Gents,

I am sharing...................
I was testing the starter relay M ( Green relay ) on my engine , because I wanted to know how its relay coil 85/86 command protocol operates , compared soon to M271.8 engine with SIM 271 engine computer.
Our M276.xx 3.5NA including M276.8xx 3.0 Turbo and probably also M276.8xx 3.5 Turbo and surely M278 and and probably M157, uses the Bosch MED177.xx engine computer family.

In most cases, a relay coil 85/86 will be commanded either by giving or connecting ground to it while the positive feed is already on stand by, or vice versa.

This is the 5 prong special diode protected pin 87A of our starter relay............ called Relay M of circuit 50.
The flyback diode is to protect the car computers from the massive kickback voltage of starter motor big solenoid.

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The DIN german standard indicated that pin 85 of relay coil must be NEGATIVE FEED or ground ( in a negative ground car chassis ).




Sometimes MB wiring is lazy to indicate which is pin 85 and pin 86, albeit this will be good to know from wiring diagram.


My engine, M276.820 , 3.0 Turbo. ECM to Relay M wiring, the relay M is at the front SAM.


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Good thing the ECM wiring shows starter (-) and starter(+), and I did verified at the Front SAM that indeed : 85 as MINUS and 86 as POSITIVE.



W212 Front SAM are all the same wiring method for 3 engines I mentioned using MED177.xx ECM....as far as pin 85 and 86 of the starter relay M.
Front SAM sheet 6, Connector 4i pin 8 is for relay pin 85 and is MINUS or GROUND.
Front SAM sheet 6, connector 6i pin 4 is for relay pin 86 and is POSITIVE


M276.9xx 3.5 NA. N3/10 is always Engine Computer component name




M278.xx and M157.xx. N3/10 is always Engine Computer component name.




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I am repeating : In most cases, a relay coil 85/86 will be commanded either by giving or connecting ground to it while the positive feed is already on stand by, or vice versa.
However for our MED177 based ECM, the command are both at pin 85 and pin 86 of the relay, meaning both NEGATIVE and POSITIVE is on purpose not available until you CRANK the starter key.
I guess this is safer than single 85 or 86 command only.
So when you guys troubleshoot, do remember this. Dual command.


This is Ignition Key inserted ( ECM wakes up ), voltage at relay M pin 85 and pin 86, just ghost voltage, can't even power a 1 watt test light.






Test Method, which is on video, at the bottom.






If pin 85 loaded with 100 milliamps test light, 1.2 watts, ghost voltage gone. So don't be surprised seeing ghost voltage, because the relay is operated by a transistor/driver and not a true ON-OFF switch
This is key out of the ignition, so Front SAM is partial awake and not in deep sleep yet.... but ghost voltage is lower than with ignition key inserted ( ECM wakes up ).





Yellow Fluke DMM is pin 85 of relay. Blue EEVBlog DMM is for pin 86 of relay.



Loading pin 86, blue DMM




The EMPTY relay I made a few years ago, useful to do this test......


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1 Amp fused supply as +12 volt. Must play safe when supplying +12V to Front SAM




Ground is from engine lift up hook, rear side.


The CRANK command is 3 seconds maximum when engine does not start.



WILL CONTINUE..........................

 

The video of the test. The CRANK command is 3 seconds maximum when engine does not start, even if you simply do a 1 second CRANK, its is auto 3 seconds CRANKING.

You Tube

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The DTC for this relay only appeared when I try cranking the engine with my EMPTY relay.
Quite a weird description ....LOL. Line A, what the heck they meant when both pin 85 and 85 are not connected.....



I think Relay M does not get the typical sensing like how engine sensors are, when accidental disconnect. Thus unless I tried CRANKING the engine, no DTC will appear with green relay M removed,
even with Igition ON engine OFF ( position 2 ).


That is all for today...........

 

always uncommon research!
you have not lost your edge.
👍

 

If I may add...........

So that other W212 owners will not be left out, here would be and equal of E200 and E250 with M271.8 engine starter wiring.
NOTE 1 : I am using C200 W204 front SAM wiring diagram, not W212 Front SAM.
NOTE 2 : I do YET not know if the command to the Relay M is both at positive ( pin 86) and negative ( pin 85 ) or only one being used....... I am waiting for W204 of 2010 using M271.8 to learn of it.


M271.8 ECM connector F pin 23 labeled STR ( starter ) is connected to Relay M coil 85 at Front SAM sheet 6 of W204 ( not W212 ). So by DIN standard this pin 85 is negative/ground.
It seems W204 retain some similiarity in its Front SAM N10/1 design to W212.
The Relay M coil 85 is connected from ECM at Front SAM connector 4i pin 8 , the same as our MED177 and our W212 Front SAM.

M271.8 ECM connector F pin 25 without any label, is connected to Relay M coil 86 at Front SAM sheet 6 of W204 ( not W212 ). So by DIN standard this pin 86 should be positive.
The Relay M coil 86 is is connected from ECM at Front SAM connector 6i pin 4 , the same as our MED177 and our W212 Front SAM.





============== OWNERS OF M271.8 and M276.8xx , Special Note ======================


As with latest engines like M276.8 and M271.8, the Front SAM wiring automatic link ( blue text is automatic link ) WILL NOT WORK for these engines.
Will not work as in it will not get you to our specific engine computer, you will get connected to M276 which meant M276.9 and/or M271 not turbo and not M271.8 turbo.
So becareful when using MB WIS while hoping its automatic wiring link in blue will get you to the CORRECT wiring diaghram, when your engine is M276.8 and M271.8, you WILL get screwed.

You must do manual search on the document last 2 or 3 digits by using asterix ( wild card ) and it will show everything available.



For reason unknown to me, but I suspected it was never properly updated for facelift cars ............. W212 owners with M276.8xx Turbo, be it 3.0 or 3.5 liter version, we are pariah.You need extra work.
Front SAM wiring diagram set exist as far back as 2009 when W212 starts its life. In 2013 and 2014 big facelift, Front SAM wiring did not get updated properly............. even on my 2018 and 2020 WIS.







The N3/10 in blue text if using MB WIS, will link you to a new wiring diagram. M276 owners is to use U1018 or PE07.08-P-2101DAH wiring diagram...it will link to M276.9xx and not M276.8xx.




M276.8 owners need to do manual search to get to DAQ wiring diagram.




I am repeating :
You must do manual search on the document last 2 or 3 digits by using asterix ( wild card ) and it will show everything available.
PE07.08-P-2101d*
or
PE07.08-P-2101*

From PE07.08-P2101 becoming PE07.08-P2101-97 is normal. 97 is the real final destination. The 3 alphabeth is most important. daa, dab, dac,daf

If a search is PE07.08-P2101-97* , you get 300 over documents




If a search is PE07.08-P2101-97d*



Basically MB WIS for updated or facelift cars of W212 era, is not friendly.

 

I think I must not assume on other relays 85/86 command being single wire command, now that for Relay M....I am seeing dual separate command each for 85 and 86 pins/wire.

 

it's very possible some of SAM's cube relays get controlled from both side of the coil.

This control needs dual conditions at once:

• Coil +12: chassis awake powered ON.
• Coil GND: SAM MOSFET active ON.
• SPDT relay output ON.