Can you try this with your multimeter set to V 1. Key ON (engine not running) Touch: Black probe → tach connector ground wire Red probe → battery negative Expected: 0.00–0.05 V If you read: 0.1–0.5 V = poor ground 0.5–2 V = VERY bad ground 2 V = ground not connected at all
Every time I go out there I hope to find something definitive that's wrong....other than it's not working. I'm hitting the point of giving up and just accepting that it's not going to work (I should have known better when Luigi Moto said they couldn't find the problem). I think the problem is well above my skill set no matter how hard I try.
Now try this to establish if the system is ‘open collector’ Disconnect tacho Measure the ECU tacho signal pin 24 with a multimeter (DC): If it floats, I.E. sticks at 0 V, or only rises to a tiny voltage → it is open-collector. Possible solution Add a 1 kΩ – 2.2 kΩ pull-up resistor from the tacho signal wire from pin 24 to +12 V.
Pins 26 and 35 will give a 12v supply to link into. Pin 26 is linked to the ignition switch and 35 to the main relay. A pull up resistor is just a normal resistor https://en.wikipedia.org/wiki/Pull-up_resistor
I really hope that you don't give up, look at this as getting closer and closer rather than getting further away. Between all of us, surely we can stumble on this. I think I will try researching a bit more in the week as I'm so rusty.
Just re-read the thread. As the output on pin 24 falls 0-12v then it is not open collector. So a pull-up resistor alone will not work. After a lot of reading it seems the tacho may need a buffer circuit to clean up the signal on pin 24. Sounds complicated but it is effectively a transistor and 2 resistors. The circuit cleans up the square wave signal allowing the tacho to read it. Well, that is the theory. Cheap to build. I may even have the bits in my stash of components.
I have built a prototype buffer device. It is built around a 2N7000 mosfet and two 1 kΩ resistors. The wire into pin 24 on the ECU will need cutting. Wiring Green wire to pin 24 Red/yellow to the tacho Red wire to pin 35 for +12v Black wire to the earthing bolt on the ECU case (there should be an earth fly lead attached here) The idea is the tacho output wire is boosted to correct voltage on the square wave signal to the tacho. PM me your address details.
