Ventilation Resets To Defrost with High RPM
 

Morning All,

I have a 2000 B2500 that has been giving me some odd AC problems all summer long.

The issue is, no matter what mode my climate control is set to, it will default back to defrost whenever I accelerate, or anytime my RPM is above 2500

So far the troubleshooting steps I have taken are as follows:
Replaced the climate control switch (3 times)
Checked vacuum connections going in and out of the cab and the vacuum reservoir
Sprayed aforementioned vacuum lines with carb cleaner to see if the engine would respond to a possible leak

I am at a loss. This truck has been good to me and this problem has been driving me and my coworkers up the wall trying to figure it out.

Thank you all

Ranger vent selection is done by vacuum "motors"
Default, no vacuum, setting is Defrost

The vacuum reservoir(black ball) holds vacuum in reserve when intake manifold has no vacuum, like when accelerating, if there is no reserve vacuum then vents move to Defrost setting.

The vacuum reservoir has a Check Valve, like the power brake booster, it can be built into the reservoir or be on the vacuum line coming from the intake, on the line is most common.

Vacuum is not "used up", but can be lost.
Like putting a straw down into a liquid(coke) and then putting your finger over the end of the straw and lifting the straw out of the liquid, the liquid stays in the straw, the vacuum pressure created by the weight of the liquid trying to drop out of the straw holds it in the straw.
And it will stay in the straw until you release that vacuum, remove finger, or until vacuum leaks out.
So vacuum power is not "used up" it is there until released or leaked

Pull off the vacuum line going into the cab from the vacuum reservoir, suck on it, it will hold a vacuum or you have a leak, change vent settings in cab, suck on hose again, there should be no leaks on any setting.

Pull off the line going to intake from vacuum reservoir, suck on it, it should hold vacuum, blow in it and it should pass air easily, if not check valve is probably bad.
If you think vacuum reservoir has built in check valve then do the same test but put hose on Cab port, should hold vacuum but you can blow into it easily.

Thanks for the tips.

I wouldn't suppose one of you guys would have a vacuum diagram for the 2.5l engines would you?

It will be on your rad support