Spark when key is turned on, but no spark when cranked

[That Ol 720]

I have a 1983 720 and I just replaced my distributor about a week ago due to a bad vacuum advance. I put the distributor on correctly at top dead center based off cylinder 1 on the compression stroke. Right after I got it on it started up just fine and ran all night. The next day I came out to go to a friends but its not starting. I began to mess with my timing originally thinking timing was the issue and when I adjusted my distributor it worked and started up. Later that day I went to start it up again and didn't start. Messed with the timing and the distributor a bit more and still didn't start this time. I noticed that when I turn my key on without cranking it I get 1 single spark and a small amount of gas vapor shoots up into the neck of my 32/36 weber. But then as soon as I crank it, it wont start. I don't have spark when cranking my engine, but I do have 1 single spark when I just turn my key on. My dad was out there with me and said that my fan turns counterclockwise when I get that single spark which is very weird. I'm very stuck at the moment. Any suggestions on what to look into? Could it be my ignition control unit?

[datzenmike]

If just popping one distributor out and another in you don't need to set TDC. The distributor can only go in, in one position. The one the old one was in. Same as finding your shoes in the dark. The right shoe only fits the right foot.

 

Make sure the distributor in firmly bolted down as it grounds through the mounting bolts. If you messed with the timing adjustment bolts make sure they are also tight.

 

How do you know there is no spark while turning the engine with the starter????

 

(just in case)

Check each wire separately there are 8 and easy to mix up.

 

[That Ol 720]

8 minutes ago, datzenmike said:

If just popping one distributor out and another in you don't need to set TDC. The distributor can only go in, in one position. The one the old one was in. Same as finding your shoes in the dark. The right shoe only fits the right foot.

 

Make sure the distributor in firmly bolted down as it grounds through the mounting bolts. If you messed with the timing adjustment bolts make sure they are also tight.

 

How do you know there is no spark while turning the engine with the starter????

 

(just in case)

Check each wire separately there are 8 and easy to mix up.

 

 

We don't know if my distributor that was on originally was on correctly so I played it safe and started from scratch. Me and my dad tested for spark by putting a screwdriving in the wire to spark plug #1 and holding it close to a ground looking for the spark to jump the gap while the other person was turning the engine over. All my bolts are tight.

[datzenmike]

Good, well done.

 

Check the voltage on the + positive terminal of both the coils. Got power?

 

 

You still have the old distributor I hope?

[That Ol 720]

Just now, datzenmike said:

Good, well done.

 

Check the voltage on the + positive terminal of both the coils. Got power?

 

 

You still have the old distributor I hope?

I turned my old distributor in for my core refund about a week ago...got power to both coils.

[datzenmike]

Never, never, never do that if you can afford not to. Your 'new' distributor may have a bad ignition module in it. You could swap the old one in if you had it. Old parts almost always have some use if only for a part off of it.

 

Well if you are sure that it is grounded and you have power to the coils then the ignition module is faulty. I guess you could take it back for a new, new one.

[That Ol 720]

6 minutes ago, datzenmike said:

Never, never, never do that if you can afford not to. Your 'new' distributor may have a bad ignition module in it. You could swap the old one in if you had it. Old parts almost always have some use if only for a part off of it.

Ok well Ill be shure not to do that next time. I do have a small black little box where 5 electrical wires go from and plug onto my distributor underneath my rotor. Is that my ignition module?? Or could there be one inside my distributor and then that small little box be something different??

 

 

[That Ol 720]

This is that little black box I was tellinig you about

[Veraciousreasoning]

39 minutes ago, That Ol 720 said:

This is that little black box I was tellinig you about

 

Those connections look pretty dirty. I would wire brush them to clean them up and see if that helps. Especially do that to the grounding wires and where they are grounded to. 

[That Ol 720]

13 minutes ago, Veraciousreasoning said:

 

Those connections look pretty dirty. I would wire brush them to clean them up and see if that helps. Especially do that to the grounding wires and where they are grounded to. 

Sounds good man. Thanks for the suggestion. I've been reading up on this issue and a bad ground seems to be a common issue.

[Veraciousreasoning]

7 minutes ago, That Ol 720 said:

Sounds good man. Thanks for the suggestion. I've been reading up on this issue and a bad ground seems to be a common issue.

 

Yea, a lot of times the ground can be faulty and allow it to work sometimes and not sometimes. 

[datzenmike]

 

This is the junction box. The brown should have 12v. and powers the ignition module. The Black/White should also be 12v if making good connection. Best check. The wires are not easily mixed up because there are 3 different size studs, nuts and matching lugs on the wires.

 

 The other two wires, Red and the White/Blue go to the two coil negative terminals. The module makes and breaks a path to ground to fire them. This is why it's essential that the distributor be grounded.

[That Ol 720]

15 hours ago, datzenmike said:

 

This is the junction box. The brown should have 12v. and powers the ignition module. The Black/White should also be 12v if making good connection. Best check. The wires are not easily mixed up because there are 3 different size studs, nuts and matching lugs on the wires.

 

 The other two wires, Red and the White/Blue go to the two coil negative terminals. The module makes and breaks a path to ground to fire them. This is why it's essential that the distributor be grounded.

I cleaned all of the grounds off last night with a wired brush 1 at a time to make shure that I didn't mess up the order or the different sized nuts. Still wont start. I think my next step is going to be getting my ICM tested so Ill do that today and keep you guys updated. 

Edited February 10, 2022 by That Ol 720

[captain720]

Check your engine to truck and engine to battery ground because if dizzy ground to engine but engine no ground to battery it's not grounded 

[datzenmike]

If block not grounded the starter wouldn't work (and it draws hundreds of amps) nor the temperature or oil pressure senders.

[That Ol 720]

38 minutes ago, datzenmike said:

If block not grounded the starter wouldn't work (and it draws hundreds of amps) nor the temperature or oil pressure senders.

My starter works tho...I can crank it over, it just won't start 

[datzenmike]

So this proves that the block is grounded properly. Thus if the distributor is grounded to the block and still no spark them likely the ignition module is bad. Take it back where you got it from.

[That Ol 720]

32 minutes ago, datzenmike said:

So this proves that the block is grounded properly. Thus if the distributor is grounded to the block and still no spark them likely the ignition module is bad. Take it back where you got it from.

Is there a chance my ignition switch could be bad??

Edited February 11, 2022 by That Ol 720

[datzenmike]

Not if there is power on the + side of the coils. Just to be safe check this with the starter engaged too.

[That Ol 720]

I just checked the resistance on my coils and I have about 13 ohms and i checked on my ignition and its varying all the way from 34 to 2...where is the ground(s) for them?? 

 

 

Edited February 11, 2022 by That Ol 720

[datzenmike]

The coils are grounded by the ignition module. This causes current to flow through the coil primary windings building a magnetic field. When the signal rotor turns past the stator, a small magnetic pulse signals the module to break the circuit. The magnetic field inside the coil collapses suddenly inducing a high voltage in the secondary windings that is fed to the spark plug.

 

If the engine is not turning and a signal from the stator does not occur within a short time, the module turns off the ground so that the coil does not over heat. A detector circuit turns the module bake on as soon as a pulse signal occurs.The module only turns on the ground for the coil for long enough (dwell) to build a sufficiently strong magnetic field. It's not on for four 90 degree segments as the distributor turns. There is also a current limiting circuit so a dead short past the coil or a lower ohm after market coil should in theory not harm the module, but I wouldn't try it.

[Logical1]

I am not sure about your newer distro but most of the stock ones also have a distro grounding spade lug next to the vacuum advance. I have brought many of these trucks back from horrible/barely/non-functioning by bypassing the stock wiring and direct wiring the distro and coils. Mike and I disagree about why this works and he knows his shit but I will post up a diagram of how I direct wire the ignition system that has worked for me with over a dozen of these trucks.

[Logical1]

This is a quick and dirty image of a setup. I run the switched hot to a kill switch and direct to the battery. Everything else is stock wiring taken out of that black box terminal block that is stock and run straight from the distro to the coils. Every truck I have done this to fires up and runs like a top. Ignore the small gauge red wire coming off the negative side of the upper coil, that is a direct run for an aftermarket tach on this truck.

Edited February 12, 2022 by Logical1

[datzenmike]

Normally the intake coil is directly connected to ignition switch from a fusible link. The power to the intake coil also powers the ignition module. The exhaust coil is powered from the first fuse on the left side of the fuse box turned on by the ignition switch. Why Nissan did this I don't know. Wiring them as you have works just as well and there's no fuse involved.

[Logical1]

I use a fusible link direct from the battery, as I don't like burnt wires if the sheath gets rubbed off. I am still not sure why this fix has brought so many trucks back to life, I just havent put the time into figuring it out people just appreciate their truck running strong. Several of them the truck has been misfiring or running weak and after I re-wire them its like swapping old points for a matchbox...VROOM!