High Voltage Tach

[Raphwygum]

Hi, I am (still) making an old kr engine going to standalone. At the moment I have fuel injector set and COP. So I am facing a new matter I did remove the old coil and now I have no tach anymore in car (as tach took signal from coil). No worry I tried to make it work with ms3x tach output and relay coil. I hear the relay buzzing but I still have no tach. As I hear relay buzzing , output is working so It should work? I heard that it don't work with all relay brand , is that true?

Best Regards Raphwygum

[R100RT]

A common Bosch relay will work for this. Perhaps gut the relay right down to the level that it only has the coil and no contacts operating (as that might diminish the spike developed as the field collapses.)

[Raphwygum]

Ok thank you, I'll try to buy a bosch one to see. You suggest that I wire it at both coil extremity? I've seen that some people can provide a 12-14v square signal with an other simple circuit, will that work? Or the dash requires a very spiky signal?

[DaveEFI]

Some relays include an internal diode to quench the flyback pulse that is used here to drive the tach. But are often marked as such.

You can remove the coil from the relay and use that on its own since this makes the footprint of the circuit smaller. And stops it buzzing.

Just for info, most coils of this type measure at about 80 ohms.

[440roadrunner]

Some relays include an internal diode to quench the flyback pulse that is used here to drive the tach. But are often marked as such.

You can remove the coil from the relay and use that on its own since this makes the footprint of the circuit smaller. And stops it buzzing.

Just for info, most coils of this type measure at about 80 ohms.

THIS..........make certain you find and cut the diode out if it's there, and remove the relay armature.

[DaveEFI]

Ok thank you, I'll try to buy a bosch one to see. You suggest that I wire it at both coil extremity? I've seen that some people can provide a 12-14v square signal with an other simple circuit, will that work? Or the dash requires a very spiky signal?

The flyback pulse from the original coil reads something like 40v peak. To match that easily requires an inductor. Which is what the relay coil is, in essence. Basically, a very economical way of generating the required pulse, since the parts needed are common and cheap.

The tach instrument on your car works from 12v. Somewhere in its input circuitry, it will have an attenuator network to reduce that 40v pulse. You don't need 40v to trigger a transistor running at 12v. But modifying the tach itself may be beyond most.

I've got quite a variety of makes of 30 amp auto relays here. All interchangeable, and all fundamentally the same basic design.

[Raphwygum]

Thank you to all for your answer. I did remove the connecting part and there is no diode insde the coil, or it's very well hidden. I did measure the resistance between both and it have 90ohm. Should I remove the part inside the coil? I broke the part that make contact between pin 87 and 30 but it changed nothing

[440roadrunner]

Thank you to all for your answer. I did remove the connecting part and there is no diode insde the coil, or it's very well hidden. I did measure the resistance between both and it have 90ohm. Should I remove the part inside the coil? I broke the part that make contact between pin 87 and 30 but it changed nothing

You've done this and still not sure one thing you can do is to apply 12V and then reverse polarity. if there's a diode, the wrong polarity will either blow it open or short it, most likely open I believe, not sure, that some coils have a varistor or other clamping device. unsure how to deal with those but you SHOULD be able to see either one if you can see the coil magnet proper

Here's a read......

https://electronics.stackexchange.com/q ... ack-diodes

[DaveEFI]

Measure the resistance then swap the connections and measure again. With a plain coil both readings will be the same.