Yet another A/C thread - where is pressure switch?

[VW1300]

My A/C worked 2 weeks ago, but when I turned it on again today I get nothing - no cold air - compressor never seems to kick on, because that little dip in engine rpms never happens when I move the temp dial to min temp. That hose that runs up top behind the radiator does not get cold like it used to either.

The obvious answer is that I'm low on freon - when the system was charged at the end of march, mechanic said I had a very slow leak and put a dye into the freon to help diagnostics in the future.

However...just to make sure, I would like to try to jump the low pressure switch to see if the compressor turns on. If so, then I will really believe I'm low on freon.

Where is the low pressure switch? I've scoured the board and no one really talks about where it is, the Haynes manual is of no use.

Second question - regarding that dye they put in the freon - a co-worker told me that the system has to have pressure inside so they can see it squirting out. Therefore, since my pressure is low it won't be visible anymore. I always thought it was visible under UV light, and that it will sit there around the leak for a while whether or not the A/C is running at the time. What's the story?

Thanks.

The pressure switch is on the receiver/dryer behind the right headlight. It has two wires connected to it, try jumping them together and see if the compressor kicks on. Don't run it if it's low on pressure.

The low pressure switch is located somewhere on the low pressure or suction line or return line (it can be called any of the three). Maybe I missed it but I did not see what car you are working on. As far as the leak detector dye goes it depends if they used the red dye that is visible by the naked eye or one that needs a black light. Either way it will stick around long enough to see. I will assume it was injected with the freon during the last re charge.

[compress ignite]

Dye into system...ANY substance out of system contains dye.
(I.E. refrigerant or lubricant)
U.V. light will illuminate the dye (and hopefully the source of the leak)

[VW1300]

Thanks. After I posted, I realized I have a catalog that shows the whole system with everything labeled and found it. I know it's there for a reason and won't keep it jumped for very long, just long enough to determine whether the compressor kicks on.

I haven't seen any red stuff around the engine compartment so I'm hoping they used the UV stuff. To the responder who asked whether they injected it with the last freon charge, the answer is yes.

Question - will any UV/poster light work to make it visible? Maybe I can pick up a bulb at a craft store and put it in my drop light and find the leak...

[dannym]

Originally Posted by VW1300

.........The obvious answer is that I'm low on freon - when the system was charged at the end of march, mechanic said I had a very slow leak and put a dye into the freon to help diagnostics in the future.

However...just to make sure, I would like to try to jump the low pressure switch to see if the compressor turns on. If so, then I will really believe I'm low on freon...........

Maybe you made that assumption too fast. Did you check the connector at the compressor for power?

To really be sure get a set of gauges. Would you do any other work on your car without the proper tools?

Originally Posted by VW1300

Second question - regarding that dye they put in the freon - a co-worker told me that the system has to have pressure inside so they can see it squirting out. Therefore, since my pressure is low it won't be visible anymore. I always thought it was visible under UV light, and that it will sit there around the leak for a while whether or not the A/C is running at the time. What's the story?

Thanks.

The Dye DOES NOT squirt out. If it did that you would not need dye to see the leak.

The pressure switch looks like this:
http://catalog.worldpac.com/mercedesshop/sophio/wizard.jsp?partner=mercedesshop&clientid=catalog.mercedesshop&baseurl=http://catalog.peachparts.com/&cookieid=23L0I4M6723L0I4S77&year=1985&make=MB&model=300-DT-001&category=R&part=A%2FC+Pressure+Switch



Danny

[VW1300]

OK, with A/C on I can see the compressor turning now...disconnecting the pressure switch turns it off, so I'm going to conclude I still have enough pressure to satisfy the pressure switch, but not enough for the whole system to generate cool air. Does this sound logical? For the responder who recommended getting a set of gauges, how much pressure do I need to get cold air? (FWIW, when I dial in "min" temp on the pushbutton unit, the fan comes on and the air gets directed to the correct vents, so that seems to be working OK)

This is my first experience with A/C troubleshooting. I bought this car knowing the A/C had a small leak, so I can't complain if that's the problem. I just want to rule out the Klima relay or pushbutton unit, so if I have to take it to a regular shop they can fix the problem rather than spend hours dealing with a weird ancient control system.

Thanks,

[jbaj007]

I think your plan sounds like you're on the right path. Since all control functions appear within normal limits and the previous mechanic stated that you had a leak; the logical next step is to check your pressures with a gauge set. ~1500 rpm, Min wheel(which is maximum cool)on climate control, door/windows open (not that important in this case). Post high side, low, ambient and vent temps. Since I doubt you have a large shop fan to set up some air flow across the condensor, just jumper the leads to the aux fan to get some consistant air across while taking the readings.

I haven't seen red dye used in many years; most all shops would have used the yellow/puke chartruese UV dye in your car. Any UV or even a psychodelic "black light" will usually work. The greater the intensity, obviously, the better. The 120v. ones made for this are probably the best; I use a $10 LED UV flashlight and works well enough in the deep shade.

The low pressure switch is NOT on the low side as some one stated, but is on the receiver (high side) as Craig said. It is set at a really low #, something like 30 psi; meaning it will not shut off the comp unless you are really low on refrigerant, so it only comes into play when there is essentially no liquid refrigerant left, so your logic seems good.

[VW1300]

I threw away my black light several years ago. Today I stopped by my friendly local head shop but they don't stock the screw-in bulb type anymore. What's the world coming to? They did have a 16" fluorescent type, but I don't have a lamp setup for that kind of bulb.

Where do I get a UV flashlight? This will work in my garage. About testing those pressures - if I open up the system to put the gauge on, won't the freon come gushing out? Or is that only when the system is on?