Tach signal from distributor - 1974 Mercedes first start

[merz450sl]

Over the winter I built a MS ECU to replace the D-Jetronic system in my 1974 Mercedes 450SL. This is my first experience with MS, or with doing anything with EFI beyond just driving it, for that matter.

This afternoon I finally got the MS connected to the engine for the first time and gave it a try. The engine fired right up, but very rough. I got the TPS calibrated correctly, and everything else seems to be working as it should, except for the tach signal from the old breaker-points distributor: the Tuner Studio program sees the RPM as jumping wildly all over the place, 0 to 6000 to 500 to 3000 to 0 again. I assume with this kind of an RPM reading the ECU won't be able to get its bearings.

In making up the harness, I connected the white wire from Pin 24 to the (-) terminal on the ignition coil; there's also a black wire and a shield, I didn't do anything with those.

Have I wired this wrong, or is there something else I may have done in building the board that is causing this problem? The RPM signal seemed fine when running from the STIM (and I have removed the jumper over the Zener diode when moving from the STIM to the car).

Many thanks for any suggestions... I'm very pleased to have as much success as I've had today.

https://www.flickr.com/photos/26262745@N08/16938918758/

rick

[Matt Cramer]

It could be a settings issue; could you post a copy of your MSQ? A tooth log will also show us what signal you're getting.

[merz450sl]

Thanks Matt! I guess this is it......

rick

[Matt Cramer]

You're using B&G code, which does not support the tooth logger. You could try increasing Skip Pulses.

If that doesn't work, I would recommend switching to MS2/Extra, as it has both more options for troubleshooting ignition noise and more filtering options to get rid of it.

[merz450sl]

Thanks Matt!