• Hello Guest, welcome to the initial stages of our new platform!
    You can find some additional information about where we are in the process of migrating the board and setting up our new software here

    Thank you for being a part of our community!

Making a Volvo 740 tachometer work with a stand-alone ECU easy.

totallyKei

Member
Joined
Nov 8, 2020
Location
Maryland
Hey there guys! While working out the bugs of going Haltech in my wagon, the last bit of seamless integration I ran into was the tach. I absolutely couldn?t get it to read a pulsed signal. After talking with a good friend and breaking down the tachometer driver board, it was really simple. This applies to megasquirt as well! Anything that gives a 5-12v pulsed tacho signal.

On the back of the tachometer, after it?s been pulled from the rest of the cluster, there are 3 posts for mounting and connection. Positive, Negative, and Signal. The signal post has a resistor as it?s first component, and all you need to do is simply bridge that, and bring its value to nothing.

The factory tachometers don?t read the factory pulsed ECU signal, but the voltage spike after the current is dropped. The ignition coil works as an inductor, and once current drops there is a pretty large voltage spike. That is what the tacho reads. Dropping the resistor value increases the sensitivity of the circuit.

I hope this helps someone! For a crude drawing over a photo of the PCB showing you what you need to do/modify, shoot me a message at caidenw11@gmail.com
 
Hey there guys! While working out the bugs of going Haltech in my wagon, the last bit of seamless integration I ran into was the tach. I absolutely couldn?t get it to read a pulsed signal. After talking with a good friend and breaking down the tachometer driver board, it was really simple. This applies to megasquirt as well! Anything that gives a 5-12v pulsed tacho signal.

On the back of the tachometer, after it?s been pulled from the rest of the cluster, there are 3 posts for mounting and connection. Positive, Negative, and Signal. The signal post has a resistor as it?s first component, and all you need to do is simply bridge that, and bring its value to nothing.

The factory tachometers don?t read the factory pulsed ECU signal, but the voltage spike after the current is dropped. The ignition coil works as an inductor, and once current drops there is a pretty large voltage spike. That is what the tacho reads. Dropping the resistor value increases the sensitivity of the circuit.

I hope this helps someone! For a crude drawing over a photo of the PCB showing you what you need to do/modify, shoot me a message at caidenw11@gmail.com
Here are the images he sent me. Can't wait to try it out! Thanks again!
bAmiuD1V_o.jpeg

9SvkQpv8_o.jpeg
 
I am happy to announce that this modification works. I just hooked it up.
'88 744, '00 LS1 w/ factory ECU.

Thank you very much, my integration is almost finished. :cool:
 
Back
Top