Clutch Safety Switch Stops Engine?? - 1984 720

[Stewart]

So I tried to clean up after an oil filter leak and probably either got oil or water somewhere I shouldn’t have.  I was going down the highway after ~20 miles or so and suddenly engine dies.  I figured out it would start and run fine but die when I took my foot off the clutch.  
 

It’s the neutral safety switch.  I pulled it out of the bracket so the clutch pedal wouldn’t press it when the pedal is released and no more issue.  But isn’t this tied to the starter relay ground?  Why would pressing it cause the running engine to die?  And that will probably tell me what I need to pull an clean better.

 

Thanks!

[datzenmike]

If the 720 has a safety switch it only interrupts the start signal. The 720 does have a fuel cutoff function.

 

In a nut shell, then you decelerate the throttle is closed but the air going past still sucks an idle mixture that is just blown out the tail pipe and wasted. The fuel shut off disconnects the idle cut solenoid which shuts off fuel to the idle circuit under certain conditions.

 

A switch detects a high manifold vacuum

Transmission must not be in neutral

Clutch must be engaged

 

What happens if the transmission is in neutral with the clutch pedal up. That should also defeat the fuel shut off.

 

 

[Stewart]

With the transmission in neutral if the switch is pressed it kills the engine.  If the transmission is in gear when the switch is pressed it tries to kill the engine but momentum tries to keep it going resulting in a bucking ride.  

 

I know that’s not how it’s supposed to work.  I tied giving it gas (in neutral) when I hit the switch (now hanging next to the clutch pedal) and it cuts out immediately still.  Could it be the fuel cut off circuit somehow?

[Charlie69]

We lived for decades with out neutral safety switches.

 

You should look for damaged wiring. 

[Logical1]

[datzenmike]

Connect the idle cut solenoid to the + terminal of the coil. This will keep the solenoid on when the engine is running and shut off when the engine is turned off. This is how Hudson bypassed the fuel cut off.

[Stewart]

Thanks for the suggestions, I have narrowed it down a little. The switch leads to the inhibitor relay mounted on the front right fender near the battery.  I can hear it click when the pedal switch is pressed.  I don’t see any damaged wiring anywhere down there but can anyone tell me how this circuit is supposed to work?  My wiring diagram isn’t great on this.

 

I thought that it would cut the ground to the starter when the pedal was up and in gear but the starter still cranks when the switch is pressed so I think it ties in to the coil? What I’m not sure is how it normally works so it stops the engine from starting but doesn’t kill a running engine? Any thoughts to help me trace this.  I know I can bypass but I’d rather figure out what happened and do it right.  Also not sure if this is related to a poor cold idle that started at the same time.  
 

 

[datzenmike]

There is no starter interlock on a standard transmission 720, or not for the '84 year at least. It's to prevent the engine turning and possibly starting on automatics only.  It would start and being possibly not in park or neutral the truck would jump forward or back into something. It only prevents the starter from working. Has nothing to do with a running engine.

 

 Bear with me. You mentioned a poor idle started about the same time???

 

Check that both coils are firing. Intake and exhaust side.

[Stewart]

Thank Mike, it looks like I only have spark on the plugs on the exhaust side.  But doesn’t the intake side only fire at certain rpms or throttle levels?  Not sure the best way to check that?  Or would it have spark when cranking?

 

I do have a relay of some sort that is connected to the clutch pedal switch, I can hear it clicking when that switch is operated with the key turned to start and when the engine is running and it clicks the engine dies.  It’s blue in a clear plastic cover next to the battery.  It’s not the fuel pump, I can hear that separately.  https://photos.app.goo.gl/TAafADstjRxnqE8BA

https://photos.app.goo.gl/ohcRc4W2Uk9eG8Uj6

 

 

[datzenmike]

Check the first fuse on the left (closest to the driver's door hinge) it's probably blown.

 

Both coils fire together at all times. There is a function that shuts the exhaust side off under heavy throttle to reduce 'noise' It also advances the timing during single plug operation so that power is not lost.

 

The very front relay is the fuel cut.

 

 

 

[Stewart]

Thanks Mike, I think I’m making progress but really haven’t dealt with ignition stuff much.

 

The fuse is good, but I swapped it with the horn fuse anyways.

 

I switch coil wires on the distributor and got spark on the ignition side and not exhaust.
 

I checked both could for continuity to ground and they are both getting 13+ volts when the engine is running.  Both coils are also sending voltage to the distributor (measured voltage at the end of the cable on the ignition side coil and the engine was running off the exhaust side).  
 

So now I’m confused why I got spark on the ignition side when I swapped coil wires but both coils seems to be functioning?  Distributor problem?
 

And I still haven’t figured out why my pedal switch is triggering the fuel relay.

 

Thanks for your continued help!

[datzenmike]

Well the idle cut solenoid needs to be on to let fuel down to the idle circuit. It gets it's power from the ignition switch but is grounded through the clutch switch, the neutral switch in the transmission and the fuel cut function when in it's off condition. These two switches keep the idle cut on when down shifting or coasting in neutral IF the fuel cut has been activated to turn the idle cut off. These two switches provide the ground needed to prevent stalling.  

 

 

[Charlie69]

I am curious was the truck and automatic transmssion and changed to a standard?

[Stewart]

I should have mentioned earlier, this has a Weber carb on it, electric choke but I don’t think it has an idle cut solenoid.  The clutch pedal switch isn’t just cutting the idle fuel, it kills the engine even with throttle applied.  So I must have an electrical fault in there somewhere since it just started doing that.

 

Any pointers for diagnosing the distributor?  I’ve never dealt with that before.  I have power to both coils but still no spark at the plugs on the ignition side.  Switching the coils wires on the distributor makes the ignition side fire but not exhaust.

 

And no, it’s always been a manual 5 speed. I think manufactured 1/84 as a dlx cab and chassis 2wd.

[datzenmike]

Better check. If not, the fuel cut would have no effect.

[Charlie69]

The reason for my question about a trans swap is the only time you would have a inhibitor relay is on an automatic transmisson truck.

 

Idle cut solenoid installed.

 

[Stewart]

Hey guys, thanks for sticking with me!  I think I am slightly closer to solving this puzzle.

 

1. I traced that relay near the front passenger side fender and it connects to both the neutral switch on the transmission and the clutch pedal switch.  Both switches can be heard activating the relay.  The relay is the same blue on as my heater relay by the fuses so I switched them to check and it seems to be fine.  My Chilton manual in the wiring diagram for z24 1981-84 shows both switches going to an "inhibitor relay" on both MT and AT.  It clearly labels an "inhibitor switch" as AT only and the clutch and neutral switches as MT only but the relay is shown for both.  I don't have an idle cut solenoid (deleted with weber install by PO and running fine for several years).

 

2. I found this: https://www.justanswer.com/nissan/13dpw-not-spark-exhaust-coil-z24-dual-co.html https://www.justanswer.com/nissan/13dpw-not-spark-exhaust-coil-z24-dual-co.html Which seems to show the clutch and the neutral switch being involved in switching from 8 to 4 plug mode.

 

3.  I am only getting spark on the exhaust side plugs.

 

4. Thus I think the problem is my intake plugs are not firing.  The exhaust side plugs are being switched off, by design, to go in to 4 plug mode but since the intake plugs aren't firing this kills the engine!

 

Does that make sense to anyone else?  I know that doesn't seem to sync with your FSM and stuff but it seems to fit this other info and symptoms (truck is 1/84 and there were some mid 84 changes I believe). 

 

So now I just need to figure out why I quit getting spark on the ignition side a week ago. 

[stingraylx1]

Interesting thread, any updates on your fix to resolve the

issue?