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AEM IG1A Ignition Coils with aftermarket Tach

Practical Motorsport Wiring - Club Level

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I'm using AEM ignition coils, Link Fury ECU, and an aftermarket Tachometer (speedhut)

I've used an output from the Link ECU to drive the tachometer, it kind of works but kind of doesn't (e.g was working but is currently reading x2), besides I could use the output for something else.

The speedhut manual says to wire this up onto a coil negative to get the signal, so my question is as anyone done this using AEM coils?

I'm assuming its pin D which is the high side current ground, this should then free up my ECU input and get a more reliable tach?

Check the documentation that you should have received with the tacho' - you probably need step 4 to correctly set the number of impulses per revolution.

If you don't have them, you can download a pdf with the fitting, and setting, instructions from their site.

You can also change the multiplier in the ECU tacho settings to calibrate the tacho if it doesn't use the normal 1 pulse per TDC or isn't accurate. You don't have access to the "coil negative" on those coils, they have internal ignitors.

Yes I had this set up as working, not sure on the multiplier off hand but it was reading accurately.

However, every now and again it would blip up to twice the value, as though it had skipped a tooth (although reading within link is fine)

And on idle it would read fine at approx 1000, then after like 10 secs would just read 2000 until you pressed the throttle and it would read accurately again

Which is all liveable but now I'm having other issues (suspected fuel pump) and its reading x2 all the time now after not changing anything in the ECU.

Also the key sweep just messed it up, it didn't like it at all, so turned it off and worked much better.

Not being able to use the coil negatives means maybe fault finding further on the Link.

I'm unsure if this is a hardware issue since the link manual says:

"An auxiliary output configured as 'Tacho' produces a 0-12V pulse to drive a low-level tachometer. There will be one output pulse for each time a cylinder reaches TDC. Connect an Auxiliary Output directly to a low-level tachometer.

G4X ECUs will NOT drive a high-level tachometer. High-level tachometers must be triggered by a coil’s negative terminal. Using a high-level tachometer on a multi-coil engine presents some problems, as each coil is not firing as often as a distributor engine’s coil would. In this case the preferred solution is to modify the high-level tachometer to accept a low-level signal."

So I don't know if the speedhut gauge is a high-level tach since it wants a negative coil connected. but as you elude to Gord, the speed hut manual suggests using a pull up resistor, my assumption was that this was built into the Link ECU but perhaps this is not the case.

speedhut manual extract below, my original question was if I could wire up as per type 2 but this appears to not be the case for AEM coils?



Also screenshot below of my link settings


A "High level" tacho is one that is designed to run off the high voltage inductive spike from the ignition primary (~50-100V). A low level tacho is one that is designed to run off a 5V or 12V square wave from an ecu or ignition module.

Your tacho is a "universal" type that is designed to work off either type of system. Unfortunately in my experience being universal often means its not particularly good at either.

Our aux outputs have a 1.5Kohm pull-up built in so it should give the same signal as what they suggest for "Type 4".

So it should not be jumping around like you say yours is at idle. You could try changing the duty cycle as I have seen some OEM tachos that are sensitive to that - try say 30/50/70 to see if there is any difference in behaviour.

My only other thought is you could try a "tacho booster" - this is a device used to generate a simulated "high voltage" signal, possibly your tacho will work with a high level signal more reliably.

I have used this one with good success in older cars: https://www.ebay.com/itm/142171581596

Thanks Adam, that's really helpful. Yes that's typical Jack of all trades master of none!

I'll try playing with the duty cycle and might order that repeater too.

Still need another output though any chance Link coming out with a G4X thunder or an IO expander?

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