My '88 Van has had this problem from the 1st year I owned it ('89). When on Max A/C, the door that changes the direction of the air from cabin to defroster opens the defroster vents when I accelerate and closes (directing air to the cabin) when I decelerate. This does not occur when I use the Vent (economy for A/C). When I look at the vacuum hose diagram in the manual, the only difference between using Max A/C and Vent is the use of the outside air/recirculating air vacuum motor. I am thinking this is the culprit.

I've changed the pushbutton control and all vacuum lines. There are 2 check valves on the supply line and both work. Suggestions welcome.

An additional problem with the HVAC system is the blower switch gets really hot when it is on maximum and has been so hot as to melt the area where its fuse was originally. I've had to place the fuse in-line and use a circuit breaker instead of a fuse. I've replaced several switches and in 20 years the blower motor once. I've added an extra grounding line for the blower motor, but that didn't help. Thanks for any suggestions.

 

I agree, the diaphragm is probably ruptured causing the HVAC system to lose vacuum. Loss of vacuum will cause the air flow to divert to the defrosters. This is only noticeable during acceleration because the vacuum in the intake can drop to zero.

 

Thanks for the quick reply.

The vacuum motor that operates the recirculate/fresh air door has two vacuum lines to it. I'm assuming one operates the door in one direction and the other in the opposite. It would seem if I disconnect and plug one and then the other vacuum line it would show whether this is where the problem is. Does this seem feasible? Thanks for any suggestions...

 

Since the problem only occurs when you switch to MAX A/C then the leak is only on the side that pulls the recirculate door closed.

 

Thanks for replying. Does anyone have a suggestion about the problem I have with the blower switch getting hot? I might mention that the circuit breaker I used to replace the original fuse has a metal outer shell that gets quite hot even on the next to highest blower position. I can put the switch on the highest and it will work OK for a while until the breaker opens the circuit. Thanks for any suggestions.

 

The current blower motor may be getting old and requiring more current to run which will overload the circuit and cause the wiring, switches and such to get hot. This would be caused by the bearings becoming dry and the motor working harder to turn the fan.

 

Yes Sir, that would be a cause, however, when I did put in a new blower motor some years back (the one installed currently) the problem did not go away. I also replaced the resistor block, and as mentioned previously I had replaced the switch. At one time I thought I might have a grounding issue and added a wire from the ground point where the blower motor connected to a point right behind the battery, still no change. This has been a mystery to me....Thanks for answering.

 

Is the 'behind the dash' wiring heating up as well, or just the breaker and the switch?
If you remove the breaker and connect a DC amp meter in place of the breaker, start the engine, what is the amp reading for each of the blower speeds?

 

Sorry for the lapse in responding, other issues taking my time. As for measuring the current on the blower, it would seem I would need an amp meter that could handle more than 30 amps since that is the fuse rating. Further, since the circuit breaker I am using gets really hot, and has tripped at least once that I remember, it most likely is pulling more than 30 amps. I have a VOM but it is rated at 10 amps max. Any suggestions where to get one that can handle up to 50 amps?

I haven't addressed the vacuum problem yet, but am becoming convinced the problem is with the vacuum motor that operates the recirculate/fresh air door. Unfortunately, it resides behind the drier and expansion valve so it won't be easy to get to.

Thanks to everyone who has replied.