'86 720: Headlights switched on, truck dies. (solved)

[MISTER_Gunnz]

Hello y'all!

My name's Luke and I drive a 1986 720 King Cab 5-speed(approx 233k miles).

 

Here is my situation. I drove my truck around town last night. And when I got home, I shut off my headlights (truck running), so a friend could pull out of my garage without the light in his eyes. When I went to turn the headlights back on, the truck died instantly.

 

I shut off the headlights and the truck fired right up again, so I was able to pull it in the garage.

 

Today, I went down to check it out and the problem still persists. Let me say that the truck turns over and starts right up. I'm able to turn on the parking lights, run my A/C and stereo without issue. But, if I turn on the headlights, they do not light and the truck dies.

 

I have to disconnect the headlight harness from the lamps, let it sit and it will start up again without dying. I tried plugging in each headlight individually and in sequence to see if it made a difference, with no change that I can see.

 

My truck was in for a service yesterday (radiator flush/fill, new thermostat, and oil change) and I'm wondering if this is just a coincidence.

 

One last detail. When it dies, it make a sound, like a fuel pump running for a second afterward.

 

When the headlights are disconnected, I can run this thing around without issue. Any ideas would be appreciated!

 

Here is a pic of my truck:

[datzenmike]

Do you have the very late 720 with throttle body injection?

 

With the lights ON will it crank over?

[MISTER_Gunnz]

It is carburated, with an electric fuel pump. Z24 w/ eight spark plugs.

 

(edited)

Actually, it will turn over with the headlight switch on (headlamps plugged in), start for a couple seconds....and then die.

[datzenmike]

The only place where the headlights and the ignition are even slightly close to each other is at the battery. Inspect the positive terminal and specially the Green fusible links. Check them all.Very odd.

 

 

 

Before I forget ... how many wing nuts hold the air filter on, on you carb???

[MISTER_Gunnz]

Thank you for your responses!

 

There is only one wing nut holding the air filter on. And I'll be sure to check all the connections on the positive terminal.

[Nissan_Boy85]

Before I forget ... how many wing nuts hold the air filter on, on you carb???

 

[THREAD JACK] Wait.....was that something Mike didn't know? My world, its all upside-down and stuff....[/THREAD JACK]

 

Nice truck. The only thing I can think of is that there's either a bare wire somewhere or there's wires touching each other that shouldn't be touching. Or some bizzare anti-theft divice?

[moparvwfreak]

i have the same issue with my 73 620. headlights on and my truck will backfire and die, as though the ignition is cutting out. maybe the alt isn't putting out enough to cover the HL but enough to cover the rest of the stuff. thats my theory anyway.

[datzenmike]

[THREAD JACK] Wait.....was that something Mike didn't know? My world, its all upside-down and stuff....[/THREAD JACK]

 

No offense but sometimes owners don't know what they have. I was hoping this was an EFI model but he said it was carb. Quickest way to tell was ask how many wing nuts hold the air filter on. The EFI has two. Anyway, it's for sure a carb.

 

 

 

 

i have the same issue with my 73 620. headlights on and my truck will backfire and die, as though the ignition is cutting out. maybe the alt isn't putting out enough to cover the HL but enough to cover the rest of the stuff. thats my theory anyway.

 

Could be. But it happened suddenly....

[izzo]

I would guess what mike said earlier, seems as if its not getting enough power to make it run. Time to get out the test ligh & volt testing tool and test some stuff out.

[datsunaholic]

In addition to not having enough alternator output, having bad chassis/body/engine grounds will do the same thing. And bad connections (particularly at the fusible links).

[moparvwfreak]

i will check those out on my truck today. before i go to work.

[MISTER_Gunnz]

Well, I tracked down what I think was the problem. Was a simple fix, thank goodness!

 

The harness that carries the headlight wiring was sitting too close to my radiator and had cut through a tiny bit (mostly from rubbing), shorting out the wires inside. I will be taking the harness apart and insulating everything properly soon. But, in the meantime, I put some more electrical tape on there and added a ziptie to help keep the harness away from the radiator.

 

Here is a pic.

 

It may be worth the time, for any other 720 owners out there, to give this spot a look....just in case.

 

****************************

 

Now, the only other thing on my truck that is giving me trouble is the speedo. It started jumping up rapidly, when I would take-off after a cold startup. And now it has wound itself back so far that when I'm doing 70-ish mph, it looks like I'm doing 15mph.

Anyone out there know where I might be able to track down a replacement meter panel? My truck does not have the tach or clock, so it would be great to find one with those in there! Don't mind if it reads in kph.

 

Thanks again for all your responses!

[elmerfudpucker]

dont see a location below your avitar, so I dont know where to begin...

[MISTER_Gunnz]

I suppose that info would help. :blink:

 

Profile updated: Burbank, CA.

[datsunaholic]

It says "Burbank, CA". Apparently added after the question.

 

Not sure of an '86 720, but I know all late 83-85 gasoline 720s used the same dash panel. Ones with tachs were fairly common in KCs and ST type trucks. Early 83 and all 80-82 used a very different, square-gauge type panel.

[MISTER_Gunnz]

I will search more on the subject, but if anyone knows and would not mind answering the following questions....I would appreciate it.

 

Is the speedo cable driven? Is the wiring for the tach tucked behind my not-tach cluster?

[Sealik]

I will search more on the subject, but if anyone knows and would not mind answering the following questions....I would appreciate it.

 

Is the speedo cable driven? Is the wiring for the tach tucked behind my not-tach cluster?

 

 

Yes....and yes.

Looking for the whole cluster....?..... Or just the tach and clock?

You would just have to drill a hole in your lens for the clock 'adjustment rod'.

[MISTER_Gunnz]

^^Thank you for answering my questions!

 

Was able to adjust the speedo needle, so that is back on track. The tach is most important to me.

 

You have a tach you want to sell?

[MISTER_Gunnz]

Hey y'all! Got a new issue and figured I'd post here, instead of starting a new thread.

 

I've got fluid leaking above my feet, in the cab. I replaced the clutch master cylinder because I saw fluid coming in near the pedal connection. But, after more leakage and inspection, it seems there is fluid leaking near the speedo cable...behind the cluster.

 

Any ideas what this could be about? Thanks for your time!

 

edit: I took everything apart and the fluid is definitely coming out of the speedo cable. Is there a seal somewhere that has gone bad? At the transmission?

[bilzbobaggins]

hey man...i found a thread that mentions leaking at the seal for the speedo cable My link

 

its on page 4...last few posts....

 

maybe its climbing up the cable(tranny fluid)

 

hope this helps

 

jeff

[datzenmike]

 

See the black thing on the end of the cable just behind the crimped on thing? Looks sort of like a top hat? That's the seal that keeps the oil in the tranny. They can tear and the oil is lifted up the cable by the turning inner cable.

 

 

 

 

.

[MISTER_Gunnz]

Thank you both, very much!

[ggzilla]

So that missing seal caused your engine to die???

[MISTER_Gunnz]

^^ my apologies for the confusion. The leak was a separate issue.