Megasquirt Tacho / Rev Output
Turbo Mini Forums

I'm going to be using Megasquirt 1 to directly drive the EDIS coil packs.

Which tacho setup are people using?

Looking at the MSextra site
http://www.msextra.com/doc/ms1extra/MS_Ext...al.htm#tachoout

I think I need the bottom setup, which is for rev counters that were fed from the coils -ve. Is this right?



using only spark A and B

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

Essentially, yes, that should work.

Providing your rev counter is the original type driven off the coil LV side.

As you say, you will only have spark A and B in that diagram.

There are other ways of doing it - well certainly in MS2-Extra, so I presume MS1-Extra too - like programming one of the CPU's spare ports to send the signal direct.

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

The alternative is to get a rev counter out of an MPi mini and just connect a single coil ouput.

Saul Bellow - "A great deal of intelligence can be invested in ignorance when the need for illusion is deep."
Stephen Hawking - "The greatest enemy of knowledge is not ignorance, it is the illusion of knowledge."

Rod, do you have a web link to programming one of the CPU's spare ports to send the signal direct?

This is what I was looking for when i found the above. I couldn't find any other info.

I would rather do that if possible. Sounds a bit neater.


Also, (on a slightly related topic) what dwell setting have you guys successfully run when driving EDIS coils from Megasquirt?

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

You can do it from within whichever tuning software you're using, at least I think that's what Rod is referring to.

It comes under the Misc menus as tacho output

Dan,

I assume it's in here in MS1-Extra...

(MS2-Extra is so much easier...)

Codebase and output Functions

"When running ignition you must choose ONLY ONE code type and select as many spark outputs as you have coils
A distributor based setup must have SparkA ONLY;

A 4cyl wasted spark setup must have SparkA, SparkB ONLY, so LED18 MUST be set to anything other than a Spark output, e.g. Warmup. Output3/SparkD MUST be set to output3, pin10 shift / Spark E MUST be set to Shift light and Knocking / Spark F MUST be set to Knock input even if they are not going to be used for those functions."

In MS2-Extra, D15 is the logical one to use, when you load the MS1-Extra code hopefully D15 will become available for tacho.

Then you use the circuit above the one you posted to make it work.

Re. the coilpack settings, as I mentioned in another thread.....


"Spark Output Inverted Choosing spark output inverted YES or NO is VERY IMPORTANT. This is determined by the way you drive your coil(s) When the spark coils are directly driven from the ECU (VB921 FETs are used) you will choose Inverted = YES. Get it right or you will risk damaging components as well as getting wildly inaccurate timing."

The best settings are in the MS2-Extra manual, I don't understand why they are not obvious in the MS1 version......

I'll post up my settings next time I'm out in the garage to read them......

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

Ah ok cheers Carl, found it in tuner studio.

It lets me pick an output pin.

What is the output like? is it suitable to connect directly to a standard -ve fed mini tacho?

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

Well, it didnt work with my tacho, but you may be lucky.

I will say that my tacho did work with MegaJolt's tacho output so i'm not really sure where the difference is.

Posts are a bit out of order......

If you use the CPU pinout direct, you need the circuit that replicates a coil (ie, including the transistor that protects the CPU) to provide the same voltage spike as a real coil would.

It's the one above the diode solution in the Extra manual.

Hope that makes sense......

Edited by Rod S on 17th Nov, 2010.

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

Edited by Paul S on 17th Nov, 2010.

Saul Bellow - "A great deal of intelligence can be invested in ignorance when the need for illusion is deep."
Stephen Hawking - "The greatest enemy of knowledge is not ignorance, it is the illusion of knowledge."

ah ok Rod,

like so



So if I set tacho to come from output 3. Then I should get a pulse from pin 15 to drive the transistor?

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

Essentially yes....

I'm running a bit blind as I use the MS2-Extra software but the principle is the same - set the output ports so the output that was the middle LED (D15) becomes a tach signal and then put it through a transistor (which isolates the CPU from any damage) and an inductor (like a relay coil) to mimic what the rev counter expects.

By using the CPU output direct rather than the diodes off the coilpack drives avoids any possible problems with interference or external faults on the coilpack wiring.

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

Edited by Rod S on 18th Nov, 2010.

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

Well the reason I ask is because my tacho did work with the MJ tacho signal, but it doesn't work with the MS output, which is why i'm scratching my head over it, hence why I wondered if there was a difference.

I think the pin out on mine is set to the old FIDLE pin, but I may switch it to D15, as like yours, it's currently just used to show WUE.

Maybe the FIDLE pin is unsuitable for use?

P.S I've had the MS unit built for quite a while now, I just havn't installed it as I was getting ignition issues. I will have another go at getting it running this weekend.

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

I was getting an extra missing tooth appearing on the tooth logger. I believe it's to do with the VR sensor's polarity but I tried switching it around to no avail. It would either run and present the extra tooth, or not run at all with the polarity reversed.

I was however guessing wildly what my actual missing tooth offset was as it's not precisely 90 degrees. That and a combination of not always fully reseting the MS unit when making the changes probably gave me missleading results, so i'll have another crack with a more methodical approach.

I was using the FIDLE pin on the DB37 plug so maybe the FIDLE circuitry was interfering with the signal. I'll try another pin for the tacho signal to confirm it.

Edit - Typos.

Edited by Carl S on 18th Nov, 2010.

Re the missing tooth, it may be the VR pot settings - on mine they are very sensitive and have to be different on the car's real VR sensor compared to the JimStim simulation.

For the tach signal, did you add the extra circuitry when you used the DB37 output or just send it direct ???
I've looked at the V3.0 schematic and although it doesn't invert (compared to the recommended circuit) it does have a 22V zener clamp to prevent HV spikes - a conventional tacho needs the HV spikes, hence the use of an inductive relay coil in the suggested circuit...

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

I had a fiddle around with the VR pots but they didn't make much difference, if any. I read the MS manual about how to adjust them, but it just talks about the actual function of what the pots are for, rather than how to get settings which are right for the VR sensor. Maybe i'm missing something here.

I'll open up the MS unit tonight and have a look to see if I installed any of the FIDLE circuitry. If it was optional on the MS build manual then I highly doubt it, as I knew before I built it that I wouldn't need the FIDLE functionality. What you're saying about it makes sense though, so maybe I installed that zener diode as part of the circuit by accident? Does that zener serve any other purpose?

Carl did you get to the bottom of this ive been trying to do the same as you. I built the extra circuit external to ms (inside an old relay) with everything connected as per circuit diagram no tacho but i happened by accedent to bridge a wire whist messing about with it and i got a tacho signal perfect

but after ive looked what i did basically i had one side of the relay coil to +ve the other to fidle and tacho input now this gave a good stable reading but obviously missing the protective transistor
you mention about the zener built into ms do you think thats enough to protect from the voltage spikes or should i try again?

Yes i moved to the darkside

Instagram @jdm_brett

No sorry Brett, the MS is still out of the mini at the moment, and i'm also 200 miles from the car so can't do a great deal at the moment. The extra tooth issue is crushing my progress but I have a few ideas to try when I get home to Devon again.

Make sure the polarity of the diodes are correct, that's all I can think of really.

Just think in it over( im at work ) i could us an ignitor chip for now wired up as a coil driver i dont think the transisters im using are working as they should (or more likily wired up wrong)
it basicaly the same circuit as the coil drivers anyway

Yes i moved to the darkside

Instagram @jdm_brett