I have a 93 240 and I just charged the R134 system with coolant, but the clutch on the compressor clicks on then off, clicks on then off, all of the time.

What could be causing it not to stay engauged when the air is turned on.

The air coming out of the vents is cool, but not cold.

Thanks in advance,

Frank

Thanks so much for the help guys. I will do more investigation asap, but right now I have more improtant things to content with....kidney stones!!!!

Frank

The clutch is supposed to cycle.

Search here and with google on "CCOT" (Cycling Clutch Orifice Tube system).

There are many posts from Chris Herbst on this in the archives. The cycling is based on "Low Side" pressure, trying to keep it between about 23 and 46 psi at the dryer. You could have a bad pressure switch at the dryer, or a pressure too high or low (a guess).

--
Bruce Young, '93 940-NA (current), 240s (one V8), 140s, 122s, since '63.

Rechecked the pressure and is at 45 and the clutch still cycles in every two seconds.

Where is the pressure switch located. Didnt see one anywhere near the aluminum canister.

Thanks
Frank

On my '93 940 the "cycling switch" (pressostat) is on the top of the dryer can, pointing back 45° right toward firewall, a seen looking from the front.

I'm no AC guru, but the 45psi sounds OK, assuming the compressor was running (see #1 below). Here's how I think it should work:

1- In the inert state the LS should be around 75psi (varies a little with ambient temp) and the switch should be CLOSED*

2- That allows the compressor to work and start driving the pressure down as the evaporator gets cold.

3- The pressure should keep dropping in sync with the temp at the dryer (where sensing switch is).

4- At around 23psi the switch opens the clutch circuit to stop the compressor (to continue might cause icing at the evap).

5- With compressor off, the pressure starts to rise. When it gets back up to 45psi the switch closes and the cycle begins again at #2.


*As I understand it the LS switch (pressostat) is OPEN below 23psi and CLOSED from 23 up.




--
Bruce Young, '93 940-NA (current), 240s (one V8), 140s, 122s, since '63.

That does not sound like a clutch failure, normally a clutch has no engagement at all or all sorts of noise when they go bad. Sounds like an issue with either the pressure switch or low pressure inside the system.

Put a jumper across the two wires on the switch, start the car up and see if the compressor runs all the time. If yes you either have a bad pressure switch or some sort or issue with pressure. To replace a switch you will need to open up the system. I would replace the Dryer, low pressure switch, and the orifice tube all at the same time if you have to go that far.

Suggest trying a 2nd set of gauges before you go that far. This really sounds like a low pressure issue (on the surface at least). A system with slightly low pressure can actually cool quite well. But that comes at the expense of the compressor running all the time and is not efficient.

If the jumper wire does not solve the cycling issue pull the jumper out and with the engine still running measure the voltage on both wires to chassis. One of the wires should have a steady 12 volts on it. If you see the voltage swinging on and off that would head you inside the car.

If the voltage is steady, turn the engine off and measure the resistance of wire without 12 volts on it to chassis. You should have a very low reading of say 3.8 ohms, that is your clutch resistance. Also measure from the compressor frame to the engine block, that should be zero ohms. The compressor has to have a solid chassis ground connection to run.

Hope this helps and Regards,

Paul

"To replace a switch you will need to open up the system."

I believe there's a shrader valve under the switch so it won't loose pressure when removed.

Good info on:
"slightly low pressure can actually cool quite well. But that comes at the expense of the compressor running all the time and is not efficient."

Mine's still getting as low as 37° at the vents on a hot day (e.g.yesterday), but no cycling. Been like this for the past 3 years at least. The shop gauge (then) was in the 50 psi range, running with r134 amount measured to spec. I'll have to get a cheap gauge to see what it is now.

EDIT:
I should add that for the 2 years prior to the non-cycling, the system worked as intended, and LS pressure(s) agreed with cycling pattern. I don't know what prompted me to check it the next year, but I wired a tell-tale light from the clutch to confirm continuous compressor operation.

--
Bruce Young, '93 940-NA (current), 240s (one V8), 140s, 122s, since '63.

Possibly it still isn't charged enough, or the pressure switch is opening when it shouldn't.