Ac Problem - Coincidence Or Consequence?

[LobsterThief]

Hey guys,

 

I had my AC recharged about 6 weeks ago in my 91 MJ 4.0 -- I have an AC buddy who vacuumed it and refilled it with R12 (the old stuff). He also tested the system for leaks.

 

A few weeks ago I decided to replace the bumper vacuum reservoir (football type) that was completely missing and capped off. Around the same time, the AC started blowing warmer and warmer -- sometimes it blows slightly cool (just barely cooler than the outside temperature) and other times it blows warm.

 

Do you think it's possible to have messed the HVAC system up (like the vac system) by replacing that canister? Or is this a coincidence?

 

I live in Florida and it's still super hot out :ack: Any advice is much appreciated!!

 

Thanks,

Zach

[cruiser54]

I can't think of how.

[danbyrambler]

Can't speak with authority for a 91 Comanche but, on my 87 MJ factory AC blend air system, a vac leak, especially in the vac resevoir will cause problems - if the blend air door is positioned-allowing flow mostly through the heater core then slightly cooler air until the engine comes up in temp & then warm - - - - - - Could very well be coincidence in that your ac has a slow leak - - - - - - Have U checked anything other than it doesn't blow cold? - - - - - - Fast & Dirty - - - with AC turned on is the compressor clutch engaged and turning? - - - - - - - A 91 MJ/XJ FSM & prob other 91 Jeep FSMs will lay it all out.

[cruiser54]

Blend air door is cable controlled. Defrost/floor/vent are vacuum controlled.

[LobsterThief]

Can't speak with authority for a 91 Comanche but, on my 87 MJ factory AC blend air system, a vac leak, especially in the vac resevoir will cause problems - if the blend air door is positioned-allowing flow mostly through the heater core then slightly cooler air until the engine comes up in temp & then warm - - - - - - Could very well be coincidence in that your ac has a slow leak - - - - - - Have U checked anything other than it doesn't blow cold? - - - - - - Fast & Dirty - - - with AC turned on is the compressor clutch engaged and turning? - - - - - - - A 91 MJ/XJ FSM & prob other 91 Jeep FSMs will lay it all out.

Here's what I've checked so far:

 

- Checked all HVAC vacuum lines (including to the vac canister in the bumper) and they all seem to be sealed.

- Checked the connections behind the HVAC controls since I removed my radio the other day and may have bumped them. They're good.

- Compressor clutch engages when I turn the AC on but it won't cycle off (pretty sure it's supposed to cycle, right?)

 

The most important thing I just noticed is that it'll blow cold when I turn the engine on and after a minute or two it starts blowing warm. If I turn it off for a bit and start it it'll blow cold and then get warm again. There is a lot of frost on the lines coming out of the block on the firewall (the lines going through the firewall into the cab). Maybe the compressor not cycling is causing it to freeze the lines up?

 

I'm gonna check the blend air door as soon as it stops raining

 

Edit: Just realized the vacuum line going into the heater actuator in the engine bay was broken off; I snipped the end off and reseated the vacuum connector but it didn't make any difference. This one (not a photo of mine obviously):

 

[LobsterThief]

Checked the blend air door and it's functioning properly

[danbyrambler]

Blend air door is cable controlled. Defrost/floor/vent are vacuum controlled.

AND? - - - - -If the blend air door is positioned-allowing flow mostly through the heater core then slightly cooler air until the engine comes up in temp & then warm - - - The plastic cable clips, both ends are known to break & cause probs on Chrysler blend air systems - PITA on some like FWD L body - - Wish I didn't know - - - - - - "Defrost/floor/vent are vacuum controlled" as is the HEATER CONTROL VALVE - - - Here's the AC operation chart from a 87 FSM - 91 info may be the same - 87 FSM was from pirate4x4 IIRC - Think it's on this site also - U should/might be able to find 91 FSM here or Pirate - Good luck

Image Not Found

[danbyrambler]

Here's what I've checked so far:

 

- Checked all HVAC vacuum lines (including to the vac canister in the bumper) and they all seem to be sealed.

- Checked the connections behind the HVAC controls since I removed my radio the other day and may have bumped them. They're good.

- Compressor clutch engages when I turn the AC on but it won't cycle off (pretty sure it's supposed to cycle, right?)

 

The most important thing I just noticed is that it'll blow cold when I turn the engine on and after a minute or two it starts blowing warm. If I turn it off for a bit and start it it'll blow cold and then get warm again. There is a lot of frost on the lines coming out of the block on the firewall (the lines going through the firewall into the cab). Maybe the compressor not cycling is causing it to freeze the lines up?

 

I'm gonna check the blend air door as soon as it stops raining

 

Edit: Just realized the vacuum line going into the heater actuator in the engine bay was broken off; I snipped the end off and reseated the vacuum connector but it didn't make any difference. This one (not a photo of mine obviously):

 

Frost is not good - Could be a number of probs, but frost would make these 2 my 1st to ck - Low on freon & faulty low P switch can cause evap freeze up - - - - - - The thermister monitors cab temps & cycles the compressor - Thermister is a pain to go after - Try to get a 91 FSM so you can follow the AC troubleshooting section - I Can send the 87 info - dunno if it's same for anything beyond/before 1987 - - - - - Be nice to your AC buddy.

[LobsterThief]

Frost is not good - Could be a number of probs, but frost would make these 2 my 1st to ck - Low on freon & faulty low P switch can cause evap freeze up - - - - - - The thermister monitors cab temps & cycles the compressor - Thermister is a pain to go after - Try to get a 91 FSM so you can follow the AC troubleshooting section - I Can send the 87 info - dunno if it's same for anything beyond/before 1987 - - - - - Be nice to your AC buddy.

 

My FSM disappeared during my move a few months back so I'm going to order a new one -- until it gets here I'm going to move forward with troubleshooting. I'm assuming the thermistor is device with the small wire loop mounted under the passenger side of the dash? It seems to just be stuck up under there (no longer mounted to anything), though it is plugged in. Any idea how to test the thing? I think if the compressor isn't cycling it might be responsible for freezing my lines up.

 

I'm going to order a new LPS and try swapping it in as well. Can I change the LPS without losing coolant?

 

AND? - - - - -If the blend air door is positioned-allowing flow mostly through the heater core then slightly cooler air until the engine comes up in temp & then warm - - - The plastic cable clips, both ends are known to break & cause probs on Chrysler blend air systems - PITA on some like FWD L body - - Wish I didn't know - - - - - - "Defrost/floor/vent are vacuum controlled" as is the HEATER CONTROL VALVE - - - Here's the AC operation chart from a 87 FSM - 91 info may be the same - 87 FSM was from pirate4x4 IIRC - Think it's on this site also - U should/might be able to find 91 FSM here or Pirate - Good luck

Image Not Found

 

I checked the door that the cold/heat slider pushes using the cable -- the controls are pushing the cable all the way (in other words, opening/closing the blend door properly). This is located on near the passenger side footwell -- is there anything that might still be keeping it from closing fully? Or another door I should check?

 

I've noticed that though the AC gets a bit warmer after driving a few minutes, it is still a colder than the outside -- but no matter what the top left vent blows warm. Under no circumstances does the top left vent blow cold. Maybe a sign?

 

Thanks guys!

[LobsterThief]

Had a bit of a eureka moment!

 

Dropped the bottom dash panel and saw that what I assume is the thermistor had broken from its glue and the temperature probe was measuring the cabin air rather than being seated in its hole.

 

Glued it back to the box, gonna wait a few hours for it to cure before giving the AC a shot. Will keep you guys posted.

 

Glue came off

 

You can see the hole where the probe is supposed to go

[danbyrambler]

Been meaning to reply & keep getting distracted/interupted/forgetfull - sorry - - Anyway you've probably found it - The thermister monitors evap temp-I goofed when I said cabin temp - - - You'll lose the charge changing the LPS maybe - Have your AC buddy look at your set up - - If your compressor has shut off valves then while system is running - - 1st - close high pressure valve - 2nd - after 2-3 min close suction/Low P valve - - All freon should now be in comp/condenser - - Now change LPS - Evac the lines/evap & then open comp valves - may/may not have to add freon - Your AC pall will understand - Really not difficult when U go at it & U seem to be doing pretty good with the troubleshooting.

[LobsterThief]

Been meaning to reply & keep getting distracted/interupted/forgetfull - sorry - - Anyway you've probably found it - The thermister monitors evap temp-I goofed when I said cabin temp - - - You'll lose the charge changing the LPS maybe - Have your AC buddy look at your set up - - If your compressor has shut off valves then while system is running - - 1st - close high pressure valve - 2nd - after 2-3 min close suction/Low P valve - - All freon should now be in comp/condenser - - Now change LPS - Evac the lines/evap & then open comp valves - may/may not have to add freon - Your AC pall will understand - Really not difficult when U go at it & U seem to be doing pretty good with the troubleshooting.

 

Definitely seems to be working properly now :) I'll keep an eye on it and use your troubleshooting tips if it becomes a problem again. You never know with these 20+ year old trucks.

 

There is still a very tiny bit of water dripping onto the passenger floor from directly under the dash -- and I know the drain has been cleaned out so that's about all that's left to check.