Soo a little while back I noticed that sometimes the A/C would take a long time to switch from defrost to max face flow.

Now the truck will work ok a little bit at first, then when driving at over like 1400 rpm constant (short acceleration does nothing ) the blend door inside will switch to defrost on me and wont go back unless i decel HARD by engine or pedal braking.

I took apart the control thing in the dash cleaned it up put some new grease on the sides of the tubes worked for a day now same problem.

Is it the electronics part of the module that is not working? Originally I thought it was the tiny vacuum lines not seating property but now im lost.

Also I noticed that now It stays on defrost only when the engine is off, before it would work on any setting.

 

sounds like a vacuum problem check the lines in engine bay first for a leak in the system

test to see if the system has an inside vacuum leak or the one way vacuum valve is bad with engine off open hood and pull off the vacuum line at the T that goes to the inside of cab if you hear a suction of air as you pull it off its working ok first test.

Pull off the one way valve and check for goop muck in it sometimes oil gets into it and stops it from working correct.

93 thats cool for AZ we are hitting 112 for the next few days

 

Pulled the T line off that goes to cab, No suction, whats next? You should be titled dakota A/C expert btw

 

Next check the vacuum connections in your cab you may of accidently disconnected one or two of them look here http://www.rp95.com/vac1.pdf these are in the cab look under your dash for them on both sides some are by your feet and can get kicked off.

Did you look at the one way vacuum valve to see if it was dirty?

You may also have a engine vacuum leak look at all your vacuum lines.

 

Didnt get the 1 way off, almost forgot to ask, is it the large round fitting on the brake booster or the tiny rubber piece on the hard tiny line into the cab?

Also im unsure if something is disconnected because it will go on and off kind of like a electrical short rather then just not working like a disconnected wires.

Sounds like im gona have to remove the dash trim, control for the ac, and all the panels on the bottom middle and bottom drivers side plastic panels. Right?

 

I’m Bushed... sounds like the one way valve is not working so good like its sticking so look at it clean it out but use soapy water not brake cleaner that may destroy it but it also sounds like an engine vacuum line may have a crack in it. One other thing it could be is the vacuum pressure tank may be cracked or the line to it may be cracked/bad the tank is under the cowl.

Does your temp control lever feel sloppy lose your cable may have become lose so the blend door is not moving so good

Grab a manual and learn all the vacuum lines and check them

There is a dozen small things this could be

 

The vacuum check valve 98 is pointing to in the picture has the hard plastic line attached to it and the valve itself is about 3/4" in diameter. It's not the part attached to the brake booster with the large vacuum line.

The check valve can fail and restrict the vacuum used to operate the doors.

Blend doors shifting to defrost is loss of vacuum, that's the default position. Check the vacuum at the T. If it's good, you have a leak somewhere and need to find it. It may be the vacuum reservoir or it could be in the lines, or an actuator.

The only electrical is for the fan and a/c clutch. It has nothing to do with where the air comes from.

You noticing it works on braking verifies a problem with the vacuum source. At idle you'd have the most vacuum available.

 

00DakDan thanks correct

I think he has a few problems not just one

First big one is vacuum a leak in the HVAC vacuum lines you may have more than one leak

second is vacuum but I’m thinking its possibly a vacuum leak for the engine and it’s so bad as to be robbing the vacuum power for the HVAC operation I think it should be checked out anyway just to be sure.

Third is vacuum but its storage or holding ability such as the vacuum tank or the one way/check valve

Forth I suspect a problem with the blend door this is minor and not the main problem but contributes to the problem.

One more possible thing to check

You may need to pull your HVAC control unit out but not off everything still attached and with the engine running stick your ear as close as possible to it and listen for any air leaks at the controller as you change settings you never know you may have a leak in the control unit its self


I forgot one more possible problem reading from 00DakDan is one of the vacuum actuators has a leak or is stuck I always wondered how long those things last

 

#1 The T were you pull off to see if the system is holding a vacuum with engine off if you pull this off it should hiss at you if so you are good if no hissing you have a leak in the system or a bad dirty sticking check valve

#2 is just to show this vacuum line goes to the HVAC box inside




This is the one way/check valve

 

Ok soo my truck's 1 way valve looks alot different then yours and it looks like a pos, where can I get a new one?

Heres some pics, Pics help alot.





Im jealous of your so clean engine bay.

Also one thing, I've had a slight oil leak the whole life of the truck that Ive had it, Its the valve cover gaskets, dont those use vacuum to remove the vapors or does that not affect anything because one line from the right bank goes above the throttle body and the left goes directly into the intake?

I looked at all the large engine bay vacuum lines they seem good, some are kinda gettin stiff, not brittle but not nearly as flexible as the one to the brake booster.