Ok.
Step one... get gauges.
Step two, get nitrogen, or co2, or argon, or helium dry gas and pressurize the system.
The R4 compressor has an oil bleed screw that will NOT HOLD VACUUM LONG.
if the car was sold to you in a vacuum, the dryer dessicant is ruined anyway, get a new one, and plan to do this job all at once.
Post pictures of the cans of r12 you have.

__________________
John HAUL AWAY, OR CRUSHED CARS!!! HELP ME keep the cars out of the crusher! A/C Thread
"as I ride with my a/c on... I have fond memories of sweaty oily saturdays and spewing R12 into the air. THANKS for all you do!

My drivers:
1987 190D 2.5Turbo
1987 560SL convertible
1987 190D 2.5-5SPEED!!!

1987 300TD
2005 Dodge Sprinter 2500 158"WB
1994GMC 2500 6.5Turbo truck... I had to put the ladder somewhere!

Search here on this forum for threads from me with ac or a/c in them, and the info is repeated there hundreds of times.

I hate it when someone checks for leaks under vacuum, or vacuums a system and leaves it under a vacuum...
Mobile ac should be under a vacuum for the charging process only.
Never for testing or left in a vacuum...

__________________
John HAUL AWAY, OR CRUSHED CARS!!! HELP ME keep the cars out of the crusher! A/C Thread
"as I ride with my a/c on... I have fond memories of sweaty oily saturdays and spewing R12 into the air. THANKS for all you do!

My drivers:
1987 190D 2.5Turbo
1987 560SL convertible
1987 190D 2.5-5SPEED!!!

1987 300TD
2005 Dodge Sprinter 2500 158"WB
1994GMC 2500 6.5Turbo truck... I had to put the ladder somewhere!

I did notice that when the car was running with the ac on, and I quickly pushed in the valve on the low-pressure side (for a split second), I hear a hissing sound. So, there must be something in there. What it is, I am still trying to find out.
Doesn't that suggest it is not under a vacuum?

It is common to draw a vacuum on the system to be sure it will hold for an hour or so to prove that you have a sealed system, BUT it is not going to hold a vacuum for weeks. Air would seep in over time. If you had pressure at the low pressure valve you must have some sort of refrigerant in it.

__________________
'97 E 300 D

Pressure in the system will rise and fall with ambient temperature. If anything exited the valve yes there is pressure not vacuum at that port.

Vacuum is used to create a lower boiling point in the system which causes moisture in the system to boil out. If a system holds vacuum that is a positive sign but it does not mean the system will hold pressure. Systems are best checked for leaks using nitrogen under pressure.

__________________
"Rudeness is a weak man's imitation of strength" - Eric Hoffer

x2

You will hear a hissing sound if it is vacuum ( air rushing in ) or if pressurized ( freon rushing out ).

__________________
Not MBZ nor A/C trained professional but a die-hard DIY and green engineer. Use the info at your own peril. Picked up 2 Infractions because of disagreements. NOW reversed.

W124 Keyless remote, PM for details. http://www.peachparts.com/shopforum/mercedes-used-parts-sale-wanted/334620-fs-w124-chasis-keyless-remote-%2450-shipped.html

1 X 2006 CDI
1 x 87 300SDL
1 x 87 300D
1 x 87 300TDT wagon
1 x 83 300D
1 x 84 190D ( 5 sp ) - All R134 converted + keyless entry.

Thanks for the help everyone. I have read everything I could on this issue.

1) With the engine running and a/c engaged on high, the low pressure port is reading 40psi, ambient air 50 degrees.

2) The compressor is not coming on, even after jumping the a/c pressure switch on top of the receiver dryer.

3) Something is in the system, it is hissing when I push in the valve on the low pressure port while the car is running and the a/c has been turned on. Possibly it was charged (seller thought it wasn't) and there is something else wrong with the system that will not allow the compressor to engage.

4) Besides relays, where else should I check?

My first check would be to disconnect the wires from the compressor clutch and do a couple tests:

- resistance of the clutch coil, should be a couple ohms
- is the car supplying 12v to try to engage the clutch
- try to jumper 12v to the coil and see if it engages

If you do jump it running, don't run this way for very long as nothing else like the condenser fans will be running. Also you don't know what's in there and considering it was supposed to be vacuumed then you know the history is questionable now.

~40psi is a reasonable low side compressor off pressure for either R12 or R134 which is a common swap. Take a look at an R12 P-T chart and you'll see it should be at around 45 psi at 50F. At least you know it's not R290 in there as it would be closer to 70 psi. You can use the charts at different ambient temperatures to try to identify the refrigerant as they all have different curves.

40 psi should be enough to make the pressure switch, though.

Without a high side gauge, the only thing you can check is to see if the low side draws down when the compressor starts. It should sag a bit, but if it drops rapidly then it's likely undercharged. A cheap set of gauges can be had on Amazon or a discount tool shop for $50-100 and I would really recommend buying a set if you're troubleshooting this yourself.

__________________
1993 300D 2.5L Turbo

Thank you. I will pick up the gauges, and start at the compressor to try and determine if it is being supplied power, check ohms and jump 12V.
How about relays on the 240d, is it the klimer relay or any specific relay that could cause the compressor not to come on?

If your compressor is not turning on the low and high pressure ports will have the same pressure. It seems like 40 pounds should be enough to get the compressor to turn on, but it is quite low. It is common to attach the fill to the low pressure port, after bleeding the air out of the fill line, and leaving it for a few minutes to bring the pressure up enough for the compressor to turn on.

__________________
'97 E 300 D

I am trying to determine if it may be a relay causing the compressor not to come on. I can't find any reference to an OVP relay for a 1980 240d 4 speed. Does this car have an OVP relay?

All I could find was 2 silver relays under a black box on driver's side fender area.
Is one of those a Klima relay?

I can't find any relays that are associated with the ac system on this car. Does anyone know if any relays are associated with the ac system on a 1980 240d 4 speed. Thanks.

Had a look at my wiring diagram collection but unfortunately they don't go back that far. By 1986 there was already an "A/C compressor control unit" and by 1990 it appears to have been moved into one of the computers. In neither case is there a separate relay.

The best you can do is try to find a wiring diagram for a 1980. It should probably have some means of increasing the idle, turning on the condenser fans and protecting the compressor from high/low pressures.

Check ohms on the low side pressure switch (should be closed when pressurized) and the high side pressure switch (probably should be normally closed on this year) and make sure all the spade connectors are well attached.

Try to turn the AC on and off and measure the voltage at those pressure switches. You might even just have a fuse out. Note though that sometimes things are energized by applying 12v, and sometimes 12v is constant and the ground is switched instead.

I believe you should have an electric condenser fan, trace the wire back from it as well and you might find a control box. There is no technical need for any relays in a simple A/C system but it's fairly common to find one or two.

__________________
1993 300D 2.5L Turbo