The Stovebolt Site Forums The Shop Area The HiPo Shop Air Condition question.

a friend of mine has a air conditon installed in his truck. running about 20-30 mph it cools good, when he gets out on the road, it does not cool, the fan is mounted unter the bed at a 45 degree angle, he thinks the hi pressure switch is taking it out, is the fan in the wrong place or got any ideas,

When you say the fan is under the bed, I assume the condenser is also there.

The condenser is normally place in front of the radiator. In this possition, the condenser is going to have air flow at road speeds. It may be that with the condenser under the bed, it is not getting any air flow. It is possible for the road speed air to cancel the fan air, and have no air flow through the condenser at all.

A couple of questions and suggestions. Does the a/c completely quit working or does the air temperature out of the vents increase some? Stick a thermometer of some type in the vents to get a real reading of the air. If the high pressure switch or trinary switch is opening due to high or low pressure you can check that. When it operates it kills the 12 volts to the compressor clutch. Use a voltmeter or 12 volt bulb with a couple pieces of wire to check it. Clip one wire to the wire going to the clutch and the other to ground (don't interrupt the circuit). When the a/c is running you will get 12 volts. If the trinary opens the voltage will drop or the bulb will quit burning. Remember that with a working a/c the thermostat will interrupt power to the clutch also. Good luck.

wh says that the high press switch cuts off at about 35-40 mph. the fan is mounted underneath the bed at a 30 degree angle with the highest point nearest the cab and lowest point nearest the tail end. the fan is on the bottom blowing up .The thinks the cab is leaking causing the air to leave, but if that was the case the high pressure switch would not cut out. he states that its no room at the engine due to the grill configuration to put it up there. running around town the air works good he says. he says he mounted it like that to keep gravels or stuff from being throwed into it. any comments

If you are sure the trinary switch is opening when it shouldn't, like the thermostat is giving problems. Then you need to check the pressure on the system. You didn't say if it is a r-12 or -134a system. If it is/was a r-12 system and someone has retofitted to a replacement gas. That's a 'can of worms'. The experts say the pressure of a r-134a is much more critical and unforgiving than r-12. Did the guy who gassed the system really know his stuff?

We run a bunch of school buses with the condensors mounted underneath. They have a vent cut in the side skirt but I doubt much air is blowing across them at road speeds. Each condensor has atleast two fans for cooling.

Have your friend take some heavy cardboard and make a air scoop to force as much air as you can throught the coils. If it still shuts down, you are looking at a different problem.

Joe

Yah, if the hi-pressure switch is cutting out then either

A: You're not getting the cooling air you need
B: there's a blockage somewhere. (Not probable or it wouldn't work in town)

Air flow is hard to understand, what seems logical is sometimes waaayyyy off.... Heck, you could be getting reverse (back to front) air flow patterns over that coil at highway speeds for all you know!! Takes alot of trial and error (or engineering if you have the skills) to get a setup like that going correctly.

Something I would try is to put a disconnect switch to the fan. When you are at road speed, turn off the fan and see if the unit blows cold air. Sometimes two fan sources can cancel out each other. The result is no air flow.

As you can tell, with out a complete discription of the unit, we are all just guessing.

Some questions:

Is the top of the condenser to the front or is it to the rear, or is it to one side?

Is the fan blowing up through the condenser or down through the condenser?

Get a set of A/C gauges and hook them up to the system. You may need to borrow some hoses to get the gauge manifold in the cab with you. Get going down the road and observe the pressures--the pressures will tell you exactly what is going on. If you build high side pressure, you will see at what pressure the trinary switch cuts the compressor out and can see if it is functioning properly.

trinary switch..........hmmmmmmmm.

how about racecarl is on to something, and probably closer to right. my guess, (like everyone else's) is undercharge or over charge, and probably undercharge. at road speed, the air movement blows off more heat from the condenser, and causes a low pressure condition on the low side, where the clutch cycling switch is located, and causes the switch to shut down the compressor. another possibility is the increased pressure at higher rpm's causes an increase in the high side, and the high limit kicks it off. in either case, the only way to tell is with a set of gauges being read at road speed. i have a set of gauges i use for thermoking work, and the hoses are 9 feet long.

sort of duct taping yourself to the fender and hitting the interstate, how about putting a good sized blower fan positioned to where the condenser is, and running the r.p.m's to highway speed. nah......... do the duct tape thing.........and take pictures.......