Hey everyone. My truck's A/C started acting weird lately. It works fine in the standard A/C mode but after being switched to MAX A/C it sometimes switches to defrost all on its own without rotating the dial. It's really strange because all of a sudden, air is pumping out the vents and then the cold A/C starts pumping out of the vents above the dash near the windshield. Has anyone ever seen this happen? Is the HVAC electrical starting to go in the temp unit. I was thinking about just replacing the temp control electrical unit but I wanted to see if anyone else has ever had a problem similar to this and how you may have remedied it. Any help would be appreciated.

 

If the air flow diverts from the dash outlets to the defrost and floor outlets your losing vacuum somewhere. Since your saying it only happens in the MAX AC position, I'd suspect the vacuum/selector switch may be the problem. If this is happening in other positions, I'd recommend you check the vacuum lines under hood, vacuum reservoir, heater hot water shut off valve, etc.

 

Check the vacuum supply line that runs from the engine to under the air box. You will find the vacuum reservoir under the air box area inside the inner fender. Check for holes or cracks in the lines and res.


Sometimes the hose melts near the pass side manifold, or is damaged after a plug change.

 

It only happens in the MAX A/C position. That's why I think it's an electrical problem with the selector switch itself. I'll check the vacuum lines as well just to be sure though. Thanks.

 

So I ordered and replaced the switch that moves the air from A/C to defrost and all that good stuff. Now my air only blows through the defroster vents on top of the dash. I replaced the small wiring harness that runs from the switch to the area just behind the glove box. I'm kind of stumped on this one. I'm pretty sure it's not a vacuum issue. It seems like more of an electronic issue somewhere.

Could it be a blown fuse possibly?

 

Upon further investigation I managed to find a couple of things that seemed very strange or out of place.

Here's the first one. This gray piece wasn't connected to anything. I'm assuming the black rubber piece at the end here is supposed to be connected to this other part. I have no knowledge of the part names but I just want to know if this is properly connected.



The second thing I noticed is this bit against the firewall. It has nothing coming out of it but when I run the air vents I can feel air pouring out of it. Is this a vacuum outlet of some kind? Should it have a tube or line connected to it? Both of these issues are on the passenger side near the firewall.

 

first pic is your hot water bypass for when MAX AC is active.

Second pic should be the drain for moisture, hard to tell though as I dont know if your looking up or what.

Air out the defroster is a classic vacuum leak.

 

As Blhde said, the first is your heater control valve (hot water valve). The vacuum line should be a grey plastic line that runs back and through the firewall in close proximity to the blower motor. It should have vacuum only when the selector control is in the Max AC and in the off position.

The second picture is the evaporator condensate drain tube. This allows the water that accumulates in AC box to drain to the outside.

What your going to need to do is a full vacuum line inspection starting at the engine. There is a line that comes off a vacuum tee that should run down to the vacuum reservoir somewhere on lower right fender. It may be under the air intake box. Here is an example:



Once you find this, ensure the line coming from the engine is in good condition. Next, Inspect the reservoir for cracks or damage. To test the reservoir, disconnect the two vacuum lines, noting which line was connected to which fitting. You'll need a vacuum pump and or gauge. If you have a pump with gauge, attach it to the inlet fitting and plug the outlet and apply vacuum. The reservoir should hold vacuum. If you don't have a vacuum pump, you can leave the vacuum line from the engine connected and attach a vacuum gauge to the outlet fitting. Run the engine for a couple of minutes until the gauge shows vacuum built up. Shut the engine off. The reservoir should hold vacuum as indicated by the gauge. If not, replace the reservoir. Next, follow the other vacuum line to the firewall (should be a black plastic line), checking every inch of it for breaks or melted spots. Here is an example:



If the above checked good. You'll need to move inside the cab. Drop the glove box. Look for the two vacuum lines coming through the firewall (black and grey vacuum lines). These should run to the function selector switch in the center dash. Make sure their in good condition (no breaks or pinched lines). Note the black is the supply line. The grey line comes from the function selector switch and runs back out to the heater control valve (hot water valve).

From there, the selector control branches the vacuum lines to their individual control devices, i.e., vacuum motors that switch the outlet doors for the vents, defrost, floor, recirculation, etc. Any leak in these vacuum lines or motors will cause the system to either not work in certain positions or not work at all depending on how large the vacuum leak is.

Here are some links you can read through that may help:

link 1

Link 2

Link 3

 

Thank you for the answers everyone. These have been most helpful. I'll keep you posted on the problem after I've had time to troubleshoot further.

 

Just wanted to give everyone an update if they ever have a similar problem. My issue began with just the Max A/C only working intermittently. I finally was able to narrow the problem down to this little tube being disconnected. The gray one pictured here:



After hooking this piece back up, the max A/C works great and there are no vacuum leaks whatsoever. The best thing to do is check to make sure all of the hoses are hooked up properly. This would have saved me some grief for the last couple of weeks.

As soon as I started the truck back up and it produced a vacuum, all the vents were cycling properly in the proper position and the Max A/C wasn't kicking on and off.

Thanks to Rev and Blhde for the prompt replies.

 

Glad to hear you got it fixed.