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1991 Cadillac Deville R12 , Ecessive clutch cycling

bondgeo on Mon August 08, 2011 9:32 AM User is offline

Year: 1991
Make: Cadillac
Model: deville
Engine Size: 4.9
Refrigerant Type: r12

We have a problem with vent temps not cold enough (60 deg f). The compressor clutch cycles frequently - like every 10 seconds, even on startup on a 90 degree day. I replaced the accumulator, compressor, low temperature sensor thermister, and expansion device ( it's a pancake type filter / orifice). The cycling is controlled by the car's body computer using the low pressure temperature sensor thermister. According to the diagnostics available on this Cadillac, it cycles off at - 2 degrees centigrade and back on at 10 degrees centigrade (which is how it should be according to the shop manual. The system was evacuated properly and held well , so no leaks are present. System had r12 charge weighed in to spec. When gages are in place and system turns off, both high and low side equalize, so there shouldn't be any clogs in the evap or condensor. Does anyone have any ideas? I am baffled!

iceman2555 on Mon August 08, 2011 9:41 AM User is offlineView users profile

No operational pressures were posted....a best guess would be the system is undercharged. Some of the older r12 type recharge machines did not allow for the refrigeant necessary to pre charge the serivice hose assembly (app 4 oz) or if a standard manifold/hose assembly was uses with a metered scale, the sytem could be undercharged by the same amount.
What type metering system was utilized for this service?

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The strongest reason for the people to retain the right to keep and bear arms is, as a last resort, to protect themselves against tyranny in government.
Thomas Jefferson

bondgeo on Mon August 08, 2011 9:52 AM User is offline

The compressor cycles off at approx 25psi and back on at 50psi. When it cycles off the high side is approx 180psi. Outside air at 85 degF. Charge was weighed in using Tiff refrigeration digital scale.

GM Tech on Mon August 08, 2011 10:32 AM User is offline

Where did you get the idea the expansion tube is a pancake device?-----the OEM is not-- it is in evap inlet----do you have an aftermarket OT device installed? If so, there-in lies your issue....why not go with the original OEM designed device.....if all you are checking is the in-line OT, then the OEM one is probably still in there and plugged up,,,,or if you are only checking the aftermarket OT, then it is probably plugged up.....You only need one OT, and I suggest you use the OEM one.....

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The number one A/C diagnostic tool there is- is to know how much refrigerant is in the system- this can only be done by recovering and weighing the refrigerant!!
Just a thought.... 65% of A/C failures in my 3200 car diagnostic database (GM vehicles) are due to loss of refrigerant due to a leak......

bondgeo on Mon August 08, 2011 10:59 AM User is offline

Yes, the original was a regular orifice tube. However, awhile ago, the line developed a leak and the oem replacement line was revised to include the new " pancake style filter/orifice unit" It is the only one speced for this vehicle now. See picture attached:

http://www.genmotorinfo.com/Picture.aspx?ccId=43880319&ppId=275723060&ppInfo=1991++++++C%2cA%2fC+Refrigeration+System%2c-V8+4.9L+(4.9B)%2c+(L26)+-+6C09021

The orifice/filter is item #13.


It is all that GM now offers for this vehicle. When I replaced the line, the filter/orice came with it. The old style line(which takes the orifice tube) is no longer available from GM.




GM Tech on Mon August 08, 2011 11:06 AM User is offline

I still say you don't have enough flow through your filter-- even if it equalizes-- a cycling system with a full charge at 95 degf ambient- has a serious restriction- put a new filter in and see the difference-- the old one is most likely plugged up from original failure...

-------------------------
The number one A/C diagnostic tool there is- is to know how much refrigerant is in the system- this can only be done by recovering and weighing the refrigerant!!
Just a thought.... 65% of A/C failures in my 3200 car diagnostic database (GM vehicles) are due to loss of refrigerant due to a leak......

bondgeo on Mon August 08, 2011 11:31 AM User is offline

Thanks for the reply. I did change the filter/orifice. The only thing I can think of as a solution is adding more r12 than the accumulator label specifies due to the change from the tube OT to the "filter/orifice device". The filter looks like more refrigerant might be contained in it than when I just had the regular OT in the old evap line. Problem is, how much r12 to add?

bondgeo on Mon August 08, 2011 6:18 PM User is offline

GM Tech, what would I look for to determine if the evaporator or condensor had a blockage?

bondgeo on Tue August 09, 2011 2:01 PM User is offline

Just wanted to give everyone an update! Problem solved! It turned out to be a dirty / clogged up evaporator. I took out the blower module, looked in the hole and saw a blackish muck coating the blower side of the evaporator fins. With a shop vac and then Simple Green, followed by a clean water flush , the surface came out pretty clean. Upon reassembly, it not only blows better through the vents, but the compressor now cycles correctly and I'm getting 41F at the vents on an 86F degree day.

GM Tech on Tue August 09, 2011 3:47 PM User is offline

Sorry to burst your bubble but that blackish muck is the result of a leaky evap-- the refrigerant and oil have leached out onto the air inlet face of your evap and collected dirt and debris in the airflow path- by your cleaning the muck off you have great airflow but you still have the thousands of pinholes in the evap face- you will soon lose your charge and be right back where you started.

Acid rain kills evap that have not been heat aged properly-- late 80's early 90's car were known for this-- always on eveap air inlet side, always block air flow....I changed 20 or 30 in one year alone. W-cvar, C/H car- were especially bad......anything built after '94 was good---

-------------------------
The number one A/C diagnostic tool there is- is to know how much refrigerant is in the system- this can only be done by recovering and weighing the refrigerant!!
Just a thought.... 65% of A/C failures in my 3200 car diagnostic database (GM vehicles) are due to loss of refrigerant due to a leak......

bondgeo on Tue August 09, 2011 9:10 PM User is offline

Thanks for the tip about the evaporators on the early 90's cars. I hope it doesn't apply to mine as the Cadillac dealer did change my evap to a "2nd design" part around 1994 or so. I did use an electronic "sniffer" and didn't detect any leaks. I will , however, keep checking for problems with it in the future. Thanks again.

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