1990 300E a/c compressor/clutch gremlin?

[Benz Mondi]

What do I need to do to engage the a/c clutch while this new compressor is on my bench?

Genesis: Suddenly, my a/c compressor won't engage again, In my previous post, when I pushed the a/c button on my climate control, the #7 fuse would blow. I thought I had narrowed it down (by disconnecting various items in the system) to a bad MAS. I replaced it with a used one and all worked again.

Now, though the #7 fuse doesn't blow, the clutch wont' engage. I'm on my 3rd MAS and 2nd OVP and still nothing.

I even have a new (rebuilt) compressor on hand (ordered for my SL) to use as a test bed. Unlike other compressor clutches on my other non-MBZ cars, I could just apply 12v to the clutch plug and get the clutch to move in-ward to engage. I can't seem to get even the clutch on the new one to engage after applying 12v to 2 of the 3 poles in the socket, in any combination.

Any ideas?

Thanks.

[Hit Man X]

Well, two wires are for the speed sensor and one for the clutch.

Try putting the ground off your test rig to the compressor body and hitting the clutch wire (probably blue) with 12V.

[Benz Mondi]

Originally Posted by Hit Man X

Well, two wires are for the speed sensor and one for the clutch.

Try putting the ground off your test rig to the compressor body and hitting the clutch wire (probably blue) with 12V.

On the bench, the remanu'd compressor has three wires and poles at the plug (looking at the compressor from the front and left to right) red, white, black. The two wires from the clutch magnet are black. One wire is grounded to the chassis and then to the right-most pole. It is also joined with the other black wire directly from the clutch magnet. The red and the white wire go to the back of the compressor. While grounding the chassis, I applied 12v to each of the other two poles, one by one, then across both poles- no reaction from the clutch.

With the compressor that is still mounted on the car, I applied 12v directly to one of the other two poles, then across both the left and middle poles- no reaction.

I tested for 12v while the engine is running and the a/c HVAC control button depressed and at the lowest temp setting- no 12v detected.

I jumped the two leads across the low pressure sensor- no change.

Replaced OVP with a brand new one, replaced MAS with two used spares.

No Trouble codes present.

What am I missing?

[97 SL320]

The red and white leading to the back of compressor are the speed sensor, Don't apply power there.

Use the ground on the compressor body and positive to the black wire at the compressor. You might have a capacitor that is there for radio noise reduction.

[Benz Mondi]

Originally Posted by 97 SL320

The red and white leading to the back of compressor are the speed sensor, Don't apply power there.

Use the ground on the compressor body and positive to the black wire at the compressor. You might have a capacitor that is there for radio noise reduction.

First, I got confirmation that the remanuf. compressor was wired incorrectly. There should not have been a pigtail connecting the wire from the plug pole to the same bolt that the ground wire from the magnet goes to. Once disconnected and 12v applied to the pole, the clutch moved-in slightly and made a click- the proper and expected result. so the remanuf. compressor is good to go.

As for the compressor on the car, I traced the power wire (blue/green) from its pole (left-most) back to the MAS plug socket. I used a volt meter to check its continuity and all is good. Once I reconnected everything, I then connected a test light to the same line, turned on the car and switched on the a/c- test light did not light- no power detected. I then applied 12v directly from the battery to the same pole and got no clutch response.

So it seems that I have two issues at the same time: 1) magnet doesn't energize for some reason (bad magnet, loose wiring at the poles) and 2) something is not telling the MAS (I've already tried 2 MAS units) to energize the line to the compressor (low pressure switch, etc.?)

Next is to take apart the plug at the compressor to make sure the ground wire is properly attached or that the power wire from the magnet is properly attached. Then jump the low pressure switch. After that, re-test with direct power from battery to pole.

I'll report back. Thanks.

[Hit Man X]

You will not receive 12V at the plug without the speed sensor in the loop AND the vehicle running.

Look at the ETM for a schematic of that circuit on the 1990+ cars with the MAS relay or search 'KLIMA relay' on the earlier cars. Same deal.

[Benz Mondi]

Originally Posted by Hit Man X

You will not receive 12V at the plug without the speed sensor in the loop AND the vehicle running.

Look at the ETM for a schematic of that circuit on the 1990+ cars with the MAS relay or search 'KLIMA relay' on the earlier cars. Same deal.

I assumed as much which is why I tested with jumped leads from the plug and socket for the two sensor wires and then added 12v direct from the battery- no response. This lead me to believe that there is something wrong with the plug.

It's a bit cramped in there to unbolt the plug from the holder while the compressor is still in place, but when I managed to do it, I noticed a problem... the wire from the magnet to the plug/pin that comes from beneath the plug bracket had worn out insulation over a 1" area and the exposed wires started to fray! It must have been grounding out as it would touch the case from time to time. This must have been the reason why my #7 fuse would blow at times.

After adding some solder to the exposed wires and covering the bare area with electrical tape, and before the bolted everything back up again, I re-tested with 12v direct from the battery... the clutch clicked and moved!

It's a PITA to re-install the ground wire beneath the plug bracket. The other side of the bracket, a wire with the resistor that is bolted to it. It is impossible to re-attach so I tastefully attached an extended ground wire that I attached to it.

Fired it up again... no clutch action....what?

So know I knew that the problem had to be on the MAS relay end.
After testing with 2 more MAS units (I've accumulated 5 spares overall), I found a MAS that worked!

Lessons learned:
1) if you don't get the clutch to move after applying a direct 12v line from the battery, check the plug itself (assuming that your clutch is still freely moving when turned by hand)

2) Check the continuity of the power line from the plug end to the MAS end, if it's good than it's likely a MAS problem

3) you can test across the tabs from the low pressure sensor to make sure that there is continuity or jump the leads themselves.

Thanks all.