Need help with the curse of the 91 A/C

[Guest waxbgone]

Here it is warm weather in the North, and the bogus "service A/C" light on my 1991 coupe has begun to appear again. The car has the original R12 refrigerant. Reading former posts, it seems that this is a common occurrance in the 1991 A/C system. It is (I think) being turned on by the low refrigerant pressure switch. This switch is supposed to be looking for a minimum of 10 psi + or - 2psi, per the manual. The gauges show my pressure to be 22 psi to 35 psi (with the compressor cycling). That means that the system charge should be OK, doesn't it?

When the outside temperature is between about 55 and 75 degrees, the light comes on intermittently. It usually shuts itself off after 4 or 5 minutes, but sometimes sets itself permanently.

In another thread, someone suggested putting a 55k ohm resistor in series with the "low side thermistor" to solve this problem when caused by changing to 134a refrigerant. I assume the thermistor is what the manual calls the "Low-side Refrigerant temperature sensor.

1. Is this an appropriate fix for the problem of the 1991 system?

2. Are there any other fixes?

3. How can I reset the error code when it blocks the function of the compressor from the IP? I can't seem to follow the steps in the manual, so I just disconnect the battery, but I am tired of resetting radio stations.

[JOHN07]

If I still had r12 in my car and had this problem and if the car cooled ok then I would add a 1/4lb-1/2lb to fix this problem. I would guess that if you drove your car and watched the pressures they would fall below the pressure to set the low refrig code. You can see you temps while driving by looking at your a/c control panel in service mode. I have only added resistors to cars after they have been converted to 134a to fix this problem. Also the resistors are in parallel and not in series with the low side thermistor.

Hope this helps, JohnT

[Guest kennyw]

How about my 88 reatta. Had to put r12 can in hot water to get it empty. I would think system is full. Still get the low ref. message, and of corse the compressor is off> can this be over riden with the touch screen? I think that if I got the compressor to run for a while it might just work o.k. ken

[Shelby]

Make sure you check the Evaporator to see if it is cover with crud and lint.

This will insulate the cold air and block the air flow.

[Barney Eaton]

I feel for you......my '91 started the cooling season by sometimes working and most of the time not working. No warning lights, just a problem with the compressor kicking in. I worked through the diagnostics and the compressor clutch was ok, Freon ok, all sensors were within spec, so why did it not work.

The final test was to replace the ECM (I had already unplugged and replugged every connection).......the ECM fixed the problem.

You can get the 1990 service manual on line (a/c function is no different between 90-91) The a/c diagnostic are in section 1C. The schematics are in section 8D

[harry yarnell]

Hmm, parallel resistance across the lowside sensor for 134 conversion? Is this value of 55k correct?

[harry yarnell]

Yes, Kenny, you CAN restart the compressor from the touchscreen. Just clear codes, and you're all set.

[Guest Greg Ross]

My '88 has been converted now for about 5 years. What I see typically is in that temperature range of 65 to 72 deg. F I get the low refrigerant warning.

Solution is to switch the climate control to "Heat" phase and engage the A/C when the actual temperature dictates. Bit of a nuisence perhaps but I put up with it! While in Louisiana this was almost a daily event in Spring time. In the northern climbs there are only about 1/2 a dozen A/C days a Summer!

[Guest kennyw]

o.k. Harry, I will try to find the page that clears the codes. I am not good at this. Consentration and patiants are failing.....ken

[manikmekanik]

kennyw, goto the BCM, then page thru with "NO" till you get to "clear BCM Codes" ,then press "yes". this should clear your A/C codes.

During off-season, or if problems appear, I switch the climate control to "ECON", this selection does'nt activiate the compressor to control temp.