1991 D21 4x4 died, smoked and now no spark

[ManzanoNissan]

1991 D21 4x4 just finished replacing the suspension and steering all the way around, just got back from town from a wheel alignment and she died on me a mile from home. She died just like she did when the fuel pump when out on her last year, except as I pulled over you could smell burning plastic and you could see smoke coming from the passenger side of the hood, right where the relay box is.

Checked the fuel relay and the main relay and both were fine. Switched out with another nissan's relays, no difference, she turns over but no spark. Tried starter fluid, no luck. Removed a spark plug wire and grounded it and we could see no spark.

All the spark plug wires, distributor, coil all new.

I would have a tendency to think the coil is bad, but I can't ignore the fact smoke/burning was happening under the hood.

The relay box does not show any fire damage, could not find scorch marks anywhere.

Anyone ever have something like this happen?

I'm suspecting the box/housing where the relays go in.

[whomadewho]

Some arm chair thoughts.... Check the fuseable link on the positive battery cable. Unplug the relays on the passenger side fender and with a test light check for power at the relays. Also check the relays in the fuse box. Check the distributor tower, rotor, and coil wire. Unplug the distributor wire harness with a test light check for power.

[ManzanoNissan]

Some arm chair thoughts.... Check the fuseable link on the positive battery cable. Unplug the relays on the passenger side fender and with a test light check for power at the relays. Also check the relays in the fuse box. Check the distributor tower, rotor, and coil wire. Unplug the distributor wire harness with a test light check for power.

Will do. I don't think the relay housing is the culprit, and I think the smoke I saw coming from the passenger side hood wasn't all the smoke. I'm suspecting a burned out coil/ignition module. We did a rough test on the distributor wire harness, yanking number two and there was no spark on test. Will pick up a test light today.

[VStar650CL]

Those PTU's are known to smoke up from bad distributor grounds, too. They're just big N-MOSFET's, and practically all N-channel transistors get angry and hot when their ground sucks. Make sure the distributor connector doesn't show any evidence of overheated pins.

[ManzanoNissan]

Those PTU's are known to smoke up from bad distributor grounds, too. They're just big N-MOSFET's, and practically all N-channel transistors get angry and hot when their ground sucks. Make sure the distributor connector doesn't show any evidence of overheated pins.

Hey VSTAR what is a PTU? I suffer vermin here on the mountain. Squirrels, mice and rats. I have to use those oversized stinky moth balls to keep them out of engine bays and they had at the Nissan while I was replacing the steering/suspension. I think I may have found the problem. Here's the video: It looks like melting happened here at the distributor and I think I see exposed silver wiring. If this is source of the overheating, would this suggest both the distributor and coil will be burned out as a result? I'll test later when I get a test light.

https://imgur.com/a/RTQbfrI

I'll check the cap points (the cap was new), rotor and check the coil with a test light but how do you check the actual distributor unit? That harness looks F'd so I'm concerned there's overheating damage done?

[VStar650CL]

That connector definitely did some melting. Fix that first, hopefully everything else survived the meltdown. PTU is short for "Power Transistor Unit". Some of them are external, but IIRC it's inside the distributor on a D21. It's basically a big, honking MOSFET transistor that serves the same purpose as points on an old school distributor, grounding the coil to charge it. When they have a "soft" ground they get hot enough to cause "warts" in the epoxy covering and will eventually fail from the heat.

[ManzanoNissan]

Just time for a quick inspection on the line starting with the fusible links and voila, I found where the smoke was coming from. Replacements fusible links on order. Will work on repairing that distributor connector, check fuses, check the distributor cap and rotor, run a multimeter on the coil but I'm hoping the poor ground on the distributor caused the fusible links to blow before anything else did. All of the electrical stuff in here, distributor, wires, plugs and coil are all 2 years old.

https://imgur.com/a/TAq6082

[ManzanoNissan]

And the culprit has been found. The wire harness to the distributor had been sandpapered through by the fan belt. Opening the plastic encasing back to make repairs, you could see this wasn't the first time it had happened. Ground wire was torn up and had to be replaced, the rest of the four wires were exposed but in good shape. Used uninsulated butt connectors for the repaired wire then heat shrink tubing over each, proper insulated wrap, then new plastic sheath and finally the electrical tape wrap. I've zip tied that harness to the front engine eye bracket to avoid that happening again.

While in the bay and fixing wires I noticed the original plastic sheath around the wire harness to the relay box looked broken into and suspect so I opened it up and voila found a 3 inch section of 10 gauge wire that had been spliced into the fuel pump relay lines. Looked like a temp fix too. A quick twist on both ends and some electrical tape. This had been fixed by the original owner probably 20 years ago. Properly fixed in the same fashion as the distributor wire harness.

Just waiting on the fusible links, haven't checked the distributor, coil or fuses. Will throw the links in first and try starting her up and go from there.