Please help me fix my A/C - PeachParts Mercedes-Benz Forum

KenF · #1 07-08-2010, 09:23 PM

I have no leaks, system is charged, Controls seem to work i.e. if I crank to lowest cool and put the fan on auto, it blows harder. As I back off the temp the fan slows. Everything seems to work except there is no cool.

When I start the car the compressor cuts in for about one second then stops.

I have tried shorting out the pressure sensor - I put a toggle switch on the cables - this allows me to start the car, then short out the connectors. Same result. I think it is setting a fault code but I don't have a reader (yet! - it's on the way). I have to turn the ignition off and restart to engage the clutch again.

The Temp cutout sensor has dropped out and sustained some damage by coming in contact with the moving parts. However it seems to still be functioning (IF I should ever find myself in 130C conditions!!!) Can't really tell but I have tried shorting it and removing it neither makes any change. I think I read 11 Ohms accross the pins.

What's left?

I hear there is a speed sensor on the compressor. My Stealership requires $350 to import this from Germany for me!! I think it will be winter by the time it gets here!

What else should I try?

Yak · #2 07-08-2010, 09:59 PM

Help us to help you: car type? (yeah, I know, it's in your profile, but then I have to go look for the info...)

How do you know it's charged?

Did it used to work, then suddenly stop?

I don't follow what you're doing electrically.

KenF · #3 07-08-2010, 11:05 PM

Sorry, 1993 400E (April)

KenF · #4 07-08-2010, 11:19 PM

We don't usually drive it in the summer. I have the feeling that it gradually stopped working but I could be wrong. It's my wife's car (she only lets me drive it when it needs gas!). Also, in our climate (Calgary, Canada) the EC setting is all we need most of the time.

When we tried to charge it (with friendly backyard mechanic) it showed about 100 psi but seemed to be all air - no fog, no condensation, no cooling of the skin as it escaped. Learned a bit since then. Took it to an A/C place who checked it out and re-charged it but couldn't do more because they are not MB experts. They powered the compressor directly to make sure everything was working before charging. The pressure switch seems to be OK so I am hoping that wasn't a waste of money!

KenF · #5 07-08-2010, 11:26 PM

Electrically? Yeah, I was looking for a couple of spades to make a shortimg wire when I came across a toggle switch - 2 spades!! Also lets me start the car, then come round front and throw the switch to observe the result - that's all - nothing complicated.

speace · #6 07-08-2010, 11:41 PM

I had a similar problem with my 92 500SL. Here is the thread where we went through it.

Intermittent A/C Clutch - A Relay, perhaps???

There is a sensor on the rear of the compressor that senses if the compressor shaft is indeed turning once the clutch is activated. If it doesn't see the pulses from the sensor coil in a couple of seconds it will cut out for safety purposes...

Some of the wires used in the wiring harnesses for 92 and 93 models had insulation that would become brittle and fall off resulting in shorts when exposed to heat over time.

Take a look at the wire harness that starts at the rear of the compressor and runs to the front and joins the plug with the wires to the clutch. These leads are probably brittle and BARE.

To fix this you can just replace the sensor, but you will have to evacuate and recharge the system. If the compressor is still original, you might as well just replace the compressor like I did and go on with life...

speace · #7 07-08-2010, 11:53 PM

Quote:

Originally Posted by KenF

I hear there is a speed sensor on the compressor. My Stealership requires $350 to import this from Germany for me!! I think it will be winter by the time it gets here!

For that much you should be able to get an entire compressor! This part will be on a replacement compressor. This is the way to go!

KenF · #8 07-09-2010, 11:49 AM

Where's the MAS controller and is it the same on my '93 400E as your '92 500?

Is there a wring diagram anywhere for the A/C?

I have all the manuals but I can't find anything.

speace · #9 07-09-2010, 12:12 PM

My MAS relay is a 011-545-8532. It is in the electronics "box" on the passenger side under the hood, in the corner, next to the firewall. The MAS relay has a black knob on top marked I/0. This is "Latched, and un-latched." Rotate the knob to un-lock the relay from the socket and it will lift out.

I expect your 93 400E will have the same electronics. I have a 92 400E. All of that "looks" to be the same, but I haven't compared it in detail to my 92 SL.

On my diagram, the compressor is marked item A9. The two wires for the sensor leave the compressor on pins 1 & 2 of the a/c clutch harness. The clutch "pick" is pin 3.

These two wires from the sensor go to the opposite corner of the engine compartment and connect on pins 40 & 41 of the MAS relay. The MAS relay is marked N16/1 on my diagram.

There should be a few ohms resistance between these two wires and neither should be connected to ground as I recall. When I ohmed mine from the bottom of the MAS socket I had virtually a dead short between the two wires. When I got under the car and physically examined the wire running from the sensor at the rear of the compressor to the harness plug at the front, it was OBVIOUS that there was a problem with the insulation...

mbdoc · #10 07-09-2010, 12:51 PM

1st thing to check on the 400E is the wiring connector at the compressor.

Remove the plug (a pain in the a**) and remove the plastic cover on the 3 wires & check to see if any of them are broken.

KenF · #11 07-09-2010, 01:28 PM

Like ESP! I just crawled underneath (I'm getting too old for this!) and removed the tray to check the sensor wiring. That part looks fine but I can see a connector on the top side of the compressor that looks a bit ratched. I can barely get my hand up there but with a little touch a red wire came free. The compressor still comes on for a second when I start the car so it's obviously not the main power (it's too small too).

Could that be my problem?

Next question - how do I get to the sucker?

I left my disks out at the lake (wife got sick and we had to come home) so all I have is a 320E haynes manual (??) and the "Bible". I see there's an air duct that might get me closer but I'm not sure I'll be able to reach the screws. It looks REALLY ugly from on top.

KenF · #12 07-09-2010, 01:59 PM

Found a backup copy of my discs. I can see instructions on how to remove the compressor - but I don't really want to do that!

At least it tells me that I can remove the Air duct from below - I'll see what else I can remove.

mbdoc · #13 07-09-2010, 02:11 PM

"Remove the plug (a pain in the a**) and remove the plastic cover."

You need a cool engine to start with. then either work your way around the belt Or remove the belt.

KenF · #14 07-09-2010, 02:37 PM

DOC,

What plug? Do you mean the electrical connector on the compressor?
I don't seem to have a plastic cover... Hmmm.

Don't think I can get my hands in from the top without removing the rad. (big hands!)

I got the air duct out and can now get a hand on the offending wire. Maybe if I knew how to unlatch the connector, I could get enough access from the top to affect a repair? Can't see if the sensor wire is going to stop me from pulling the connector up thru' the top. (I presume that's the sensor wire that seems to be going into the bottom of the connector somewhere)

speace · #15 07-09-2010, 02:45 PM

M.B.Doc is the PRO!

He has done this many times. I have only had to deal with this once.

The clutch wire is usually blue and the sensor wires are red and white according to the diagram.

You might want to follow those two wires along the side of the compressor to the sensor in the back and be sure they aren't bare like mine were.

I don't remember there being a latch on the connector, but I could be wrong.

Hopefully, he will clarify this.