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How can I test the ignition relay in my 78?


jeremy93ls

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My head is spinning from scrolling through the FSM and web forums looking for information about my problem.

This problem is intermittent but seems to be happening more frequently. This evening, the Datsun decided it was going to stay right where it is.. None of this started happening until recently.

My symptoms are:

-starter will turn over, but..

-temp, fuel and oil pressure gauge do not work

-fuel pump doesn't run/pressurize the FI system (my pressure drops down after I turn the car off, inline gauge attached)

-radio and blinkers do not work

-idle has become very rough and after warm up, the CHARGE light comes on and the BRAKE light dimly lights up (with E-brake up). 2nd rebuilt Hitachi alternator in two months with internal VR installed and seems to be doing its job. 1st one might have been ok, in retrospect.

I thought it was maybe a flaky ground cable because of some corrosion so it was replaced and connections properly cleaned. I then turned to the fusible links and briefly considered swapping over to blade fuses but changed my mind and ordered OEM links (haven't arrived yet). I did test the links and got 12.6v through all 4 as well as the green links mounted to the side of the relay box by the battery but new ones have already been ordered.

After seeing pics of another member's rust-filled ignition relay, I contorted around enough to pull mine out and take it apart only to find it looked pretty good. I put it back on the relay bracket and it clicks when the key is turned to the ON position. I can feel it click with my hand on it.

I keep thinking that these symptoms are related to a failing ignition relay but I'd like to test it if I could. If it clicks when the key is turned to ON, could it still be failing?

Electrical problems suck.

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That was probably my relay.

Could you separate out the problems that are there when it was running, and the problems that are there now? Looks like you have everything blended together.

The odd one is that the starter will turn the engine,which means the solenoid wire is getting power, but the fuel pump does not have power (this should be confirmed). That could be the fuel pump control relay or the pump relay itself. Try unplugging the small yellow wire from the starter then turn the key to Start and make sure that the fuel pump does not run. You can see in the attached diagram that the Fuel Pump Control relay gets power from the Start circuit first and powers up the Fuel Pump relay itself.

If the pump is not getting power, make sure that your car actually has these two relays. Some of the 1978 cars still have the combined EFI and Fuel Pump relay, but may still also have the Fuel Pump Control relay to power it. I had a 78 with the combined relay.

I think, that a failed Fuel Pump Control relay might explain some of your symptoms, like the back feed through the alternator charge light, since the L wire runs through it. There are others with better electrical knowledge on the forum.

Yeah, it's complicated. Unplug that yellow wire at the starter solenoid and make sure the pump does not get power. That's what I would do first.

post-20342-1415082865523_thumb.png

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That was probably my relay.

Could you separate out the problems that are there when it was running, and the problems that are there now? Looks like you have everything blended together.

Before this issue began, the everything else worked normally other than a bouncy voltage gauge. Gauges worked, radio, turn signals etc worked like normal. It did seem to idle better before this, though. Now, if I can get it to start up, the idle is low/uneven and it hesitates when the throttle is opened.

I'll get my 'assistant' to help me test the fuel pump for voltage as she turns the key this weekend.

My 78 has three relays under the box in the engine bay. Fuel Pump Control, Fuel Pump Relay and EFI Main Relay, according to the FSM. No AC in my 4-speed car so those are the only ones under there.

SteveJ, man, you're awesome for offering to help. Let me see what I can get accomplished this weekend before I call for backup.

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The odd one is that the starter will turn the engine,which means the solenoid wire is getting power, but the fuel pump does not have power (this should be confirmed). That could be the fuel pump control relay or the pump relay itself. Try unplugging the small yellow wire from the starter then turn the key to Start and make sure that the fuel pump does not run.

When I unplug the yellow wire to the starter and turn the key to START, the fuel pump does not get power. BTW, the pump is new OEM and the connections are clean.

*edit* I faintly remember having this problem a few weeks ago and while the key was still in the ON position, I was under the hood jiggling the battery ground cable when I heard a clicking from somewhere and then the fuel pump started churning. I bet it was one of those two relays under the hood. I should have mentioned this earlier but just now put 2 and 2 together.

Edited by jeremy93ls
recolection!
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*edit* I faintly remember having this problem a few weeks ago and while the key was still in the ON position, I was under the hood jiggling the battery ground cable when I heard a clicking from somewhere and then the fuel pump started churning..

Interesting clue. But that's not supposed to happen either. The pump should only run when either the alternator is spinning or the oil pressure switch is activated, with the key On. That's what the pump control relay is doing. You're getting closer though.

On looking at the relays - I think it might be easiest to just unbolt the complete relay bracket to get to the relays. I seem to recall trying to get to them form underneath and it's a pain. Don't forget to disconnect the battery first in case you have exposed wires. There might also be a fuse on the pump circuit that could have blown if you have a shorted relay or wires.

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Don't forget the ignition switch behind the key, it can wear out then cause frustrating problems when starting/running.

maybe a wire to the oil pressure sensor switch (which kills the fuel pump) is loose or corroded.

I took the ignition switch out and plugged in a spare switch from the PO with the same results. When the key is turned to ON, nothing from the fuel pump and no buzzing from the seat belt warning device like normal. The only thing that happens is a faint click from the ignition relay in the passenger side.

The oil pressure sensor connections were clean but I cleaned/sprayed them again anyway.

I got my new fusible links in the mail today but they didn't solve anything. I don't regret getting them, though. Spare parts are priceless sometimes.

When I removed/opened up the three relays under the hood, they sparkled like new. I was hoping for something like Zed Head's relay.

I plan to disconnect and clean the 2 big wiring harness plugs by the passenger firewall (again) Maybe something is screwy with a few of those connections.

Edited by jeremy93ls
typo
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I am working through a "won't start, no fuel pressure during start attempt, no sound from fuel pump during start attempt" situation also with my 78. I am in metro Atlanta too, fyi. Last weekend I tested the fuel pump control relay and the fuel pump relay per the fsm and found mine to be working correctly. Next I move to the fuel pump (mine is 36 years old so this could quite possibly be my problem).

Did you ever find a fsm test procedure for the ignition relay? I cannot find it anywhere in the fsm. While the fuel pump control relay is powered from both the ignition relay and the startup position of the ignition switch directly, the fuel pump control relay won't send power to the fuel pump relay unless both power sources are present during startup (based on my analysis of the diagrams). The combination (both power sources) allows the control relay to send power to the fuel pump relay during startup when you have neither oil pressure nor alternator functionality (again, based on my read).

Very interested to see how this turns out for you.

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  • 2 weeks later...

A replacement ignition relay solved my problem. Thanks to all for advice and suggestions.

Now, if it would quit raining down here so I could put some miles on her.. good for the vegetable garden/bad for keeping a clean Datsun

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I'm glad that resolved the issue.

If you're on Facebook, join the Georgia Datsun Owners Group. I think one member is trying to have a get-together at his house this Sunday.

Also several of us from the Georgia Z Club are going to the Po Boys car show on the 21st. Just look for the events calendar on the Georgia Z Club FB page.

Edited by SteveJ
I wanted to prove that I actually am familiar with English grammar.
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  • 5 years later...
On 5/31/2014 at 6:22 AM, SteveJ said:

It's easy to test the ignition relay with a couple of 6VDC batteries, some clip leads, and a multimeter. Let me know if you would like some assistance sometime.

Hi Steve,

I know this is a very old post, but do you happen to have instructions on how to test the Ignition Relay?  I've been able to find directions in the FSM on how to test just about every relay but that one.

Thanks,

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