Megasquirt RPM signal
Turbo Mini Forums

I'm trying to convert to electronic ignition using Megasquirt.

All the bits are in place but the rpm signal is playing up. I ran the car with the dizzy and checked for a rpm signal from the sensor. It works well above 1500 rpm. The tooth logger shows a lovely clear, clean input which is of the same rpm as my clock in the car. Under that however it struggles and either reads far too low (200rpm) or nothing.

This causes the car not to start when the dizzy is tuned off and the megasquirt set to run the plugs because the rpm signal doesn't go above zero (so it doesn't spark)


Any ideas?

1275 N/A Sprite, 998 T2 Turbo Mayfair
1275 EFi Turbo

Are the crank sensor wires insulated and the sensor end of the shield earthed?

I guessed it may be that but was hoping for something even simpler.

No they're not, but the wires removed from the ford (its EDIS system) didn't have any shielding.

Do I use single core (with the sensor -ve as the shield) or two core (two internals as the sensor wires and the external as the shield)

Thanks
Dan

1275 N/A Sprite, 998 T2 Turbo Mayfair
1275 EFi Turbo

On the van we used 2 core microphone wire and joined all the insulation at the edis (obviously not relevant for you).

As Andy has said, make sure the wire shielded all the way along from point to point, on both wires. (you can get dual core shielded cables from shops like Maplin)

You may also need to adjust the VR pots on Megasquirt as well, unless you mean to say Megajolt instead of Megasquirt, in which case ignore this bit.

Dan,

Although I'm not really qualified to say (as I apparently dont know the difference between EGT and AFR) it will most likely be the VR pots as Carl says above.

If you are using a JimStim to set it up, do NOT expect the VR pot settings to be the same on the engine as the JimStim. For most people they are the same but not in all cases. The MS2 manual has guidance how to set the pots but I found a bit of experimentation was best (start with 7 turns on each).

Also make sure you use 2 core shielded cable but only earth the shield at the MS2 end, not the VR sensor end. The two cores have to be seperate from the shield as the VR sensor sends an AC sine wave and although one side of the signal is ground at the MS2 box, it still needs shielding between the sensor and the box.

The polarity of the two wires is also important, if you search against my user ID you will find the correct connections on the junior timer plug (I think it was Tadge who asked a while ago)

Schrödinger's cat - so which one am I ???

Thanks chaps, i'd forgotten about the pots.

Played with them now and got it sorted.

(also moved the sensor a bit closer to the wheel which helped)

1275 N/A Sprite, 998 T2 Turbo Mayfair
1275 EFi Turbo

ah, not quite sorted....

Revs well when in neutral (no misfiring any more)

But now the pots stop the car from starting. I have to wind the hysteresis down to start the car and back up to get it to run.

Also took it out for a spin (almost literally too... its very icy) and it runs well except at higher boost pressures then it starts to misfire again.

1275 N/A Sprite, 998 T2 Turbo Mayfair
1275 EFi Turbo

ok looks like when i moved the sensor closer it made it harder for it to distinguish between gaps and teeth. so moved it back played with pots and all sorted now

1275 N/A Sprite, 998 T2 Turbo Mayfair
1275 EFi Turbo

Depending on which software you are using (MegaTune, TunerStudio or MegaTunix) it will be displayed in different ways but do you get the correct sequence of cranking and running displayed ??? (bottom of the screen but different in each software....)
This predominantly comes from the VR input showing that the signal is being synch'd correctly for the ECU to interpret at different engine speeds (cranking and running) - it also shows if the signal is being dropped at higher RPMs.

Remember MegaSquirt is a bit more involved than MegaJolt, especially as you are (I hope) not using the EDIS module but firing the coilpack direct - so has some settings for cranking RPM (I think they only affect injection but they may affect ignition too).

Next point - polarity of VR sensor ???

And most important of all - especially as you have what seems to be a misfire of sorts - have you set the charge/dwell settings correctly for direct coilpack ignition ??? The default MS-Extra settings will be WRONG and you may fry your coilpack and even the ignitor chips if the fuse rating on the coilpack supply is too high.

Final point, if charge/dwell is correct and the misfire only under boost, too large a plug gap, or wrong combination of resistive leads and/or resistive plugs.

Schrödinger's cat - so which one am I ???

thanks for reply Rod. All seems ok now tho. Used it Sunday no probs. Went to work today (60 mile round trip) no problems with ignition. Dwell times are based on advice from paul or yourself (sorry can't remember who it was)

Carb icing is killing me tho. going to see what i can do about it tonight.

1275 N/A Sprite, 998 T2 Turbo Mayfair
1275 EFi Turbo