Mercedes M120 Ignition Coil Testing

MercTech20 · Post by **MercTech20** » Sat May 13, 2023 6:28 am

Hey picoscope newbie here, I would like to know how I can test this particular ignition coil design. It seems the ignition drivers are inside the ecu instead of on the COP. Any methods with the basic test leads, I unfortunately do not have the COP probes.

vasek · Post by **vasek** » Sat May 13, 2023 7:34 am

Hi.
.
You need to connect one channel to a common power supply wire to measure current.
There are also two separate channels for the control wires.
If there is only one coil per cylinder, it is enough to connect two channels.
The first - is measuring the current on the coil power wire.
The second - is the measurement of the primary voltage on the coil control.
Here is the measurement of the signal on a dual coil with a diagnostic function :
.

MercTech20 · Post by **MercTech20** » Sat May 13, 2023 4:34 pm

vasek wrote: ↑
Sat May 13, 2023 7:34 am
Hi.
.
You need to connect one channel to a common power supply wire to measure current.
There are also two separate channels for the control wires.
If there is only one coil per cylinder, it is enough to connect two channels.
The first - is measuring the current on the coil power wire.
The second - is the measurement of the primary voltage on the coil control.
Here is the measurement of the signal on a dual coil with a diagnostic function :
.

Do i need an attenuator when connecting to the other 2 wires. One is secondary coil ground and the other is the primary signal circuit according to the diagram. So the primary coil signal is what i need to check really.

Post by **ben.martins** » Fri May 19, 2023 9:09 am

Hi there,

Apologies for the late reply.

As vasek has mentioned, current is the safest way to measure these COP styles if you're unsure. In my experience 3 wire COP units tend to have the primary winding in the COP unit so you will only have a supply, trigger and a feedback. Much the same as our guided test which can be seen here - https://www.picoauto.com/library/automo ... nt-3-wire/.

However, if you're ever unsure then it is best to use an attenuator regardless of what scope you have. Which scope do you have? If you can let us know we can provide the best solution to capture these signals.

Let us know.

Kind regards

Ben

MercTech20 · Post by **MercTech20** » Fri May 19, 2023 1:58 pm

Hey so I found a faulty ignition coil and got it replaced with a brand new one. Still there's slight misfire but after a scan it shows one whole bank misfiring. I was gonna try using a math channel on the crank signal and figure out which cylinders were actually misfiring, but unfortunately the belt tensioner broke and now it cannot be started until new parts arrive. Also the picoscope I use is the Xentry scope(4 channel picoscope) which is provided by Mercedes. There is also another ignition coil with the wrong part number but its new, I'll attach the file. The current ramp is different from the other ignition coils.

rayray · Post by **rayray** » Sun May 21, 2023 1:16 pm

Put 2 wire inductive distributor pickup coils or 2 wire inductive ckp sensors on the COPs using double sided tape. Do 1200-1500 rpm brake torques to find lean injectors. The spark plug burn voltages will rise up when there is a lack of fuel molecules in the cylinder, which facilitates the spark to travel thru the plug's air gap to the plug's ground terminal.

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