A/C question

[mpullen66]

I just noticed there is no difference switching from norm a/c to max a/c. Its not blowing real hard so i figured its on norm. Can anyone give me an idea where to start.

Also it blows cold at 70mph but its starts warming up at a stop light. Needing Freon? its original r12.
Its like 165 degrees in oklahoma so i need all the cold air I can get.

Thanks for any advice

[starkey]

What year truck are you driving? Lot of folks put a heater core bypass on the heater hose, because there is some residual heat from the heater core that makes it in to the airbox, regardless of what temp you have it set on.

If it's cooling at highway speeds and getting hot at idle, that tells me that your radiator fan isn't pulling enough air across the evap core to do it's job. You've either got a failing fan clutch, or a clogged up evap core.

Just some ideas.

[mpullen66]

Its an 87 lwb 5.7 tbi 69k miles. The fan clutch is less than 6 months old. Would a good pressure washing do any good? or likely to damage something

[bd]

I just noticed there is no difference switching from norm a/c to max a/c.  Its not blowing real hard so i figured its on norm....

Switching to "Max A/C" doesn't increase fan speed, it just recycles cabin air for maximum cooling instead of bringing in fresh outside air.  But, if you don't hear the doors switching under the dash, you might have a problem with vacuum, actuators or doors not working properly.

...it blows cold at 70mph but its starts warming up at a stop light....

Airflow through the condenser is part of it (fan clutch, mashed over fins, bugs & debris, oil & dirt, etc - anything that limits air flow); refrigerant charge may be part of it (Do you see snow in the sight glass at idle?); and if the inside of the sight glass appears muddy gray or brown the desiccant bag in the accumulator has ruptured, partially plugging the expansion tube, evaporator and/or condenser.  Is your fan shroud in place? 

Its an 87 lwb 5.7 tbi 69k miles. The fan clutch is less than 6 months old. Would a good pressure washing do any good? or likely to damage something

Regarding the fan clutch in "165° temperatures" (Yikes!!!  ), use a heavy-duty thermal fan clutch (like a Hayden 2797 - http://www.summitracing.com/parts/HDA-2797/) specified for a 454.  The problem with pressure washing is its tendency to bend/mash over the delicate fins of the condenser and radiator, which may already be one of your issues.  It would be better to use compressed air on a long nozzle bent 90° at the tip, though you still have to be careful not to bend fins.

[mpullen66]

The fan shroud is in place and the fan is a thermal fan. Where is the sight glass located?

[bd]

...Where is the sight glass located?

Oops!  Sight glasses were eliminated from A/C systems about the time fixed orifice tubes were introduced and substituted for expansion valves.  The sight glass was a clear glass window ~3/8" across that allowed a tech to peer into the high pressure side of the system.  Its primary function was to allow the tech to monitor the system for correct refrigerant charge while the A/C was running.  "Snow" (actually fine bubbles) would form in the line when a system was undercharged (hence, allowed the refrigerant to boil from insufficient pressure in a part of the system that should contain only high pressure liquid).  Sight glasses typically were integral with a receiver/drier or mounted in-line between the condenser and drier.

Your '87 doesn't have a sight glass unless it was added by someone later.  On your vehicle, you have to rely on system pressures and cycling frequency to arrive at the correct charge.  You won't know the condition of the desiccant (a non-serviceable bag inside the accumulator) until the orifice tube is physically removed and inspected.   

...it blows cold at 70mph but its starts warming up at a stop light. Needing Freon? its original r12.

If everything else we mentioned checks out okay, is it possible that someone added the wrong refrigerant along the way?

[mpullen66]

Found a vacume hose off that goes to the kick.panel on the passenger side.  Max A/C working good now. Think.it was warming up when stopped was due to the extreme heat. 113 degrees for several days.