Hi all,

I bought a used MS3 Basic for my 1991 Miata from a reputable member here and I think the tach driver circuit is defective. I can't get the tach to work on my Miata with the car on, or with the test mode. I've done the classic shorting pins 4&5 on the igniter connector with a jumper to connect the MS3 directly to the tachometer.

Another reason why I think it's defective is that when I do send a pulse wave through the tach out pin of the MS3 and read the signal with an oscilloscope, I see a very very weak pulse wave, with the high being around or under 1V. Where is the tach circuit on the MS3 and can I replace whatever part may be the culprit? I'm confident with microsoldering as long as it's an SMD component.

 

I think I mentioned moving it to the DB37 connector tach out. Have you already done this? This is how it worked on my car, I think I remember having to do it on another MS3 basic as well.

 

I haven't given it a shot, I'm away from the car for a while again but I brought the ECU with me so I can try putting it on a bench and testing the DB37 tach out sometime soon.

 

The Basic MS3 does NOT have the tacho output on the front connector on pin 2I by default, if you tried bridging the black/white (which is on pin 2I) to yellow/blue. On some versions of the ECU, you can enable it by installing the TACH jumper inside the ECU. Otherwise you can connect the black/white to pin 10 (ten) on the DB37, which should have a yellow/green wire already installed.

 

Which one is the tach jumper? I have a version from 2016

 

It is literally labelled "TACH".

 

Found it, it was hiding under the daughterboard(?) so I didn't see it the first time I looked for it. I've jumped it now, and I'll later connect it to a power supply and see if the tach out from the ECU plug works now.

Out of curiosity, why is that disabled by default? Wouldn't it be more convenient to have the ECU plug output that signal out of the box?

 

The circuit may be damaged if it is enabled and you still have the stock igniter module installed and working.

 

That did it! I powered it up on the bench to test the tach and I'm getting a strong square wave depending on how much supply voltage I gave it (I kept it at 8V to keep it safe).

MS3 Basic powered up

Nice square wave from tach out

power supply

On a side note, I thought I needed to give it 5V on pin 2K as it was the Vref for the MS3, but it worked regardless of it being connected to 5V or not. Is it okay that I gave it power? I'm starting to think I shouldn't have done that.

 

For a 90-93, the 5V output for the TPS is on pin 1N. 2K is the pin for the 94-95.