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High pressures

vdubnut on Tue September 04, 2007 5:29 PM User is offline

Year: 1991
Make: chevrolet
Model: suburban
Engine Size: 5.7
Refrigerant Type: r134
Ambient Temp: 90
Pressure Low: 50-60
Pressure High: 325-375
Country of Origin: United States

Wondering what the problem is...

I took my truck to a local shop to have the system vacuumed down because my friends pump was insufficient to reach the necessary vacuum...

they hooked my system to their machine, brought it under vacuum and let the machine monitor / time the sustained vacuum. It passed the "test" as the tech said.

then i had him re-vacuum the system just to make sure it was "deep" 30in range and had him disconnect the machine and cap my lines.

then i went home and began to charge the system.

i was able to put in 1.5 cans (12 oz each) of R134 prior to starting the car....then i started and turned system on high.....and continued to fill system.

clutch was cycling on and off until i got to about 36 oz installed. Then pressures got into the 50 low and 325 high range and stayed there.

i stopped everything and let it cool down and equalize and came back to try again....

i started another can and the guages went up to 55-60 and 350-375, which is where i stopped. at this point i was getting into the low 60's at the vent, with the engine somewhat revved(dont have a tach) and approx 40 oz installed.

the front/rear system calls for 84oz of R-12 so i was told around 76 of R-134 should fill the system.....

driving around i got down to 39-40 vent temps on the freeway but those pressures worry me, and the non-cycling of the compressor.....and the fact that i only have 1/2 the R-134 installed that i should.

what are your thoughts?

thanks!

JACK ADAMS on Tue September 04, 2007 7:00 PM User is offline

Well the first thing is do you have rear a/c? Next thing is what modifications have been done for this 134a conversion? You may need to add a Electric Fan to the condenser for better cooling if you did not change the condenser to a PFC.

vdubnut on Tue September 04, 2007 7:11 PM User is offline

Yes, I have rear A/C so those were the refrigerant volumes recommeded to me.

modifications: just replaced the compressor, did a system flush, added the 11oz of pag 150, replaced the orifice tube, replaced the accumulator with a r134 compatible unit.

Chick on Tue September 04, 2007 7:34 PM User is offlineView users profile

Did you remove the rear expansion valve before flushing, or did you flush thru it??

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Chick
Email: Chick

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Freedoms just another word for nothing left to lose

JACK ADAMS on Tue September 04, 2007 7:39 PM User is offline

Adding the fan will help out with in-town stop and go travel with the pressure. The PFC will help with all of that. Just remember that you have a lot of BTU space to cool down. Once this is done the pressures will drop.

mk378 on Wed September 05, 2007 8:59 AM User is offline

Many people report lackluster performance when attempting to convert a GM full-size truck like that one to R134a.

Also you shouldn't trust the shrader valves to hold vacuum in. If you look at how they are built they work like check valves and with weak springs and enough pressure difference, they can open. It's best not to disconnect the hoses while the system is under vacuum. It may not have happened here but there is a chance you have some air in the system which of course will run up the high side pressure.

iceman2555 on Wed September 05, 2007 10:00 AM User is offlineView users profile

Appears that there is a possible engine cooling problem or as mk378 states...air in the system. With out use of an identifier....to determine if air is in the system....it all becomes an assumption.
However, if it is assumed that there is no air...that the vac remained intact......go for it.
First it is utmost importance to know that the system is fully charged. Continued operation of a partially charged system will result in compressor failure.
Try charging with the engine at idle, max cool, high blower, doors open, rear system deactivated. Recharge until in evap inlet and outlet temps are the same (cool side of the orifice and the evap outlet prior to the accumulator). Temp differential is 0° to 5°. Allow system to stabilize...operate for several minutes....measure vent temps and compare to ambient temps.....a drop of 25-35 is desired. Also test pressures...they may be a bit high at this point...because of the high heat load on the evap...but this condition will insure complete recharge of the system. Activate the rear unit and retest temps.....at this point the evap temp differential should be app 5°-8°. Monitor high and low side pressures.
If excessive pressures are encountered....check the fan clutch...in fact if it has not been replaced in the last three years....or has over 50k miles of operation, replace the darn thing. The best repair would be to follow Jack's advice...replace the condenser with a more efficient unit....the Parallel Flow units are the very best for cooling efficiency. However, if working with the system in its current condition....insure that the engine cooling is working to par.
Another observation...is the high side port BEFORE the condenser....is it not located between the compressor and condenser on this model? If so expect high side discharge pressures to be app 30psi higher than liquid line pressures.
A high side pressure of 350-375 equates to a refrigerant temps of 172-177°. Seems as if the system is not cooling the refrigerant. Liquid temps need to be reduced by app 25-30°...this could simply be a engine cooling problem.
Given the ambient temp....and not knowing the humidity in your area....high side pressures with this retro could be 275psi or higher. Get the system fully charged and work with the system at this point to reduce excessive pressures.....however, once the evap floods...do not remove refrigerant.....the result will be compressor damage.
Keep in mind also..Jack's comment...once the interior cools down...and the evap is not getting hit with high temps...the pressures will be reduced....but make sure...first of all the system is fully charged....not 75%....80% or 90% of 134a to 12 specs....just flood the evap.....and that part is accomplished.
Also, a compressor is not going to cycle until low side pressures are in the 25-28psi range. This could be an undercharged system (rapid cycling) or a fully operational system....cycle off..low pressure...cycle on....app 40-45 psi....functional cycle rates.
Good luck !!!

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The strongest reason for the people to retain the right to keep and bear arms is, as a last resort, to protect themselves against tyranny in government.
Thomas Jefferson

vdubnut on Wed September 05, 2007 11:56 AM User is offline

Quote
Originally posted by: Chick
Did you remove the rear expansion valve before flushing, or did you flush thru it??

yes....removed the rear expansion valve and flushed lines seperately....

I felt confident that the vacuum held with the schraders and the caps on but after reading the posts this may be a contributing factor.

I hate to pull out the refrigerant again but i may have to. If i want to purchase a pump will the small one work? Im starting to get in deep in this project.

Not sure on the mileage on the clutch fan....just bought the truck this year. I did replace the radiator (was leaking not overheating) and i havent had any engine cooling issues this summer so i figured the clutch fan was probably ok.....will check into that....

the high side service port is after the condensor and before the orifice tube, if i remember correctly.

thanks for all the suggestions, I will do some more investigating

Edited: Wed September 05, 2007 at 12:00 PM by vdubnut

iceman2555 on Wed September 05, 2007 12:42 PM User is offlineView users profile

Thanks for the feed back.....check the front of the fan clutch for a dark greasy substance....this may indicate a possible problem. However, it is not a true and final test for clutch reliability. An over heating engine is not always an indication of a fan clutch that does not have sufficient air flow. The condenser is much more susceptible to lack of air flow than the radiator.
Check with some local AC shops...see if they have a refrigerant identifier...it will indicate how much air is in the system. May save a complete recovery and recharge. Just a strong feeling about the fan clutch. Considering the age of the vehicle and normal fan clutch service.
Check with site sponsor....Jack will have some suggestions for a good vac pump....just stay away from the 'air powered' units.....not worth the money...not the initial cost....that is small....it is the parts replacement cost for the parts that fail after these darn things are used.



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The strongest reason for the people to retain the right to keep and bear arms is, as a last resort, to protect themselves against tyranny in government.
Thomas Jefferson

john,??? on Wed September 05, 2007 1:47 PM User is offlineView users profile

Quote
Originally posted by: iceman2555

Check with site sponsor....Jack will have some suggestions for a good vac pump....just stay away from the 'air powered' units.....not worth the money...not the initial cost....that is small....it is the parts replacement cost for the parts that fail after these darn things are used.



not to hijack, but I have one of these air vac pumps, what's bad about them?

vdubnut on Wed September 05, 2007 5:46 PM User is offline

my understanding is that they use a ton of air to make them work and that they dont work all that well....

what have your experiences been?

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