S-Prihadi

Gents,

This is a help request, but the electrical/mechanical benefit is all yours, not mine. I only want the knowledge.

I need your kind help to do PULL OUT FUSE 25 at the Font SAM.

With FUSE 25 removed.
1st - Ignition ON, Engine OFF. Scan for codes.
2nd - CRANK the engine and tell me if it can idle properly ? Kill engine and scan for codes.

I did this REMOVE FUSE 25 test few years back, but back then I only have iCarsoft MB V2.0 which is slow and not friendly to use. I get NO DTC at all and my engine can start and idle OK.
I did not test further with on the road test drive.

Today I did the exact same REMOVE FUSE 25 because I have Launch and Xentry to scan for codes/DTC. I am revisiting this test.
The result is the same , NO DTC at all and my engine can start and idle OK, but I believed it did not do the fast 1,300 RPM idle as it used to do when starting from COLD.
I did not monitor RPM sorry, but by ears I can tell it is probably only 1,000 RPM.


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Why am I so keen to test FUSE 25 Removal ?

Because this is one of the fuse for the ECM, among the 5 or 6 fuses the ECM uses.

M272 and M273 uses 6 fuses for ECM. They are :F16, F27, F25, F7, F22 and F23. The ECM family is ME 9.7
M276 3.5NA, M276 3.0Turbo, M278 and M157 uses 5 fuses. They are F27, F25, F22, F23, and F24. The ECM family is MED177

All these engines shared some common wiring strategy from MB, because the fuses are from Front SAM and Front SAM is the same for entire W212 production in the terms of their fuses.

F27 is the always HOT or special circuit called Circuit 30z ( 2nd Circuit 30 ) , which is a stand alone power and has its own wire and its own fuse from and at F32 prefuse box
There is no relay at all from battery to F27. I think this is the Keep Alive Memory power if US cars techy will call.

Source is Circuit 30 ( always HOT ), but these Fuses are via relay N ( Circuit 87M ), they are : F22, F23, F24 and F25.
So it is not a true always HOT for the ECM itself. The power source is indeed always HOT at the Front SAM.

The other fuses of M272/M273, the F16 is via K2 relay (Circuit 30g ) at F32 prefuse box but no relay at Front SAM and F7 is via relay J ( circuit 15 ) at Front SAM.

Front SAM itself get 3 power wires from F32 prefuse box.
1st one is Circuit 30 ( always HOT ) , so not via K2 relay at F32, its own wire , very big red wire16mm.
2nd one is Circuit 30z ( always HOT ) for Fuse 27, 6mm red wire.
3rd one is Circuit 30g ( switched via relay ), which is via relay K2 at F32 and using 10mm red wire.


I believe if this fuse 25 is blown and you guys do not know about it, and since there is no DTC, it can create bad drive-ability or weird problems but as we know from the test I conducted,
Fuse 25 does NOT turn-OFF / KILL my engine.


Now here is the unusual bit.
When MB choose terminal size on the connector, we know that big ones (SLK 2.8 ) will carry higher power than the common small one MLK 1.2.
When MB give a fuse connection into ECM using double or triple wires into ECM, that fuse must be of high importance. That is the logic.

Here is how Fuse 25 is wired into the ECM F connector. F connector is the smaller 56 pins and not the longer 96/98 pins called connector M.

From Fuse 25 single wire, its spliced to make 3 wires into ECM.




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For M272 and M273. the fuse 25 get spliced to 2 wires into ECM. The same pin 3 and 5 is in use.






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If Left Hand Drive cars / LHD, the "FRONT" orientation of the Front SAM N10/1 is as per the black arrow of the WIS.





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I was under the impression that Fuse 25 is powering all the 5V devices/sensors which they all do not get direct fuse like other 12V devices/sensors.
ECM has to use its 5V drivers to supply those 5V devices/sensors.

For my engine M276.8 3.0T , these are the devices/sensors which does not get power from F22, F23 and F24, but from ECM itself.




All devices/sensors not mentioned above are using Fuse 22, 23 and 25. If for M276.8

These are the devices/sensors using Fuse 22, 23 and 24 direct from Front SAM for its +12V supply.





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Its very confusing if Fuse 25 removed as a simulation of a blown fuse, but there is no DTC at all and at least for idling it does not show any bad effects.
I will find time someday and drive with Fuse 25 removed.


Thanks you all for those who would do this REMOVE FUSE 25 favor me.

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S-Prihadi

BTW, I did a test too for devices/sensors I know using 5V volts, I set Ignition ON Engine OFF and Xentry standing by watching the parameters I chosen.

Not all below are using 5V, but I thought it will be related, hence I watch their values too.
NO change of values when I pulled out Fuse 25.

JettaRed

iTrader: (1)

Sorry, but I'm a little lost as to why Fuse 25 is important (other than existing) if no symptoms are showing. What stops working if Fuse 25 is blown or removed?

I agree that it must do something, but what?

JettaRed

iTrader: (1)

This isn't very helpful. Does the WIS offer a better explanation?

CaliBenzDriver

Master Surya, is it okay if I have my chicken-feet for fuse 25 live testing?
If we don't know exactly what circuit 25 powers and the ECU is working why chance the ECU scavenging power internally...

I may have something of interest here: a GLOW FUSE!

Not the best fuse, not the best light but easy to spot an open fuse circuit by the road side

seeing is believing: glow fuse!

Replace your F25 with that and you'll see when it has gone open circuit.

You makeba good point when you say F25 uses 3 distinct path to power the ECU circuit whatever it may be. This circuit is not casual vanilla, it's got power, it must be important or need true redundancy for 3x parh.


> POWER FLIP SIDE
I know today an "ECU Power FUSE" day ... can we make tomorrow an official "ECU GND Path" day?

W212 has decent fuse boxes to supply every circuit branches - This stands in contrast of their painted GND. I have only checked my two engine front GND screws. There are more screws nested in bad aluminum/magnesium alloy oxide...

CaliBenzDriver

MS is trying to figure WHAT circuit the F25 powers. The 3x wires make it look important.

I can see this as puzzling...... and me nuts, love clean GND, all of them!
So I wonder where is the painted post associated with F25 circuitry

This may well be a case of "If it ain't broke don't f... with it"

S-Prihadi

[QUOTE=CaliBenzDriver;8915066]Master Surya, is it okay if I have my chicken-feet for fuse 25 live testing?
If we don't know exactly what circuit 25 powers and the ECU is working why chance the ECU scavenging power internally...


You makeba good point when you say F25 uses 3 distinct path to power the ECU circuit whatever it may be. This circuit is not casual vanilla, it's got power, it must be important or need true redundancy for 3x parh.


/QUOTE]


yes, it is OK to chicken out Cali.


WIS does not mention anything for power methods to ECM.
I looked at ECM clone/repair specialist for MED177 pin out.




From : https://ecu.design/ecu-pinout/pinout...1797-mercedes/
Above image is ECM connector side view.


So they use Pin 5 too as +12V, that is the Fuse 25. Also pin 15, which is Circuit 15 from Fuse 6, via Relay J.
Below image is wired connector side view.




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This was my fuse pull out test result 3+ years ago. Original test date was 3rd Aug 2021. iCarsoft scanner only.



I know now why the idling went bad for 20 seconds back then and not yesterday's test.
3 years ago, I pulled Fuse 27 first, because it was easier by position if for RHD car. Fuse 27 is the main power for the ECM and that will remove fuel mixture learned trim and some other learned values of
my driving profile. So the bad idling of 20 seconds I believe it is because I pulled out Fuse 27 first , before fuse 25.


If Fuse 27 pulled out, this is total DEAD ECM and learned fuel trim gone.
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Hope someday I will know what actually is Fuse 25 powering ?