MoTeC Global Forum

I tested a suspected bad mag sensor (crank position sensor) with an oscilloscope. I tested it while cranking with the sparkplugs out - first at the sensor plug, and then at the Motec ECU plug to hopefully account for possible bad wiring.

Do these waveforms look OK? Two observations:
- The center line looks a little fatter than I expected.
- One of the waveforms in the first screen shot looks mis-shaped. Could this represent some interference that could cause issues in the signaling to the ECU? I'm not sure if this is within the realm of normal to see in the output of an oscilloscope.

Thanks.

Scott

The signal at the ECU looks OK apart from 300mV peak to peak seems a little small.

Is this pattern for one tooth per cylinder?

What sensor and teeth combination are you using?

What is the gap from the tooth to the sensor?

Thank you Sprinter, appreciate your input.

I should've mentioned the scale in the screen shots. The yellow marker on the vertical axis is 50 mV, so the peaks in the waveform are 25-35 mV.

The sensor is a Motec M 3025 SS13 / #52002 "small mag sensor" similar to the threaded one that Milspec Wiring now sells. https://www.milspecwiring.com/4-APX-001 ... p_427.html
The recommended air gap for these sensors is .012-.025". I had set it to .015" and recently checked for a full rotation and found some variation in the gaps with each of the teeth .009-.015"

The crank sensor wheel is a 6 tooth wheel, no missing or extra teeth. Not sure why this setup choice was made. There is a synch sensor on the cam. It's a 4 cylinder engine. I can say the engine ran well previously, and old logs show no synch or ref errors.

You can see the tooth height difference in the wave forms but 25 - 35mv is too small on the REF input.
Are you using x10 scope probes which would mean 250 - 350mv?

What revision is the ECU PCB?
Are you using V6.2 software?

If so you could change the REF input type setting to one of the low voltage ones. Press F1 to see the options.
On the other hand you could simply have a bad sensor with shorted winding's which could lower the output voltage of the sensor.

I just checked the oscilloscope probe, and you're right it was set to 10x. So I guess the sensor output is indeed 250-350 mV. You mentioned that this may be too small. Does it indicate a potentially bad sensor?

I believe the ECU is v6.02. I did see an option in the setup for "mag sensor very low level" for REF. It's currently set to to "mag sensor low level". For a 250 mV input, which is the appropriate setting?

Ok, That makes sense but the voltage still seems too low at cranking speed.

The low level & very low settings may not work as expected, It all depends on the PCB revision of the M4.

As I noted earlier, If you press F1 more information/help should be displayed when that parameter is highlighted.

If the ECU is a late version then changing the setting to very low level may help, you would need to try it and see.

I would measure the resistance of the sensor and see what that shows.

Thanks Sprinter. When I looked at F1 for the REF sensor settings, I recall it gave some general guidance about the sensor setting options and went into some detail about setting for an M4e model. I'll look again, perhaps I missed something. This is v6.02 on the ECU. Perhaps there were more options on later versions,

I measured the resistance of the mag sensor (M 3025 SS13 DTM (#52002), small mag sensor) and got 280 ohms. The new sensor I just got (M 4-APX-001 DTM), not installed yet, registered 820 ohms. Is the difference meaningful?

Yes, I mentioned this in a earlier post, If you have shorted winding's in the sensor the output voltage will be smaller.

Thanks. I put the new mag sensor in today and measured it with the oscilloscope. The amplitude of the waveform is the same as the old sensor. The waveform may be a little sharper, and the center line is thinner - perhaps less noise? I was hoping for more of a smoking gun, but perhaps I'll be lucky and something about the old sensor was causing the poor running condition.

I'm still suspicious of EMI causing the REF sensor interference I've been seeing, so for the next track outing I'll be installing new sparkplugs, and will have a spare set of sparkplug wires and a spare coil to swap in if the problem persists.

Scott

Are you using resistor sparkplugs? If not then they can cause noise issues.
Same with the leads, use suppressed ones.