Need Wiring Schematic for A/C Relay Switch in 82 240D Mercedes Sedan - PeachParts Mercedes-Benz Forum

teddyd49 · #1 08-25-2014, 07:07 PM

Mechanic took out a/c relay switch incorrectly (didn't press button to release easily from base) and the whole thing came apart a few weeks back. Have purchased replacement switch (Meyle 001-542-02-19) that has 5 prongs but I have only four wires feeding into the switch. Have tried and failed to figure out with mechanic which wire goes into which hole/slot on this switch so that the switch works correctly (connector rod comes up when engine a/c starts and retracts when it is shut off).

Have tried for hours putting these wires in different holes without success and have blown a half dozen fuses in the process. Presently the a/c is wired directly and comes on when engine in turned on without ever shutting off...this is a short-term solution to this problem.

Was able to download a schematic from MB in Germany but as is the case almost always, the wiring colors in the schematic don't match the colored wires in my sedan. I have four wires feeding into this switch: brown: ground; blue: direct feed to compressor; red/black stripe and black/red stripe wires...one which is positive (+) and perhaps these two wires are parallel wires (don't know)...

This is a 1982 240D Sedan and these switches (a/c and aux fan) are located on a metal bracket directly above the driver's side front tire in the engine compartment.

This is now driving me to insanity! Does anybody have an answer for how to get Humpty Dumpty back together again????? Thanks in advance for any leg up on this front. Cheers!

jay_bob · #2 08-25-2014, 09:19 PM

The relay has 5 terminals but you are only using 4 as you noticed.

The relay terminals are:
86 - coil supply
85 - coil ground

30 - common
87 - normally open
87a - normally closed

The relay passes electricity between 30 and 87a if the coil is not energized.
When the coil is energized, the electricity goes between 30 and 87 instead.

You have 4 wires, and you have already figured out two of them.
Brown is the ground, it goes to 85 (pin 5 on the base)
Blue is the compressor feed, it goes to 87 (pin 1 on the base)
That leaves the black/red and red/black wires.
One is the power for the compressor, the other is the command from the climate control a/c thermostat (the rotary wheel in the dash).

From my diagram I kept from my 83 the black/red wire is the power feed from fuse 14. That should go to 30 (pin 3 on the base). Check that you have voltage when fuse 14 is in place and no voltage when it is not. (Key must be in run position.)

That leaves the red/black wire (my diagram says it is violet), it should have voltage if you turn the a/c wheel to max cold and no voltage if you turn it all the way to warm. That goes to 86 (pin 4 on the base).

Hope this helps.

teddyd49 · #3 08-25-2014, 09:38 PM

outstanding!

this is the very first CLEAR and CONCISE direction on what to do with this switch and the wires feeding into it that I have gotten on several forums over the past several days.

thank you...thank you...thank you....

now let's see if this solves the puzzle! Cheers!

teddyd49 · #4 08-25-2014, 09:45 PM

one more question and this is going to sound really stupid......

I want to make CERTAIN that I leave no stone unturned to direct my mechanic to undo the screw up on this front.....and neither of us is conversant in electrical things....

I also know that there SHOULD be a diagram on top of switch unit that delineates these holes/connections...87, 87A 30, etc.

Can you please tell me which prong is which in terms of the numbers provided to insure that we get this right...we have five prongs with five holes/connectors in this switch...need to know where these numbers are on the switch?...one hole/connector will not be wired up...which one is this? thanks.

jay_bob · #5 08-25-2014, 09:52 PM

Thanks

I'm an electrical controls engineer so I work with stuff like this every day. Sorry if I wasn't clear enough with the pin assignments, I'm on a plane now flying to a customer site. Somewhere over Texas I think.

87 is pin 1 (blue wire for compressor)
87a is pin 2 (no connection)
30 is pin 3 (black/red power wire from fuse 14)
86 is pin 4 (red/black control wire from thermostat)
85 is pin 5 (brown ground wire)