I recently had to replace the low pressure cycling switch on my 85. When I did I got the surprise that there was no schrader valve. So I lost some Freon doing the swap. The A/C is now cycling when the RPMs go above idle. By the way, the system was converted to 134a.

I asked a local shop if they could add Freon. The answer I got is that they have to evacuate the system, measure the amount of Freon in the system and then refill with the correct amount. The price tag will likely be $300. Does this sound like BS? I thought they could add until the low pressure side holds at 28-30psi at 2000 RPM.

 

yes and no its best to know a measured amount for optimum performance, but lets be realistic go to your local parts store get a good recharge kit (has a gauge)follow the instructions don't over fill and put what's left in one of your other cars of a friends.

if you drained and opened the system I would recommend a draw down but it sounds like you just swapped the switch real fast.

 

I always try to duplicate driving conditions while I add refrigerant. After a short drive(engine hot), put a fan in front of car, to push air through front of car. Add just enough to get compressor to stop cycling. Check pressure on gauge. If gauge is el cheapo do not add much more after compressor stops cycling. Drive for a while, then recheck.

 

It seems like a lot of money for an evacuate ant refill. However, I am wondering if the repair shop was uncomfortable with the condition of the A/C system. You say it has been converted to R134. Do we know what components were replaced when the conversion was done? Many folks believe you just change the freon and you are done. It may work for a while but typically things begin to occur. First; was the system evacuated before adding R134a? Was the receiver/dryer replaced at the time? If the system was not charged before adding the R134A then there was air in the system which contains moisture. The receiver dryer is there to remove moisture but has a limited capacity so the receiver dryer may be saturated and non effective for removing moisture. Moisture in the system causes ice to form blocking passages and rendering the A/C system unable to cool properly. Also, the R12 oil is different than the oil used in R134a systems. They are not compatible. Another factor is the R134a is corrosive to certain system components where R12 is not. This maybe the reason you had to change the pressure switch. If it is a fair repair shop and in the process of evacuating your A/C system he is going to change the receiver/dryer abd refill with the correct amount of R134a then maybe the cost is not to far off. Systems using R134a refrigerant typically require 10% less R134a than factory specs for R12. Your repair shop may realize your system has been converted and he may find by just "topping off" the refrigerant would not improve your systems operational performance. So he may figure he has to check much more than you think. I know this is a little long winded but I thought I would share some of what I have learned about auto A/C systems so you don't get a fast recharge and still encounter performance issues.

Sledge_78

 

The entire A/C system was replaced 3 years ago. I replaced the condenser, evaporator, compressor, hoses, and dryer. The system was professionally evacuated and charged with the correct amount of oil. The system has worked fine since then until the switch started failing. if it hadn't been for the missing shraeder valve, it would still be working.

At this point, I think I'll attempt a recharge using a can. I suspect it won't take much refrigerant to get it back to the correct pressure.

 

Did you get the conversion done while you owned the car. If you did, did a shop do it?
Or did you but the car that way.

But what I am confused about is, if there is no Schrader valve fitting, then how is the shop going to work on the system. A price of $300 is normally way too much to vacuum it down and add Freon. But perhaps they are going to perform work on the system and add a Schrader valve.

Also, under normal circumstances you should be able to remove the low pressure switch without losing and Freon due to a Schrader valve in the pipe. Could it be missing or stuck. If you removed the switch and Freon came out real fast, that sounds like a problem

 

Sledge I can agree with what you said, and procedurally it is the most correct. but if the conversion was done correctly and system was working correctly other than the switch failure I tend to believe its just a little overkill, and yes there is risk of the unknown a/c short coming but way the risks with evidence on hand take it from there. as for the valve when I was working at the dealer I replaced a few pressure switches and had a few leaky Schrader valves face it the switch has a pin that holds down the valve 20 years of that it could be a little sticky and have some crud on its seat

 

Drive to St Louis, and I will do it for free.

 

There is normally a Schrader valve where the low pressure switch is installed. This allows the switch to be replaced without losing Freon. In this case the valve wasn't there. So I lost Freon while swapping the switch. This has nothing to do with a shop's ability to evacuate and charge the system. And no they were not thinking of adding a valve.

My question was whether Freon can just be added until the low pressure side of the system stays in its range. If this is not possible, then why do auto parts stores sell cans of Freon with gauges. I think the shop opted for the quick and expensive answer.

 

exactly and to the point, but do keep and eye on things for a while

 

*EDIT*

I believe I have miss-read or misunderstood some of the thread. Let me just say at this point there is no reason you can't add Freon to the system. Do you have any idea if you lost any substantial amount of oil.

 

I had to lose some oil. But the cans of Freon have oil in them.

 

If all the procedure you mentioned was done correctly and there is still refrigerant in the system you can go to an auto parts store, [ ie auto zone or advanced ] Buy the recharge kit add the refrigerant slowly till you feel the suction line chill down to about 50 degrees and you will be real close to a correct charge .