75' 280Z Intermittent Injector Pulse - ZDriver.com


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Old 04-06-2015, 07:14 PM   #1  
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75' 280Z Intermittent Injector Pulse

Hello, I know this is another "injectors not firing" thread, but I have tried all the trouble shooting I can think of, and I am out of ideas. I usually don't post unless there is no other option.

Using a noid light, I am not getting any pulse out of all 6 injectors.

I have been through the EFI bible several times now.
So starting down the list of things I have tried.

1. Fuel pump runs, good fuel pressure
2. Ignition lead wire tests good
3. Injectors did not click which led me to power relay check
4. Power relay clicked on "On" and "Start" position
5. So I performed 2-(2), 2-(1), 1-(9a-9d), these all tested good
6. The last test was 2-(3a-3f). None of these tests were good. None returned voltage when tested.

The manual states that if these tests bad, check wiring, resistor, relay and injector. I performed the drop resistor checks and it tested perfectly with 6 ohms on all circuits. Injectors had continuity, and the wiring also tested ok.

I then pulled the power relay and would not test good during any of the checks that are listed in the FSM. No click, no continuity, nothing. I put it back on the car however and it will click fine when cranking the engine. So I don't understand that part. I then replaced a fusible link that was burnt, that I didn't notice earlier. After doing this however I started getting a weak pulse with the noid light on all 6 injectors. I hooked the injectors back up thinking I had fixed the problem, but the injectors were still not clicking. I removed the injector plugs again, hooked the noid light up, and nothing. Nothing was changed other than hooking the injectors back up. So after that the injectors would only pulse 1 out of 10 starts. The light looked very weak too. When I would put the noid light on the cold start injector it would light up steady and bright.

So from here I don't know what to do. I am hoping that there is something else to check before I spend $130 on a relay that I may or may not need. I keep hearing something about a resistor connected to the tachometer that could keep the injectors from firing but the search function is not coming up with good leads on that.

Any thoughts?
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Old 04-07-2015, 07:41 AM   #2  
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The tachometer resistor you are looking for is in the wiring diagram. http://www.atlanticz.ca/zclub/techti...80z_wiring.pdf
It is also documented on page BE-33 of the 76 FSM. (That section of the 75 FSM wasn't available online.) It is in the engine compartment.

However, it just appears to be a branch off of the ignition circuit. As a matter of fact, the tachometer circuit was a re-design from the earlier Z cars where a malfunctioning tachometer could kill the ignition.

What relay are you referring to as the $130 relay? I have had good success using standard SPDT relays to replace OEM relays.
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Old 04-07-2015, 08:52 AM   #3  
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Quote:
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The tachometer resistor you are looking for is in the wiring diagram. http://www.atlanticz.ca/zclub/techti...80z_wiring.pdf
It is also documented on page BE-33 of the 76 FSM. (That section of the 75 FSM wasn't available online.) It is in the engine compartment.

However, it just appears to be a branch off of the ignition circuit. As a matter of fact, the tachometer circuit was a re-design from the earlier Z cars where a malfunctioning tachometer could kill the ignition.

What relay are you referring to as the $130 relay? I have had good success using standard SPDT relays to replace OEM relays.
Yeah from looking at the diagram I could not see why the resistor would stop ignition, but I am not an electronic whiz so I wasn't sure if I was missing something.

This is the relay. Called the Power Relay Control Unit in the EFI bible.

Datsun 280Z Fuel Injection Relay, 1975 1976 1977 Datsun 280Z, new AM

The cheaper Beck Arnley replacement is discontinued or people want the same price for it too.



I can tackle most basic electrical projects, but without a step by step walk through I wouldn't know how to put two standard relays in place of this one. I know it should be easy with the right electrical know how.
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Old 04-07-2015, 09:23 AM   #4  
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Okay, I usually see that called the EFI or fuel injection relay. If you have a multimeter, a couple of lantern batteries, and some clip leads, you can test the relay instead of just replacing it.

I don't have enough time to explain how it works now. If I'm not too tired after getting off work, I may write it up.
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Old 04-07-2015, 04:57 PM   #5  
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Thanks, yeah the checks I performed made to the relay confused me more than it helped.
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Old 04-07-2015, 05:43 PM   #6  
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Pin - Function
71 - Gets 12VDC from BW wire when key in Start or Run. Energizes a coil.
72 - Ground pin for the coils
70 - Gets 12VDC from R wire coming off of a fusible link. When 71 has voltage, Circuit closes between this pin and pins 43, 39, &10
76 - Gets 12VDC from BY wire when starting. Energizes a coil and connects to pins 47 and 4.
73 - Gets 12VDC from WR wire. When coil connected to pin 76 is energized, it completes a circuit to pins 74 and 48.
74 - Sends 12VDC to fuel pump via GL wire.
43 - Sends 12VDC to dropping resistors
39 - Sends 12VDC to AFM. Voltage returns to EFI relay via pin 36.
36 - Energizes the fuel pump coil when car is running. Also sends 12VDC to pin 20
20 - Sends 12VDC to ECU
47 - Sends 12VDC to Cold Start Valve while starting
4 - Sends 12VDC to ECU while starting.
48 - Sends 12VDC to Air Regulator

So how do you diagnose?
Pull the relay from its socket.
Wire the lantern batteries in series so that you have 12VDC.
Connect the negative to pin 72.
Connect the positive to pin 71.
Use the multimeter to verify that you have low resistance between pins 70 & 43. Also check between pins 70 & 39 and 70 & 10.
Disconnect the positive from 71 and move it to 76.
Check for low resistance between 73 & 74. Also check 73 & 48.
Disconnect the positive from 76 and move it to 36.
Check for low resistance between 73 & 74. Also check 73 & 48.

Report your results.
Please do the tests as I described. If you have questions, please post them.

If your relay passes these tests, your problem is elsewhere.
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Old 04-08-2015, 12:46 AM   #7  
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Thanks man I'll try it out.
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Old 04-09-2015, 08:00 AM   #8  
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Quote:
Originally Posted by Steve260Z74 View Post
Connect the negative to pin 72.

Connect the positive to pin 71.
Quote:
Originally Posted by Steve260Z74 View Post
Use the multimeter to verify that you have low resistance between pins 70 & 43. Also check between pins 70 & 39 and 70 & 10.
70 & 43 = 1.4
70 & 39 = .6
70 & 10 = .6

Quote:
Originally Posted by Steve260Z74 View Post
Disconnect the positive from 71 and move it to 76.
Check for low resistance between 73 & 74. Also check 73 & 48.
73 & 74 = 4.5
73 & 48 = 3.1

Quote:
Originally Posted by Steve260Z74 View Post
Disconnect the positive from 76 and move it to 36.
Check for low resistance between 73 & 74. Also check 73 & 48.
73 & 74 = 1.8
73 & 48 = 8.7


I am not sure what constitutes a low reading, but the numbers above are what my meter read.

Thanks
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Old 04-09-2015, 08:01 AM   #9  
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And thanks again for the procedure. It was much easier to understand, over the FSM.
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Old 04-09-2015, 09:26 AM   #10  
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So after testing the relay again. I hooked the noid lights back up and all 6 injectors lit up. So i hooked them back up and turned the motor over. Nothing. I pulled the injector plugs back off, put the noid light on #1 injector plug and it had no pulse. All six went back to not working.
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Old 04-09-2015, 09:53 AM   #11  
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So, you think $130 is expensive for an EFI relay...Um, I think you might have an issue with your ECU.

Of course, you may want to see if your car will fire with starting fluid. Also check the fuel pressure in the line. If both of those are good, then it would look more like the ECU.
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Old 04-09-2015, 10:06 AM   #12  
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It purrs like a kitten when you throw some starting fluid to it. Its also getting good fuel pressure with a new fuel pump and cleaned out tank. So did those numbers look good to you for the relay?
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Old 04-09-2015, 10:54 AM   #13  
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As long as the meter wasn't auto-ranging, the numbers are good. If those were in the Megaohm range, I'd call it toast.

You have spark and fuel pressure (another indication the EFI relay is good.). So, you could start checking the voltage throughout the FI circuit. You might need an insulation piercing probe to do this.

1. Check to see if your ECU is grounding out properly. Detach all 6 injectors. Determine which side of the connectors is positive and which is negative. Connect the multimeter to the negative of the FI connector and to ground. Measure resistance. You should see the value fluctuating between infinite & zero when the car is trying to start.
2. Conversely with the FI connectors pulled and the key in the ON position, you should see voltage to ground on the positive side of the connector.

My guess is that you won't see the fluctuation in resistance but you will see the voltage at the connector. Likely you either have a bad or intermittent path to ground. The path to ground is through the ECU.
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Old 04-09-2015, 04:53 PM   #14  
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I am receiving voltage, but I will check for fluctuation in resistance.

Thanks so much for your help!
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Old 11-19-2015, 10:16 AM   #15  
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So several months later, all tests showed good. I repeatedly removed grounds cleaned and replaced. I eventually pulled the efi harness and inspected every wire and found no issue. I eventually started working on some other projects, and the Z hit the back burner. Then I got some free time.

So I was reading through the FSM a few nights ago and noticed a blurb about unplugging the thermo time switch and that should put the ECU into running the base map. I actually unplugged every connector in the vicinity of the thermo time switch. First crank I got a strong consistent flash on my noid light.

So I threw some gas in my temp tank and started the motor on starter fluid, it eventually came to life. Once I burnt the cobwebs out and got good fuel throughout the system, the car will start on the first hit.

It's pretty amazing what one bad sensor can do.
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Old 11-19-2015, 10:19 AM   #16  
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