A/C won't work....how do you check EPR valve........

[bandit67]

Guys, trying to get my stock 72 ac going. Got the system cleaned, pulled a vacuum, and gassed. Old unit would not pull down. Expansion valve checked and it is open, not sure how much it should be open, but some.  Actually, suction side would increase slightly when clutch pulled in . Swapped to another used compressor and same thing. Was told reed valves prolly stuck or broken. Bought a remanufactored unit. They do not come with EPR valve , I assumed I needed to installed the valve in the new unit.  I could not blow air thru it. It appears to have an inlet, but no outlet. Cleaned outside and installed in new unit. Charged new unit and the same thing is happening ........suction side will not pull down and also has a small increase in pressure.  Any ideas as what I am NOT doing or checking?  I am thinking that if the expansion valve is stuck wide open , it should at least try to pull down on suction side. Also thinking that if EPR stuck close and some pumping by compressor , that would cause an increase in suction side , if that is where blockage is.............any thoughts or comments welcome.......John

[bandit67]

Ok, for those who will walk this way  later, this is what I have learned. After speaking with Ron at Classic Air in Tampa, they do sell replacement EPR valves for around 70 bucks.  You only need one in your systen if you are staying with R12 in a totally stock systen.  The EPR valve somehow prevents the cooling coil from freezing.  I f you are changing over to 134a as I am , they recommend , removing the EPR valve and using a temperature switch to cycle on and off the compressor.  They sell them for about 41 buck each.   He could not tell me  how to check an EPR valve or how to unstick one that is stuck.  If a stock compressor sits for a long time........how long , no one can tell me, chances are that if the unit will not pull down and the EPR valve is in there.........that is problem.  You can remove it, replace it with the temp switch and use ANY gas  made for the application, R134a, R 12 or the drop ins.  Ron did not recommend the drop ins.
         The EPR valve is inside the suction side of the compressor, about as big as a quarter and about two and a half inches long.  I used two screwdrivers to insert into the valve and twisted counter clock wise and it pulls right out. There are no threads that it is screwed into , just an o-ring seal.   Hope this helps..........J

[John_Kunkel]

The service manual shows how to test the EPR valve, it requires a third gauge that isn't commonly used when testing/servicing A/C systems.

Like Ron noted, Mopar is kind of unique in that they don't cycle the compressor clutch to control the evaporator temperature.

[bandit67]

John K, if the valve is stuck closed , you will never get to the  point of using the third gauge.  The compressor will not pump at all and suction gage will actually show a little increase when the clutch pulls in. I am looking for a way to test one out of the compressor. I gassed it and got it going a bit ago with the EPR valve removed, as I wait for the temp cycling switch.  I got some cooling  and began to  drip water but I was not any where near the cool I am after.  Don't think the switch is going to get me there.   I watched the suction pressure move down to around  30 and then back up to 38.  This was with the rpm being held at a set speed.  This tells me the expansion valve is working , but , I think I will need to buy one for 134a.  I will wait till the temp switch is installed and play with it somemore.

[John_Kunkel]

Related story:

I once inadvertantly replaced a compressor with one from a '73 Imperial that had Automatic Temperature Control, unknown to me, the ATC doesn't use the EPR valve so the temperature at the center outlet was reading in the high twenties and blowing icicles on a hot day.

Bottom line, without the EPR valve (or one that's stuck wide open) the evaporator temperature will drop dangerously low; lucky for me the ambient temperature was high so I didn't freeze the evaporator and possibly damage it.

If yours isn't blowing really cold with the EPR valve gone, the coils is frozen or there's some other problem.

[bandit67]

Good tip, thanks John.  That was the consensuses I got from Ron, if you run a unit without the valve, you will freeze the coil.  I have had coils to freeze before due to what I think was low gas. They would get really cold for a few moments and began to warm until most all flow was blocked. Cut off the compressor for a bit , let it thaw , and a little bit more cool till it froze again.  My temp never got more than a little bit cool.  But , in hoping for the best , maybe this temp switch cycling unit will fix  me up right.  I use to have to learn this type of stuff alone, but , thanks to this great forum, and all the good guys who share their wealth of knowledge, and experiences, much time , effort and money is saved.  Many people  have helped me much , and try to help anyone , anytime , in anyway I can , who suffers with this same addiction and passion we have for our cars.  The quest for knowledge never stops.........to be continued........