I'm new to the forums but just wanted to pick everyone's brain.


I've got a 1979 ford F-150, I've swapped out the compressor (reusing the old clutch), the drier and the condenser. Still original are the evap and expansion valve and all the lines.












System is charged with 2.5 lbs. of R-134A, but after a little bit of running the compressor starts making an awful chattering noise and then the blower motor AND Compressor disengage. when I move to other climate controls the blower will kick back on but the AC will only come back on after I drive for a few miles.
Ac is getting cold it just won't last for any amount of time. At higher RPM's the compressor smoothes out and doesn't make much noise, it just gets really loud at low rpms.
When sitting in the driveway the compressor will get louder and louder and then the clutch and blower will disengage, but when I rev and get past some arbitrary RPM point it will reengage but then begin to slip and finally disengage as rpm's come down.


Sorry if this is rambly, but looking for answers, as my local AC guy is stumped.

 

Could be freezing up the condenser. If the fan is not moving enough air over it, it can cause the pressure to go up, and blow out the safety valve in the back of the compressor...like my brown truck did!

 

The head pressure is getting to high. When you increase the engine speed and or road speed air flow across the condensor increases and lowers the head pressure. Would like to know some pressure readings and why the amount of freon. Some Ford cars in those years used a valve simular to GM's POA valve. I thank they called it by a different name but you could have one of them hid in the dash some where. Then some switch and wiring issues possibly.

 

The pressure was like 40 on the low and at idle sitting in the shop the high side got up around 250, strange part though is even going down the highway say 50 mph, the whole thing will kick off including the blower motor, The guy only had specs for an 82 so he assumed that # and then reduced to 80%. Do you guys think that's too much R-134? I've seen some other stuff online that say 32 oz. of R-12 so that would make me think maybe overcharged?


To complicate it even more, when it gets loud and shuts off, If I hotwire to the battery the compressor will start working and cooling again.


Thoughts?

 

There are conversion tables for r-134 conversions, but that seems like a bit too much gas, and also a very high pressure

The shop that did the brown truck said my fan clutch was weak and wasn't drawing enough air over the condenser. That may have been partially true, but I think he also over-filled it

On mine, I think the pressure got so high that it blew out the safety valve in the compressor and let the magic blue smoke out

 

I can't find the original # for the 79 and have seen stuff all the way from 1.4 lbs up to 3.


If the pressure was getting too high I would blow out a valve which hasn't happened yet.


Do you think the compressor disengaging is a diff problem that is electrical?

 

If you can straight wire the compressor and everything works well then it sounds like an electrical problem.
As for freon amount the conversion does not always work out. Start with a low amount, system on high, fresh air, doors open. Put a thermometer in the vent and add freon about 4 OZ at a time. When the temperature does not decrease that is the best amount for your system. If it is not cold enough you have some other problem.

 

I can straight wire the compressor and it will cool, but the whole compressor seems chattery,
I'm using the original compressor clutch from 1979 even thought he compressor itself is new.
I can track down the wiring problem (was just making sure there wasn't some sort of feed back loop that caused the whole system to shut off when pressure went high.


Only high RPM's will fix the noise the compressor/clutch is making, spraying water on the condenser or driving even fast doesn't seem to help its only rpm related, that's why I'm doubting the pressure is getting to high for operation.

 

Will the safety valve open before the compresser goes out? I'm trying to deterimine if I have an old clutch problem or a bad (out of the box) compressor problem.


Thanks,

Dave

 

It should.
Safety switches that disconnect the clutch should not cut off the blower motor. Sounds like to or three different problems.

 

When I replaced the "original" compressor on my '76 F-250, 460, I HAD to reuse the clutch as I could not find a replacement for the clutch. Even when "they" had a listing, the aftermarket part(s) did not match the original.

 

too much draw on small wires perhaps?


Grasping straws. I know the wires themselves to the AC switch on my truck get physically hot because I don't think they're a heavy enough guage for the amps traveling through them.

 

So I replaced the vac switch in the dashboard and now I have constant power that keeps the compressor/clutch engaged. The thing is cooling, but I'm still getting a pretty bad noise when the compressor is running and its putting a pretty big power draw in the motor, belt also jumps also has some wobble its pulling so hard. It's a new compressor but could it be faulty? or could all the noise be clutch related?

 

The noise could be from either one or both. When nothing is running try to turn the driven part of the clutch by hand. It should feel smooth with resistance due only to compression of the gas. The belt will be "jumping" some when the pressure builds, but there should not be any abnormal noise; just a bit of pumping noise.

 

if I pull the belt off, the should I be able to turn the driven by had and I should only feel some small resistance that is smooth from the compressor?