Driving home tonight with the defrost on, all of a sudden my 98 (sport, 5 speed, 4.0) starts cycling the aux fan and AC compressor. Like...every 5-10 seconds. Change from defrost and the issue persists.

Anyway, I gather that I need to recharge my AC system? It's had a pinhole leak for years that has meant no cool air during the summer. Can't find the darn leak. Anyway, can I bypass the low pressure switch?? Or can I just do an easy VatoZone recharge to keep it happy?

 

...no thoughts??

 

I'm not helping answer your question, but I have the same type of issue with my 97' sport.

For me, it turns on and off every 5 to 10 seconds when I'm stopped with the defrost on. Also makes the XJ idle kind of rough. The fan and compressor seem to turn off if I'm moving, and if I turn the defrost off they cut off.

Same thing happens in the summer if I have the AC on. Although it does push out some decently cold air.

I've been having a difficult time finding much info about it.

 

5 of the 7 vent positions engage the compressor.....defrog/defrost is 1 of the 5. Low refrigerant can cause compressor cycling. Compressor on turns on the efan.

 

Why in the world does defrost run the compressor?? AC systems are refrigeration cycles...not heat pumps.

Anyway. What two positions DON'T engage the compressor, as evidently I didn't find them the other night.

 

The AC compressor kicks on during defrost to pull the moisture off the windshield.

In the summer when you have your a/c on, do you ever notice the water that accumulates under your rig when stopped? That's the evaporator core getting condensation on it from the moisture in the air passing over it.

When in defrost, it will turn on to remove any condensation.

If it's kicking on and off that fast it probably is low on charge. I just charged mine at the shop I work at and it still cycles pretty fast. Mostly because of the low temperatures. If you don't want it on, you could just disconnect the connector on the low pressure switch, but it won't pull condensation during defrost.

 

Right, I understand that having the defrost on will kick the fan and compressor on, and obviously if the AC is on they'll definitely kick on, but should it really kick on that often? At stops mine will literally kick on and off ever 5-10 seconds. No exaggeration. This never happened on my 00' xj. Also, why would I get a slightly rough idle when it's on? Just doesn't seem right to me.

I guess I could get it charged and see what happens...? Is it better to do a DIY recharge from Autozone, or should I have a shop do it?

Really not trying to hi-jack this thread, just was surprised to actually find something on this and I have questions myself.

 

Page from the Owner's manual. Read about Window Fogging.



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Mine still cycles even when it's below 30F. It's been below 30F (as low as -5F) for like 18 of the last 20 days here in Michigan. Even though the compressor didn't cycle as often, it still cycled a good amount.

Again, still not sure why the compressor kicking on would cause a little bit of rough idle in my case.

 

The Idle Air Controller is supposed to compensate for loads on the engine, like the AC compressor.

Try cleaning the IAC pintle and its port in the TB.

 

Here's what the FSM says:

LOW PRESSURE CYCLING CLUTCH SWITCH

The low pressure cycling clutch switch is connected in series electrically with the high pressure cut-off switch, between ground and the Powertrain Control Module (PCM). The switch contacts open and close causing the PCM to turn the compressor clutch on and off. This regulates the refrigerant system pressure and controls evaporator temperature. Controlling the evaporator temperature prevents condensate water on the evaporator fins from freezing and obstructing air conditioning system air flow.

The low pressure cycling clutch switch contacts are open when the suction pressure is approximately 141 kPa (20.5 psi) or lower. The switch contacts will close when the suction pressure rises to approximately 234 to 262 kPa (34 to 38 psi) or above. Lower ambient temperatures, below approximately -1° C (30° F), will also cause the switch contacts to open. This is due to the pressure/temperature relationship of the refrigerant in the system.

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Your system may not be charged properly and causng the Evap inlet (suction) pressure to hover around 34 psi even when the outside air temp is below 30*F. Or the calibration of the Low Pressure Cycling Switch is off. You may want to change this switch if the system charge is correct.

 

I'll give that a try. I've been wanting to do that anyway, but it's been so damn cold here, my fingers will freeze before I can get a bolt off the TB.

Anyway, thanks for the help! I'll check that my AC is fully charged, and clean the IAC when it warms up a bit.

NM-XJ, sorry to jump on this one so much. Been trying to figure this out for a bit now.

 

Have your system pressure checked at an AC shop where they can check the high and low pressure.

 

All cars engage the compressor when defog/defrost is selected. Cold/dry (refrigerated) air removes the fog/condensation from inside the glass.

The 5 positions circled in yellow engage the a/c compressor.....the other 2 positions do not.

Again, this is the way it is on all cars with a/c.

U need to understand the difference between defog and defrost......defog is moisture on the inside of the glass, defrost is frozen moisture on the outside. The heat selector must also be used when defrosting the windshield.

 

TFitz, no worries about jumping on this thread. You're heading the direction I wanted to go anyway! About the rough idle - 2nd checking your IAC, but also remember that the compressor loads the engine, so with it kicking on and off so often I'd expect a "wandering" idle. I fluctuate about 50-100rpm with the current problem.

Where would I find the low pressure switch? If I disconnect it and problem solved, then that is a positive diagnosis of low charge, correct?

I've been suspecting a leak for awhile, as the AC hasn't worked for a couple years. Here in New Mexico, it's only towards mid July that you really wish you had cold air...