Loss of Ignition Pulse Signal

Nissan 300ZX technical discussion forum: Maintenance, performance, installations, modifications, how-to's and troubleshooting.
peterpilot9
Posts: 42
Joined: Thu Jun 05, 2014 10:29 am
Car: 1990 Nissan 300ZX NA

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1990 300zx NA Automatic with 310,000 miles; about 90,000 on a rebuilt engine. All stock, no mods.

Issue is a misfire/dead number 4 cylinder. I will apologize upfront for the length of this post but maybe someone who is technically knowledgeable will read this and can make sense/use of the information. This is a description of a fault and the testing, using the on-line service manual provided here as a reference.

The complaint is as follows: Engine starts and runs with no issue when cold. After a period of time, which appears to be temperature dependent, the number 4 cylinder begins to miss, then goes dead. The time to failure appears to correlate to the outside air temp with a time to failure of about 10 minutes with an OAT of 80 degrees increasing to about 14 minutes with an OAT of 60 degrees. The failure occurs in the same manner whether the engine idles for the period of time or if it is driven. The engine is at operating temps (per the gauge) for more than a few minutes before the failure occurs so the failure does not occur at the point the engine gets to temp.

There is no code. I started with the ignition system and ultimately found an ignition related failure. I have tested the issue per the instructions for a code 27; ignition system. All of the tests have been done at the time of failure since the engine operates normally when cold. Coils and plugs from 3 other cylinders were swapped and the results remain the same; the failure occurs only in cylinder number 4.

Diagnostic Procedure 27 EF & EC 118:

All items in Step A test OK.

Step C, using a logic probe, pulse signal between coil connector terminal C and ground exists until the misfire starts then the pulse signal becomes intermittent. After about 30 seconds or so,the pulse signal stops and the cylinder goes dead. If the engine is allowed to cool for a few hours, the pulse signal will again exist and the cylinder fires normally for 10 - 14 minutes, depending on OAT at which point the problem repeats itself.

So, with Step C a No-Go, I proceed to Step D.

Step D(1). The Power Transistor Unit in the car at the time of failure was only 10 months old. I completed the inspection per the service manual and found one circuit that should be open at about 60 Mega Ohms resistance. Not sure that this was a problem but I went ahead and put in a new PTU, tested and passed prior to installation. The failure remains the same.

Step D(2&3). All harness continuity has been checked and found OK. The continuity tests on my DMM defaults to resistance <50 ohms and I am working under the assumption that this is an acceptable resistance. Continuity has been verified between the ECU/TPU including the sub-harness and between the coil pack connector and the TPU.

At this point, the SM testing sequence dead ends as Step D does not flow chart any further. I went ahead and completed Step E and found all ground circuit continuity exists as required.

I made an attempt to measure signal voltage using my DMM, but given the signal is pulsed, I'm not sure of the value here. I did note that the DMM returned a voltage reading of 0.52 on the number 4 input to the TPU and at the ECU output, which falls to 0 volts at the point the pulse signal goes dead. The other operating cylinders have the same .52 volts readout and remain at that value even after number 4 falls to 0 volts and the cylinder goes dead.

I sent the ECU to SIA Electronics who provided a function test and found the ECU to have no faults. In addition, I have installed and ran a known good ECU with the very same failure results.

Sorry for the dissertation, but the issue is a bit complex and I am at best a You Tube Certified Mechanic. Loss of the ground from the TPU to a particular coil along with what appears to be a loss of signal voltage to the TPU from the ECU, with a correlation of some sort to temperature while all components and harness continuity being tested are found to meet standards.

As a caveat, while the harness continuity has been tested and found OK at the point/time of failure, it is in very poor condition. It just seems that if the harness is the cause, it would be demonstrated in continuity failures.

If anyone has had this experience with an issue like this or has any ideas, I would greatly appreciate the input. Thanks


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VStar650CL
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2004 Nissan Altima 2.5 S

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Sounds like maybe a missing reluctor tooth inside the Crank Angle Sensor (CAS). Would be a weird sort of failure but I think it would produce the symptoms you describe. If you can scope it, the output should be 5 narrow pulses with a wider pulse for number 1. If number 4 is weak or missing, that will be your culprit. Not sure if those old ECU's would throw a specific code for that, I think not. Code 11 was pretty much for a flatline condition, the ECU might not flag it if the sensor "works but".

peterpilot9
Posts: 42
Joined: Thu Jun 05, 2014 10:29 am
Car: 1990 Nissan 300ZX NA

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Thanks for the idea. I have been considering the CAS but as you mentioned I would need a tool to read the square wave pulse.

Not sure how it would work when cool and fail when hot but I'm not really that familiar with the physical internals of the CAS but it might have to be my next step.

Thanks again.

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VStar650CL
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2004 Nissan Altima 2.5 S

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It'll most likely have inductive coils inside with a spinning magnet, and any coil can open with heat if a winding is broken. It will be either one coil with 6 magnets or one magnet with 6 coils. Never had one apart but from the outside appearance, I'd say it's most likely 6 coils. If so, one defective coil that opens when warm would cause your issue.

peterpilot9
Posts: 42
Joined: Thu Jun 05, 2014 10:29 am
Car: 1990 Nissan 300ZX NA

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Thanks VStar. I've found some pretty good info on the CAS and I'm going to go ahead and test it starting with the SM procedures. I'll see what the results are and take it from there.

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VStar650CL
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You're most welcome! If you dissect one, post some pics. I'd be very interested to see how it's constructed.

peterpilot9
Posts: 42
Joined: Thu Jun 05, 2014 10:29 am
Car: 1990 Nissan 300ZX NA

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I have pulled the Crank Angle Sensor and separated the housings. Looks like I might have found my problem, at least I am hoping.

I have some pictures but here is a link is to a write up I found that has great pictures and a detailed system description along with tips on maintenance. It is on the Twinturbo.net forum and was posted by user DVDBURN (MD) all the way back in 2010 I believe.

http://twinturbo.net/nissan/300zx/forum ... -info.html

I will try and post my pictures later as mine were taken with the CAS still dirty and contaminated. His pictures are of a unit that has been cleaned.

First thing I noticed on mine is that the shaft does not rotate smoothly. It moves a few degrees and then drags a bit through the entire range of the disk. After pulling the housing apart, I can tell it is lacking clearance somehow as it passes through the LED fixture.

As with the example in the link I posted, my CAS is very dusty inside. The six 120 degree slots are somewhat distorted appearing on the edges and the outside ring of one degree slots is pretty much so full of dust/debris that they appear to be clogged in some places. My thought that the CAS might be my misfire issue is that the single degree slots on the outside rim appear to be most contaminated in the arc over the slot for the number 4 cylinder.

It makes sense that the ECU is not supplying voltage to the TPU if the CAS isn't signaling the ECU properly. It could be that the contamination expands with some heat and blocks/distorts the light from the LED sufficiently to disrupt the voltage generation in the CAS. If the ECU gets no signal, neither does the TPU and the coil doesn't ground.

In any case, I'll get the CAS cleaned up and installed to see if it corrects the issue. If not, I'll pull it and test it per the SM and see if it passes the check. I'll update as soon as I get it done.

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VStar650CL
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2004 Nissan Altima 2.5 S

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Great info! Freaky that it's basically a distributor-type opto wheel stuck on the camshaft. Guess I should've known, but leave it to Nissan...

peterpilot9
Posts: 42
Joined: Thu Jun 05, 2014 10:29 am
Car: 1990 Nissan 300ZX NA

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Image Image

Issue resolved!! Idled and drove the car for an hour today and no misfire.

These are before and after pictures of my CAS. While the lighting is a bit different, you can see the amount of red dust on all of the components before cleaning. The outer slots were particularly clogged but the edges of the 120 degree slots were also well contaminated. I cleaned the unit using electrical contact cleaner, q-tips and a soft cloth. It did not spin very freely before cleaning, but would spin with no resistance when I finished.

So, if you are experiencing a misfire in your 300, consider cleaning the CAS. It does seem that most CAS issues are not normally cylinder specific, but in my case, it looks like a dirty CAS manifested itself in a single cylinder misfire after warm up. When diagnosing this problem, I did find a loss of signal voltage to the TPU and thought it to be a bad ECU. Instead, it looks like the CAS wasn't sending signal voltage for number 4 to the ECU which in turn couldn't provide signal voltage to the TPU. So the coil wasn't grounded and the plug wouldn't fire.

To be fair, this CAS has >300,000 miles and 30 years on it and I doubt it has ever been opened, so hard to complain :)

Thanks again to everyone for their input.

itsa300zx
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Car: 1990 300zx NA W/TT swap
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Thanks for posting the solution and nice work on not giving up.

That red dust is a good sign of metal particles, typically resultant in dry bearing applications.

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VStar650CL
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itsa300zx wrote:
Tue Dec 01, 2020 2:36 pm
Thanks for posting the solution and nice work on not giving up.
Seconded!

peterpilot9
Posts: 42
Joined: Thu Jun 05, 2014 10:29 am
Car: 1990 Nissan 300ZX NA

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Appreciate that. Hopefully this will be of use to someone one day :)


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