I'm working on a 1978 280Z that someone started to convert to Megasquirt and cut a lot of wires. I've purchased a used ECU, engine wiring harness, and appropriate parts to put the car back to stock. I have it to the point where I have spark and the fuel pump and relay are working properly. However, it won't fire the injectors. I'm pretty sure it's because there isn't a wire for the revolution signal on the negative side of the coil.

The coil has a black with blue tracer wire going to the positive side of the coil. It comes from the body harness. The positive side also has a wire that feeds the distributor module. The negative side just has one wire that also feeds the distributor module. Looking at the factory wiring diagram, it looks like there should be a blue wire on the negative side of the coil that ultimately ends up at pin 1 on the ECU. I can't find the blue wire on either the body harness or the engine harness.

I'm hoping someone here might be able to point me in the correct direction. Can you confirm I'm looking for a blue wire and tell me which harness/area it is in?

Thanks,

Carl

Sounds like someone did a bunch of hacking on that car.

So you mentioned a "distributor module"... Not exactly sure what you're talking about, but if you are talking about an ignition module on the outside of the distributor, then that car is not supposed to have such a thing.

Can you take a couple pics of the distributor and the coil area?

2 hours ago, Carl Heideman said:

I'm pretty sure it's because there isn't a wire for the revolution signal on the negative side of the coil.

2 hours ago, Carl Heideman said:

Looking at the factory wiring diagram, it looks like there should be a blue wire on the negative side of the coil that ultimately ends up at pin 1 on the ECU.

Have you checked continuity from Pin 1 to the negative side of the coil? And have you checked for spark?

The blue wire splits three ways after it leaves the coil - the ignition module under the dash by the fuse box, the tachometer, and the ECU's Pin 1. The blue wire at the coil is the same wire that allows the ignition module to create spark, in the factory setup.

There are other reasons that the injectors won't work, with Pin 1 connected correctly.

5 hours ago, Carl Heideman said:

I'm working on a 1978 280Z that someone started to convert to Megasquirt and cut a lot of wires. I've purchased a used ECU, engine wiring harness, and appropriate parts to put the car back to stock. I have it to the point where I have spark and the fuel pump and relay are working properly. However, it won't fire the injectors. I'm pretty sure it's because there isn't a wire for the revolution signal on the negative side of the coil.

The coil has a black with blue tracer wire going to the positive side of the coil. It comes from the body harness. The positive side also has a wire that feeds the distributor module. The negative side just has one wire that also feeds the distributor module. Looking at the factory wiring diagram, it looks like there should be a blue wire on the negative side of the coil that ultimately ends up at pin 1 on the ECU. I can't find the blue wire on either the body harness or the engine harness.

I'm hoping someone here might be able to point me in the correct direction. Can you confirm I'm looking for a blue wire and tell me which harness/area it is in?

Thanks,

Carl

Yup, L (blue) from the Electronic Ignition Module - sorry, Transistor Ignition Unit - (mounted to the right kick panel under the glove box - behind the relay cluster). That L also tees off, goes through a resistor, and on to the tachometer. Megasquirt doesn't use any of that so it may have all been removed or abandoned.

The stock distributor also does not connect directly to the coil. Rather there's a terminal block near the coil with two wires (that probably also come from the Transistor Ignition Unit - yup, R and G from the TIU...) and the R and B (maybe I remember the colors right) from the stock distributor connect there. It's all in the Factory Service Manual.

If you only look at the sections that describe the individual modules they don't necessarily describe the entire circuit. For instance the Transistor Ignition Unit diagram shows the L going only to the coil "-". The ECU diagram only shows the L from pin 1 going to the coil "-" and nothing else. Which is NOT how the wiring is actually done in the harnesses and you would also have to go to the Tachometer wiring to discover the L is also the tach signal wire AND that it needs an inline resistor... The FSM wiring diagram in my .PDF copy is poor and fairly hard to follow but it does show all the connections and how the harness wiring is really constructed.

I'm more familiar with the ECCS system of the ZX Turbo - but - as the TIU doesn't receive a separate signal from the ECU, I believe the L is a signal FROM the TIU TO the ECU (just as it's a signal to the Tach and Coil) rather than the ECU sending a controlling signal to the TIU. Correct me if I'm wrong. Nevertheless, the BLUE (L) connects all four as @Zed Head said above : Transistor Ignition Unit, Coil "-", Tachometer, and ECU Pin1.

Thanks for the replies and details. This is a customer car for the shop I own and it's a background project between more pressing work. I'll snap a few pictures and follow-up within a few days when things quiet down.

Carl