[PSIG]

HI,,,i'm new here. I understand the relay coil "stabilize" the sig output from the ECU....

Actually the opposite. It takes the DC voltage supplied to the coil inductor, and creates a high-voltage spike for the tachometer to read. This is for specific types of tachometers that are designed to read the negative terminal of the ignition coil.

If you cannot connect directly to the the ignition coil(-), then the circuit above creates a similar voltage pulse for the tach to read. It operates in very similar fashion to your ignition coil, only smaller, to create that spike pulse. More info in this thread about the circuit design and relays often used: Challenges getting old school tacho working

I have used several of these wrecking-yard micro-relays to make tach adapters, soldering the transistor inside, and cutting all terminals except 1 and 2 off (86 and 85 on a Bosch relay). This provides a convenient package with housing and handy terminals, for almost free. Use whatever you like that works:

20-30A_Micro-relay_12V.jpg (31.8 KiB) Viewed 4110 times

if i want a smaller device, what can i use instead of the relay coil?

Only the coil inside the relay is needed, which is much smaller than the relay housing. However, any inductor of sufficient potential may be used, such as an inductor from an old TV or computer, but it will likely be of similar size to the micro-relay coil. I would test with a relay or something handy for function (prove it works with your tach), then look for a smaller version of similar capability, such as the inductor below that sells for $0.80USD at electronics suppliers.

Inductor_27mH_80ohm.jpg (6.53 KiB) Viewed 4110 times

[Logicaluk]

Which transistor do you use? Asking more package and voltage rating than the exact one.
Dan

[PSIG]

All the background transistor info is in the link within the previous post. I suggest reading more than just the first few posts for more background and more options:
viewtopic.php?f=17&t=2853

[Logicaluk]

All the background transistor info is in the link within the previous post. I suggest reading more than just the first few posts for more background and more options:
viewtopic.php?f=17&t=2853

Thanks PSIG, that was not the way I wanted my first post here to go.
I did find the info in the link, just after I'd posted, but then got lost down a rabbit hole of equivalents then, whilst having my dinner, remembered I'd left the post and didn't get back to it till after you had replied.
Thanks for all the work you do here.
Dan