1977 Cylinder #1 Mystery

[Zed Head]

I would unplug the dropping resistors (you might be able to do it without removing the resistor, can't remember), then check continuity to the injector #1 plug from the resistor plug.  I don't know if the wires at the dropping resistor are color coded or not.

 

[Zed Head]

Looks like they are not color coded.  So, your best bet is to unplug them all and confirm continuity for each power wire to each injector.    

Then check the wires in the dropping resistor itself if you find that all injectors have continuity.  You'll have to remove the resistors for that.

The problem would be either in the dropping resistor assembly itself or in the harness on the way to the injector plug.

Edit - just noticed that #1 should come from the resistor with five wires.  Just keep narrowing things down...  If the drawing is correct you can even tell which pin is #1 by its position.

Edited April 3, 2022 by Zed Head

[ckurtz2]

Update: So I checked voltage at the pin on the injector connector. It gets the proper voltage. I also checked it at the pin on the dropping resistor. Gets good voltage.

        So it's my understanding that the fuel injector plug gets 12v constantly correct? And that the injector fires by the ECM alternating the ground to the injector plug, thus firing the solenoid in the injector. 

        So, in theory, if my connector is getting 12v than the reason for it not working is that the ECM is either malfunctioning and not pulsing the ground, or the ground in the harness is bad.

        But then that begs the question, if my injector connector is getting 12v, why would it not be reading 12v at the ECM?

 

1975 280z #1 Injector Circuit.pdf

[Zed Head]

Did you disconnect all of the injectors before you measured voltage at the injector  plug?  And did you measure both pins of the injector plug?

You are correct about power and the ECU grounds.  But the ECU grounds several injectors at the same time, in two separate groups.  If you left the other injectors plugged in and saw voltage on one pin it would be because the injectors are wired in parallel as CO explained long ago.

You need to unplug all six injectors before checking for voltage.  Actually, that should have been the first suggestion, before digging in to the dropping resistors.  Unplug all six to break up the parallel circuit, turn on the key, and measure voltage at each pin of each injector plug.  That will tell if #1 is getting power.  You should see battery voltage on one pin of each plug.

[Zed Head]

But you do bring up a good point about the grounding part of the circuit.  You could have power but no ground.

You can measure voltage at the ECU connector also.  That will test the whole circuit.  If you have voltage there then your theory about the ECU fits.  Then you can get out your magnifying glass and check the trace back to the transistor.

[Zed Head]

[Captain Obvious]

I was thinking it was an intermittent break inside the wiring harness (somewhere further up into the harness than right at the connector). But I didn't suggest that because I could not reconcile that with the following:
 

On 3/31/2022 at 11:21 PM, ckurtz2 said:

• Disconnected both #1 and #2 injectors while car was running. RPM dropped
  • Swapped #2 injector plug to #1 injector. No change in rpm.
  • Swapped #1 injector plug to #2 injector. It improved rpm.
    • however I did this test many times, and sometimes the rpm would not climb. It is not consistant.

I could explain how connecting the #1 connector onto the #2 injector might cause that injector to work*, but the part I really couldn't deal with was when you put the #2 connector onto injector #1 and it did NOT make that injector start working. That's the part that caused me not to pursue that avenue more thoroughly. 

*small break inside the harness causing an Intermittent connection. 

[Captain Obvious]

Oh, and the brightness of the noid light getting brighter at higher RPM's is perfect.

It's because the duty cycle of the injector pulse got wider. More wider. More gas at more RPM.

[Dave WM]

I thought at some point you had swapped the injector connector and the problem stayed with #1?

[Captain Obvious]

11 hours ago, Dave WM said:

I thought at some point you had swapped the injector connector and the problem stayed with #1?

He did. (Along with a bunch of other fantastic trouble shooting efforts!) That's why I'm thinking the break is somewhere further up inside the harness. If I were there, I would stick the meter in the connector and start retching the harness around and looking to see if I could see the issue.

I'm thinking that it might be a green corroded crimp connector somewhere inside the harness?

Edited April 4, 2022 by Captain Obvious

[Zed Head]

You guys are reminding me of something I see on some of these youtube repair videos.  You can see proper voltage but it's best to put a load on the circuit to make sure the current is there.  The guys that seem to know make up their own test lights with a headlight bulb.  A cruddy connection might show proper voltage, especially with today's digital meters.  But it disappears when a load is on.

Edited April 4, 2022 by Zed Head
- on

[Dave WM]

10 hours ago, Zed Head said:

You guys are reminding me of something I see on some of these youtube repair videos.  You can see proper voltage but it's best to put a load on the circuit to make sure the current is there.  The guys that seem to know make up their own test lights with a headlight bulb.  A cruddy connection might show proper voltage, especially with today's digital meters.  But it disappears when a load is on.

and I have a video for that!

https://www.youtube.com/watch?v=wsaP95p4q88&ab_channel=DaveWM