More '93 240 A/C Questions - Electrical

I have been trying to figure out why the A/C doesn't work properly on our newly acquired 240. So far I have determined with a pressure gauge
that the freon was a little low and I added enough to get the low side running pressure to around 40 PSI.
The problem now is that the compressor cycles on and off rapidly. I tried bypassing the low side pressure switch as some suggested and that made no difference.
Then I used a sharp probe to check voltage on the compressor clutch wire at the white plastic connector under the intake manifold. Voltage is ~13 volts when the compressor is engaged, but drops to zero when the compressor disengages. So, I conclude that something upstream from that point is cycling on and off. Unfortunately the Bentley manual is no help at all on this.
I deduce that there must be sensors, switches, logic and one or more relays which determine when the clutch engage line is powered or not. Can anyone help point me in the correct direction? Is this a likely relay problem? Is there a circuit breaker in this line somewhere?
TIA for any help!
John
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I'd exercise some caution at this point. Go and burn up the compressor and you'll have compressor debris spread throughout your system and need to start replacing major components.
Do you have R134 or R12? If R12, i'd consider doing an R134 conversion with new oil, expansion valve, drier.
To me if the compressor just started working with a little freon added, you have enough to cycle but not enough to meet requirements. Go and add the wrong amount of freon and you'll fry your system. Cycling the compressor is bad for it and over filling will kill it.
You need to decide what you have on your hands and where you want to go. If the freon was low, there is a reason for that. maybe just dried o-rings, or maybe a leak.
On my 1987 740 I had a drained system and R12 system. While the system was dry I did an R134 conversion myself, replacing all o-rings, new drier and expansion valve, draining and refilling the compressor. My plan was to replace all of this and address leaks as needed. Oh, I flushed the condensor too.
Be careful just jumping in and trying to get it running on the cheap -- it may cost more down the road.
John Horner wrote:

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As far as the cycling goes, it's controled by the air conditioner knob (on my 89) that goes from low air to high (the blue range) or max air (the orange range). When the contol is set to high or max there is very little cycling. when it's set to low temp, it cycles to keep the temp down. there's a thermostat kind of device in the cabin that controls this and sends the 12v DC to the compressor. So the whole thing is dependant on air temperature and the setting of the air conditioner switch. You may have a problem with the thermostat control or you may just have a low outside air temp. and the switch set to minimum which would cause cycling (but not rapid cycling I would think). I tend to think you're still low on freon. Seems like I read the low side pressure should be 30 or 35. Don't have access to a manual right now. I agree you should have it checked by a pro for leaks etc.
Jamie wrote:

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A correction to my post, I ment low inside cabin temperature and switch set to minimum and be carefull not to overfill with freon! I ruined a compressor on a chev truck that way. Also check this post
http://www.volvo-forums.com/t10268-air-conditioner-help.htm
Doug at Lakeview has a good description of how the system works.
fastblue wrote:

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