91 jeep grand cherokee AC?

[barbee6043]

i'm not an ac guy, maybe some of you guys can advise. 91 jeep grand Cherokee, Her vehicle! it is fully charged, the compressor is not kicking on. I took it to good local mechanic, he said the problem is in the "computer", he says it will have to go to dealership.
so I call the dealership, and talk to service mgr. he said in 20 years he has only seen the "computer", ( he called it some else) be the problem. if so, they are EXPENSIVE! ( no doubt!)..... they want $89 to charge it and diagnose it. I hate the idea of taking it to the dealership! you know their rates and their track record for costs! should I try another local mechanic? any input will be appreciated!

 

[340dartley]

89 dollars sounds cheep . My shop charges 135 an hour and it pays 1.4 hours out of the Chilton guide. So if you brought it to us it would me minue priced at 180 plus refrigerant. Also that is probably R12 so that is even more expensive. I doubt it is the computer. It should be fairly simple to diagnose for any decent tech. Find a shop that you trust and take it to them.

 

[RustyRatRod]

Do you have a set of AC gauges? A 91 should still be R12 unless it has been converted.

 

[magnummopar]

Ok, I will try to keep it short and simple. The way the AC works on a magnum,which you may or may not have, is that the PCM(computer) controls the AC compressor clutch relay by grounding one side of the relays coil. The other side of coil will have 12v start/run. The terminal marked 30 is 12 Batt + and terminal 87 is the dark blue with yellow stripe that powers the AC compressor clutch. The PCM wont ground the relay UNLESS there is continuity through the high and low pressure switches circuit. There will be two "sensors" in the AC lines, one on larger line and one on the smaller one. You can bypass each sensor with a paper clip. The sensors are just a switch that is closed with pressure allowing continuity through it. Only when both are closed will pcm look at AC request(dash switch) and apply ground to relay, turning on compressor clutch.
Did tech say if the system was charged? Was he manually able to get compressor to run? While it may be true that pcm was not turning compressor on, it is quite possible and more likely that the pcm didnt receive the proper command to turn relay on. Low pressure will have light green with white stripe and a light green wire, while the high pressure will have dark blue and a brown wire on it.
This is off the top of my head, not out of the book but if its a magnum(multi port) then the colors should be accurate. The only difference between R12 and R134 will be 7-10 degrees on a super hot day. This info should give you enough to diagnose if its the pcm or not. Important note....dont jumper relay or switches unless you know its charged with Freon or you will toast the compressor. There should also be a thermistor(temp probe) in evaporator that shuts down compressor if it ices up during hard use, also not likely to fail.
If your 91 is indeed a pre magnum throttle body, it should still have very similar system, just not sure about wire colors if it is a 91. hope this helps.

 

[krazykuda]

Do you have a set of AC gauges? A 91 should still be R12 unless it has been converted.

92 was the first year of the Grand Cherokee and it was the first of the Jeeps to get R134A.

It's a clutch cycling orifice tube system. (No expansion valve...)

Unplug the switch on the accumulator and jump it with a paper clip to see if you can get the compressor to engage....

 

[barbee6043]

thanks for the the info. it is a straight 6. I will try to check it out. it was fully charged.\ with R134A.

my take on AC is: if its too hot to run down the road without air, its too hot to do anything, period....!!! ha so far its very been too hot for me to work ( outside). but htios is Missouri, not s. az!

 

[Ferdinandbites]

i'm not an ac guy, maybe some of you guys can advise. 91 jeep grand Cherokee, Her vehicle! it is fully charged, the compressor is not kicking on. I took it to good local mechanic, he said the problem is in the "computer", he says it will have to go to dealership.
so I call the dealership, and talk to service mgr. he said in 20 years he has only seen the "computer", ( he called it some else) be the problem. if so, they are EXPENSIVE! ( no doubt!)..... they want $89 to charge it and diagnose it. I hate the idea of taking it to the dealership! you know their rates and their track record for costs! should I try another local mechanic? any input will be appreciated!

Like 340 said go to someone you trust. I once worked at the dealership and we were good, honest, fair, 8 other technicians to collaborate with and had a wealth of resources at our disposal including technical helpline for those more challenging problems. No doubt the labor rate was high but our success rate was high too. With all due respect.
Like the idea of closing the circuit on the pressure switch. Should be a yes/no for triggering the system, compressor to work. Make sure there's power(or ground) at that connector. And I agree check the charge. A present charge means control side fault.
Have a child that attends high school? Give it a shot and have them go to the auto shop and bring up some wiring diagrams/ diagnostic procedures. Better than buying a specific manual for your vehicle then never touching it again.
Good luck with your adventure!