I have done some searching, and some troubleshooting, but can't figure out how to make the stock tach work.

Details:
-'90 1.6L car (now has 1.8L VVT engine)
- Custom standalone ECU - uses 0-5V digital outputs for spark control. Stock ECU gone.
- GM LS coils - the badass ones with the heatsink on each coil. Standalone wiring between the ECU and these coils - no stock wiring interaction.
- I have the ECU driving both the stock 1.6L coilpack through the factory wiring and the GM LS coils in parallel. Ignitor and factory wiring harness has not been touched. Jumper harness used for ECU.
-Pin 2I on the ECU side is not connected to anything (BLK-WHT wire Pin E on the ignitor) - Ignition sense?
- I just physically unplug the set of coils I am not using - unplug the 1.6L coils at the 3 pin plug, and the GM coils at the stock GM 16? pin plug. Plug in the coils that I want to use.
- This provides me with the ability to use either ignition source, and no factory wires have been hacked. Yes - it was probably more work than required, but I really dislike hacking the harness if I don't have to.
-The engine runs just fine with either set of coils


The problem:
-The tach works fine with the 1.6L coil plugged in.
-The tach does not work at all with the GM coils plugged in, and 1.6L coil disconnected.

Based on some reading on here, it looks like I might need a 12V pullup for the ECU pin 2I. But, I put a 1K resistor in the diagnostic connector between B+ and IGN- and no help there. Also, since the tach works fine with the 1.6L coil plugged in - this wire isn't needed??

So, I'm looking for some advice. I understand that I could use the tach out pin on my ECU and run that to the diagnostic connector in order to drive the tach. Maybe I will do that. But, I still don't understand why the tach doesn't work - when the only difference is the fact that the 1.6L coil isn't plugged in...

Why would the coil need to be plugged in if the ignitor is still wired up?

 

If I recall (and I'm still a noob of all noobs) the 1.6L equipped cars never needed the 1k resistor?

...or am I thinking about the capacitor on the +12 and gnd.

 

I'm running a home brew LS3 coil setup in a 1.6 car , I just used FM's big spark installation instructions to make up my harnesses and its worked prefect for the get go .The instructions are available on their web site ,product support /ecu and peripherals/big spark. Cheers Fred

 

Yeah, I read the instructions here: http://www.flyinmiata.com/support/in...ra%20coils.pdf

It looks like they connect the Black/White wire (ECU pin 2I) to the tach wire (Yel-Blu). But, that doesn't help me, 'cause the Black/White wire isn't connected to anything in my setup. What does the Black/White wire do on the Hydra? Does it go to the tach control pin?

 

I'll have to check the pin out on my set up but you're probably right about the black/white wire ,I check and get back to you. Fred

 

What ECU do you run? On a 1.6 you have to have the ECU outputting the tach signal signal when you run the LS coils.

 

Just checked the pin out, Mazda pin 21 is connection to the rpm signal on the the Hydra . Fred

 

I'm not sure if I understand, but I recommend cutting the wire at the cluster, then running that to the computer for the 21st century tach signal.

 

I thought that maybe the ignitor needed to see a pullup on the coil outputs. So I took an old 1.6 coil side harness and hooked up some resistors to simulate a pair of coils. Tried 1k, and some 10 ohm resistors. Still no luck.

 

Well, I finally gave up on troubleshooting, and took your advice. 21st century tach to the rescue.

On another note - it seems that the stock tach is pretty optimistic. At a commanded 5800 rpm it reads 6000. POS.