I need help with a coil wiring issue

[BabbitMike]

I've had this little rig for about 3 years and she has been pretty decent. A little back story here . It's an '86 D21 4x4 with the Z24 engine. The head is from a Z22 from what I am understanding. The EFI and ECM, ECU, PCU whatever it is for this rig has been removed and the carb is a re-jetted Holley. The distributor is a vacuum advanced electronic unit and I think is from a 1981 Z24(?) Datsun. I just rebuilt the head from having the #4 exhaust valve burning a chocolate M&M sized hole in it. I did get her back to running again except I am not getting any power to the exhaust coil pigtail.  According to the wiring diagram the intake also sends a signal to the tach and the tach is working. The exhaust is supposed to be on a 10 amp fuse. I am not finding a burned fuse and don't know if that would be a fuse or a fusible link. I am not familiar at all about how this dual coil setup is supposed to work and I wonder if that didn't have something to do with it burning a valve. Being the purist I normally am I wish the guy I got this from had left all of the electronics alone. Any help at all to get power back to the exhaust coil will help. Just for S & G's I am going to take the distributor cap off in the morning and make sure I didn't knock a wire loose from the electronic unit because 1 of the wires for that coil go back there.   

           Thanks,

                       Mike 

Edited November 15, 2020 by BabbitMike
correction

[datzenmike]

Check the fuse box, far left or rear position. 10 amp fuse probably blown. These engines use two plugs so the burn time is shorter than with a single plug. Because of this the timing has to be retarded. Shorter burn tome, light it later. If the exhaust side plugs are not firing the engine will be running retarded. That might over heat the exhaust valves.

 

The Z24 was introduced in November '82 for the '83 model year. The '81-'82 was a Z22 and the head is identical but for the shape of the intake port. It's round while the Z24 is square with rounded corners. It'll still work.

 

The D21 Hardbody uses a Z24i engine with throttle body injection called 'Electro Injection'. This same engine (Z24i) was available on the April '85 720 trucks and till about December '85 or a few months into the '86 model year. It was a half year model and so was the Hardbody.

[BabbitMike]

  Thanks Mike,

The intake ports are round with the notch for EFI and the chamber size is several CC's smaller than the Z24. I took a look at the fuse boxes and I am not seeing a blown fuse or anything that I would relate to that circuit. I got a pic of the fuse box under the dash and the relay boxes on the passenger side of the fender well. What conditions make the exhaust coil cut out?  

 

https://imgur.com/AO8wyG0 fender well

 

https://imgur.com/LsmNQ6q under dash

 

 

Mike

 

[datzenmike]

 

HEADS

I have cc'd Z20, Z22 and Z24 heads twice each and all were exactly 57cc. Both Z20 and Z22 use the same head and available with carburetors or EFI depending on the vehicle. It might be possible your Z series head has been milled down for flatness.The one exception is the #W 04 casing Z20 for the mileage option 720. This is about 46cc and the combustion chambers are visually smaller and can't be mistaken.

 

COIL

There are 2 conditions where the exhaust side plugs do not fire...

 

1/ There is a vacuum switch that senses low vacuum under heavy throttle conditions and this electrically tells the distributor to deactivate the exhaust side coil. This is to reduce engine noise under load. Your distributor has three wires covered with a black sheath and a single wire bundled with it having it's own connector. In effect 4 wires total. Unplug the single wire. If the vacuum switch is faulty the coil will start working ALL THE TIME.

 

 

 

2/ The fuse is blown or looks ok but faulty just the same. Use a test lamp on both the exposed fuse test points on it's top. In the picture there's one on each side of the 10

 

 

 

[BabbitMike]

I took a look at the pigtail coming from the distributor and took the cap off and there are only 3 wires coming out. I also pulled each and every fuse from the box and they are all good. The transistor unit only has 3 lugs. I have another distributor that I picked up from my storage unit and it does have the 4th wire. I have been trying my damnedest to get a hold of the guy that I got the truck from but still have not heard from him.

   Mike

[datzenmike]

Never trust a visual check. Electricity is invisible, use a meter or test lamp. Both sides of a good fuse will have 12v or battery.

 

 

If this can't be resolved jumper both coil + terminals together.

 

 

Now it's up to the module in the distributor to switch both coils to ground to make a spark.

 

 

[BabbitMike]

I checked the fuses with a multimeter, they were each pulled and tested.

[datzenmike]

The fuse may be good but is there 12v on both sides? connection may be bad or burned. The output from this fuse goes to the exhaust coil. You said there was no power at the exhaust coil +, so something isn't right and we can't make any assumptions.

[BabbitMike]

I did check all fuses on both sides and was getting power to the proper keyed fuses. I did find the wire for the temp sensor was dangling and plugged it in. I jumped the power from 1 lead to the second coil and both were firing but weakly since it was pulling for both coils. I then hooked up the old pigtail and I had power to both. I knew both coils were working but the feed for 1 wasn't. I don't know if I jiggled a loose connection or what but the exhaust coil almost knocked me on my ass. I think the next order of business is to clean up all connections and secure what splices there are.

   Thanks for all your help.