I bought a 1990 Nissan 300zx about 5 months ago, and it wouldn't start, after replacing all spark plugs and installing new style injectors it fired right up first try. I drove it down to the gas station filled about 5 gallons and drove it home, on the way home a coolant hose clamp was loose and the hose on the front of the engine was spraying coolant everywhere, didn't have the tools on me to fix it so drove it home leaking coolant for about 2 minutes. When I got home I topped up on coolant and started it back up, let it idle for about a minute, and I started driving and it instantly stalled.

I'm now not getting any spark, I have tested the CAS/Crank angle sensor, PTU as per factory service manual, and I have tested each coil pack(all read 1.6ohm), and I have tested the wiring from the PTU to each coil pack plug by testing continuity from the back of the 7 pin plug on the PTU sub harness to the far right female connector on the plug, http://www.twinturbo.net/nissan/300zx/forums/main/view/821754/To-check-ignition-coil-wiring-when-one-coil-is-not-firinggtgtgt.html This is the forum I used to test the wiring from the PTU to the plug for the coil pack. I ran diagnostics on the ecu and got code 55.

I believe I have tested everything involved in spark now and everything checks out but I'm still not getting spark. I'm really at a loss here and if anybody has any information on this I would really appreciate it.

https://www.nicoclub.com/service-manual ... 2Fefec.pdf
If you look at the WD on EFEC-118, only the firing signals for the coils come through the 7-pin connector. They need power and ground to operate, and those come from completely different places. Ground comes from F39 on the block and F28 on the chassis, and power comes from one of the Green fusible links by way of the Coil Relay E5 and wire-to-wire connectors E12:F25. The best tool for checking those is a bulb-type test lamp. Start at one of the coils. I can't read the wire color for the power leads, but ground is Black and the triggers are a different color on each coil, so power will be whatever is left over. Check from that to the Black wire on any coil connector. The light should light up brightly with the key on:
1) If it lights brightly, the problem isn't coil power or ground. Post back and we'll check some other things.
2) If there's no light, check from the power wire to battery ground. If that lights then your ground path for the coils is open. If it doesn't, then the coil fusible link is blown, the Coil Relay is bad, the ECM isn't turning on the relay, or there's an open in your wiring.
3) If the light is dim, do the same test to battery ground as (2). If that's bright, then you have high resistance somewhere in the ground circuit. If it's still dim, then the resistance is in the power side of the circuit, most likely a pitted Coil Relay or wiring problem.

Hello thanks for the quick reply. I have tested the 2 wires on the plug with a DMM and I'm not sure if this test works the same, If your holding the plug with the locking clip on the top the left wire is the battery/chassis ground, and the middle wire is the 12v from the battery or fusible link, I have continuity tested the left wire by putting a lead from my multimeter into the spade connector and the other lead on the plenum bolt, and have continuity confirming that's my ground, next I tested voltage by putting the black lead in the left connector, and the red lead into the middle connector. I got about 12v. I also tested continuity from the positive terminal on the battery and the 12v connector on the coil pack plug. If this test does not work the same as what you have told me to do I will try that next, I don't have a bulb-type test lamp but can purchase one if necessary.

When testing a power circuit a bulb is always a better tool than a meter simply because it draws power. High-impedance tools like DMM's and LED's won't tell you if there's resistance in a circuit, a bulb will. But if you're seeing 12V with your meter then most likely it isn't a power issue. How did you go about checking the CAS? That has to work right, even a few blocked slots or missing signal transitions can send the ECU into "duh" mode with no spark or fuel. The 1-degree wheel (G/Y) is fairly easy to test while cranking because the signal is uniform. It's optical internally and it's a 5V square wave, so it should give you a flat "middle voltage" around 2.5V when spinning. The exact voltage isn't important but uniformity is, the reading shouldn't be jumping around in the slightest. For the 120-degree wheel (G/B), the most reliable test without a scope is generally to take it loose and spin it manually while watching the DMM. You should see a single 0V/5V transition every 60 degrees with one wider slot indicating #1. If the CAS works right, check one of the trigger wires between the ECM and PTU while cranking. You should see about 3V, if not the ECM's driver chip may be blown. If the signals are there, check for actual coil pulses coming out of the PTU, if there aren't any (the voltage stays flatlined at 0V or battery voltage) then the PTU is shot.

It seems that I haven't tested the CAS properly, I pulled the CAS, and spun it while listening for the injectors to fire, I heard all 6 injectors fire. Also when I pulled a spark plug after cranking the motor it was wet of fuel, so I definitely have fuel. I will be able to get back to testing on the Z tomorrow thank you for this information.

You're most welcome. Then maybe you should check the firing signals to the PTU first. If they're there, then the PTU is looking like your culprit. It's a transistor array with a common ground path internally, so if the ground blows out on the circuit board you lose all 6 cylinders.

Okay was able to test the trigger wires coming from the ECM, and I got .6v out of all 6. Does this mean I have a problem with the ECM's driver chip as you stated before?

That should be a 2V~4V sawtooth waveform. Your meter will average it, but 0.6V sounds awfully low. However, I'm used to scoping stuff like that and not relying on a meter, so I'm not comfy with definitive statements. Maybe one of the Z mavens in here can shed some light on what a DMM should make of it. Are you seeing any change in voltage on the PTU outputs when you crank, or are they flatlined?

How do I go about testing the PTU outputs? The PTU outputs a ground pulse correct?

I believe that's right, the system was a hybrid and I recollect the signal polarity was opposite a normal 3-wire COP (which is a positive trigger). Your meter should show a change in voltage while cranking either way. I think you'll see a 12V level with the key on, if so, check with the DMM common on 12V instead of ground. That will cancel out the voltage drop from cranking and allow you to see relative voltage changes on the PTU outputs.

Have you tested the PTU it self? Do you have the phase 1 PTU (aluminum finned), they are known to fail when hot.

Yes I tested the PTU as per service manual, I ran all 40 tests, everything checked out. I should be able to test output signals today.

Tested PTU outputs, with key on my DMM read .5v, while cranking it jumped around 1.5v. I tested again on the same cop plug and with key on it read 1.2v, and while cranking it read 2.2v. Any idea on why it would change like that? I didn't touch anything in between tests.

While cranking, I'm not sure if a DMM is fast enough to catch the max output, you need an oscilloscope with high frequency to capture/log the voltage. Maybe that's why you have different Volts both tries on the out put signals.

1) key on engine OFF, you have 12V at the center terminal(B) of the coil connectors. ground at (A)?
2) PTU Ohm tests are good
3) ground points are good (especially the two at the top of the plenum near the firewall)
4)All connections are confirmed good,(CAS, PTU etc)
5)wire integrity btw coil and ECU via PTU/CAS are good. Not just the wire but the pin connection too.

If all above is good then the last issue is the ECU. This is a tough one, thats all I can think of right now.

When testing the PTU I put the key in the on position, and used wire tappers to tap into the wires behind the plugs on the PTU. I put positive lead on "3" and my negative lead on "1" and it reads -0, and when switching polarity it reads .700, according to the FSM this should be the other way around, it reads every connection backwards. When I continuity test with the PTU unplugged my DMM reads nothing with connections "1" and "3", "3" and "2", and "1" and "2" read normally. Also I have a Phase 2 PTU. What am I doing wrong?

Sounds like the ptu is dead. With resistance readings you always get a reading from 0 to infinity. Maybe the DDM is set to the wrong setting or range.
With my ptu disconnected I get roughly 500ohms at connection (“3+to 1-”) ie terminals d1,d2,d3,d4,d5,d6. Infinity with the polarity reversed.

(3+ to 2-) ie terminals ad,bd,cd,ed,fd,gd I get ohms between 460-1120. And same with polarity reversed.

Thanks a lot! I ordered a new PTU from Z1 last week and it arrived today, threw it in the z and it started first try! I'm a little bit concerned that whatever caused my old PTU to go bad could cause the new one to go bad as well, not sure how possible that is though.

kylemendelow wrote: ↑

Sat Jul 30, 2022 6:49 pm

I'm a little bit concerned that whatever caused my old PTU to go bad could cause the new one to go bad as well, not sure how possible that is though.

Congrats! I'd suggest running voltage drop checks on your PTU and engine grounds. Any device that drives coils will be very sensitive to poor grounding, no matter what the rest of the circuit looks like or how it's constructed.

Heat, bad grounds and fray wires are the killers for the ptu. That sub harness strapped near the water pipes can rub and short out the ptu also.

Awesome I will double check all that. One last concern I have, when I have the car in Park or Neutral the idle is rough bouncing around 500rpms from 1500-2000 RPM. Once I put the car into drive it sits flat at 1000 RPM, what could cause the RPM to bounce around only in park and neutral?