Normal for AC/vent to cut off under engine load?
 

On a long drive this week, I noticed that my AC was cutting out. My Ranger is a 1997 XLT, 4 cylinder, 5 speed manual. I played with the controls and found the cutting out happened everytime I was driving up a long grade. Blower on high would cut off. Once I reached the peak of the grade and started down, blower would come back on. I switched the AC off and to the vent position, I observed the same symptoms. Is this normal? If so, I assume there must be a vacuum activated cut out switch. This would make sense in the AC mode but why also in the vent mode.
Or, Is this a symptom of a problem?

I bought the truck new in 1998 and it has about 105k miles. I normally don't drive it on long trips but I don't recall seeing this behavior previously.

its normal

I can see why it would be normal to cut off the compressor under WOT, but why the blower fan?

Talked to the tech from the local Ford dealership (over a beer at a bar - only cost the price of the beer!) and learned it is not normal but is a symptom of a small vacuum leak. The vent damper door position is vacuum controlled. If there is not enough vacuum, the door defaults to the defrost position. It could be expensive to find at the dealership, as they may have to remove the dash. I'm going to use my Mighty Vac and start under the hood at the recently replaced heater control valve.
Any helpful hints or is there a thread on vacuum leak issues?

I wouldn't even bother but if it bugs you that much then try to find the leak. mine went to the defrost setting also going up a high grade or WOT but the truck kept cool, tranny never got over 170 so it didn't bother me

If I can find it, I'll fix it as I have purchased an aftermarket cruise kit that also connects to vacuum. I assume this leak would also affect the cruise.

Places to check for the vacuum leak:

Vacuum line between intake manifold and check valve.
Bad check valve.
Vacuum reservoir could have a crack or disconnected hose from check valve.
Vacuum line between check valve and HVAC plenum under hood PS firewall area.
Vacuum line to the heater water valve.
Vacuum lines that go to the 3 vacuum servos under the dash in the interior.

It is highly unlikely that the dash would ever need to be pulled to fix this problem.

Very helpful! I'll let you know what I find.

"Vacuum line between check valve and HVAC plenum under hood PS firewall area."


Help, Bob, where does this line enter the plenum??
The line from the check ball is laying on the PS frame rail, with a tiny bit of plastic stuck in its end. I guess that is what on the plenum it was supposed to go to, but HOW do you get to it??

John- I can't tell you the exact location. The vacuum schematic for your 97 is laid out a little different from mine because the 2003 has the vacuum reservoir and the check valve in one part but yours are separate pieces. There could have been other changes over the years as well.

I'm confused by the term "check ball". Do you mean the vacuum reservoir or the check valve?

Here's a diagram showing the vacuum reservoir (tank) and check valve. Unfortunately, it doesn't give the exact locations for connections.

Attachment 82146

The ball I was referring to is the reservoir on the PS fenderwell, with the manifold line w/checkvalve going to it, and a line coming out heading toward the HVAC plenum. This line is still full length, but whatever it used to connect to at the HVAC plenum is stuck in the end. I can't reach or see the bottom of the plenum, but I'm sure if I could access it, I could repair the vacuum going in and restore function to the ducts.

The truck is a 95 btw, and I was wondering if I could just bypass the line going into the plenum and attach full vacuum to the back of the control head, which is MUCH easier to reach by just removing the radio. That is, IF I knew which line on the back of the control head was vacuum IN.

Anyway, I appreciate any help you can give. While the AC does cool the cab eventually through the defrost ducts, it gets uber hot in Phoenix and having cold air hitting your face and chest makes the ride much more pleasant.

The line from my reservoir/check valve passes below the heater blower on it's way to the interior controls but the actual location cannot be seen from above. You may be able to see more with a mirror and flashlight. Probably the best way is to remove the passenger side plastic inner fender so you can look at the bottom of the plenum. I believe that your 95 inner fender is similar to mine - held on by a series of small screws along the fender lip plus a few bolts here and there.

OK, I'll try that! Thanks for the tips!

I think its normal. The reason is this is a heavy vehicle and its needs to carry the load during the peak, so its get cut out and load on the engine gets reduce. And again when its on normal road again its back to the normal.

Yes, it is normal for the compressor to shut off at WOT in a Ranger. It is not normal for the airflow to be redirected to the defroster registers when the mode selector is in other positions - that indicates a vacuum leak in the HVAC controls or the vacuum supply to the HVAC controls.

found the broken hard plastic line by taking out the PS inner fender. every time I touched the line to splice it, it broke again, one last time it broke flush with the vacuum reservoir.
Ran a vacuum hose from the check valve straight over to the HVAC housing where the hard plastic was far away from the PS exhaust manifold to not be so brittle.
YAY! I have cold A/C blowing out the vents, the Max Cool vent door does it's thing too. A happy camper here in too hot PHX! Thanks for the tips along the way!!