Weird defrost a/c problem.

[239]

Hey everyone, so a few days ago I turned my max a/c on and I noticed that the blower was running but my ac wasn’t coming out of the dash vents. Upon further testing I noticed that it randomly switches between max ac and the defrost vent while driving. I can also have my ac completely off and the windshield will get foggy from air leaking through the defrost vent while I’m driving. It seems that the defrost vent is being switched on when the ac system is engaged and its also staying on when the ac system is off. Has anyone had this issue before? I’m going to search for vacuum leaks but would like some input, thanks.

 

[koda2000]

A small vacuum leak will be apparent when the engine is under a load (like when climbing a hill). At idle, when engine vacuum is highest, the HVAC controls may function normally. I larger vacuum leak will typically throw P0171/P0174 lean codes. There are many places to loose vacuum under the hood plus the in the evap system in the rear of the vehicle.

As you are apparently aware, the default direction for air flow inside the cabin is to blow air out of the defroster vents. To control air flow there are multiple vacuum motors under the dash. They control air direction (floor, dash, defrost) and recirculate or fresh air. It could be you have a bad vacuum motor.

Depending on which HVAC controls your truck has (manual or EATC) you can have an addition source for vacuum leaks. The EATC unit has tiny vacuum solenoids inside of it. These solenoids have O-rings that wear out over time, creating another place to loose vacuum.

I believe @Turdle has posted a diagram showing the vacuum motors, HVAC control lines and their approx locations under the dash

Happy hunting.

 

[1998Exp]

+ 1 with Koda. Just one comment: except in vent, the A/C should be running all the time. It's actually the A/C that helps defog the windshield, not the other way round. If air is coming out of the defog vents and the windshield is not clearing up, your A/C isn't working, and that is a separate problem from your vent control altogether. See if the compressor clutch is even engaging. If not, check power to the clutch. If no power, the system could be very low on refrigerant (which disables it through the low pressure sensor), or perhaps the A/C relay is bad.

 

[239]

+ 1 with Koda. Just one comment: except in vent, the A/C should be running all the time. It's actually the A/C that helps defog the windshield, not the other way round. If air is coming out of the defog vents and the windshield is not clearing up, your A/C isn't working, and that is a separate problem from your vent control altogether. See if the compressor clutch is even engaging. If not, check power to the clutch. If no power, the system could be very low on refrigerant (which disables it through the low pressure sensor), or perhaps the A/C relay is bad.

My compressor and blower work fine, the system is just randomly switching between max ac and defrost by itself. My normal ac works ok, so it must be a vacuum leak causing an actuator to move by itself.

 

[239]

It’s amazing what looking under the hood can reveal. I’m 99.99% sure that this broken vacuum line running to the heater core control valve is to blame for my ac problem.

Quoted from ranger-forums:
“When vacuum is applied the actuator shuts off the heater core. This is only in the OFF and MAX A/C positions.

When the MAX A/C position is selected the recirc door is closed, front vents are actuated and the heater core is shut off.

Since all of these are on the same vacuum "circuit" a loss in vacuum will cause them all to default. Front vents default to defrost vents, recirc door defaults to open allowing fresh air in, heater core isn't bypassed.”

Is fixing this line as easy as replacing the elbow with the broken line in it? I’m thinking of replacing the old vacuum line with silicone line. Is 3/16 the correct size?

 

[MAS Tequila]

Snip a piece off and take it to the parts store.

Is there enough slack to get it to connect to the vacuum motor?

 

[239]

Snip a piece off and take it to the parts store.

Is there enough slack to get it to connect to the vacuum motor?

Yeah there’s about 6” of extra line. But I’m going to replace it with 3/16(5 mm) silicone line. Hopefully it’s not difficult to access the other end under the blower motor.

 

[239]

I went ahead and removed the elbow from the heater control valve and it just pops right off. I managed to get the now solid vacuum line out of the elbow and will be reinserting the old line back in tomorrow as it has plenty of slack.

I was going to replace the old vacuum line with a silicone version, but it’s buried underneath the blower somewhere and I don’t want to mess with the blower as it works fine. The line measured 3 mm in diameter for anyone who’s wondering.

 

[Turdle]

Were you able to find this and was it of any help?

How to: - 2nd Gen NO AC ( or heat) FROM VENTS!!! A "How to" what to look for thread

 

[239]

Were you able to find this and was it of any help?

How to: - 2nd Gen NO AC ( or heat) FROM VENTS!!! A "How to" what to look for thread

Yes, it was very helpful. Vacuum line sizes would be great for those who need to replace lines, especially since that info can’t be found on this site. I don’t have a vernier caliper or I’d measure them all. Other than that, it’s a good source of info.