A/C not working properly

Tiny
EDWOOSR667
  • MEMBER
  • 1988 JEEP LAREDO
  • 4.0L
  • 6 CYL
  • 2WD
  • AUTOMATIC
  • 19,999 MILES
Hello when I turn the A/C selector switch on I hear the blend door directing air to the vents. The A/C was working very good then one day it started blowing hot air, it still had good air flow coming through the vents, looked under the hood, the low side line was freezing up, so I changed the low side pressure switch and it started cooling again and no freezing up. Then after a few days I turned on the A/C it started cooling for about 4 minutes, then it stopped cooling again, but it still has a good air flow through the vents. I don't hear the vent doors change their position to change the air flow. What can be the problem? (Thanks)

Vehicle listed above is a 1998 Grand Cherokee Laredo.
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Wednesday, October 13th, 2021 AT 3:29 PM

8 Replies

Tiny
STEVE W.
  • EXPERT
Being an 88 the first item to check would be the vacuum lines that operate the HVAC doors. There is a single line that comes off of the intake manifold then to the vacuum reservoir, it comes out of the reservoir to the control head. Inside the control head are a bunch of ports that direct the vacuum to the different vacuum motors that move the doors. If any of the lines fails, which is common as they are rubber tubing that doesn't age gracefully, you end up with erratic operation. Your description sounds like the issue might be under the hood as the vacuum line is easy to knock loose if you breath on it wrong. The underhood lines commonly get rotted and when you touch them they feel like goo and fall apart. The lines under the dash can be a pain to work on but if you pull the trim and then the controller you can get to the block on the back that the multi-connector lines run to. From that you can use a hand vacuum pump to test each of the ports to find the problem.
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Wednesday, October 13th, 2021 AT 6:14 PM
Tiny
EDWOOSR667
  • MEMBER
Thanks for responding to my question, in your answer you said (Being an 88) this is a 1998.
Jeep Grand Cherokee Laredo 10 years older, I don't know if this makes a deference or not I will check my vacuum lines. Thanks a lot.
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Wednesday, October 13th, 2021 AT 8:37 PM
Tiny
STEVE W.
  • EXPERT
Hmm, two different years listed. Doesn't make a lot of difference though, the 98 still uses the same vacuum controls for its heater. Jeep didn't change much over those years the Grand used a lot of the same parts as the lower Cherokee and shared them up until 1999 when the new version came out. These are the 98 vacuum lines; the connector is a different shape, but it still uses vacuum for all the doors. One change though, they relocated the vacuum reservoir to the heater box behind the dash instead of in the fender.
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Wednesday, October 13th, 2021 AT 11:18 PM
Tiny
EDWOOSR667
  • MEMBER
What are the correct size of the small A/C vacuum lines?
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Thursday, October 14th, 2021 AT 1:48 PM
Tiny
STEVE W.
  • EXPERT
The small ones behind the dash are either 1/8" inch nylon with rubber couplers or 3/16" rubber. I generally have rolls of the stuff on the rack and just match it up as needed but those are the common control sizes. Another area that fails is the internal seal in the control head itself. They wear and leak, usually though you would know if that was the case as the system will work in one mode then not in another, or it will work fine at idle and then slowly fail as the rpms go up. Not working in any mode is commonly the main vacuum line or the reservoir.
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Thursday, October 14th, 2021 AT 3:32 PM
Tiny
EDWOOSR667
  • MEMBER
Thanks again for the info. I'm going to try some auto stores around here and buy some.
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Thursday, October 14th, 2021 AT 4:07 PM
Tiny
STEVE W.
  • EXPERT
That year is around the time they started switching to the nylon stuff with fittings. If it doesn't have it, I would convert it, the plastic seems to hold up better than all rubber. Most parts stores will have a rack of the various fittings and line. Just drive it there and match things up.
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Thursday, October 14th, 2021 AT 8:33 PM
Tiny
EDWOOSR667
  • MEMBER
Okay, thanks again.
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Thursday, October 14th, 2021 AT 9:05 PM

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