Continuous Compressor Cycle

[indoorcomfrtsl]

I am working on a Snyder General Unit. M/N VSC020B. The Compressor sits in the air handler. There is 2 compressors.1st stage runs normal until the call for cooling is satisfied. It then closes the liq. line solenoid valve sending the system into pump down. Once the suction reaches 20# the 24v to the contactor opens shutting off the compressor. Within 10 secs the pressure starts to rise and the compressor cycles on and off rapidly. I disconnected one lead from the low pressure switch and started the second stage compressor. It ran and upon removing the call for 2nd stage cooling it proceeded to shut down. Is it possible that I have a sticking liq line solenoid?
Once the Liq line solenoid closes shouldn't my suction pressure hold at 20# .
Am I missing something obvious? lol
thanks

[AHRteechur]

What kind of compressor is it? If it's a recip, you may have bad valves or head gasket is cracked allowing high side to leak back into suction on shut down. Try closing off suction service valve, if available, and pump down #1 to see if it holds to verify if compressor or system is at fault.

[indoorcomfrtsl]

It is a Scroll compressor. It runs within the rec. amp draw,isn't very noisy until shut down. Come to think of it, I think the compressor is 6 months old.

[AHRteechur]

Just did a Google on this. Check out this link about pump down and scrolls.
http://www.hvacrinfo.com/scroll_trouble.htm

[indoorcomfrtsl]

I read the info. thanks.I am wondering if they put in a scroll if i need a valve to hold the pull down.I do seem to remember that you can't pump down a scroll. But...if the liq line solenoid valve was remaining closed wouldn't it stay pumped down until it became energized?

[AHRteechur]

Yes, the liquid line solenoid is probably OK. A scroll compressor will not hold back the high side from the low side on shut down like a recip will. That is why you need a check valve in the discharge line. Try this - close off the suction service valve after it pumps down. If the pressure in the compressor builds up much faster, then the check valve is bad. If the pressure doesn't build up, then something else is going on.

[just_opinion]

It is best that you have a company out to install you a "non-recycling pump down control circuit". It is the cheapest and most efective way.

Adding any other check valve or to replace the compressor because of its bad (worn)check valve will lead you to the same road as you have today.

[indoorcomfrtsl]

My boss told me the same thing. I don't think the liq line solenoid is bad,so we will install a nonrecycling pump down relay. Will let ya all know how it turns out.
thanks