AC compressor not engaging

[sytruong]

OK, I know this topic has been beat to death and I did search the other topics here. I just had a few questions regarding this.

1995 E420 -

So the system is completely full. I'm reading about 110 psi static at the low pressure side. The car boggles a tiny bit when I do turn the AC button on which means that the car is responding to the input. Both switches at the dryer, there's a low pressure red colored switch and another black switch, the wire is going to those are receiving voltage and I jump both of them.

I'm checking voltage at the harness going to the compressor. And I do not get any volts going to the connector female pins. I'm curious if there's some kind of complicated feedback mechanism or are the pins just supposed to get voltage? There are three pins. There's a red and blue one, white and blue, and green and blue. There is about nine ohms or resistance between two of the pins, which, according to AI means that the clutch is still functional. So I'm just curious if I'm supposed to get voltage at all to those pins or is there an issue of the car not sending power to that harness?

Also, I did read in another thread that the face of the compressor should be turning easily by hand. Mine does not turn easily. According to that thread, I will need a new compressor.

But I'm just curious about the voltage at the connector. I would jump the AC compressor, but the location of the compressor side connector is nearly impossible to pin a power supply to.

I'm just curious if I have two different AC issues or is a bad compressor doing something to the system causing me not to get voltage at the harness side connector

Thanks again in advance

 

[emerydc8]

One sensor on the receiver/dryer controls activation of the low-speed fans. If there is insufficient pressure the fans will not activate. The other sensor does two things: if there is insufficient pressure it will not allow compressor engagement. If the pressure is too high it will cut the compressor out.

Two One of the three wires going to the compressor controls the clutch solenoid engagement and I think the third wire other two are sending a voltage signal generated by the slip sensor in the compressor back to the base module. Incidentally, a bad base module can cause the compressor not to engage but that’s usually far down the list of suspects. Rather than testing voltages to/from the compressor, I would get a blink tester, clear all codes and see which come back after trying to engage the compressor. If the belt is slipping you should get a code for it and the compressor will cut out. Turning the engine off and back on rearms the system but the code will still be stored. You say that you detect a boggle when you turn the compressor on. If the belt is slipping this is what you would expect and a code should be stored. If you can’t turn the compressor by hand this could be causing the belt to slip. Just to make sure, the belt is removed when you try this test, right?

If you don’t know how much refrigerant is in your system, sometimes it’s easier to evacuate it, check for leaks and recharge. If there is too much pressure, including being overcharged, the sensor on the receiver/dryer will cause the compressor to lock out when you try to engage it. I think three cans is all it takes. Walmart was $10 per can when I bought some this week so it will cost around $30 in R-134a but then you will know that it’s correctly charged.

 

[gsxr]

The compressor has 3 wires. The large one is the +12v feed to engage the clutch. If you apply +12v to that wire, the clutch should engage. The two smaller wires are for the speed sensor output on the rear of the compressor. The speed signal goes to the BM/GM, which monitors compressor RPM vs engine RPM and will cut out the compressor if the speeds are out of synch (indicating a slipping belt, or seized compressor).

The compressor face should rotate by hand. If not... something is wrong with the compressor.

Have someone engage the AC by pressing the pushbutton switch, while you watch the compressor face. If it engages briefly and then stops, the BM/GM is likely cutting the signal on purpose, and won't try to engage again until the engine is restarted. A fault code will be stored as well.

 

[sytruong]

Thanks guys. No in my brain dead frustrated moment, the belt is totally on when I try to turn the compressor. I'll take that belt off and turn it. Amongst the other things I will test.

 

[gsxr]

The belt / pulley normally spins freely. The clutch is what connects the pulley to the compressor shaft.

Either way with engine off, you should be able to rotate the face of the compressor with your hand, with minimal / moderate effort. If it's so stiff you feel like you need large pliers to turn it - that isn't normal.

 

[sytruong]

You guys are right on the freaking money again. I watched the car compressor and ran the AC and it twitched a little bit and stopped. So I suppose the clutch is seized. It also doesn't spin freely.

Just FYI, and for the record, I spent about three hours on AI going back-and-forth trying to diagnose this lol.

Anyway, it seems like the easiest thing to do is replace the entire compressor rather than change the fan clutch

 

[gsxr]

If the compressor doesn't move freely by hand, something is wrong internally and most likely the compressor needs replacement.

If the compressor turns by hand normally but the clutch is being disengaged, that could be a different problem (belt slipping, clutch slipping)... or compressor has excessive resistance and needs replacement.

Definitely want to make sure BEFORE you replace the compressor, that's a big job.

 

[emerydc8]

The compressor has 3 wires. The large one is the +12v feed to engage the clutch. If you apply +12v to that wire, the clutch should engage. The two smaller wires are for the speed sensor output on the rear of the compressor.

My mistake. Dave is right—there are two wires (red and white) coming from the sensor in the rear. I’m wondering what the green wire is for. There are only three pins on the connector but if you add the single wire for the clutch you have a total of four wires.

 

[gsxr]

The 3 pins at the connector go to the clutch (+12v) and the speed sensor. The 4th wire going to the loose component is, I believe, a capacitor or something like that? There is no 4th wire for the clutch as the negative/ground lead is the compressor itself, bolted to the engine block which is grounded to the battery negative terminal.

 

[gerryvz]

OEM (Denso) compressors are still available and relatively inexpensive. Just shop around with the appropriate product number for the compressor.

We have a detailed HOW-TO on replacing the compressor, here on the forum.

Thread 'HOW-TO: Replacing 500E/E500 A/C Compressor'

Jul 13, 2021

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 used in the W124 MB models is generally very reliable. However, "accessory" items such as the electromagnetic clutch on the front of the compressor, and the RPM sensor on the rear, do tend to go out with age, mileage and usage. The clutch is considered a "wear" item, and with use will wear down and...

• gerryvz
• Replies: 19
• Forum: "HOW-TO" Tutorial Articles

• 

 

[emerydc8]

Anyway, it seems like the easiest thing to do is replace the entire compressor rather than change the fan clutch

If you mean the compressor clutch you are correct. As the @gerryvz alludes in his HOW-TO, supra, the clutch kit is more than half the cost of a new compressor so it makes more sense to just buy a new compressor with the clutch installed. I’ve bought a few Denso units through Autohausaz.