99 Cherokee HVAC Problem
 

A recent 300 mile camping trip pulling a small tent trailer afforded an
opportunity to better characterize the problems with the HVAC of our
1999 Jeep Cherokee. We'd taken it in the week before our trip with a
complaint that the ventilation would randomly shut off the vents
although the fan continued to run. Our repair shop found a technical
service bulletin that was relevant, diagnosed a failing panel vacuum
actuator, and replaced the unit.

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

99 Cherokee HVAC Problem
 

Hi Janie,
All manufactures design their computers to shut the air conditioner
off if the engine is in danger of over heating. Maybe, add an auxiliary
transmission cooler if you're going to haul a trailer again.
God Bless America, Bill O|||||||O
http://www.billhughes.com/

JB wrote:
>
> A recent 300 mile camping trip pulling a small tent trailer afforded an
> opportunity to better characterize the problems with the HVAC of our
> 1999 Jeep Cherokee. We'd taken it in the week before our trip with a
> complaint that the ventilation would randomly shut off the vents
> although the fan continued to run. Our repair shop found a technical
> service bulletin that was relevant, diagnosed a failing panel vacuum
> actuator, and replaced the unit.
>
> 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

99 Cherokee HVAC Problem
 

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

Earle

"JB" > wrote in message
ups.com...
A recent 300 mile camping trip pulling a small tent trailer afforded an
opportunity to better characterize the problems with the HVAC of our
1999 Jeep Cherokee. We'd taken it in the week before our trip with a
complaint that the ventilation would randomly shut off the vents
although the fan continued to run. Our repair shop found a technical
service bulletin that was relevant, diagnosed a failing panel vacuum
actuator, and replaced the unit.

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

99 Cherokee HVAC Problem
 

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 Horton wrote:
> Your vacuum reservoir is collapsed, or there is a vacuum leak.

99 Cherokee HVAC Problem
 

The older Jeep Cherokees (88-92) had a vacuum reservoir inside the
right front bumper. Two vacuum lines went from there to the HVAC and
other things like the speed control. The lines went under/behind the
battery. Over time the line wore out (acid ate through) and the Jeep
had both the HVAC and speed control problem. Look at the vacuum lines
then see if there is still a vacuum reservoir.

JB wrote:
> 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 Horton wrote:
> > Your vacuum reservoir is collapsed, or there is a vacuum leak.

99 Cherokee HVAC Problem
 

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:
> The older Jeep Cherokees (88-92) had a vacuum reservoir inside the
> right front bumper. Two vacuum lines went from there to the HVAC and
> other things like the speed control. The lines went under/behind the
> battery. Over time the line wore out (acid ate through) and the Jeep
> had both the HVAC and speed control problem. Look at the vacuum lines
> then see if there is still a vacuum reservoir.

99 Cherokee HVAC Problem
 

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. <g>

Janie

L.W.(Bill) Hughes III wrote:
> Hi Janie,
> All manufactures design their computers to shut the air conditioner
> off if the engine is in danger of over heating. Maybe, add an auxiliary
> transmission cooler if you're going to haul a trailer again.
> God Bless America, Bill O|||||||O
> http://www.billhughes.com/

99 Cherokee HVAC Problem
 

Neither was a line of new cars stopped in traffic leaving Las
Vegas, but as far as I could see everyone had their windows down.
God Bless America, Bill O|||||||O
http://www.billhughes.com/

JB 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. <g>
>
> Janie

99 Cherokee HVAC Problem
 

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

On Mon, 24 Jul 2006 08:58:20 -0600, "Earle Horton"
> wrote:

>Your vacuum reservoir is collapsed, or there is a vacuum leak.
>
>Earle
>
>"JB" > wrote in message
oups.com...
>A recent 300 mile camping trip pulling a small tent trailer afforded an
>opportunity to better characterize the problems with the HVAC of our
>1999 Jeep Cherokee. We'd taken it in the week before our trip with a
>complaint that the ventilation would randomly shut off the vents
>although the fan continued to run. Our repair shop found a technical
>service bulletin that was relevant, diagnosed a failing panel vacuum
>actuator, and replaced the unit.
>
>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
>

99 Cherokee HVAC Problem
 

And the winner is.. Howard! Yup, battery acid on the vacuum lines. New
battery, new vacuum lines, new shocks. Rides great! Thanks!
Janie

Howard wrote:
> The older Jeep Cherokees (88-92) had a vacuum reservoir inside the
> right front bumper. Two vacuum lines went from there to the HVAC and
> other things like the speed control. The lines went under/behind the
> battery. Over time the line wore out (acid ate through) and the Jeep
> had both the HVAC and speed control problem. Look at the vacuum lines
> then see if there is still a vacuum reservoir.