W140 A/C Cool and Heat through Defrost Vents

[rsmcmahon]

It seems my Air-conditioning systems has developed a random occurrence of blowing heat through the defrost vents while blowing cold air through front dash vents. The new feature (AC/Heat condition) doesn’t occur consistently.

One other condition that has puzzled me regarding the A/C is when the engine is under load (going up a hill) the air flow will cease.

Last, it seems the car has a noticeable “whining” noise when the A/C is operating.

I’m unsure if all these condition are related, but if anyone else has had similar problem or can offer me insight into my problem – your help would greatly be appreciated!

92 400SE W140

Thanks in advance.
Russell

[EC93SE]

My 93 300SE does the same thing, but only on very long runs. If I stop and switch the car off and on, it will operate normally for a while but eventually I have to close the side vents to keep the hot air out. I had been having trouble with the door closer pump and I believe that pump also controls the operation of the vent flaps. Hopefully someone else can chime in here and offer some insight.

[rsmcmahon]

I just checked the Air Conditioner Diagnostic codes on the LCD display.

I'm getting a

E0 76 (Heater Supply Unit Circulation pup (A31m1) over load
E0 77 (Heater Supply Unit Circulation pup (A31m1) over load

E0 91 A/C cmpressor ground Activation, open or short circuit.

This is the 1st time I've ever checked codes on my A/C. Since I was unsure what codes might have been "old", I cleared all codes and will recheck codes again later tonight.

What is the Heater supply unit circulation pup?

[KenP]

Yes, the "door closer pump" (the PSE pump under the back seat) is also used for other functions in the car... including manifold vacuum assist for some aux systems. like the climate control.
You may have a leak in the vacuum system allowing the actuators to move to the "no vacuum" position when engine vacuum is low (like going up a hill), or a weak PSE that can't keep up with demand, or both.
You need to vacuum check your system.
Hope this helps

[JimF]

Originally Posted by rsmcmahon

Last, it seems the car has a noticeable “whining” noise when the A/C is operating.

92 400SE W140
Thanks in advance.
Russell

I think KenP has diagnosed a vacuum leak . . . someplace in the system.

The whining noise is probably the bearings in the blower motor. Try this; move the blower speed pot in the dash and see if the noise goes away at a lower speed. If it does, it's the bearings in the motor. You might try to 'lube' the bearings? Can't hurt!

[rsmcmahon]

Since I replaced the PSE pump just in the last year, and all other functions of the PSE closure assist are working correctly, I’m leaning towards a vacuum leak in the climate control system – as KenP suggested. I just reviewed the procedure for testing the “vacuum valve block” (front passenger footwell ), valve block (behind the glove box) and vacuum reservoir (passenger firewall).

Wow.. this is a complicated system!

Just a side note… After clearing AC Diag codes (Special thanks to Jim F for posting the procedure on his website.) I continue to get E0 76-77 indicating a Heater Supply Unit circulation pump overload. Since my current problem doesn’t involve a lack of heat (only A/C) I’m going to review this issue at another time.

Has anyone performed the “Individual Flap Test” via A/C pushbutton control? The procedure doesn’t look clear.

I'll try lubing the 'lubing' the bearings on the blow motor in an attempt to quiet the "whining noise" too!

Geeze! Never a dull moment for W140 owners....

Thanks!

[KenP]

If you are going to wait to chase the circ pump "overload", I'd suggest you locate the pump and detach the power to it. I think if the pump seizes completely, the short may fry your Climate Control unit in the dash.

And, yes, you're quite right. The W140 has a complex Climate Control system.