AEM Smart Coil and tacho

[Timbo_021]

Hey all,
Have wired in an AEM Smart Coil (IGN1A) and have the car running and driving well but tacho not working.


https://drive.google.com/open?id=1yNF1n ... amixccE9QO


Car is 1987 Z31 with corresponding ECU and a bucket load of mods which I won't get into at the moment.

Here's a quick rundown on the wiring from the ECU:
Pin 3 is a Blue (L) wire which originally goes to the negative on the coil and the power transistor.
Pin 5 is a G/B which triggers the transistor.
Pin 34 is a G/W which provides power to ecu and coil via 15A fuse.

I'm aware the original trigger of the coil is negative via the transistor.

I have used G/B as the 5v TTL trigger for the coil and L as the reference signal. I think the ECU isn't receiving the trigger from the transistor anymore hence why the tacho isn't moving. If I connect the original transistor in parallel the coil won't fire. If I earth out the coil to chassis and connect L wire to the transistor how it was originally, the coil won't fire. Is the only way to make it work finding the wire to the tacho and using the original transistor to give it a pulse or am I missing something. There's also fault code 21 Ignition Signal Primary Circuit that won't clear, probably because it isn't receiving the trigger? Electrics aren't my strong point

Cheers,
Tim

[Matt]

Can't help with the electrics, but code 21 is expected if the feedback from coils is not the same as factory. It can usually be ignored with aftermarket coils

I always thought the tacho output comes from the CAS 1 degree signal. The coil output is determined by dwell time + ignition time (charge start and release) which comes from CAS 1 + 60 degree signals

[Timbo_021]

Hey Matt,
Your right, the tacho output for the ECU is from the CAS. The tacho output for the gauge cluster is a signal from the coil. I may need to get an adapter to make it work.

Thanks for your reply

[bsdkllr]

im just spit-balling here.

transistor has blue wire green with black and a black wire

the black wire is a common ground
green with black is the trigger
and blue is the switch.

blue goes to the negative on the coil and back to the ecu.
green with white is the 12v feed to the coil.

the ecu triggers the transistor which switches the ground to cross over to the blue wire.

the tach signal and code 21 is most likely being generated from the blue wire. not being pulled down to ground.

[Timbo_021]

Yeah, that or its expecting a high voltage feedback from the coil discharging to run the tacho and tell the ECU the coil has fired. There is 2.2k resistor inline from the blue wire to the tacho so I think it'll be a high voltage signal.

But this is the kicker. If I try run the coil without the blue wire connected and earth the coil out to chassis ground it won't fire. Is there a logic or sensor ground I need to use? That way I can get rid of the code and get the tacho working without being worried about frying the transistor in the coil.

[bsdkllr]

try leaving the transistor wired in like factory.and tie the 5v ttl from green with black connected to your aem coil and the transistor. but grounding the coil directly to the body.
the ecu should only be reading the triggering pulse from the transistor as sort of a feed back. and the tach must use the same logic.
with the key on and the blue wire unplugged. do you get a voltage to ground?

if you ground the blue wire with the transistor unplugged does the tack peg at redline?

unfortunately i have the digital dash so i cant go try this out.