freeway on an on-ramp with the A/C on Max, the air from the dash vents will default to the defrosters. Then after cruising speed has been reached the air will come back to the dash vents. I have researched this for hours. Some people say the problem is a vacuum check valve somewhere between the vacuum source and the vacuum tank and the A/C-Heater Function Selector Valve. Tonight, I removed the engine cover, the plate on top of the dash (where the control vacuum motors are), the battery and battery tray, The right headlight assembly ( Why? because some people say the Check Valve would be under the A/C evaporator case under the hood. I thought I could look with a flash light thru the hole and locate it). I also removed the vacuum tank and vacuum line assembly that travels up the frame rail toward the engine. I checked the vacuum tank and vacuum lines. I replaced a couple of rubber nipples and everything check out good.
I have downloaded a couple of vacuum diagrams, but none of them match what I have. I have traced this black line from the control unit under the dash to the far right side at which point heads down the right side of the heater core case and goes thru the firewall. It comes out under the blower fan housing and then up between the inner fender and the fan housing and over the evaporator case toward the engine. At this point it connects to a RED vacuum line with a rubber nipple. That line runs with a few other colored lines and ends up at the intake manifold just below the throttle body. Now this Red line is on the same circuit as the fuel pressure regulator which is also RED. This doesn't make sense to me. So I guess what is happening is when the manifold vacuum drops, the A/C control looses vacuum and the system goes to the default defrost position until vacuum returns.
What I need is a vacuum diagram ( color coded) for this vehicle. Someone also said that the vacuum check valve is between the heater core case inside and the evaporator case under the hood. I can't believe Ford would do this. So does anyone have some good solid information on a fix for this. I've read on-line that this is a common problem but no one seems to know where this check valve is or how it is connected to the system and vacuum lines. This is a used van for me and there is indication that the engine cover has been removed more then once. One other thing, the vacuum tank I have only has two ports. A Black source vacuum line. And another port that has two lines connected to it, Tan and white. Any HELP would be great on this, I'm out of ideas !!! THANKS

 

Well, my friend took it to a shop and they said the compressor went out, without blowing any relays or fuses. The DEFROST / acceleration weirdness was due to blockage in the vacuum tubes. Unfortunately, Ford apparently hid a lot of this inside a welded box inside the engine compartment, so you have to cut open an area of the firewall to get to it and work on it.

My friend was not too happy. No idea what the cost is / was.