Megasquirt and older tachs questions...

[targa44]

Since converting a 3.0 liter (in a '73) to Megasquirt 3.0, I've gone without a tach.

The literature for the MS3 says the tach out signal is a 12v pulsed. However, my '73 tach apparently needs high voltage coil signals to work properly.

Ques: Are there later model tachs (83-84?) that could accommodate the 12v pulsed signal. I'm not sure when CIS/CDI systems converted to more sensor related stuff and I'm hoping a later model tach would work with the MS3 signal.

Any experience out there?

thanks,

['76 911S 3.0]

My '76 tach worked fine with a 12v square wave.

[jpnovak]

The 12V square wave from megasquirt should work all the way up through the 911SC tachometers. If it does not then you likely have a burned out capacitor inside the tach.

[targa44]

thank you - did you both use the IAC1a jumper connections or some other 12v pulsed output from MS?

[jpnovak]

Use the Tachout pin wire.

what ignition are you running? There may be another way to connect it.

[targa44]

I have searched high and low in the HW manual and not found a pin reference for "Tachout" on the DB37.

I'm running MS3 3.0... so, not the MS3X
Clewett trigger wheel
My old CIS had a 3-pin CDI, but I don't know where the tach voltage signal came from (CDI or Coil)

[safe]

The "modern" 12v tacho signal did'nt work on my 69 tach, not on my 77 tach. The 77 I could get to work with a diod. The 69 needed an adapter (TACH-ADAPT).

[targa44]

Magnus,

I have the TachAdapt, but I would still like to know what people are using for MS output signal (other than the IAC1a group... maybe a stepper in my future). I'm stumbling on the "tacho" tach out pin reference. I can't seem to find one.

There's a reference in the MS manual about building a special circuit, but I'd much rather find something someone else has made work.

I have pieces of the original tune from the seller (used) as I screwed up and didn't know to save that original. Looks like the former used FIDLE output to work his tach (tacho out). I've noticed it has a .1A rating vs. the other outputs at .5A. I don't know the significance of the difference.


thanks,

[dap930]

kj,

You can try hooking up the tach wire (pur/blk, pin 14 on the engine bay connector) to pin 2 of the EDIS ignition module to see if that will work. It may make the tach jump around when the key is turned on before the engine is started, but after the engine is running, the tach signal should be stable. Other people have done this success.

If that does not work, since you are saving the IAC1A, IAC2A connections for the stepper idle control circuit, then you will have to add the tacho pull up circuit shown in the manual, to the proto area of the V3.0 main board connected to the D14 LED circuit and bring the signal out on a spare pin (SPR4). I did this when I had MS2 before going to MS3/MS3X.

If you want to add the tacho pull up circuit, perhaps "billr" can help you with that.

[Tippy]

My 87 tacho reads just fine from the 'tacho out' of MS 2. Ran the wire directly into the old DME harness, and voila.

[jpnovak]

Let's clarify...

Do you have an MS2 - V3.0 board or an MS3 board? They are different and the tach signal output changes.

[ngeorgiou]

Hi i have a 1980 911 sc when i turn on the lights and turn the knob to get the dash light bigter my speedo dosnt work when i dim it down it work could any one help me out

[targa44]

Oh - this came alive again.... thanks folks.

Jamie - MS 3.0 v3, not MS3X
Clewett trigger
No EDIS
'76 Euro 3.0 liter

Tippy - thanks, but no DME

When people say just use the Tacho out - apparently there is no designated pin - it's generic? and one should use any of the outputs that support a signal... (or create the special circuit Dennis referenced)

thanks,

[brighton911]

For my MS 2 V3 setup, tach setup was as follows:

Under Basic/Load settings select Tacho output Set it to ON then choose which output pin (wire) you are going to use. Then connect that wire to the black with purple trace wire that feeds the signal to the tach.

For your MS3, I looked at a tune from my buddies MS3 pro and it was much the same procedure as the MS2 except there is a designated tach out wire OR you can choose any number of tach output wires if you choose.

If none of those work, is the tach a known working unit?

[jpnovak]

Ok. That's part of the information.

What ignition system are you running? Do you trigger the stock CDI and distributor? Is there an MSD? Do you have Clewett coil packs? Are you even controlling ignition from the MS2 (this would be pin 36)? Are you using any of the other circuits and spare I/O pins?

@brighton911 has it correct. Find an unused programmable pin on the CPU. Assign this pin the "tachout" function in TS using the "basic load settings >> tacho" dialog box. Then solder a wire from the CPU or PCB pad to an unused pin in the wiring harness. Then connect this wire to the black/purple wire in the harness. The easiest place is the CDI connector. HOwever, if you are using the stock CDI or its connector, you can open the 14 pin main engine harness connector underneath the CDI mounting point and solder to the back of the factory pin with a small jumper wire.

It is possible that the internal jumper wire from the MCU to the harness pin location might have to route through the proto area on the V3.0 PCB. First try a 12V pull-up and if it still does not work you will need to add a switching transistor. If it does not work at that point then you have an internal problem with your tach.

[Tippy]

Originally Posted by targa44

Tippy - thanks, but no DME

I probably confused the situation, but I was simply stating I hooked the tacho out from my MS2 straight to the tachometer of my 87 without any other circuits needed.

Disregard the DME acronym, which was simply referring to the harness.

[dap930]

Jamie,

kj has MS3 with V3.0 board. No MS3X. He is saving IAC1A and IAC2A for his stepper idle control motor. He is using Ford EDIS for ignition and trigger wheel.
For MS3 tacho, he has J11, D14, D15, and D16 available. JS10 is used for EDIS. All these require a pullup circuit for tacho.

For a tach signal he could try pin 2 on the EDIS ignition module, I read some old Pelican posts where some people used this and it worked, but the tach might jumper around until the engine is started due to a test signal EDIS generates.

[dap930]

Jamie,

I was wrong, he is not using EDIS. I was going on (bad)memory from some posts he made in the Megasquirt forums a while back. So disregard all my remarks concerning hooking up a tach signal from EDIS.

kj,

Do you have some sort of igniter box to drive your coil pack?

[targa44]

I want to thank everyone for chipping in and taking the time to comment and make recommendations.

It's time to stop soliciting info and just start trying various output signals and see if it works with my old (somewhere between '69 and '76) 911S tach. I'll start with FIDLE because I haven't incorporated a 2-wire PWM idle valve I've had for a while. I've gotten used to some quirky behavior with warm starts - but cold seems decent.

If FIDLE doesn't work, it's jumpering the IAC1a group and go from there.

I haven't had a tach in about a year and a half since starting MS - I've just marked the speedo where 6500 rpm comes in and used ignition retard for a limiter (that works). This is a 90% autocross car and I can't remember taking the time to look down at the tach - it's mostly by ear.

thanks again,

[targa44]

Update:

I went with FIDLE output from MS (I have the relay board - so it's easy). I set up the TachAdapt(er) per instructions and initially, no joy with low voltage signal. Then, I pulled the simple jumper as directed and voila. It is now a very close representation of the MS tach.

Magnus was spot on in that I probably have a '69-'71 tach (referenced in the instruction sheet also) and needed the higher voltage signal provided by removing that little jumper.

thanks all,