Troubleshooting MS261CM with ohm meter

[Robnjay]

I’m trying to verify that I actually have a bad coil on this saw before I spring for the new part, here’s what I have done so far:
Replaced the solenoid with a new repair kit (white solenoid + orange filter)
Disconnected the 2 wire harness from the coil and measured both wires to ground - black is open until the lever is moved to the off (up) position, then shorted - red measures 36 ohms to ground. This is probably the resistance of the solenoid coil.
Measured the coil:
Primary is 1.87k ohms
Secondary is 2.4M ohms
Primary to secondary is 24.7k ohms
Zat 4 no spark…
Buy the part???

 

[stihl #1]

This is an early CDI:

This is a M-Tronic:

You can't ohm out between the primary and secondary because the circuit board is in between them. I have never tried to compare readings from one known good module to another, so maybe there is something there but to my knowledge STIHL never documented it.
The only thing I have run across is a break in the wiring harness that would give an intermittent open, so test the wires from end to end for continuity while flexing and stretching them and if they are good, and you have a new solenoid, then it probably is the module, and the only way to verify would be with the MDG1. I think if the wiring harness is open you get one spark on the tester and then no more, but that was a long time ago when I saw that.

 

[Robnjay]

I managed to get ahold of a service manual and Stihl shows the primary reading to be 1.5 - 12k. No spec on the secondary (red terminal to black terminal). I’m not sure that my primary reading ensures a good part…

 

[Robnjay]

I managed to get ahold of a service manual and Stihl shows the primary reading to be 1.5 - 12k. No spec on the secondary (red terminal to black terminal). I’m not sure that my primary reading ensures a good part

 

[stihl #1]

Sorry, I put the wrong pic up for the M-Tronic. Here are 3 views of the M-Tronic:

The charge winding is on the left, the primary is the large gauge wire with the secondary wound around it. The main capacitor is in between. See how the primary wires solder to the board. The secondary connects to the electronic components on the board so that is why an ohms reading doesn't tell you anything. The second terminal is for the solenoid and has no connection to the ignition windings. So any ohms reading between them is also of no value. Ohms reading on a coil only tells you something if it is old school points/condenser type, not capacitor discharge electronic like I guess everything being made today.

Notice this side of the circuit board is covered with components, plus the big microprocessor on the right.

Even the back side of the board is covered with components as well.

This schematic shows you how all the bits connect. I had forgot that there is a switch and diode on the choke shaft that tells M-Tronic when you are in the triangle (start) position. That is why it will stay running in the triangle position, because the module knows the choke is shut and adjusts fuel flow accordingly, for when you do a reset. The choke shutter has a big air bleed compared to non M-Tronic carbs so it does not load up with fuel. It is rare for that switch to fail but if it did I don't think it would cause the problems you are seeing. Again this is where the MDG 1 will check all of these things. I support right to repair and struggle with the fact that I want to be able to troubleshoot everything myself, but having worked for STIHL see their side where the R&D and development cost money to make something, hopefully better, and so they want to protect their investment. My 2018 Traverse is doing a bunch of intermittent, weird computer behavior and I think about my 66 Riveria with a Quadrajet and points that I could work on with some basic tools and basic knowledge. Even my 93 Suburban is fairly easy to work on and very reliable with 189K and never had the engine apart.
Sorry for the rant...
Eddie