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AC Question Expansion Valve

LowBikeMike

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69 Roadrunner factory AC, converted to Sanden compressor, 134a and Expansion valve with sensing bulb, factory AC box in car, newer hoses. AC system works good and cools properly.

Reading all the recent AC threads I started looking at my system and noticed the sensing bulb for the expansion valve is clamped on the evaporator inlet line instead of the outlet (suction line). How would this impact system performance? If I put the sensing bulb on the correct line will my system work better?

I have not checked pressures. Assumed they are fine based on satisfactory performance of the system.
 
Clamping the TXV (thermostatic expansion valve) bulb to the inlet line would keep the expansion valve open all the time. This may work as a fixed restrictor (if properly sized) and when used with a cycling thermostat. The TXV bulb is normally mounted on the suction line and insulated from engine heat. You will need to re-check system high and low side pressures (and possibly adjust the refrigerant charge) if you change sensing location.

A couple of questions:
1. Is the expansion valve new and designed for R-134a?
2. Do you have a compressor clutch cycling thermostat? They are usually mounted on the firewall and it’s bulb is usually clamped on the suction line. This is designed to prevent icing up the evaporator by energizing or de-energizing the compressor clutch. Sometimes this is used in conjunction with the TXV when systems are converted to R-134a.
Ron
 
Last edited:
Clamping the TXV (thermostatic expansion valve) bulb to the inlet line would keep the expansion valve open all the time. This may work as a fixed restrictor (if properly sized) and when used with a cycling thermostat. The TXV bulb is normally mounted on the suction line and insulated from engine heat. You will need to re-check system high and low side pressures (and possibly adjust the refrigerant charge) if you change sensing location.

A couple of questions:
1. Is the expansion valve new and designed for R-134a?
2. Do you have a compressor clutch cycling thermostat? They are usually mounted on the firewall and it’s bulb is usually clamped on the suction line. This is designed to prevent icing up the evaporator by energizing or de-energizing the compressor clutch. Sometimes this is used in conjunction with the TXV when systems are converted to R-134a.
Ron

The system was plumbed this way when I purchased the vehicle a couple of years ago, so I can only assume the Expansion valve is correct. The TXV only has the sensing bulb. Upon inspection about a year ago I noticed there was no compressor cut out of any sort (either low or high pressure) in the system. I installed a cycling thermostat, but attached the sensor in the same spot as the sensing bulb. To my knowledge the cycling thermostat is not triggering compressor shut off, probably because it is installed on the wrong line. If I put the thermostatic switch sensor line in the freezer connected to an Ohmmeter for a few minutes it opens the circuit so I know the switch actually works.

Sounds like at a minimum I need to reposition my thermostatic switch sensor the suction line.
 
The system was plumbed this way when I purchased the vehicle a couple of years ago, so I can only assume the Expansion valve is correct. The TXV only has the sensing bulb. Upon inspection about a year ago I noticed there was no compressor cut out of any sort (either low or high pressure) in the system. I installed a cycling thermostat, but attached the sensor in the same spot as the sensing bulb. To my knowledge the cycling thermostat is not triggering compressor shut off, probably because it is installed on the wrong line. If I put the thermostatic switch sensor line in the freezer connected to an Ohmmeter for a few minutes it opens the circuit so I know the switch actually works.

Sounds like at a minimum I need to reposition my thermostatic switch sensor the suction line.
IMO.....
The TXV sensing element needs to be in contact with evaporator's OUTLET CONNECTION and insulated to prevent ambient temperature influence. The TXV controls the amount of SUPER HEAT that is added by modulating the liquid refrigerant entering the Evaporator. Putting it on the liquid line to the evaporator controls nothing except flooding the Evaporator with refrigerant. The same is true if you are sensing evaporator OUTLET temperature to control the compressor clutch. The person that converted the system to R-134A and put the TXV sensing element on the liquid line was totally wrong.
BOB RENTON
 
Thanks for the advice. I'll make the adjustments and see what happens. The suction line has a very short pipe section but I think I can get the probes on it.
 
Thanks for the advice. I'll make the adjustments and see what happens. The suction line has a very short pipe section but I think I can get the probes on it.
Perhaps consider using "ty-wraps" to secure sensing element to the suction line, as close as possible to the line as it exits the evaporator. Then wrap the sensing element with material called "ARMAFLEX", a self adhering, easily conformable, comes in a 2" wide roll, at any Home Depot or A/C equipment supply house.

ArmaFlex Black LapSeal Tape​

Flexible tape for ArmaFlex end seams​

Self-adhering 2" tape specifically engineered to cover end joints of AP ArmaFlex Black LapSeal tube material. Provides added joint seam security and a clean look to pipe insulation applications. For use on end seams that have already been sealed with ArmaFlex 520 adhesive. Not intended for longitudinal seams.

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BOB RENTON​

 
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