My '98 5.9L AC compressor is short cycling, (runs 5-6 sec, off about 15-20 sec) and I just wanted to check with the 'brain bank' here to see if I'm overlooking something. This is the first time trying the AC since I got the vehicle. I'm an HVAC/refrigeration tech by trade but I do resi/commercial work & have almost no vehicle AC experience. Front & rear fans are working correctly. System has enough if not full charge. Can't run it long enough to know if it's %100, but standing pressure is about 85psi. Ambient temp is about *66F. Low side only gets down to about 30psi before compressor kicks out, high side to 110-120. I jumpered out the high & low pressure switches to eliminate them as the problem. According to the manual in the FAQ sticky above, I seem to be looking at a faulty electronic clutch cycling switch or PCM. There are no AC related fault codes on my reader. Anything else I should check before ordering a new cycle switch? Thanks in advance!

 

Update: after much giggle searching & testing, I jumpered the two blue wires at the cycling switch plug & the compressor runs steadily so it seems that cycling switch is shot. So far I have been unable to locate a replacement. Anyone have a source for one?

 

Sounds like it's low on refrigerant. The only way to know how much is in there is to evacuate and refill measuring how much goes back in.

 

It might be a little low, but I don't think that's the issue. At my outdoor temp I should have been at around 30psi so the charge is probably close. The cycling switch is a temperature activated switch mounted on the expansion valve and the compressor is not running long enough to drop the temperature enough to activate the switch. My local Dodge dealer says that the cycling switch is obsolete & local parts stores & online stores have been a no go so far. Anyone out there with a lead on a source?

 

The cycling switch is a pressure switch. If pressure drops below threshold value, switch opens, compressor stops, pressure comes back up (assuming low side of the system here), compressor runs. 99% of the time, the compressor cycling on/off fairly quickly, is low on charge.

 

It is a temperature controlled switch, NOT pressure controlled. I''ll copy the description from the manual just to clarify for anyone else with this issue:


The Electronic Cycling Clutch Switch (ECCS) is
mounted to the outboard side of the thermal expansion valve in the engine compartment. A thermistor probe within a capillary tube on the switch is
inserted into a well on the side of the expansion valve filled with a thermally-conductive grease. This arrangement allows the ECCS to monitor the temperature
of the expanded refrigerant entering the evaporator.

The function of the ECCS is to prevent refrigerant temperatures from becoming so low that condensation
freezes on the evaporator fins. This condition is referred to as evaporator icing. Evaporator icing
reduces air conditioning system air flow and cooling performance.
The ECCS is connected in series electrically with the heater-A/C mode control switch, the low pressure
cut-off switch and the high pressure cut-off switch between ground and the Powertrain Control Module
(PCM). The ECCS will open when it monitors a refrigerant temperature below about 2.9° C (37.2° F),
and will close again when the refrigerant temperature reaches about 4.3° C (39.7° F).
The opening and closing of the ECCS signals the PCM to cycle thecompressor clutch on and off through the compressor
clutch relay.

The ECCS is a factory sealed and calibrated unit and cannot be adjusted or repaired. If faulty or damaged,
the ECCS must be replaced.

 

I know the one you're talking about now. I've been watching your post but didn't know what switch you were talking about.

I don't know, you could go to the salvage yard and grab a couple of them. I don't imagine it's a common item to go bad. You might need some sort of thermal gel or paste to go with it. When I did my A/C rebuild, which was due to my rear expansion valve being faulty from purchase, I didn't use any thermal paste or anything aside from what came out of the old expansion valve and mine works fine. Cycles a little in the 60's to keep my evaporator from freezing up but it never cycles other than that. If your pressures look good I would just bypass it until you get a new one that works right, assuming the temperature is high enough outside to keep it from freezing up. Like you've mentioned, you have a high and low pressure cut off switch that will keep the A/C system safe. But I understand it being hot outside and not having adequate A/C, two summers ago I had that, it was terrible. Finally figured out it was the rear TXV. A/C works great now ??

 

Ok, things come clearer now.....

I can find the expansion block everywhere. The switch, however, seems impossible to find...... according to theory, 98-00 are the same......

 

Sounds like time to hit the junkyard then.

 

Ya, it looks like it's time to comb the junkyards. My little town doesn't have any early D's at the moment so I'll have to wait for my next trip to the city. FYI, I jumpered the switch and ran the AC for about an hour or so while driving around today (it's about 72-74*F outside) and it cooled fairly well so the charge seems pretty good. I'll throw my gauges back on later to verify pressures & temps and top up if need be.