Why did my A/C blow air to my windsheild randomly? - JeepForum.com
 
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post #1 of 13 Old 07-07-2014, 10:59 PM Thread Starter
ChopperX
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Why did my A/C blow air to my windsheild randomly?

Hey guys. So I took my 97 ZJ 4.0L out for a nice 7 hour road trip. On my way home my A/C started to blow air through windshield defrosting vents and not the front vents. It did this for about a minute and then it went back to the front vents for the remaining hours of the trip. I was driving up a decent hill in the desert having cruise control set to 75mph. Did the A/C do this automatically to reduce heat in the engine bay? My temp gauge was reading normal and wasn't moving up. Should I be worried? Figured to ask you all first to prevent something that night happen. Thanks!

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post #2 of 13 Old 07-08-2014, 12:00 AM
ElCaminoManT
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did the transmission downshift or was it in overdrive for the climb up the hill? if it stayed in overdrive, sounds like when the engine is under heavy load, there is almost no vacuum to be had from the intake manifold. since the blend doors in the HVAC box are vacuum operated, they default to the defrost position until enough vacuum is restored as the load decreases on the engine and the blend doors return to the desired positions.

would be a good idea to double check the vacuum hoses/tubes/lines that go from the intake manifold to the firewall where the line goes thru with the heater hoses for any cracking or possible leaks just in case.

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post #3 of 13 Old 07-08-2014, 02:10 AM
kg6mov
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Vacuum leak is very common, under heavy load the vacuum reservoir can't keep up and the climate controls will flip to the defrost.


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post #4 of 13 Old 07-08-2014, 04:37 AM
elijahpr
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Mine does this when I "have" to slam on the gas sometimes. Another forum member suggested my exhaust was clogged, or a faulty accessory switch.

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post #5 of 13 Old 07-08-2014, 10:48 AM
Oldfrog
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needs a check valve at the firewall vacuum line.

TSB....dont know the number.

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post #6 of 13 Old 07-08-2014, 11:16 AM
xcaliber81
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Quote:
Originally Posted by Oldfrog View Post
needs a check valve at the firewall vacuum line.

TSB....dont know the number.
I have the same problem, nose up or down it changes from vent to defrost for a few seconds and changes back. I was told a check valve on the vacuum needs replaced.
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post #7 of 13 Old 07-08-2014, 11:56 AM
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Quote:
Originally Posted by Oldfrog View Post
needs a check valve at the firewall vacuum line.

TSB....dont know the number.
Quote:
Originally Posted by xcaliber81 View Post
I have the same problem, nose up or down it changes from vent to defrost for a few seconds and changes back. I was told a check valve on the vacuum needs replaced.
I think this is what is being discussed:

NO: 24-01-96 Rev. A

SUBJECT: Heater A/C System Changes Mode To Defrost When Accelerating

DATE: Oct. 18, 1996

NOTE: THIS BULLETIN SUPERSEDES TECHNICAL SERVICE BULLETIN 24-01-96, DATED FEB. 2, 1996, WHICH SHOULD BE REMOVED FROM YOUR FILES. 1997 GRAND CHEROKEE WAS ADDED TO THE MODEL LIST AND A CLEARER DESCRIPTION OF CHECK VALVE LOCATIONS WERE ADDED. ALL REVISIONS ARE HIGHLIGHTED WITH **ASTERISKS**.

NOTE: THIS BULLETIN APPLIES TO VEHICLES EQUIPPED WITH MANUAL A/C

SYMPTOM/CONDITION:

The vacuum supply to the Heater A/C system may drop when accelerating or when speed control engages. This may cause the mode vacuum motor to switch to the defrost position.

DIAGNOSIS:

Use the procedure outlined in the appropriate Service Manual (Group 24) to verify that the Heater A/C system does not have any vacuum leaks. If no leaks are found, perform the Repair Procedure.


REPAIR PROCEDURE:

This bulletin involves installing an additional vacuum check valve.

**Locate the vacuum source line to the Heater A/C system in the engine compartment (Figure 1, Figure 2, and Figure 3).** Install the vacuum check valve in the Heater A/C system vacuum line. Make sure that the check valve is installed to allow vacuum to pass into the Heater A/C system, but block any vacuum drop when **accelerating.**

**NOTE: DO NOT INSTALL THE VACUUM CHECK VALVE IN THE VACUUM LINE GOING TO THE RESERVOIR OR THE SPEED CONTROL SERVO. INSTALLING THE VACUUM CHECK VALVE IN THE LINE GOING TO THE VACUUM RESERVOIR OR THE SPEED CONTROL SERVO WILL HAVE A NEGATIVE EFFECT ON THE SYMPTOM/CONDITION AND POSSIBLY CAUSE THE SPEED CONTROL SYSTEM TO DROP VACUUM WHILE ACCELERATING. **

POLICY:

Reimbursable within the provisions of the warranty.


NOTE: The part specified in the TSB was 04677204 - Valve.
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post #8 of 13 Old 07-08-2014, 11:30 PM Thread Starter
ChopperX
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Thanks for the replies everyone. According to what JeepCares posted, is that a recall?
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post #9 of 13 Old 07-08-2014, 11:38 PM
jnicewan
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Quote:
Originally Posted by ChopperX View Post
Thanks for the replies everyone. According to what JeepCares posted, is that a recall?
No, not a recall but a technical service bulletin. Bulletins are published by the manufacturer to aid technicians on information or repairs. Its just their way of acknowledging a known or common problem and letting techs know about it, this works much like we do here by sharing known information.
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post #10 of 13 Old 07-09-2014, 12:43 AM
Uniblurb
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I'd still check the vacuum line from the vacuum reservoir under the battery tray/box going along the fender, the T line going into the firewall, and the intake vacuum hose connection. The hose going to the underside of the vacuum reservoir is known to come loose/rot.

Granted it could just be the cruise servo taking the vacuum away from the switch as the TSB spells out the need for a check valve. Personally I don't like using cruise to go up fairly steep hills/inclines since the trans will kick down a gear too much causing loss in mpg. Seems like I can keep my speed up w/o using the kickdown by manually controlling with the gas pedal. But sometimes there is a need for speed.

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post #11 of 13 Old 07-10-2014, 03:09 PM Thread Starter
ChopperX
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Well I usually turn off cruise control if the car downshifts using more mpg. Then I take control of the throttle to get a much mpg as I can. I will say that I got 437miles in range on one tank using cruise control about 75% of the time. When the AC went to the default vents the car was on cruise control but I don't remember if it downshifted since I was going 75 and I think it had enough speed to where it didn't need to downshift. The problem hasn't happened since then. Just that one time.
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post #12 of 13 Old 07-10-2014, 03:12 PM Thread Starter
ChopperX
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Quote:
Originally Posted by ElCaminoManT View Post
did the transmission downshift or was it in overdrive for the climb up the hill? if it stayed in overdrive, sounds like when the engine is under heavy load, there is almost no vacuum to be had from the intake manifold. since the blend doors in the HVAC box are vacuum operated, they default to the defrost position until enough vacuum is restored as the load decreases on the engine and the blend doors return to the desired positions.

would be a good idea to double check the vacuum hoses/tubes/lines that go from the intake manifold to the firewall where the line goes thru with the heater hoses for any cracking or possible leaks just in case.
Well isn't overdrive on full time? Cause I have that button that has the overdrive symbol and then the word off. When I drive up big mountains I turn off overdrive to prevent the car from making so many gear shifts in a short amount of time. But to answer your question, yes overdrive was on the entire drive.
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post #13 of 13 Old 07-10-2014, 03:16 PM
kg6mov
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OD Off is really overdrive lockout.


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Quote:
Originally Posted by Texas ZJ1 View Post
I heard someone yell "mall crawler" from the back porch. I'll go see who said that. Be right back.
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