AC pressure switch

[J. M. van Swaay]

1994 E500, 129K miles:

AC not blowing cold with compressor engaged. Leak trace dye visible on top of pressure switch at receiver dryer. Switch was replaced about 4 years. Need to replace switch again but am wondering if I should replace with genuine MB part or Mahle/Behr which is supposedly OEM. Comments, Suggestions?

Thanks,

J. M. Van Swaay

 

[gsxr]

J.M., get the OE/dealer switch(es). These will include a new O-ring, and I believe both are made in Germany.

Is the leak from the red switch (004-820-68-10) for the electric fans, or the larger high/low safety switch (124-820-83-10), or both?

The aftermarket Mahle/Behr red switch is now made in Taiwan, and is not much cheaper than OE. The aftermarket Mahle/Behr safety switch is also made in Taiwan but at $10 vs $75 (MSRP), I wouldn't take the risk. A/C service work is too costly to gamble on cheap parts. Edit: @M104-AMG reported failures of the $10 Taiwan switch - buy OE/Genuine only!!

ALSO, while the system is open, I would replace the high-pressure service valve/port (002-830-54-84) proactively. I've found this valve to be leaking on at least 3 of my 124's. Very cheap insurance. Buy OE/dealer only, this also should include the required O-ring. $20 list, $16 discount, and takes 5 minutes to replace with the refrigerant evacuated. When this valve is leaking, it's typically a very slow leak, where the system will be cold for 1 season and then not working the following summer.

Finally - double check there is no trace of UV dye around / below the AC compressor pulley. This is another common leak point, and often leaks refrigerant oil without losing much (if any) refrigerant. The cause is the compressor shaft seal. If you find UV dye here, replace the compressor with a new Denso (currently ~$215 at Amazon). If the compressor needs replacement, get the Denso ND-8 double end-capped oil (made in Japan) if possible, avoid using Chinese oils if possible. If only replacing the switches you may not need to add oil.

 

[J. M. van Swaay]

Once again Dave, thanks for your expert advice. I’ll order the genuine MB switch (it’s the larger one 124-820-8310). I’ll also order the high pressure service valve. A local shop quoted me $670 to replace the switch and evac/recharge. I think it’s finally time to buy a vac pump. I already have a set of gauges. Time to learn something new…

 

[gsxr]

J.M., if you have a set of quality gauges, you can buy a vac pump and do the work yourself. But you also would want a scale and a 30-lb tank for filling. I don't think it's possible to get a truly accurate fill from cans. I bought a 30-lb cylinder of Chemours (Made in USA) from the local McParts (O'Reilly?) a couple years ago. Lifetime supply for several 124's as you use <2.5 lbs per charge.

This assumes the system is empty, otherwise you need a $$$ recovery machine to evacuate the remaining charge before doing the work...

 

[gerryvz]

You can get a pretty good approximation of a fill from cans if you have a digital table / postal scale. You weigh the full can, then weigh it as you discharge refrigerant, and the difference is the amount of refrigerant that went into the system. If you have three cans of R-134a, that should be MORE than enough.

You definitely need a good set of gauges, hoses, and a vacuum pump. I recommend this pump, which I got off of Amazon. It has worked quite well.

I did a write-up when I replaced my compressor last year. That write-up is located here:

HOW-TO: Replacing 500E/E500 A/C Compressor | "HOW-TO" Tutorial Articles

This HOW-TO is focused on replacing the A/C compressor in the V-8-based W124 Mercedes-Benz models. Since the compressor and key components of the A/C system are very similar on other W124 models, most of this HOW-TO should also apply to the inline-six models. The Nippon Denso compressor as...

www.500eboard.co

I did not replace the system components as none of them were bad, but it is an opportune time to do so if you need to.

 

[J. M. van Swaay]

Having a little trouble finding an online vendor that has both the genuine MB valve and the genuine MB pressure switch. Any suggestions?

 

[gsxr]

Try MB of Covington, LA or Laredo, TX:

1985-1998 Mercedes-Benz HVAC Pressure Switch 124-820-83-10 | MB of Covington

1985-1998 Mercedes-Benz HVAC Pressure Switch 124-820-83-10

parts.mbofcovington.com

Factory Direct OEM HVAC Pressure Switch Part# 124-820-83-10 Fits 1985-1998 Mercedes-Benz: Up To 35% Off On Every Order And Guaranteed Fit When You Enter Your VIN | MB Parts Source

1985-1998 Mercedes-Benz OEM Part # 124-820-83-10 - HVAC Pressure Switch

www.mbpartsource.com

You'll need to go into the last page of checkout to see how bad the shipping cost is. Alternately, find a couple other items you need to buy to push the total over $100 and get free shipping from MB of Naperville (MB OEM Parts) with code 500EPROMO:

1985-1998 Mercedes-Benz HVAC Pressure Switch 124-820-83-10 | MB OEM Parts

1985-1998 Mercedes-Benz part # 124-820-83-10 - HVAC Pressure Switch

www.mboemparts.com

 

[gerryvz]

Mercedes-Benz OEM Parts | Get Mercedes Parts

Shop OEM maintenance parts, replacement parts, and accessories for your Mercedes-Benz ah low prices and nationwide shipping.

www.getmercedesparts.com

 

[J. M. van Swaay]

Thanks! Ordered from MBpartsource.com, good pricing and cheap shipping.

J. van Swaay

 

[J. M. van Swaay]

Have the parts, vac pump should be delivered today. Do I need to lubricate the o-rings on the switch and service valve prior to installing? If so with what?

Thanks!

 

[gsxr]

J.M.,

Use a bit of Nylog on the O-rings & fittings as needed:

Refrigeration Technologies RT201B Nylog Gasket/Thread Sealant, 30 ml (1 Pack): Nylog Blue: Amazon.com: Industrial & Scientific

Refrigeration Technologies RT201B Nylog Gasket/Thread Sealant, 30 ml (1 Pack): Nylog Blue: Amazon.com: Industrial & Scientific

www.amazon.com

 

[J. M. van Swaay]

Update: replaced pressure switch, evacuated and recharged, pressures good, cooling performance normal for 6 or 7 drive cycles. Now all of a sudden compressor clutch will not engage. Tried jumpering new pressure switch, clutch still won’t engage. Checked fuses, swapped base module, still no clutch engagement. Cooling stopped abruptly so don’t suspect problem with too high or too low pressure. Wires to pressure switch appear OK, any ideas or suggestions? Is there a way to force clutch engagement by bypassing speed sensor, pressure switch, pushbutton, etc? Also checked for codes on pin 8 and 16, no blinks…

 

[gsxr]

If you've bypassed the pressure switch (big one, not the red one)... AND tried a different BM/GM... that is strange. You could try another pushbutton unit if you have a spare, but that's a long shot.

It should be possible to force clutch engagement by connecting directly to the 3-pin connector on the compressor (bypassing the car's clutch control) however this is much easier said than done. Accessing that connector is a PITA. I'd at least measure clutch coil resistance on the odd chance the coil died.

 

[J. M. van Swaay]

Thanks. I’m not looking forward to it but I guess I’ll have to try to access the 3 pin connector. I’m assuming I’ll need to supply +12v to one of the pins. Any chance you know which one?

 

[gsxr]

Here's the schematic of the electrical system:

https://www.500eboard.co/forums/misc/Manuals/Servicev3.2/W124/w124CD1/Program/ETM/2483600.pdf

Looks like pin #3 has a larger wire and goes to the clutch. Pins 1+2 are for the speed sensor, and have smaller wires.

You could also try measuring resistance from socket #20 below N16/1, the BM/GM module in the CAN box, to ground. If open or shorted, that would indicate something wrong with the clutch. Easier to start with before diving into the oily grime on the compressor.

 

[J. M. van Swaay]

Thanks! I’ll get my DVM and have a look at resistance from pin 20 to ground…. Guess I could also supply +12v to pin 20 with the BM removed and see if the air gap on the clutch closes.

 

[J. M. van Swaay]

Finally had some time to spend on this, here’s the update: Removed Base Module, check with ohmmeter across pin 20 and ground revealed continuity through clutch coil. Applied +12 volts directly to pin 20, clutch activated with very noticeable click. Supplying/removing +12 volts consistently activated/deactivated clutch.

Curiously, the first activation resulted in a slight puff of smoke right above the clutch. No smoke noticed with subsequent activations. Pulled codes from diagnostic module, 6 blinks. 6 blinks notes ac clutch jammed or serpentine belt failed. Cleared code, reinstalled Base Module, clutch now operating normally. Don’t know if direct cycling via +12 to pin 20 “unstuck” the clutch or if clearing code 6 removed a base module “lock out”

Let’s hope it keeps working….

J. van Swaay

 

[J. M. van Swaay]

Here’s an update, hope it’s helpful to someone else in the future…

Clutch and AC worked normally for maybe 8 or 10 drive cycles then would not engage anymore. Tried jumpering via base module pin 20 again but clutch would not engage. Suspect maybe mechanical function of clutch jammed so removed clutch hub to inspect. Noted green refrigerant dye present around shaft of compressor. Originally considered replacing just clutch components but with evidence of leak decided to replace compressor. Was inspired by Gerry’s write up, never done any real ac work before but Gerry’s excellent write made the job very doable. Evaced and recharged system, everything working normally now. I wonder if the leaking refrigerant and oil was responsible for the small puff of smoke noted in my earlier post? Could it be that the oil and refrigerant leak caused the whole clutch assembly to get gummed up and stop engaging? The clutch hub and friction disc had lots of crud and buildup…. Just a theory…

 

[gsxr]

Glad you got it fixed, J.M.! Bummer it required a replacement compressor, but you made the right decision. I have this exact problem on at least 1 car - green dyed refrigerant oil leaking from the shaft seal. I have a new Denso compressor and switches ready to install but haven't gotten around to the swap yet. The AC is still working normally, but I don't like the PAG oil leak.