expansion valve testing?

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pishta

pishta

I know I'm right....
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one of my cars 92 Lexus SC400 A/C will not cool. Pump is good I believe as I can see pressure change from high to low when the compressor kicks in. I have 50 psi of R134 in the system when the compressor is running and about 130 psi when its not running from the low side port. I have very slight cooling from the vents but no where close to what it used to produce. Could my expansion valve be blocked by moisture or just go bad? Dryer has not been serviced since forever to my 4 years of ownership knowledge. Dads car from Texas, not sure what he did with it but he was a car guy so he loved this car and treated it well. It looks to be an older generation type valve: I dont even know where this would live in this car as everything is very densely packed in this model. Do these go bad?
FS_39232_Ang.jpg
 
Try to make temp checks even if only "quick" and "by feel." The liquid line not far from the txv should be warm to quite warm. On many systems that are working hard, you can actually "feel" the flow as there is often pockets of boiling refrigerant on a hot day in the line. Yeh!! I know!! It is supposed to be liquid, but not always. If you can get to the evap side of the txv it should be quite cold and forming condensation. If you can remove the sense bulb, this should cause the txv to go wide open.

So far as the dryer, both lines should be "reasonably" within same temp. If there is a dramatic change, this indicates a blockage or restriction. "Use your head." The tube leaving the compressor is hot gas, and you normally cannot touch it anywhere. If you have an infra-red/ laser thermometer, you should be able to see cooling as the refrigerant passes through the condenser, and the liquid line coming out should be markedly cooler. It may still be quite warm, depending on ambient T and engine bay conditions and airflow, etc

Don't discount mechanical door problems, especially nowadays with stepper motors and "more plastic". Some of the Mopar trucks are plagued with this problem.

This is where "I am spoiled." On the older, "built in place" systems I started life on, there were always lots of service valves and access ports so you could isolate parts of the system--as well as make pressure checks.
 
educate me on the sense bulb. Its the bulb at the end of the capillary tube but at what temp does the thing do something? Hot or cold to open the valve? Im not versed in A/C so Im assuming the valve is shut until the bulb changed temp and then the valve opens to start letting the refrigerant into the condensor? Im guessing if the valve doesnt open, no cooling (ie. a ice or debris clog would render it inoperative?) It pisses me off more than my son who drives it all the time. I hope the thing is not behind a 2 hour dash removal....
 
On a car that new it may have a fixed orifice style and not even have an expansion valve. There is usually a filter located just before the fixed orifice.
Pic for reference.
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The bulb "tends to" close the valve as the suction line cools. This helps regulate flow back to the compressor. This is superior to an orifice/ capillary tube setup which has no regulation, therefore it better handles variable conditions, the worst of which EG is a got condenser (hot day) which causes more refrigerant flow into a cooled down "box" (passenger compartment) which let's say you are in a shaded area and lower heat load, would tend to flood refrigerant. The TXV better regulates this

Since this the bulb/ diaphragm is a relatively small, enclosed system, and has a moving part, it can fail, and it doesn't take much of a "molecular level" leak to allow the pressure in the bulb to drop
 
Just looked it up. The 92 sc 400 does in fact use an expansion valve with a capillary tube located right next to the evaporator.
 
pishta said:
educate me on the sense bulb. Its the bulb at the end of the capillary tube but at what temp does the thing do something? Hot or cold to open the valve? Im not versed in A/C so Im assuming the valve is shut until the bulb changed temp and then the valve opens to start letting the refrigerant into the condensor? Im guessing if the valve doesnt open, no cooling (ie. a ice or debris clog would render it inoperative?) It pisses me off more than my son who drives it all the time. I hope the thing is not behind a 2 hour dash removal....
Click to expand...
Chilling the bulb should cause the valve to open more, reducing the restriction, & increasing the low side pressure. Warming the bulb should do the inverse. Use Minnesota Freeze or refrigerant from a tapped can of r134a bleeding from a charge hose to chill it if it is accessable, should be seviceable, but I am not familiar with this model so...
 
Killer6 said:
Chilling the bulb should cause the valve to open more, reducing the restriction, & increasing the low side pressure. Warming the bulb should do the inverse. Use Minnesota Freeze or refrigerant from a tapped can of r134a bleeding from a charge hose to chill it if it is accessable, should be seviceable, but I am not familiar with this model so...
Click to expand...
Other way around. Warm bulb opens the txv, colder bulb closes the txv
 
WTH, this looks accessible! Im not sure where this is but it may be behind the glove box as that plenum looks like a floor heater duct.
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Go to the engine side of the firewall and follow the hardlines where they go through to inside the cabin. You’ll find the evaporator, and with that the expansion valve.
 
I think I know exactly where this is as I replaced the cabin filter once in this , its right above the passenger foot well. Son has car so Ill have ot check it out this weekend.....first week back to college dorms.....:-(
 
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