Curious if any one has had problems identified- when I switch my Ac on and put it on max it will not operate. , over to norm works fine(and blows out vents cold) .I can slide the lever(norm max etcc) to defrost -vent and it works correct-so no lever door issue . When on max I hear the compressor kick on but it will not blow unless I slide it back and forth a couple of times.. As stated every other position works as it should vent defrost etcc… and it only seems to be the max position .I did change the blower switch a few years back and it works correct on norm def. etc. TR

 

Mine if going from off to max AC will not blow out vents I have to go to normal then max AC to blow out vents.
This is just in testing as I don't have truck on the road or the AC charged.
I believe it is the vacuum part of the switch on the back side of the control.
From what I have seen posted this is hard to come by and IIRC there are different ones f=thru the years.
Dave ----

 

Yep your best bet is to hunt a NOS unit down. they can be hard to find but they do pop up on Ebay though. I got one for my '78 years ago. I also got a whole NOS hvac panel for my truck but mine was a bit easier to locate as I just needed a plain non air panel which oddly enough is easier to locate than the factory AC panels.

 

Just to confirm, your blower stops completely when moved to max AC? What year is your truck?

On my 80, the electrical switch on the vacuum assembly passes electricity through to the blower speed switch in all vent positions but OFF. It does this by opening a contact within the switch via a cam on the vacuum diaphragm itself. This cam only engages opens the switch at the OFF position and never comes into play at any other vent selector position.

Does your blower work when the selector is in the VENT position?

 

My '84 acts up just like that on occasion, and only in the MAX position. Usually cycling the lever a few times takes care of it.

I've ordered a new vacuum control switch (Motorcraft YH380) but have been too lazy to install it:




Not sure if your '85 takes the same switch.

 

The truck is a 1985 ; (as listed in my signature )if I understand correctly it operates the cam from off and the vacuum portion of the switch itself swaps it from max defrost etc . Nos units are hard to find ( been searching ebay) and before posting I did find the motor craft yh380 on eBay ,Amazon and Rock auto .
Years back I did do the heater blend -vent door swap,(had a thread on here )purchas3d the motorcraft part and that didn’t last long , so 2nd time I used industrial grade tape .That fix has lasted till present .
I would rather wait until cooler weather before looking into it and making sure the Yh380 part is correct or which it may be .Thanks for the reply’s Long Live The Bullnose Trucks !

 

It is hard to tell if the blower motor stops if the truck is running as air flow stops coming out other than defrost IIRC.
For me my 300 is vary quite and no glass in it so was Abel to tell / check where air was going and the the blower motor was spinning.
For some reason the vacuum switch will not let air pass as it should th I'll I go to normal AC as then max AC.
Dave - - - -

 

I haven’t notated these pics yet so bear with me. This shows the vacuum switch pieces. I know the later year Bullnoses use a different switch because the electrical harness exits the passenger side of the switch. I’m unsure of any other differences.



The two tabs you see below the switch’s black housing are the contacts for the AC compressor (background) and blower power supply (foreground). The cams will rotate toward the contacts as the vent selector is moved toward OFF. The small cam (1/2 width) is the AC compressor kick off cam and, on my unit, opens up the circuit when the selector is in the (square) position. The large cam kicks off the compressor and blower at OFF. I can’t recall if it kicks the compressor off at VENT, but looks like it could. All this was determined on the bench as my AC system isn’t functional.

It’s odd that you lose blower function at MAX AC - not sure what would cause that contact to go open. The power supplies into this switch are hot in key RUN position (book also says ACCESSORY). On my unit, the bottom two switch contacts are power in, and the top two contacts are power to blower motor and power to AC pressure switch.

I write all this knowing that our units aren’t identical, but hope this may help in your search for a fix.

 

I don't think he looses blow but looses air flow.
If he was to check I bet the blower motor is still running just no air flow out the vents or floor but if he checks the defrost air might be coming out there.

That is because that plastic vacuum switch valve is not working as it should has nothing to do with the blower motor not running as it should, he needs to do more testing and checking for motor running and air flow else where
Dave - - - -

 

Perhaps I misunderstood his issue.

I don't have my EVTM in front of me but almost certain that the only thing that changes between MAX AC and NORM AC is that the outside air door goes fully closed so all the air is recycled to/from the cab. I don't recall any other vacuum motors changing position (at least not purposely).

Of course, even after rebuild my unit directs air out the register vents at all positions. I even applied vacuum via a hand pump to the HVAC head's vacuum harness while it was out and watched all motors moved and thought I heard clunking from all doors. Luckily I live in Texas, so we use A/C out the register vents 12 months out of every year, so I didn't bother chasing it any further. Sometimes good enough is better than another dash disassembly.

 

This sounds right, and is how the doors SHOULD react with the lever to MAX. It's been a while, but when my truck acted up, all of the outlet doors somehow closed. I don't know what the fresh/recirc door was doing. The fan was still running as commanded, but the airflow had nowhere to go. Other than a little leakage, nothing much came out of the defrost, dash face, or floor grill.

This struck me as very odd because with a loss of vacuum, the various actuators are spring-loaded to send fresh air to the defroster. For whatever reason, the defrost door closed at the time. I chalked it up to an unusual internal failure of the vacuum control switch, sending vacuum to the wrong places. If you couldn't hear the fan, you might think it had stopped if you only noticed the lack of airflow.

 

You are right on the door going to recirculating cabin air on max from what I have seen posted also.

Now I could be wrong on air out the defrost, it has been a wile since I tested this so you could be right again (don't let get to your head ) that the air just had no where to go.
I will not be able to test mine till the weekend.
Dave - - - -

 

Here's the vacuum hook-up for a 1986. You can see the defrost selection does not direct vacuum anywhere. That is why it defaults to defrost when you lose your vacuum source from the engine. I have never messed with one of these, I do not know if there are different layers to the switch or that is just the way they have it drawn in the diagram.

 

Here’s the vacuum “switch”. I did not dare to enter the maze. Seems like our trucks only port vacuum to four different motors (really two single stage and one dual stage motor) so I’m not sure why the switch has 9 vacuum ports unless different models have different harness setups.



This is the lever driven by the vent selector linkage and that drives the vacuum switch. You can see the wear in the leg that is driven by the selector linkage. That wear makes the linkage deflect a bit under load - not sure if it disturbs vacuum porting, but not ideal. That deflection makes this linkage skip over the “detents” in the housing, resulting in that continuous selector stroke most of us have.



Detents on switch housing. There’s a thin male projection on the lever that fits into the female slots in the housing.

 

It may have a lot of nipples for hoses but IIRC if you look at them closely not all of them are open on the ends.
I will have to check that leg for wear to see if that could lead to something not working?
Dave ----