w201 m103 ac low pressure switch wiring
 

So I need help finding the problem to the ac turning on. I have narrowed it down to the low pressure wiring. Everything works fine and ac is freezing. When pressure gets to about 250 psi or so, fan turns on and then turns off at about 180 psi. Low side pressure is about 30 psi - 35 psi. The way I get the ac to turn on is running 12v to the compressor clutch. If I jump the 2 high side wires (switch with red cap and brown wires) the fan turns on, fan also turns on if I ground one of the wires to the chassis. The low side pressure switch (2 prong switch in receiver drier) is brand new, but jumping the 2 wires (blue/red & blue/red) does nothing. I tried grounding the wires to chassis and got nothing. Checking the resistance of the wires shows one wire as having .070 when checked against chassis and the other wire does not register.

Just checked the wiring from the low pressure switch (blue/red): one switch goes to the push button climate control unit to pin 6 (or whatever pin the blue/green wire is) which shows continuity, the other goes to pin 3 of the MAS unit, which also shows continuity. The 3 connectors on the ac compressor (red/blue,white/blue,blue/green) all show continuity to their respective pins on the MAS relay (pins: 22, 5, 6). I swapped out both the MAS relay and the climate control unit to see if there was any change and there was none.

You need to check to be sure the speed sensor on the compressor is putting out a voltage when the compressor is running. The MAS relay needs this signal to keep the clutch engaged. The value should be greater than .25v at idle.

How do I test the speed sensor?

To test the speed sensor you need to connect a volt meter to the two wires in the plug on the compressor that go to the sensor. The plug has three wires in it, one goes to the clutch and the other two go to the speed sensor. You said you applied voltage to the clutch to make it come on so you need to do that and then check the voltage on the other wires.

Applying voltage and grounding wires randomly on this system can cause damage to the ac pushbutton panel so proceed with caution. A wiring diagram is a must have when trying to sort these kind of problems out.

Best of luck, Robert

Before you spend time on the speed sensor...does the A/C stay on once started? if so, then both sensors are fine.

Just so I understand...do you have continuity all the way from pin 3 on the MAS to pin 6 on the PBU? If not, the new switch may be bad, or your pressure really is low.

I connected a switch that runs 12v to the clutch to engage it on the clutch pin. The regular plug that goes to the compressor is not connected. Yes, I checked all the pins relating to the MAS which are pin 3, pin 5, pin 6, and pin 22. However pin 9 which shows a/c compressor cut in signal to (KE control unit) I did not check because I don't know what that is. So all wheres for the connector that plugs onto the compressor have continuity to their proper termination points. All wires that go to the low pressure switch have continuity to their proper termination points, and the high pressure/refrigerant temp sensor is working as it should. Whenever the pressure gets up to about 245psi - 250psi (I made a mistake previously and said a much lower number) the fan turns on and cycles off at about 180psi.

I don't think you have a single function low pressure switch. I think it's dual function high/low. The question is whether you have continuity across the switch contacts. Test without the engine running...just unplug the switch and measure resistance across the switch. If it's not closed, either the switch is faulty or you have low pressure.

Correct. I do have the dual function switch according to the packaging as well as the fsm. Contact has continuity with engine off. What I did notice was when I grounded one of the wires that goes to the pressure switch, the clutch engages. If I ground the other wire nothing happens.

I know nothing about the W201 system, but I'll share my experience troubleshooting the W126 Gen II system which probably has some similar control circuitry to it.

1: The voltage source to engage the clutch originates in the Klima relay on the diesels (apparently the MAS relay on the gas cars). Not only does it contain the relay contacts for the clutch itself, but it monitors the call for the compressor from the pushbutton unit, monitors RPM from the sensor on the compressor, and monitors the Tach signal from the engine.

2: The control circuit runs from the Klima relay (MAS relay apparently on the gas cars) through the pressure switch and back to the pushbutton unit.

3: The control circuit is GROUNDED by the pushbutton unit which then allows the Klima relay (MAS) to close the compressor relay and engage the clutch.

If the RPM sensor on the compressor is bad, the clutch will not engage. If the tach signal is not present, the clutch will not engage. If the pressure switch is open, the clutch will not engage.

You have ruled out the pressure switch by showing it has continuity.

You have tentatively ruled out the wiring since you show continuity from the MAS back up to the pushbutton unit.

You have proven that the Klima (MAS) relay functions, the RPM sensor is good, the Tach signal is present, and the control signal is present by grounding the wire at the pressure switch.

That leaves a poor connection from the pressure switch back up to the pushbutton unit.

Or:

That leaves a pushbutton unit that has some sort of problem and is not grounding the circuit when the AC is called for. Look for broken grounds, broken traces, poor solder joints (especially around the pushbuttons), or burnt components.

Interesting....I swapped out a push button unit and put another one in to no avail. It is possible that both push button units are faulty and not grounding the circuit for the ac when the 'A/C' button is depressed or the 'dehumidifier'. Since I already have one push button unit out I'll take that one apart and see what I can find out. Any way to know where to check to see which joint should be grounding out or is it just visually checking the joints for broken, missing, or burnt parts?

There was another forum member earlier this year going through the same issues with a diesel W201. He swapped in something like 2 or 3 different PBU's including a programma-rebuilt one. His issue ended up being poor solder joints on the PBU, despite going through several.

Inspection is visual. The solder tends to crack around parts with mass, so inspect around the pushbuttons first. If you see a "ring" around the solder pad it's broken and needs resoldering.

Excellent. I'll give this a try and report back.

I am getting no voltage at the rpm sensor with compressor running.

Are you reading AC or DC? The sensor output is an AC voltage.