AC questions on 420sel
 

I am experiencing an intermittent problem on my A/C. Charged it up last year. Low side 40 mm Hg, high side 220 mm Hg at idle. The A/C seems to cut out at higher speed and would mysteriously come back some of the time once speed is slower. Sometime would stay on at freeway speed. I am thinking that it could be the speed sensor. Is it also possible to develop high pressure at increased speed due to an overcharge condition, therefore causing the pressure sensor to cut out. I think the cut out pressures are 40-50 psi for low and 150 psi for high. I can put a manifold guage on and take pressure reading while flooring the gas.

What is the conversion from psi to mm Hg?

Thanks,

John

P.S. A/c blows cold once it is on.

The 420SEL's don't have a compressor speed sensor!

OF course excessive pressure will cut the compressor OFF.

The evap temp sensor would be a starting point...

Thanks
 

I did look at the compressor and did not find the speed sensor. No wonder the Klima did not have pins for the rpm. I did order a ETR, but it didn't seem to work either. I did a continuity test with a multimeter and there was no continuity . I am waiting on a replacement.


John

mm. HG or PSI? All my gauges are psi or Kg/cm2. I think your pressures given are really psi.

1 mm Hg is ~ .01933 psi.

Typical cutout pressures are 2 bar low; 30 bar high. (~30 psi ; 450 psi)

http://catalog.eautopartscatalog.com/mercedesshop/sophio/wizard.jsp?partner=mercedesshop&clientid=catalog.mercedesshop&baseurl=http://catalog.peachparts.com/&cookieid=1RX11DXS31S01A6ZA0&year=1986&make=MB&model=420-SEL-001&category=R&part=A%2FC+Pressure+Switch

A/C is still working intermittently
 

Well my AC is till not working properly. It would start up almost everytime after the car is restarted, then would cut out at 1500-2000 rpm. I changed the ETR sensor twice. After the car slow to idle, AC sometime would come back and would cut out again at increase engine rpm.

Guage showed 30 psi low, and 250 psi high side at 2k rpm when AC is working or when jumping pin 5 and pin 7. Static pressure on high side is 130 psi with the engine turn off. The pressure switch is closed by multimeter testing. Bypassing it doesn't help. So does bypassing ETR either (I bought it here at Fastlane).


Is there other switches/relay that need to be check? Or is this a bad Klima? OVP? Throttle microcircuit?

Help, it is getting hot here in Texas.

John

The "Klima" relay needs a ground signal from the A/C pushbutton at pin # 10 to engage the compressor.

Only other signal is a RPM signal fron the ignition module at the pin marked TD.

How do I check this with a multimeter?
 

So if the PBU is activated, where do I measure voltage across to check for this signal?

Thanks,

John

P.S. I check the new ETR for continuity. I set the multimeter in the lowest ohm setting, I still see an infinity reading, not zero (which would mean that the ETR is open). Is this normal for the ETR?

All testing is done at the klima relay... Again the pushbutton sends ground signal to pin # 10 of that relay to turn the compressor ON.

Use voltmeter between pins 5 (power w/key on) & pin 10 to see if there is a command signal.

The ETS should measure 5K-15K ohms.....

Ok, some number
 

I check the voltage across pin 5 and 10, got less than 0.5 volt

So there is no ground signal. So do I need to check the PBU next and replace if necessary? What else to check and what should I look for in the PBU? I did mess around the area a while back while replacing a switch.

Thanks,

John

P.S. the new ETS measure >5K ohms, old one .35 ohms

update and need help
 

I also check the current between pin 5 and TD, showing less than 1 volt with A/C on and engine on.

What does this mean?


Thanks,

John

bump
 

MB DOC? Anyone?

You need to measure the TD signal between ground & the TD pin. & that needs to be around 6-8 vdc.

Got 8.5 v
 

Checking TD and pin 10 showed 8.5v. So rpm signal is good. Then the next step would be to open up the PBU to check for ground and/or replace the PBU, is this correct?

Thanks,
John

I feel your pain. My W126 has similar problems. Mike R. thinks maybe the monovalve? Only seems to work if proper buttons on the dash are set before starting the car. Then sometimes quits 10 mins. later. Only way to get it running is pull to side of road, wait a few minutes, start it up again and all is good.

Seems being so far north, the Germans never understood airconditioning very well. This is not new.

The story I have heard was that this was a VERY serious problem at BMW back then also. So, four top BMW engineers were put into a BLACK BMW sedan with BLACK seats and NO AIRCONDITIONING and made to drive across the entire state of Texas in the middle of summer.

The BMW airconditioning problems were solved soon thereafter.

So is the problem in the PBU?
 

MB Doc,

As above. Just want to make sure before I start disassemble things.

Thanks,

John