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A while ago I was scoping my COP working parameters.
I noticed the voltage drop does not look pleasant at all, 1.523 volts at peak load of near 20 amps at COP pin 3 which is the +12V power supply.
I added what I call extra resistance caused by interconnection terminals and put it as 20cm worth of extra wire length.
Any interconnect is not a good thing, as the copper core is no more non-stop compared to a copper cable with no interconnect in between its entire wire run.
Copper core crimped to female terminal, female terminal mating male terminal, copper core crimped to male terminal = all these added extra resistance, and hence I put their value as though as
wire length get extra longer by 20cm or 2/3 of a foot.
On my M276 3.0 Turbo, the wire length estimate and size below :
Too bad MB on purpose will use a small 1.5mm cable size for fuse 24 and bigger 2.5mm for fuse 23 even on M276 3.5NA.
Loading is equal for fuse 24 and fuse 23 on my car and would be the same for M276 3.5NA. Diesel engine I do not know.
The last time I scoped COP 4 ( fuse 24 ), I misread the data as to why COP 1, 2 and 3 which are from Fuse 23 has much lower voltage drop seemingly only 0.38 volts and no where near 1.5 volts.
THE REASON WAS :
Albeit the scope Session 1 is only for Fuse 24 ( COP4,5 & 6 ), shown as Zone 2, since everything is interconnected at Front SAM which is equal to Main Positive Bus Bar for most power consumption of the car's
engine and drive train related power consumers........ I can also see Fuse 23 ( COP 1,2 & 3 ) activities.
However, if I truly wants to measure accurately Fuse 23 or COP 1,2 and 3 voltage drop, I must do another session which I marked as Scope Session 2 below :
If I rely on Scope Session 1 reading, I am reading fuse 23 ( COP1, 2 & 3 ) before it get loaded by its COPs as I can only read up to zone 1, hence its voltage drop look so low at 0.38 volts
The reason I can't improve much is the connector in use for the COP. The Kostal Connector. Its cable hole is a limiting factor.
It won't be able with ease to take a 2.5mm wire, to replace its 1.5mm one for pin #3. I also can not yet get the silicone seal for 2.5mm fatter wire and I do not want to loose
waterproofing integrity.
So, I shall put on-hold the voltage drop improvement for COP +12V supply, till I can find an easy to do solution. I will not tear open Front SAM to replace the wire of fuse 24 to X26 of 1.5mm to 2.5mm... no no no.
Since my alternator LIN is disconnected and my car is always at a beautiful 13.9 to 14.1 volts all day long , loosing 1.5Volts is not a big deal as lowest voltage for
the COP +12V power during peak load , which is still at 12.45V. If it is under 12V, I would by hook and crook modify the wiring.
Hi Surya.. Perhaps I am confused, but to me there is no such a thing as an "unexplained voltage drop". Here is my interpretation:
1 - Every voltage drop is due consumption by a component
2 - Every voltage drop must be shown in the schematics, or associated to a component in the schematic (black box)
3 - Unexplained voltage drop are nothng more that gremlins in the installation, or hidden components in the schematics.
It is a like a water distribution network: pressure drop are losses at components, and unexplained water amounts are leaks or hidden consumers.
Well, voltage drop always happen when there is a load, no way to avoid it.
However, we want by bigger wire size to reduce voltage drop.
For cars COP , since the peak load is so short, only 1 millisecond in a single spark event, designer doesn't really worry about abnormally high voltage drop of 1.5V from 14V or = 10.7%.
If a constant load device, I will not allow voltage drop higher than 5% at peak load for sensitive equipment.
The interconnect voltage drop is like a mild soft bend 45 degrees elbow in a water pipe, they are not a good thing if we can avoid it, but better than 90 degrees hard bend.
I forgot to add, I tested my engine COP voltage as in COLD condition.
Every increase of temperature, there is a de-rating factor for any electrical conductor.
Hot is never good as resistance always increase with heat if for copper but for carbon brush at alternator hot is good.
Surya, I do not think saying 1.5V drop out of 14V is unacceptable is a fair statement. There is a cost to transfer something from A to B. The only way the theoretical value can be 0 is for the distance from A to B to be identically 0, in this case no wires/connections, and in my water network analogy: no pipes.
Once the ECU is not directly connected to the COP (which is impossible), the voltage drop is linearly proportional to the distance from ECU to COP (plus connecting losses) as you have described previously. No amount of thicker wire will make it 0, but only a more tolerable drop, correct?
A known intended voltage drop is (to me) just the cost of doing business between A and B. As with any transaction, there is a cost-benefit ratio to be considered before making a decision keeping in mind cost is not about money only.
I would aim for 5% voltage drop at COP peak load if Front SAM is easy to re-work its connector number 3M pin 5 to fit a 2.5mm wire size.
And if I COP connector can take 2.5mm wire with ease too, 0.75V voltage drop would be awesome.
Imagine any car in ECO mode and LIN is still connected, they get 12.7V at alternator often.
That means the COP at peak load is a tiny but over 11.x volts only and that is not a good thing for any device which uses 12V as nominal operational voltage and expects 13.3V as minimum standard
alternator voltage during engine running.
Good thing now our sensors are more of 5V type, so more headroom for voltage regulator to keep it at a nice 5V always even at cranking voltage which could be 10V or less for very short duration.
New cars trying to save money and weight by using so small sized wires or borderline sized wires is a sad thing.
======================
This won't happen to a typical Japanese cars in the tropics using the same 5 watts load bulb.
Below is from a C200, W204. Under sized wires & not high enough temperature rating wire .......... for parking/standing lights and day light at engine bay hot temperature + incandescent bulb hot temperature
0.5 amps only if 5 watts load, but temperature de-rating for wire ampacity has to be massive due to up to 90C+ ambient temperature of engine bay
MB even failed to de-rate or upgrade the plastic material used for the bulb holder , if for the tropics and more so when used as a daylight service for headlights sharing engine bay air space.
Even at night as standing light, in the tropics with my city traffic jam, ambient temp of engine bay is only cooler by 3C compared to day time.
All these small wires are scary for longevity and reliability long term.
There is another de-rating factor for when a lot of cables are bunched closed together and in a conduit.
I am really tied up with guest plus uncle Murphy's 4th Of July gift... emergency bathroom plumbing: The Pump!
bathroom pump 45° elbow by the leader Zoeller!
My 15 year old pump whent into full stop... it won't restart. All new models are 2In. A new Zoeller quiet check valve is now available. Original valve always banged hard at every cycle.
So I must swap the pump and repipe the new valve in earth quake county.... new PVC design better have some flexibility because it's not as strong as current cast iron pipes.
The big question is how to allow flexibility with rigid pipes?
The incentive is I don't like repeat grey-water jobs.
It needs to be serviceable, not all glued solid...
not fun under the sun!
COILS mixed with cam SENSOR
They are couple opportunities for improvement:
what causes voltage loss us called OHM'S LAW:
A copper wire occurs a resistance based on its diameter and length.
The relationship with voltage is based on the amount of current: VOLTAGE = RESISTANCE X CURRENT
Voltage drop remedial:
The current is dictated by coil loads. You can split in half, wire ea. bank for the distance of travel between source and coils.
Noise reduction:
Reuse the "hall sensor" power to feed coils, same fuse circuit, right!
Move clean cam sensor power to Fuse #24: great match with clean ECU power.
Scope the cam sensors for extreme noise from COP+ VVT Coils:
POWER to Reference
(OUTPUT signal to Reference)
It's amazing BOSCH can filter out the noise to read a hall sensor in an artificially extreme noise environment. Why not screw up signals with noise
-- Let remove the noise from this poor setup :
- 4x 12AWG wires from SAM to COP/VVT banks
- Cam sensor moves to clean F24 power
Priority is move Sensor out of noise curcuit. Rewiring coils very likely won't improve engine power. It will make it more reliable.
Speaking of reliability... "dual path power": leave existing coil feed circuits in place, add to it!
No need to slice the whole harness loose, make your own distribution - Around Linux, this called "distro"
Noise reduction:
Reuse the "hall sensor" power to feed coils, same fuse circuit, right!
Move clean cam sensor power to Fuse #24: great match with clean ECU power.
Haha haha seems easy in writing ..... I don't want to tear open my front SAM as wiper arm need to be removed for my case.
From Front SAM its 2 output wire bundles are already in a wire conduit , part of wire assy and the rubbberized cable gland is waterproof type, any extra wire need to be inside this conduit .
Also how to attach 12AWG ( 4mm ) to connector 3M of front SAM ?, the female terminal at best can only handle 2.5mm by its crimping claw.
If this is my boat fuse box, easy peasy..............
That voltage drop calculator is a simplified Ohms law calculator, better than I do manual calculation.
When one day I am comfy to tear open my front SAM like you did, I will re-visit this matter.
I think I will venture into Front SAM this time
Need to follow your foot steps.
End of this month or early Aug, I will open up my wiper plastic cowling for HVAC work. I want to install a thermocouple at the fat output/vapor pipe at TXV.
I already ordered a new cowling, because mine is a tiny bit dull already and to look good I need to coat it with those plastic trim protection/conditioner spray.
It is 9 years old anyway and I want to make it new looking.
When I open up/replace this plastic cowling, I can then access the wiper mechanism to remove it, grease it and yada yada, there then the Front SAM module can be lifted out properly.
20% of my front SAM is blocked by the wiper mechanism.
I have ID-ded the connector Number 3M. Its terminals are not SLK 2.8 of Kostal, but MCP 2.8 from TE and is a custom ordered 7 pin connector for Mercedes, as the 1st pin is super big using MCP 6.3.
However, Pin 1 MCP 6.3 is only for a car with air injection, and mine does not have such device.
I do no need to worry about the MCP female terminal. I have it already when I bought many brands/type for fun sake, but for 1.00 mm2 maximum wire size.However, I will not mess with the female terminal anyway.
I will peel the wires of Fuse 23 and Fuse 24 wire and solder an add-on AWG14 (2.5 mm2 ) wire to them, perhaps 10cm away from the female terminals.
Make new hole and use waterproof cable gland on the black plastic body of Front SAM where there are now 2 wire harness exited at. item 30 at above parts drawing. It is do-able.
1 cable gland can fit 2 of AWG14 (2.5 mm2 ) using plastic tube to maintain roundness so waterproofing seal of cable gland can work. Tube only need to be 2 inch long and then
the tube gets heat-shrink and extra layer protection from nylon-PET wire harness jacket.
These two new wires of AWG14 (2.5 mm2) goes into the plastic box ( cable tray ) under my ECM where all wire splices are there.
Each 1 of AWG14 is then spliced to have 3 output wires of AWG16 (1.5 mm2) and each of them goes to each Bank's COP pin #3.
I then peel-&-solder again this AWG16 ( 1.5 mm2) wire to existing 1.5 mm2 wire of COP pin #3, probably 5-7cm before the female terminal.
Will need to de-pin all 4 of SLK 2.8 female terminals of each COP to allow use or slide in of a new slightly wider wire jacket .
So each COP basically its pin #3 is 95% equal to getting 2 of 1.5 mm2 wire from splice to COP and extra 2.5 mm2 in parallel between Fuse 23/24 to the splice.
Voltage drop should be 0.6V to 0.74 volts estimated and no more 1.5V ish at peak 20 amps load. 5% voltage drop class, acceptable.
The new COP extra wire set must be waterproof and must make Front SAM box stay spray-proof
Those whitish thingy is hot glue. This hot glue DIY waterproofing a cable entry is actually submersible , I tried already in a bowl with water for 2 hours a month ago before I pull the trigger to do this mod.
Food coloring is used for the water, so I can see if any trace of water actually enter the hot glue section into the tube.
Test success...........
=========================
The wire harness
The use of the translucent plastic tube is to make ROUND, this way the cable gland watertight round seal will work properly.
Cable gland can not work on 2 x 1.5mm wires side by side.... we need to make it round. Hence the translucent plastic tube is a must to use.
Extra protection layer. I think it is called a PET material, some sort of plastic which does not burn like regular plastic bag.
How to make the hole entry at Front SAM box ?
Burn the sharp tip with my lighter to heat it up and easy to do a bore hole at the black plastic.
Can't use powered drill. No space. Go Manual.
Must make hole bigger to fit the cable gland assy
Hole size good and tight. Use test cable gland, OK.
=============================
Now splice & solder for fuse 23 and fuse 24 output.
Not the greatest result..... Dughhh
The 200 Celcius silicone wire I use has super super fine strand , super soft and flexible and is already tin plated. Its not the easiest to "marry" to the MB copper wire.
Pulling new thick wire into existing harness is no fun............
Silicone spray helps smooth the cable-fishing
My useful small plastic tube serves me well as the cable puller
Some Silicone on your burger anyone ?
Use my old soldering trick........... Forced Marriage
Damn this silicone wire strand are so fine.................. no wonder they are super flexible. Like high quality welding cable strand.
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Triple layer electrical and abrasion protection
3rd defense layer. Good I have the Tesa. Matt non-shiny black is indeed nice.
Not too bad...........
North meet south....................
Believe it or not, I can't find small OD heat shrink with adhesive in Jakarta. They have the big OD ones for commercial cable sizes 50 - 200 mm.
Good thing I already got it from USA - Amazon.
Sometime its a pain to want to get good stuff locally. Tesa tape from Ebay-USA and genuine one.
And then my usual forgetful me, I forgot to pre-sleeve the BANK 2 wire harness with the adhesive heat shrink...LOL. I ended up using 3M Super 33+, see it is shiny
OK Done...............
Will update scope reading of reduced voltage drop in a week or so.
I will also use the red 100% D100L De-Oxit on all these COP and sensors. https://caig.com/product/deoxit-d100l-2db/
Voltage drop improvement on average less by 0.3 volts.
I was expecting a bit better result
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Below, Channel A blue, probe is to pin 3 and pin 2 of COP.
Channel B red, probe is to pin 3 and battery ground at suspension tower.
I forgot to measure COP amperage ....duggghh.
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Something good to know.
The small copper stud on my COP and its brass rings are the body ground of the COP.
So W11 ground wire supporting pin 2 of COP carry only so little current. Engine cylinder head carry the bulk of the current.
Come to think of it 19ish amps for a COP is a lot of power.
.
This beefed up COP positive wire and the new oxygen sensors been very good for the engine throttle modulation speed and ease.
I did not know (forgot) you went through that.
Now do you have the diagram of what relay on board of F-SAM switches the coils & injectors +12V power side ?
These are hard working contacts with nasty sparks from inductive loads at each open/close - Well worth swaping new.
Also the 15A fuse on coil.... (upgrade to 20A for less drop).
Pwr supply: GND:#2 / +12:#3
Lets sanitize 100% of these ignition/injection circuits including W11 that feeds ALL the power GND!
Last edited by CaliBenzDriver; Apr 30, 2026 at 05:25 AM.
It is Relay N, powering fuse 23 and fuse 24 for COPs of both banks and some other devices.
Too bad the relay is not the very heavy duty one like the grey relay, but it is Black Relay.
.
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Latest 2026 verification below :
Black relay N is easy a 30Amp relay
=========
I like the grey relay. The pin 87 and 30 is bigger, that meant this is heavier duty.
Only Relay J and K get the grey one.
Below is relay J pins tested.
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The micro relay is the worst and had TSB ( you informed me of it, remember ) . It is so small.
Front SAM, Relay R and relay O is micro.
Rear SAM also quite a few micro relays, but I don't remember them well.
Front SAM works at hotter temperature, is much worse ambient temp than rear SAM.
Again you have turned up an ESSENTIAL TOPIC to recognize built-in issues.
We have a vested interest to put these design defects to rest.
The COPS must always have identical voltage without significant change so ECU can time ignition precisely.
Here Surya gets 1.5v... huge delta: OMG !!
200mV Max may be acceptable ripples
8x times that... not for us
This is very significant because funky ignition timing is related to some of the engine extreme heat. MB really hands out short sticks.
The ignition coils sevondary spark plug voltage is directly proportional to the primary winding voltage. When primary changes a little secondary changes a LOT by an X- factor near 3000 times more.
This has a huge significance !!
40kV secondary over 12v Primary = 3,333 factor
We need a steady clean primary voltage that ECU can adapt to - Not moving target with marginal performance.
Thus we've got to deliver clean stable voltage without ups/downs GDI ECU is unable to deal with.
The list of items used to deliver unstable voltage is long ...
We need to rework everything that's causing this poor electrical circuit:
SAM (relay + fuse + oxidation)
Harness undersized
Prefuse oxidation /links
Connectors + splices
Shared power GND
(ALT-LIN:14.15v)
...
I dont have a lot of time today to dig in every item screwing voltage supply (ign + inj) one by one.
because drops add up in serie.
Firing A/F mixture at not precisely exact time causes unnecessary top cylinder wear + high heat + lean misfires.
Testing shows less randomness makes our engines tick more reliably.
The bottom line is variable voltage spark at variable times. Hence the importance of controling voltage to get precise timings.
Last edited by CaliBenzDriver; Apr 30, 2026 at 04:32 PM.
The reading of lowest voltage using a scope is a blessing and a curse.
The blessing is we can see the truth , which is the voltage drop or COP firing .........in faster than 1 millisecond or 1/1,000 of a second. About 0.8 millisecond.
The curse is, when compared to regular DMM, we now see true voltage drop a DMM can never see, super short duration, but voltage drop nevertheless.
There are bound to be voltage drop/s.
The Front SAM bus bar is not the best.
Every relay connector has voltage drop.
Every wire connector to and from front SAM has voltage drop.
Every intermediate connector the engine uses, like the X26, there will be voltage drop.
The COP female connector itself, has voltage drop.
ECM connectors has voltage drop
Add engine bay temperature of typical 80C to 90C / 176-194F for me in bad traffic jam = more voltage drop.
I have done my best with clean and beefed up grounds and cleaned F32 prefuse box and Front SAM.
Use your scope and test the same like me, see what voltage drop you got at your COP terminal 3, power at 50ms per division. So we can collect more data.
So far only me done all this kind of voltage drop test, no one else is this forum will do this for the sake of knowledge and how many here has a proper scope ?
I will not do anymore improvement, I already max out.
Now its maintaining for connections to be clean and tight...which is a must and this is on-going as car is aging.
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I am trying to review the steps you went through here in this thread.
scope traces
I find it interesting how the voltage in the circuit you're scoping goes between ALT (13.97V) and BATT (12.5v).
I dont know what is this exact circuit location before/after what rework ?
What does it drops to Battery voltage ?
It looks as if this is evidence of prefuse 350A fuse drop... its easy to reach wrong conclusions so I ask
> RELAY UPGRADES...
If we are facing relay issues there are many upgrades to pick from.
Apparently we are dealing with 30A/12V relays.
I see there are 40A Bosch relays (0986332030) and 30A with 2x extented life cycles (0986332302) as found here.
> Noise mix injection... engine V-banks split in two
> scope opportunity...
I still find it wicked to mix the noise from VVT "Magnet" solenoids plus COPS to feed the clean Cam SENSOR (CPS).
Last edited by CaliBenzDriver; May 1, 2026 at 09:50 PM.
There is no MB design I seen, which the COP/s uses stand alone fuse/s and not sharing with other sensors.
Even other car makers would share COP fuse/s with other small loads/sensors too.
The voltage sensing at COP is using custom DIY connector adapter for COP wire side and naked male terminal .... since I can't find COP side equivalent connector, no one sell such connector.
This DIY custom test wire set, which in itself add extra resistance due to 2 pcs more female+male terminals and the banana plugs (2 ),
total 4 extra terminals per wire and the extra 20cm of wires too. If I probe directly using offensive wire piercing version, the voltage drop would be less.
COP wire side connector, we can buy spare one. Label A, is my DIY COP wire side connector.
COP body side connector, no one sell it, so I use naked male terminal. Label B.
Label C is COP wire side original to the car.
I hope this is clear.........
This is the DIY test wire for COP.
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I will only use piercing tool , when it is on my DIY test wire and not on MB original wire. See the two yellow wire piercing tool.
===============
If only I can buy the COP side connector as stand alone connector set, it will be awesome.
No one sell such connector with MALE PIN side. I call this COP side connector. I asked my indie if he has bad M276.8 COP, but he does not have any......... I want to canibalize the connector
You're running a nice test setup for COPS with pico sensing clamps. dual clamps sensors
You then were able to evidence a 1.5V power drop in your test set: significant drop in shared power
This power line feeds one bank of 3x cops + 2x VVT oil solenoids ("magnets") PLUS ... the 2x CPS reluctor sensors.
You're saying it is a standard MB engines (M27x) setup to supply Camshaft reluctor sensors with noisy power.
No question this setup works! Noisy CPS signal can be filtered by firmware.
I got here because I found my M276 ignition coils setup was super voltage sensitive. That is way abnormal.
Seeing all this... you rewired part of your COP circuit.
-- What were the results of beefing up the supply line ?
-- Did that zap the 1.5V ripple ?
There is something big to be discovered:
How to cancel that nasty voltage drop that detunes GDI performance on M27x ?
You have the perfect test gear to capture this.
Voltage gets lost somewhere between the engine bank splice and the prefuse.
> How about this quick test setup:
Couple test points ladder may be...
Prefuse input
Prefuse output ??
(SAM input)
SAM fuse 23/24
Shared splice
COP Pin2
It is essential to find where is that voltage being dropped ?
Until we know, I keep my scary prefuse strap.
Arigato gozaimasu
Last edited by CaliBenzDriver; May 2, 2026 at 03:15 PM.
as I can only read up to zone 1, hence its voltage drop look so low at 0.38 volts
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