99 Cherokee HVAC Problem

However, on our trip we had the same problem but now were able to observe that the ventilation shuts down under conditions of high manifold pressure (e.g., going up hill) but the ventilation was restored on level or downhill grades. Rapid acceleration on level ground also caused the ventilation to shutdown.

This behavior was consistent regardless of whether the air conditioner was selected or just simple fresh air ventilation. We did not check whether it occurred when the selector was positioned for defrost.

A possibly related problem is occurring with the "factory installed" cruise control. It used to be quite solid, keeping the speed within ± one mph--now it has a good five to ten mph slop and can take tens of seconds to resume the set speed.

Fuel consumption has remained constant, we continue to get 20 mpg, in-town and on the highway with or without the trailer. I have noticed that the engine seems to have some hesitation and not quite the same acceleration lately but it is approaching 90,000 and is time for service.

Any help would be appreciated.

Janie

My wife and I have twin 1999 Jeep Grand Cherokees. Mine has no problems, as you describe, but her Jeep has the same problem. Since they are both over our 60,000 mile warranty, I found a cheap (meaning no money) fix. I put her control (on the right hand side) to the upper vents, for air conditioning (in the summer) and leave it there, never turning it to the off position. It takes maybe 10-20 minutes for the baffles to "set" but once they do, there they are. When heating season comes, change to the down (or up and down) position and it will take another 10-20 minutes to "set' but there you are. Never put it in the "off" position.

I don't know if the Grand Cherokee system is the same as the Cherokee, and I don't know much, but that's what know, and hope it can help.

Your vacuum reservoir is collapsed, or there is a vacuum leak.

Earle

However, on our trip we had the same problem but now were able to observe that the ventilation shuts down under conditions of high manifold pressure (e.g., going up hill) but the ventilation was restored on level or downhill grades. Rapid acceleration on level ground also caused the ventilation to shutdown.

This behavior was consistent regardless of whether the air conditioner was selected or just simple fresh air ventilation. We did not check whether it occurred when the selector was positioned for defrost.

A possibly related problem is occurring with the "factory installed" cruise control. It used to be quite solid, keeping the speed within ± one mph--now it has a good five to ten mph slop and can take tens of seconds to resume the set speed.

Fuel consumption has remained constant, we continue to get 20 mpg, in-town and on the highway with or without the trailer. I have noticed that the engine seems to have some hesitation and not quite the same acceleration lately but it is approaching 90,000 and is time for service.

Any help would be appreciated.

Janie

Hmm, this sounds the most plausible. I'll bring it to the attention of the mechanic so he can look further.

Many thanks to the others who responded, too.

Janie

Earle Hort> Your vacuum reservoir is collapsed, or there is a vacuum leak.

My husband looked and couldn't see a vacuum reservoir. Looks like we'll have to take it in and have somebody take a closer look. Thanks!

Janie

Howard wrote:

Thanks, Bill. It wasn't overheating. That's the neat thing about my Jeep - it pulls my little tent trailer like there's nothing to it, and no loss of gas mileage either.

Janie

L.W.(Bill) Hughes III wrote:

Or the vacuum check valve has failed... the symptom indicate that under conditions of low manifold vacuum (uphill, accelerating) there is insufficient vacuum to keep the vacuum motors in their selected position. The system was engineered to default to Defrost since that is a safety condition. Lots of Chrysler products had this problem back when they ran vacuum systems on their HVAC - ask a Dakota driver with the 4 cylinder - they were famous for this problem.

ZJ, WJ... etc. switched to electrical actuators which have different failure modes and don't default to defrost.

reboot

I'll second that, thanks for letting us know how it turned out.

Jeff DeWitt

Howard wrote: