I have a 98 F-150

My A/C cycles on and off every 2-3 seconds and the temp of the air at the ducts is only about 10 degrees cooler than the outside temp.

First I added one can of refrigerant to it. No difference. I didn't want to add any more for fear of overfilling it and it's tough to read the pressure with it cycling on and off so often.

I searched here and several people seemed to have problems with the cycling switch causing the quick cycling and it's a cheap part so I bought one at the dealer and thought I'd give it a shot.

Question:

I see the old switch that I need to replace on top of the dryer. Does the system need to be drained in order to replace this switch or can I just unscrew it and replace it without refrigerant spewing all over every where. I can't tell by looking at it whether I need to drain the system or not to replace the cycling switch.

Thanks

 

I would first find the leak before doing anything else. Look for any wet/oily spots around fittings and the compressor. I think the switch is doing its job.

 

Oh, yeah, I forgot to mention that I also looked for a leak using a black light and found nothing. Also, if there was a leak and I added refrigerant, wouldn't the cycling decrease? It didn't.

 

It may take more than one can to get you into the nominal pressure range.

If not enough, the cycling will continue. When you start getting in the nominal range, it should as you say, decrease in frequency.

To get a good reading on the pressure, close the valve periodically and then read the guage with it closed. Although it will still fluctuate some, you should be able to get an idea.

The instructions on the stop-leak / leak detector that I have used indicate that the system must be fully charged for the leak detector to work properly.

I used a large can (24oz I think) of refrigerant with oil charge. Put me just in the green on the guage. then 8oz stop leak /leak detector. Then 12oz refrigerant put me near top end of nominal range. After 2 weeks still going. Air very cold. Never found a sign of the leak
though.

Hope that helps, Good luck

DayHiker

 

I checked the low side pressure and got between 35 and 45 PSI which is the high side of normal (green). This is why I assumed it was the switch since the pressure is normal.

Also, if there was a leak, wouldn't the pressure slowly decrease over time, too? It doesn't. A couple of months ago, I checked the pressure and it was between 35 and 45 PSI then as well.

I see the thing cycle on for 2 seconds--then off for about 6 seconds over and over.

The temp when I checked it tonight at the vent was still only about 10 degrees cooler than the outside air with the A/C on "Max A/C"

 

Let me state this clearly: You are UNDERCHARGED!

Normal static pressure (system off) should be between 70 and 120 psi depending on how hot it is under the hood. A cold engine on a cool morning will measure on the low side of the range, a hot engine just of the road will read on the higher end. During cold weather, the pressure can drop low enough so that the cycling switch never closes.

Remember the relationship between gases under pressure and temperature in an enclosed system. The expansion or contraction of the gas is directly proportional to the temperature. Since the gas is contained within a defined volume of space, the pressure of the gas will therefore fluctuate in direct relationship to the temperature.

The cycling switch requires approx 45 psi to close and ENABLE compressor clutch operation. When the compressor engages, it sucks down the low side pressure until the pressure reaches around 22 (or so) psi at which point the cycling switch opens and the compressor cycles off. If there is insufficient refrigerant in the system, this cycle occurs very rapidly, often in just a few seconds. This is referred to as "compressor short-cycling" and is a symtom that almosty always is caused by an insufficient quantity of refrigerant in the system.

A word of advice: Completely ignore the color scales on the low side-only gauges. They are completely worthless, especially since they are marketed to novices who have no understanding of how to interpret the gauge's actions. Using a low-side only gauge to try to charge a system is like trying to drive with only one eye - you will miss half the detail and cannot see the whole system's actions.


Steve

 

Aside from the big ego, I think SHO may be correct about you being undercharged. I don't know about the DIY guage being worthless though.

If I had to start messing around with the high pressure side of things , then I think I would just go ahead and take it to a good shop and have it checked out by a certified A/C tech.

Nothing wrong with being a novice. That's how boards like this get started.

DayHiker / novice

 

Thanks for the help. I had about 6 oz of the refrigerant left and I put it in there. Now the thing cycles about half as often. Obviously, I must have a leak somewhere that I need to find.

 

Find a shop or someone who has a sniffer, it will help find the leak.

 

[QUOTE=GrnXnham;2670941]I have a 98 F-150

My A/C cycles on and off every 2-3 seconds and the temp of the air at the ducts is only about 10 degrees cooler than the outside temp.

First I added one can of refrigerant to it. No difference. I didn't want to add any more for fear of overfilling it and it's tough to read the pressure with it cycling on and off so often.

I searched here and several people seemed to have problems with the cycling switch causing the quick cycling and it's a cheap part so I bought one at the dealer and thought I'd give it a shot.

Question:

I see the old switch that I need to replace on top of the dryer. Does the system need to be drained in order to replace this switch or can I just unscrew it and replace it without refrigerant spewing all over every where. I can't tell by looking at it whether I need to drain the system or not to replace the cycling switch.

They sell a pretty solid product at Auto Parts stores and it sole purpose it to stop all but the biggest leaks in compressors. I am having the exact same problem as you are re: cycling and air being only 10 degrees colder than outside. You can do a good job with just a low side gauge, if done properly. If you had the mechanic two gauge setup, you would still focus on the low side. The summer started with it working fine, had to turn the AC down for it was too cold in the 90 plus degree heat. Its a ten year old vehicle, hence I thought it had just been slowly leaking over the years and reached a threshold where is was only blowing hot air. Compressors are 'designed' to leak over time for they have flexible gaskets and seals.

I will try a stop leak product, will with a little more R134 and check for leaks.

Thanks

Mike