AC No Worky: Compressor Short Cycles

ferg on Sat August 21, 2010 2:12 PM

Year: 2001
Make: Dodge
Model: Ram 1500
Engine Size: 3.9 V6
Refrigerant Type: R134A
Ambient Temp: 96
Pressure Low: 38 & 8
Pressure High: 50 & 110
Country of Origin: United States

Several of my recent posts were in reference to friends vehicles, those do no apply here. This is for me this time!

I just picked up this truck at auction yesterday so I don't know much of the history. I can tell you that it was a city owned work truck and appeared to be maintenanced regularly by the cities mechanic shop. It was removed from service back in january and has been parked since. The AC is not working and was stated so in the online auction, however I am not sure if this was something that developed before or after the truck was removed from service.

What I noticed:
The compressor short cycles, it turns on for about 2 seconds then turns off. It never comes close to blowing cold air.

What my tools tell me:
On initial start up and turning the system on max ac the low side reads 38 then when the compressor cycles on it drops to 8 ( which is why I gave two readings in the info section above) The high side reads 50 then jumps to 110 when it cycles on. The vent temp was reading 104.9F at idle and the ambient temp was 96 according to google. The compressor is a Sanded Model # U 4788.

What I tried:
Knowing the system was low and assuming there is a leak somewhere I put in a can of r134a with dye in hopes of seeing where the leak was coming from. (I have since read the tips and faq's on mechanic in a can stuff and now understand the problems that can cause) But after adding the can the compressor was no longer short cycling and the low side reading was now 25 and a high side of 190 with a vent temp of 78.3 and a vent temp of 77.4 at 2k RPM. HAving been on this forum recently I decided to spray some water on the condenser while the system was running and I found that the compressor would start cycling on and off again. The low side pressure was fluctuating as the compressor cycled from 45 to 20, and the highside held fairly steady at 100, while the vent temp rose slightly to 80.5

So what does all of this tell me?
How do I start to look for a leak?
How should I proceed from here?

Thanks,
FERG

GM Tech on Sat August 21, 2010 3:11 PM

On any dodge product-- I always look for an evaporator leak-- a good sniffer will confirm or reject this..

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The number one A/C diagnostic tool there is- is to know how much refrigerant is in the system- this can only be done by recovering and weighing the refrigerant!!
Just a thought.... 65% of A/C failures in my 3200 car diagnostic database (GM vehicles) are due to loss of refrigerant due to a leak......

mk378 on Sat August 21, 2010 5:32 PM

One can wasn't enough to restore a full charge, as the system was nearly empty when you started. I think they were still using the lemon evaporators in 2001.

ferg on Sat August 21, 2010 10:25 PM

Ya I added another 12 oz of jensens r134a without any dyes additives or stop leaks. It brought the lowside pressure up to about 42psi and the vent temps are good and cold. I got a vent temp of 58 at idle and with a quick trip around the block it dropped on down to 49 with the ambient temp being 95.

On closer inspection I also found that one of the ac hoses coming from the compressor to the condensor was pretty new as was the condensor itself. SO i have a feeling they had worked on it and when it didn't hold a charge they pulled it from service to sell it. I found that the low side shrader valve was good and tight but the high side was just a little loose, maybe less than a 1/4 turn...ish. So maybe Igot lucky. I will keep a close eye on the temps and pressures over the next few days and then look for a leak.

I am at work today and all night, so when I get home in the morning I will hit the engine bay with the pressure washer since I spilled a little bit of the can the had the dye in it, then start lookin for leaks after 24 hours or so.

Thanks for the advice.

Ferg

Edited: Sat August 21, 2010 at 10:29 PM by ferg

ferg on Sat August 21, 2010 10:27 PM

Also, is there any way to identify the "lemon" evaporators

GM Tech on Sun August 22, 2010 7:13 AM

Just look on the back of the vehicle....if it says Dodge, Chrysler, Jeep, or Plymouth then you have a really good chance you have a "lemon evaporator"

-------------------------
The number one A/C diagnostic tool there is- is to know how much refrigerant is in the system- this can only be done by recovering and weighing the refrigerant!!
Just a thought.... 65% of A/C failures in my 3200 car diagnostic database (GM vehicles) are due to loss of refrigerant due to a leak......

Edited: Sun August 22, 2010 at 4:09 PM by GM Tech

ferg on Sun August 22, 2010 1:49 PM

Fair Enough....

When I spray the water on the condensor and watc hthe gauges, what is the reaction I am looking for? Waht is a negative finding for this test and what does it tell me? Just curious because someone had told me to do it for a friends truck and I haven't met up with him yet to take a look.