Hi all,

New member and new owner of a 98 Cherokee Sport with auto trans. I love the thing so far and am sorting thru some fairly simple repairs and maintenance.

One thing has me perplexed and I did not find another thread about it on this forum when I searched. When the car is idling or at speed, all lights dim momentarily when I have any mode selected on the AC/Heat switch. When it happens, the idle will drop 50 RPM and the voltmeter will fall .5 volts (RPM and volt estimates from the guages). This happens like clockwork every 12 seconds or so.

When I have the HVAC system in the "Off" position, the XJ does not exhibit these symptoms.

I opened the hood to observe the phenomenon and when it happens, the aux fan turns on, followed by the A/C compressor. the compressor runs for 2 seconds and then both the compressor and fan turn off. Then the phenomenon repeats.

This can't be standard operating procedure, can it? I have turned the switch to "Off" for fear of wearing out the compressor clutch or fan or any electrical switches. But it is getting pretty darn cold driving around in Maine without heat. I would assume that setting the switch to a non-A/C setting would not trigger that system and just let heat filter into the cabin.

Has anyone experienced this and have an idea what could be wrong?

Thank you in advance for any help,
Aherngang

 

If you have the controls set to defrost, the AC cycles as part of the defrost mode. If the controls are set to normal AC and the compressor cycles rapidly that is an indication that the system is low on freon charge. If you have the controls set to vent and the compressor is still cycling then you likely have a control panel issue.

 

most systems with climate control cycle compressor on heat as well as cool, not sure on a '98, but 2 secs seems a bit quick to cycle

mine is about every 10-20secs, depending on ambient (it may have leaked some out since charge)

a probe in the outlet vent will tell you something, if it wont cool in summer, the gas has leaked out

I think they have a low pressure shut-off switch

best bet is an aircon pro, they can measure and interpret in minutes, re-gas if needed

 

There is a maximum number of cycles per minute called out in the factory service manual that indicates normal operating procedure. I want to say its 6 or somewhere in that ballpark but I can't confidently remember. My XJ displays similar symptoms and I believe its because the freon charge is low due to a leak in the system.

 

Thanks for the ideas and suggestions on this. I am surprised that the A/C & fan continue to attempt to run even when the control is set to a setting w/o an A/C icon in it. In reading the owner's manual, it is very vague on whether the A/C compressor is on all the time or only when the selected setting has the A/C icon on it.

I am tempted to try Steve Hayes' suggestion and put a new controller in there. It seems logical that it would be sending the wrong messages to the other A/C components. Hate to resort to the "keep throwing new parts at it" approach....

I was hoping that I could maintain and fix most things. But HVAC is one of the things that i don't have the skills or the tools to tackle. Guess I will take it to a pro.

Aherngang

 

If the 6 cycles per minute is SoP, then mine would be normal since it is cycling about 5 times per minute. That makes me feel better.

And does your car electrics momentarily dim or take a hit each time the compressor cycles? It is kind of unnerving to me, but I can get used to it.

And is the A/C always on when you select a setting on the control that does not have an A/C icon with it? My A/C is cycling on even when I have selected airflow at feet/face/defrost-level without the A/C icon (on the right-hand side of the rotary selector).

Thank you again for the reply.

 

Could be normal.

An electric fan does draw significant power and the voltage will drop when it kicks on. Also, the compressor adds drag to the engine when it's on which would cause an RPM drop except for the fact that the computer should compensate and actually kick up the idle RPMs when the compressor is on.

If the freon charge is low or the pressure sensor in the system isn't working right, it could be working well enough to kick the system on but the drop in pressure on the return line would be enough to shut it down again. It would be a good idea to hook up some gauges on the ports and see what it's doing when it kicks on and off again.

 

From what I understand, the compressor should not come on if the setting is on either or floor or vent heat. Defrost will have the compressor cycle on and off.

Get some gauges hooked up to it. Most likely it is low on freon, you may have a leak somewhere.

The fact that it is dimming the lights when it kicks on means that the fan and the compressor are putting a draw on the system.

How old is the serpentine belt? Is the belt loose?

How old is the the alternator and battery?

Have you cleaned the grounds recently?

 

If I remember I'll look in my manual when I get home for the exact verbiage on standard operation. Like stated, the compressor will kick on when either AC settings or defrost settings are selected. It is not usual for the vehicle to show symptoms of voltage drop and increased engine load when the compressor kicks on.

 

FSM states compressor cycling 10 or more times in a minute is a sign of low refrigerant charge.

 

NO, not at all normal, sounds like something is stalling out the compressor, needs immediate attention, if your compressor detonates the whole system is contaminated internally with metal debis

 

The AC system needs to cycle even during the winter months to keep the oil on the components like compressor seals,O rings, etc...If it didn't cycle the oil would settle and you'd get a system leak from them drying out...That's why most vehicles cycle the compressor on defrost or a combo of defrost and another location...Perfectly normal..
You've got two things happening there..When the compressor kicks in at idle it will draw the RPMs down a bit which in turn will make the alternator output drop..Add the auxiliary fan engaging at the same time and it also draws the electrical system down...So there will be a noticeable drop in the system you're seeing there...Again, normal...

 

its possible it could be normal..or abnormal..RPM should increase when A/C compressor is engaged on an XJ, as a command is sent via PCM, (described in FSM)

If I was the OP, I would physically watch the compressor engage..afaic, there should be a slight jolt but no slowing of the belt (have a look at the harmonic balancer while you are at it)

If a big change is noticed on voltmeter, that possibly means the compressor electric clutch is faulty

 

I will get this into an HVAC specialist to hook up guages and tell what is happening and is wrong with the system. After reading all of your responses about the momentary dimming of the lights, I am less worried than I was about the possibility of it stalling out or dying out. The system charge may be low which will need to be rectified before Spring/Summer. But it looks/sounds like SoP except that the AC is kicking on even when the controls are placed in "Vent" mode only.

To answer your questions, I don't know the age of the serpentine belt. It has no evident cracking and the tension is correct. There is no noises from the belt or pulleys that are out of the ordinary.

The alternator is 2.5 years old and the battery is 4 years old. My mechanic tested both and said that they are good. He cleaned those connections, but I will clean the other grounds after reading up on them in this forum

Upon observing the operation of the compressor clutch, I do not think it is faulty. It operates much like my other cars with AC except that it it is cycling on more rapidly (indicating a system that needs charging) and it is "ON" even in vent mode. The total failure of the compressor is scary, however. If a pull out the relay for the AC compressor, that would protect me from compressor failure and also allow me to get heat through the normal VENT operation. It is Winter and very cold up here in Maine, so heat is of the utmost importance right now.

Again, thank you all for your help. I am amazed by all the resources and information available on this forum. I will update this posting as new info emerges and a resolution is found.