1976 280Z No start, no pulse to injectors.

[Stangrage]

first, I have downloaded the EFI PDF file and looked it over. I have done all of the no start troubleshooting I could until it instructed me to plug in another ECU and give that a try.

A pulse is evident from coil to ECU

I get an 11 volt signal to each injector terminal (in other words, no ground signal)

I plug use my LED test light, which has green and red LEDs, red for power and green for ground and i get a pulse in that case not when an incandescent bulb is plugged into the injector plug.

I have consistent fuel pressure, spark and compression is normal.

I have used artificial fuel to confirm this along with the conventional methods

I have tested all circuitry coming from the ECU and everything is going where it is supposed to.

All signs point to a faulty ECU, but I am nervous about replacing it. I want to be sure about it and i want to test the unit itself. I swear up and down that there is a way to Test the ECU by continuity and resistance values through pin to pin in the 1975 Datsun 280Z factory service manual.

Does anyone know where to point me when it comes to finding a proper diagnosis on the ECU itself?

Thanks for reading!!

Edited October 19, 2011 by Stangrage

[Zed Head]

The ECU is connected to the negative terminal of the coil through Pin #1. That's how it "knows" when to ground the injectors, allowing them to energize and open. Make sure that you have continuity from coil negative to Pin #1 at the ECU connector.

Also, the tachometer and its resistor have to be in place for the ECU to work correctly, If your tachometer is not connected or if the resistor has fallen out, the ECU will not fire the injectors.

[Stangrage]

I do get a pulse signal directly to the ECU but Ill check to see if this resistor/ Tach is working or still connected. Thanks for the reply.

[FastWoman]

Well, the interesting thing is that you get a WEAK signal to the injectors. If I understand you correctly, you're getting voltage changes, but no appreciable current.

I assume this is true of ALL your injectors. This is interesting because your injectors fire simultaneously, and each has its own ground circuit in the ECU. For all 6 circuits to fail simultaneously would seem odd. The failure point would have to be common to all circuits. Perhaps it's a foil trace or solder joint inside the ECU, feeding the power transistors. (I assume there's 6 of them?) Perhaps it's the +12V feed to the drop resistor assembly. But between those points, the circuits are divided up, and the simultaneous failure of all 6 injectors would be very unlikely.

I wonder whether all your connections are solid. A weak connection might give you the correct voltages, but no current handling capability. Try this: Unplug your ECU. Identify the injector ground terminals in the ECU connector. Turn the ignition on, and one by one, manually ground each of the 6 injector wires at the ECU connector. You should hear them click, and if you have a pressure gauge on your fuel rail, you should see the fuel pressure drop.

If you can't actuate your injectors this way, the ECU isn't the problem. Our models are wired a bit differently. Mine has a separate fusible link supplying the injectors. I believe your fusible link would be common to other circuits. Anyway, check the integrity of the fusible links, and clean them up.

Also my ignition relay has a separate contact to feed power to the injectors. Yours might too. Your relay might have a dirty or burnt contact and might not pass current well. You might be able to disassemble it and clean it. Obviously the relay connector could be a problem point too.

The next stop is the drop resistor assembly beneath the master cylinders. I'm less suspicious of these, because I would think you would lose your injectors in banks and not ALL of them. However, this is a place where you can get lots of corrosion. Clean up those connectors too.

If all of the above works and connects, you should be able to fire your injectors manually.

If the problem is on the ECU end, be certain you have a good ground! If that doesn't work, then open up the ECU and do some wire tracing. Trace the ground to the circuit board and then to the power transistors. I bed you'd find a bad connection somewhere in there.

If it really is the ECU, don't worry too much. It's not a big deal to find one and swap it out. I bought a spare off of eBay for $20.

[Zed Head]

I missed the green and red LCD comment, which does suggest that the ECU is grounding the injector circuit. I took the title at face value - "no pulse to injectors". Sounds like the real problem is "no gas from injectors."

To add one more to FastWoman's list - 11 volts might be too low to get good current through the injectors to get them opening correctly. You should have a full 12.6 volts (fully charged battery) at the injector connectors. How long has the car been sitting? There have also been cases written of, where the injectors are so gummed up from sitting that they won't open.

Another way to check if your ECU is "seeing" the coil and triggering the injectors, is to connect a jumper to the negative terminal of the coil, turn the key to On, then repeatedly tap the jumper to ground quickly. The ECU will fire the injectors on every third tap. It's quick and easy because there're no other noises like the starter cranking or the engine turning over.

[FastWoman]

Another way to check if your ECU is "seeing" the coil and triggering the injectors, is to connect a jumper to the negative terminal of the coil, turn the key to On, then repeatedly tap the jumper to ground quickly. The ECU will fire the injectors on every third tap. It's quick and easy because there're no other noises like the starter cranking or the engine turning over.

NICE! Very clever!

I think the problem has to be electrical, though. The load of an incandescent bulb plugged into the injector connector was sufficient to kill the signal.

I didn't catch the 11V part. Yeah, that could be the problem too. I'd definitely charge the battery before drawing too many conclusions.

[Zed Head]

I actually learned the trick from some Tony D posts, but have verified that it works.

I agree on the electrical. I was intentionally vague on the 11 volts. Let the OP do the troubleshooting to find out why there's not 12.6 at the connectors. Somewhere from the battery and its charge, and through all of the various connections (fusible links, etc.) there is a cause for the lack of voltage.

[Stangrage]

Yeah I just discovered that the issue i am having is definitely part wiring. I found the ground wire on the injector had continuity with the positive wire on the dropping resistor. I would say as of now it could be a 50/50 problem for I have not corrected the wiring. I have not a clue of how this happened. I plan on getting some wire tonight and re-wire the ground wires to the ECU.

[Zed Head]

Both sides of the injector circuit will have power. The transistor in the ECU allows more current to flow at the appropriate time. With the key On you will see battery voltage on both sides of the dropping resistors, both sides of the injector connector and all the way up to the transistor in the ECU. That's how circuits that work by grounding function. Power everywhere up until the very end, where the grounding happens to cause an action.

Edited October 19, 2011 by Zed Head

[Stangrage]

Yeah it makes sense, I only get this connection when the ECU is connected, otherwise there is no continuity to the ground side on the injectors.. Apparently there is still an issue with the PCM grounding the signal itself. I have tried grounding the injector via the connector at the ECU and there is no injector click.

[Stangrage]

Here is a new regiment of tests that i did on my Z today.

All 6 injector terminals get this signal.

With ECU connected:

No Pulse

Both terminals get positive signal

I get 11.3 Volts on Pos and 11.03 volts on Neg

With ECU disconnected

No Pulse (of course)

Pos terminal gets Voltage

Negative terminal gets no voltage (i did not check for open signal)

At ECU connector gets Voltage

Remember I am checking the Injector to ECU circuits only.

I am assuming I have a short in the system somewhere but I am unsure how this could happen to all six injector connections.

[Zed Head]

Identify the injector ground terminals in the ECU connector. Turn the ignition on, and one by one, manually ground each of the 6 injector wires at the ECU connector. You should hear them click, and if you have a pressure gauge on your fuel rail, you should see the fuel pressure drop.

Did you try this suggestion from FastWoman yet? This will test the electrical circuit from the battery, through the dropping resistors and injectors, to the ECU connector, but without the ECU in the picture. Break the problem down in to chunks, then break the chunks in to smaller pieces.

Edit - It's not clear what you mean by positive and negative terminals. You said the ECU connector gets voltage but not clear if you mean the injector pins or elsewhere. Pin 10, for example, gets battery voltage to run the ECU functions. If the injector pins show voltage then your problem is either the ECU itself, or the trigger signal from the coil negative, or the resistor and tachometer circuit. You never did confirm that that the tach was working. The ECU won't work right without that circuit.

Edited October 21, 2011 by Zed Head