AC Short Cycling and maybe Black Death

Have a very well maintain 1994 F150 with over 250,000 mile. Engine running good, but now have Air Conditioning problems.

I have done some Air Conditioning research on the web and believe I am experiencing what is known as ?Short Cycling?. This is where the compressor turns on for 2 seconds and then off for 4 seconds this cycle keeps repeating.

I purchased a can of R134a with a pressure meter and connect to the low side pressure port at the Accumulator.

With AC set to Max the compressor clutch clicks on for about 2 seconds and the off of another 4 seconds. The pressure at the low side goes from 47 psi to 25 psi then back to 47 psi. The Low Side pressure drops as soon as the compressor clicks on and the pressure rises when the compressor clicks off. The pressure on the high side goes from 110 psi when the compressor turns on and then drops to 90 psi when the compressor turns off.

Thinking I had a low amount of R134a I started injecting R134a into the low side port.

Kept injecting R134a into the low side until I could not get anymore R134a gas into the system, even with a new can of R134a. Pressure stills bounces between 47 psi and 25 psi. May have a bad new can of R134a, since I cannot get the pressure to increase.

My low side engine off static pressure with is 69 psi and my high side static pressure is 90 psi. I think the low side pressure of 69 psi is too low, but don?t know what the resting low side static pressure should be.

Believe that I may be still low on R134a. Since I cannot get any more R134A into the system I may have a clog orifice tube or even the AC Black Death where the whole system is totally clogged with small particles from a bad compressor.

Thinking about just replacing most of the major components myself, the compressor, the accumulator, the hoses and the orifice tube. But not replacing the Evaporator or the Condenser. Looks like all these parts collectively are under $350 from my truck. Want to spend my money wisely. Anyone with some good advice other than buying a new Truck or moving to Alaska, I’m 30 miles outside of Atlanta, its cool here today but later this summer it will get very steamy hot!

Buy a set of real gauges and a quality AC system theory and diagnosis book. Read the book use the gagues and you will be moving forward. Have you even used the low tech but very effective technique of rating system components in regards to their temperature? You already have the tools, they are your hands. Read the book and you will know what I am talking about. Inlets and outlets should have matching temps or temp. differentials.

I suggest finding a good A/C shop. Down south they will be called A/C shops up north look for radiator shops. Chances are good they can fix it for less than you think. So far you may have caused more problems than you fixed. When it comes to the A/C fluid, more is not better.

You could also try the detailed AC diagnosis in the shop manual for this vehicle. Try your library or buy a subscription at It will only cost you about $25. YOu will probably be directed to use a couple of thermometers and a clock in addition to the gauges.

When attempting to recharge the AC, did you have the engine idle speed @ 1800-2000 RPM’s?


The static pressure is too low on the high side even if the system has not completely stabilized yet.
After sitting for a while, the static pressure on both low and high sides should be the same; generally around 120 or so PSI.
There will be a variation in static pressures when the A/C system is first turned off.

I also agree with Tester that you should check the pressures with elevated RPMs.
If you have a clogged orifice tube the high side pressure should be spiking up pretty high; much more than the 110 you have.

Just curious. How much 134 have you added?

This is my first time trying to repair an AC system I think I’ll get a subscription with and go through the rating steps.

I always get an estimate from the shop and then consider doing it myself.
Trying to gain some insight about AC Systems.

Yes, I think i will get a subscription to Thanks.

I don’t have a tachometer so the answer is no. But I’ll try getting one.

Yes the system should stablize after about an hour or so. The reading where taken after the engine was turned off. I think I added about 20 oz one can.