Hello MBWorld,

I recently swapped the wrong relays in my 2014 Mercedes E350 (base V6).

I swapped the "Starter circuit 50 relay" with the "Transmission oil auxiliary pump relay".

After I did this the car would not start or turn over.

It has an ABS light, and traction control light, and it says the power steering is malfunctioning.

The OBD2 port also does not seem to have any power as the scanner could not read anything.

Anyone have any thoughts of what could be the problem or how I could fix/diagnose it?

 

If they relays were identical (otherwise they would not fit), my first guess will be to check for fuses. Likely something was already wrong, and a short may have occurred during the swap.

If fuses are all good, and the OBD not working, it will have to be the old way of tracing backward.

 

The relays ARE NOT identical for their output pins duty !!
You should read first the diagram on the side of the relay and also do understand why relay M ( starter ) is made to be GREEN COLOR, so that people know it is special.
Never never mess/swap Relay M for starter with any other relay except genuine one for M, it is the most unique relay of all in Front SAM.




Relay Q of Tranny auxillary oil pump is a Normally-Closed for its pin 4 (output ), meaning if relay is not energized there is always power at Pin 4 and
when the relay energized the output then change to Pin 5. Pin 3 is power input, always HOT always +12V.

Relay M of starter is a NORMALLY OPEN for its pin 5 (output) not pin 4 , it is pin 5. But its pin 4 and 5 is actually parallel but the Pin 4 get a diode to reduce voltage spike ( back EMF ) from starter solenoid.




Yes, the pin physical seating is the same for both, but pin duty and actual output connection is not.










Remove both relays at once and check fuse 7, it probably blown and we hope that is the only damage you get.








Check fuse 91 at REAR SAM FUSE BOX










This is how Relay M actually works : See Pin 4 is to ground wire . 31 means ground.






Pin 4 final destination, it is ground.





Wide angle view





By right if you place Relay M green into Relay Q place, no harm will come to Relay Q circuit.




==============


If your OBD2 can loose power, because of this mistake, is not good news as it uses different fuse at REAR SAM, not fuse 91, it is fuse 73 at REAR SAM FUSE BOX.
If your fuse 73 is not blown, do a voltage test at OBD connector :
Pin 16 of OBD connector to positive lead of DMM.
Proper body metal to negative lead of DMM. Near the HVAC box there is a nice shiny metal you can use as ground.




Cross your fingers, run the check and do come back to us.

 

A candle, and perhaps a few words. . Hopefully you just blew a fuse.

 

I sure hope there is no other damage aside from Fuse 7 blown.

You see, the Relay M output serves starter solenoid which is a coil and a big one.
Upon disconnection or power removed from starter coil by Relay M being de-energized, the starter solenoid coil will produce a discharge spike to release its magnetic field.....hence there is at DIODE at Relay M pin 4 to absorb the dangerous spike which can be up to 388 volts.

So Relay M with 5 legs, has unique two outputs which is Pin 4 ( 87A )and Pin 5 ( 87) , these two output pins are not like a typical 5 leg relays like Relay Q where the 2 outputs are EITHER ONE ONLY,
so it is either Pin 4 (87A) or Pin 5 (87) and not BOTH parallel-ded like Relay M.

Also do not ever use other non MB relays for MB cars, because MB relays get spike absorber resistor for its coil which is called Pin 1 (86) and Pin 2 (85).
Regular relay does not have that resistor

See the small tube drawing under the relay coil Pin 1 (86) and Pin 2 (85). Without that resistor the spike ( back EMF ) can be 300+ volts.


85 and 85 is German DIN pin identification for Coil plus and minus 12V.
30 is power source or input, where 87 and 87A is/are the output
If 4 leg relay, you get only 87 output, if 5 legs relay you get 87 & 87A outputs.


This is a spike of a a regular relay without resistor at the coil 85/86 ( pin 1 and pin 2 ), yep 388 volts




This is MB relay with resistor at coil 85/86. Only 72 volts spike.





Different relays brand may set their physical pin position different, so becareful.


See how we can screw ourselves when we do not read the relay wiring diagram, usually found on the relay body.



Pray pray pray your Front SAM logic board does not get voltage spike damage.

 

Hello my friend,

I've followed your directions and downloaded the WIS/ASRA to look at the diagrams, but I am not good at this electrical stuff (why I am posting here in the first place haha) so I did not learn much. I checked fuse 7 (starter), and it was blown which now seems obvious as I never heard of the starter before. I also checked fuse 91 and it was thankfully ok. I also tried to locate fuse 73 but for some reason, it was missing, as are many fuses in my rear compartment, but for now I just left it alone as I don't think it is critical to starting the car.

I did not have any 20A fuses with me so I decided to take fuse 26, the fuse for the center console/radio temporarily. I will of course be ordering various fuses to replace many that have been lost.

After putting that fuse in, I crossed my fingers, lit the candles, and tried to star the car. I heard the starter click but it did not start. I just went inside and left it as it was already 3:00 AM.

The next day I remembered that it could have just had a dead battery so I jumped it and it started!

I will definitely watch out next time I mess with fuses and relays, I've learned my lesson.

Thank you for the help

 

Okey dokey..... glad I can assist and your front SAM is okey.
When you do buy fuses, please buy branded ones because unknown brand sometimes their printed rating is not their actual rating....usually bigger rating than actual and that can be damaging.

Go for Litttel Fuse or Bussman or Flosser or other reputable brands.
https://www.littelfuse.com/products/...enger-car.aspx
https://www.eaton.com/us/en-us/catal...ade-fuses.html
https://flosser.com/en/we-about-us

Here is a professor-teacher of Weber University whose channel is so awesome for automotive, watch the FUSE comparison video :


Garbage mini circuit-breakers I have seen, so simple ATO fuse is even easier to made more garbage.

 

I already ordered some, I asked on Reddit and they said any fuses that are rated the same work fine, I bought this set:


I'll be alright, right?

 

NO it is not safe buying NO BRAND fuse. I specfically post to warn you, for your sake. Did the guys at Redit is the one telling you all relays are the same as long as 5 legs ?
Get Littel Fuse .............

 

Ahhh I wish I could cancel the order, I guess I'll just keep this box for emergencies and I'll go to Advanced Auto Parts tomorrow to buy the LittelFuse brand, but damn they are expensive haha.

Or maybe AutoZone's Bussmann fuses...

 

The Amazon link I gave you shows : US$22 for 80 fuses is very cheap. US$0.27 each and if you are a Prime member, free shipping.
I want to buy one too and send it to my friend's kid who is studying in USA. His daddy can take it home for me during holiday visit to USA.
I can get Flosser easy in Indonesia and hence I been buying Flosser, but Littelfuse unless their industrial fuses the automotive ATO fuse is very difficult to get .
I got some only but not complete ratings..

I like Littelfuse sort of matte hazy finish. This brand is very good and also Bussman.
Fluke DMM for the amperage fuse which is a special non-exploding safety fuse, uses ONLY Littelfuse or Bussman.
Our car modules are VERY expensive modules, some costing thousands of US$, give it the best most accurate protection by using the
proper fuse. I mean proper is SAE qualified fuses and/or UL or TUV approved ones.

 

I would buy from that link but the problem is that the shipping says it'll take two weeks to arrive and it isn't a Prime item for me, it's ok though, I think Advanced Auto Parts has it. Thank you though.

 

Yes., yes...I see..I just bought it and it said arriving 18 - 19 July. To US east coast

 

Well, this will probably be my last reply in this thread, I've fully repaired my friend's E350 after getting good fuses from Advanced Auto Parts earlier today with your guidance. When I buy my first Mercedes soon (I hope it will be a E63 ), I will not forget this forum

Thank you my friend.

 

Thank you S-Prihadi. I was about to buy any Chinese 12v 20a for my 2003 E320. Now I know I need a MB relay because of the absorber resistor.
I hope this will correct the problem with the auxiliary fan that, when connected, is always on. My car has 108000 km.

 

FYI: your smart fan keeps spinning NOT because of a bad relay. Put back your original part in place.

-- Confirm the A/C pressure sensor has failed to cause non-stop fan.

-- Other causes involve fan not receiving ECU 10% signal to turn off.

 

Which AUXILLARY fan are you speaking of ?
The main engine radiator cooling fan is not AUX fan, it is MAIN fan and if ur W211 2003 E320 share the same wiring style as W212, there is no relay to control the ON-OFF for main radiator fan,
it is variable speed control using PWM and powered from a big fuse at F32 prefuse block. Note : I am speaking of W212 and no idea/experience of W211 actual wiring.