Is there a high pressure switch? changed the fan clutch twice, last one was the high dollar Napa, with no change. Untill the truck is warmed up , the a/c pressure will spike causing my (not cheap, and hard to find R-12) to come spraying out the releif valve! Once the truck is warmed up the a/c works perfect cooling down to 36deg

 

High head pressure is often the result of poor airflow through the condensor coil; since the pressure only seems to spike when the engine is cold, my question to you is this: is your fan clutch failing? It sounds like it is freewheeling too long, and only engaging when the engine is good and hot; by then the A/C has tried to reject more heat than the system is capable of with diminished airflow. Just for giggles try a stone-cold start with a box fan in front of the grille and see if it recurs. If I'm right you can change the fan clutch. If not you haven't spent any more than the cost of refrigerant and can move on to the next possibility.

You can get head-pressure safety switches, either manual or auto-reset, from most refrigeration-supply houses; but you will require a highside adapter to tie into the service fitting.

Have you considered the possibility of moisture in the system causing a migrating(and often hard to find) restriction?

Another source of trouble I have seen(both in automotive systems, and in the stationary systems I am licenced for) is too much oil. If I remember the specs on my old 91 the system calls for about 4 ounces of oil. If you recharge every year or so and use an oil charge every time you could be overfilled.

Just a few ideas; good luck.

 

your right about everything here,but not sure what you mean about moisture causing migrating.
moisture in the system is non condensibe causing high head pressure. and the only restrictions ive seen with moisture is freeze up's in the tx valve or metering device.Too much oil will give you poor cooling as well as high suction pressure.

and believe it or not i have seen internal failures on hoses that intermitently cause restrictions...kinda like a flapper in there.

get guages and test it the right way so you know what your dealing with.i dont understand the just cram another can of duracool in there theory

 

Thanks for the input, I'm sure the fan is not engaging enough when it's cold, but that's another issue I was just wandering why the power to the a/c clutch was not cutting out when the pressure went outta control

 

i have not seen a relief valve on a system.....i am going to check my truck though.
it came blowing cold so i havent even looked at it.


the HPCO high pressure sw. SHOULD disconnect power to the clutch. if it is not there is a problem with the sw.

 

That's my thinking, just don't know how it's posed to work