MSefi.com read only forum (Jan.15,2018)

I didn't want to hijack another users thread, so I started another one. I am datalogging my startups and getting very high tach signals on startup. Matt had mentioned a Zener diode in line with the tach signal could help with erratic tach signals while the engine is running. Could it help during startup to? I am also firing from the coil. This is happening on two MS-II projects i have going on right now. Hmmmm.....

Sure, noise can be a problem any time. I'd add the Zener diode.

Installed the diode (36 volt) and still gets high RPM on startup. I think I may need to add a cap to the MS power. I did not mention this before, but my ASE randomly turns on sometimes during throttle movements. I will rev it slightly and ase turns on then revs high for a bit. I think this is all due to the RPM signal? I may try shielded wire on the coil wire next. This problem is on a boat I'm working on with wire lengths of roughly 24', so there may be some places it's picking up noise from.

Could you post a data log showing the ASE randomly turning on?

Thanks for the reply Matt.
It looks like at about time 213.327 the tach signal drops to 0 and ASE starts over and the engine runs as it had just started (IAC opens up, idles high for a few seconds, etc, etc). This happens a few times, mostly right after I rev it up a little. I'm currently looking for a scope to try and narrow down some noise issues. I did a quick shielded wire install from the coil directly to MS-II with no change in the erratic tach signal!

By the way, I have a 4 conductor shielded cable. Can I run my 3 TPS wires and 1 Tach wire in the same cable? I don't have a 1 conductor shielded available for the tach. This is what was available from work. Thanks.

I don't see a problem with putting TPS and tach in the same shield - not ideal but there shouldn't be too much noise in there. Having the RPM fall to zero is what turned on ASE.

That's what I had noticed, which led me to these last few posts. Now I have to find out why it's dropping to zero. I will permanently install shielded cable (and reinstall the zener diode) today and re-log to see if it's any better. I will post my findings. Until I get a hold of a scope, should I even bother throwing caps in on the MS power to see if that helps? Any idea on a value?

Power is unlikely to be the problem - it's the signal off the coil, which is pretty noisy.

Update, last night: I installed the shielded cable for the tach wire and TPS wires. It didn't seem to help too much. I'm still seeing erratic tach signals during startup AND tach is dropping to 0 after a throttle rev.

This morning: Looked over my msq file and noticed and changed the lag factor for the RPM from 5 to 30. Seemed to be an improvement in the beginning, but after a few minutes and revving the throttle, it did it again. I don't know if that really made a difference but ASE did not turn on until later on. I datalogged this morning's attempt and the tach is all over the place during startups and quick throttle movements. Any suggestions on my next option/attempt?

Ford 351 v8, MS-II version 2.89 firing off coil negative

Update: I installed a 36volt Zener diode with the band pointing to the coil negative. This combined with the shielded cable on the tach wire to MS at first did not seem like it did anything. I still had high tach spikes on startup and RPM dropped to zero after hitting the throttle (causing a sync problem?) and having MS thinks it's restarting....ie, ASE kicks in, IAC opens up and idles high for a few seconds. Try pulling out of a parking spot and having the engine suddenly rev up while you're in drive! Not good.

Here's the weird part. A couple of days after installing the zener diode the IAC/ASE thing seemed to stop no matter how many times I revved the motor. The datalog shows the zero is still there, but the weird idle & ase kicking on doesn't happen.....yet. In the meantime I had my laptop disconnecting from MS - acted as if somebody pulled the db-9 cable out and plugged it back in after a few seconds. This happens on both MS projects I"m working on. I figured it may be a software corruption issue, so I reinstalled MT. No more disconnects but the Tach spikes are still there. The RPM zero drop shows up on the datalog once in a while, without any adverse affects on ASE/IAC. Would adjusting the time mask help with the RPM zero drop?

My next move will be to add a cap to the coil power to see if it helps smooth out the tach signal. I may also get new plug wires too.

Have you looked into (or tried) the Peter Florence Tach input circuit? http://www.megamanual.com/v22manual/PFl ... ch_mod.pdf

Or the similar "Dave Cap" solution that is explained in the MegaManual "Wiring and Sensors" section http://www.megamanual.com/v22manual/mwire.htm. This is also where the reference to the Peter Florence Tach circuit can be found.

Also, in the assembly guide for building up the circuit board for every MegaSquirt version there's a short discussion of triggering off the negative coil terminal.

I've had the same problems you've been having; crazy tach readings at startup and then tach signal dropouts later in the engine run. Now I've got these things mostly solved with a slightly changed version of the Peter Florence circuit. It does seem that a circuit with a capacitor or a combination of two capacitors is the key to smoothing the coil negative input.

Another thing you can easily try is resistance in series with the tach line. I have a couple of old rotary Mazdas whose coil negative was extremely noisy with a couple of ms of high amplitude ringing as evidenced on a scope. I experienced exactly the problems you describe. I placed a linear slope (non-audio) 100K potentiometer in series with the tach line as an experiment and it worked. I increased resistance just until signal was lost, measured that resistance, subtracted 20% or so and replaced the pot with a standard 1/4w resistor (50K or so I recall) under some heatshrink. The one fuel-only install was driven for more than 40K miles with zero further problems. I realize your hardware is nothing like mine, YMMV but this is cheap and easy to try.

-Mike