1991 940 Tach signal

[phuz]

Trying to dig up info on this from turbobricks, and chatted with someone on their discord channel, but I need some further clarification on what signal the tach is expecting so I can properly send a signal from my Haltech to make the stock tach functional.

I'm told it's just a 12v pulse from the #1 coil that makes the tach operate. On the haltech, I can output a 0-12v square wave with any number of pulses per cycle, and any duty cycle. I've tried numerous combinations, but cannot get the tach to move. I'm going to pull the cluster today or tomorrow and see if there are any weak solder joints, etc., but want to make sure I have a full understanding of it first.

[Laird Scooby]

Originally Posted by phuz

Trying to dig up info on this from turbobricks, and chatted with someone on their discord channel, but I need some further clarification on what signal the tach is expecting so I can properly send a signal from my Haltech to make the stock tach functional.

I'm told it's just a 12v pulse from the #1 coil that makes the tach operate. On the haltech, I can output a 0-12v square wave with any number of pulses per cycle, and any duty cycle. I've tried numerous combinations, but cannot get the tach to move. I'm going to pull the cluster today or tomorrow and see if there are any weak solder joints, etc., but want to make sure I have a full understanding of it first.

On the factory set-up, there are two pulses per revolution from the coil -ve terminal. If you are taking a pulse from 1 coil pack, that will only fire once every two revolutions so 1/4 the frequency of the expected signal. On another forum (for a different car) i remember once seeing a small DIY interface when they converted to an aftermarket engine management system so you may have to explore that option as well.

[phuz]

Originally Posted by Laird Scooby

On the factory set-up, there are two pulses per revolution from the coil -ve terminal. If you are taking a pulse from 1 coil pack, that will only fire once every two revolutions so 1/4 the frequency of the expected signal. On another forum (for a different car) i remember once seeing a small DIY interface when they converted to an aftermarket engine management system so you may have to explore that option as well.

OK, that answers my question. So it IS using the -ve signal to accomplish this. Someone else said this was a 0-12v square wave, and clearly that's not the case and explains why I can't get the tach to move.

[TonyS9]

You need a much higher (or possibly lower) voltage than 12V, its driven from the back emf from the coil primary.

When converting to wasted spark you have to generate your own tacho signal from the 5V rpm output to the the fuel computer.

You bascially need a transistor and a relay coil. Circuit is on Buchka Spark thread somewhere, you'll have to search for it.

Here's mine for my 360 Turbo
http://www.volvo300mania.com/uk/foru...15882&start=30

[phuz]

Originally Posted by TonyS9

You need a much higher (or possibly lower) voltage than 12V, its driven from the back emf from the coil primary.

When converting to wasted spark you have to generate your own tacho signal from the 5V rpm output to the the fuel computer.

You bascially need a transistor and a relay coil. Circuit is on Buchka Spark thread somewhere, you'll have to search for it.

Here's mine for my 360 Turbo
http://www.volvo300mania.com/uk/foru...15882&start=30

-12v worked just fine as someone else mentioned. Taking the haltech output (square wave) into a signal converter (Dakota Digital SGI-5) which was able to output a sine wave giving me a negative signal as well. Works great.