68 Vette Cycle Issues

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dntoearthman
Posts: 2
Joined: Tue Jun 30, 2020 12:21 pm

68 Vette Cycle Issues

Postby dntoearthman » Wed Jul 15, 2020 6:37 am

I installed a new Sanden compressor, parallel flow condenser, new evaporator and insulated heater box , hoses, drier, Expansion Valve, and a POA bypass valve with cycle switch.
All the vent controls and fresh air and valves have been rebuilt as well.
MY issue is it cycles to often and takes a long time for the compressor to come back on, my LS pressure is just to low I suspect for some reason?
I don't know if this means anything but when I turn the system off it takes 25 minutes for the system pressure to balance out.
I installed another new Expansion Valve thinking this was the problem, Another new cycle switch as well.
Could a faulty drier cause any of my issues?

Here are some readings:
Recharging the system after full vacuum:

23.3 OZ
83 deg Amb
LS = 10 PSI
HS = 165 PSI
Vent temp = 43 Deg

23.3 OZ
90 Deg Amb
LS = 12 PSI
HS = 180 PSI
Vent temp 42 Deg

39 OZ
90 Deg Amb
LS =16 PSI
HS = 200
Vent Temp 43 Deg

In Garage:
The only time the LOW side goes up is if the AMB temp Exceed 95 deg and or my electric fans are not on.

Electric Fan on with External Fan, raising the RPM up to 2000 RPM the LS pressure will drop below 8 PSI – this is with cycle switch bypassed and it will freeze up the LS lines.

I have the cycle switch set to go off at 15 PSI now because at 20 PSI it will not keep the compressor engaged, The car spends more time with the compressor cycled off than engaged, It takes several minutes for the LS pressure to go up enough for the cycle switch to re-Engauge the compressor.

Road test:
It cools relatively well 45- 50 deg vent temp but the compressor cycles way to much its off more than its on. The vent temp will get up to around 65+ Deg before the compressor re-engages.

Thanks for any advise.

Mike
dntoearthman
Posts: 2
Joined: Tue Jun 30, 2020 12:21 pm

Re: 68 Vette Cycle Issues

Postby dntoearthman » Thu Jul 16, 2020 10:32 am

I just found a NOS POA valve, once installed I'll post back. From what I have read I can not recommend anyone trying the POA bypass kit.

Thanks Mike
Al9
Posts: 242
Joined: Tue Mar 10, 2020 3:26 am
Location: Southern Europe

Re: 68 Vette Cycle Issues

Postby Al9 » Thu Jul 16, 2020 12:15 pm

A TXV and suction pressure controlled AC clutch cycling (or even compressor destroking) is a huge no no. They're like Tom & Jerry, especially with low heat loads. Unless the pressure setpoint is somehow kept outside of the TXV's control range (the pressure range in which the TXV is able to throttle refrigerant flow in order to control superheat to a specific value), that is. The proper evaporator temperature control system when the system features a TXV, a fixed displacement compressor and a cycling clutch is a thermistor placed downstream of the evaporator's air flow, that cycles the clutch off once the evaporator approaches the freezing point and allows for a certain hysteresis in order to minimize clutch cycling. Not a suction pressure cycling switch. In short. TXV based automotive systems feature no low side safety pressure switch for this reason. A (seriously) low refrigerant charge condition is only sensed on the high side or by other means (such as a temperature switch located on the compressor's body).

The TXV is by definition a variable dimension orifice. As patent US5251453 states in column 2, lines 14-21, there might be no apparent relationship between evaporator temperature and suction pressure when the expansion device is a TXV. Hence, suction pressure-based control systems (other than POA/EPR valves) and TXVs tend to fight with each other. This patent is likely referring to an ordinary TXV that will be always controlling superheat and hence tend to snap shut (i.e. hunt) once things get very cold- i might add that the only way of re-estabilishing that pressure-temperature relationship is making the TXV behave like a fixed dimension expansion device, that is, an ordinary orifice tube, near the control system's setpoint - that is, the TXV's orifice has to somehow keep a fixed diameter at low head loads for the TXV to not fight with a suction pressure-based control system, basically giving up superheat control at a certain point- but then that's a TXV that is specifically tailored to a variable displacement compressor MVAC system.
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JohnHere
Posts: 288
Joined: Sun May 13, 2018 10:20 am
Location: South Carolina Upstate - USA

Re: 68 Vette Cycle Issues

Postby JohnHere » Thu Jul 16, 2020 3:52 pm

To me, it sounds as though a couple of issues might be in play here. One is an incorrect charge (too little refrigerant), and the other is an incorrect TXV (when used with the POA eliminator).

When you install the NOS POA valve, check the under-hood decal for the original amount of R-12 it called for. My guess is that it was about 52 ounces. So an 80 percent charge of R-134a would be around 42 ounces, which is nowhere near the 23.3 ounces you charged originally and still more than the 39 ounces you put in after that. This assumes keeping the TXV you have in there now.
Dougflas
Posts: 357
Joined: Mon Apr 18, 2016 6:10 pm

Re: 68 Vette Cycle Issues

Postby Dougflas » Thu Jul 16, 2020 3:57 pm

Early Corvettes were a hard vehicle to keep cool. If I were you, I would keep it original and keep it R12.
Al9
Posts: 242
Joined: Tue Mar 10, 2020 3:26 am
Location: Southern Europe

Re: 68 Vette Cycle Issues

Postby Al9 » Fri Jul 17, 2020 9:30 am

If i remember correctly (please correct me otherwise), this system features an oil return line on the evaporator's bottom. If so, the original TXV is definitely of a type that is going to interfere with a cycling pressure switch...
Dougflas
Posts: 357
Joined: Mon Apr 18, 2016 6:10 pm

Re: 68 Vette Cycle Issues

Postby Dougflas » Fri Jul 17, 2020 10:21 am

If my failing memory is correct, the POA has an oil bleed line on it. There is a shrader valve in it that is SPECIAL. Do not use a normal replacement.

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