Ignition Coils and Modules

[jhill0014]

I inherited a 1986 720 4x4. I need help with ignition components.
After driving it for 3 days, the truck started having difficulty starting. Then it stopped starting all together.
Plenty of juice in the starter, but it stopped firing and never starts. After looking at the ignition components, I noticed that they have melted. Both of the coils and both of the ignition modules. A mechanic did tell me that the pick up coil in the distributor was going out. Are these elements connected? If someone can help I can provide pictures. Thank you.

[VStar650CL]

Melted coils and PTU's are usually from a very bad engine ground. Check that first before you make any other repairs.

[jhill0014]

I'll check on that short.

But I am pretty sure the pickup coil in the distributor is SHOT. Do you know if that might be connected to the heat overload on the coils/modules?

[VStar650CL]

Very doubtful. I'm assuming you have a Z24. What's inside the distributor is basically just a sensor that the ECCS Unit (ECM) uses for timing drive to the Power Transistors (PTU's). There's no direct link between the sensor and the coils or PTU's, so unless something is seriously screwed up in your wiring, nothing inside the distributor is likely to cause melted hardware. What does cause melted hardware is high resistance between the block or body and the negative battery terminal. That heats up everything on the high-voltage side of the ignition circuits and can destroy both the coils and PTU's.

[jhill0014]

What do you mean by PTU? I'd like to show some pics of my situation. can I send you an email?

[VStar650CL]

PTU's are the power transistors that the ECM uses to charge and trigger the coils. I take back what I said above, the first thing that fails in old age is memory. The PTU on your '86 will be inside the distributor underneath the mag pickup, so it's possible it's messed up along with the coils. It's 22020 in the diagram below. However, a complete meltdown of ignition parts is still unlikely to be caused by anything but a ground issue.

Posting pics is easy if you do it as an attachment. The only proscription is that you probably need to downsize them, the site doesn't like anything bigger than about 1MB. At the bottom of the message window, tag Full Editor & Preview. Then again at the bottom of the message window you'll find an Attachments tab. On that tab, tag the Add Files button and follow the prompts. To display the pic along with your message text, after the upload is complete, put the cursor in the message window where you want the pic to appear, then tag the Place Inline button.

[jhill0014]

Ok. I'm with you. What about 22136. Ive had suggestions to replace this part as well.

[VStar650CL]

Yep, those are all done-done. Replace all of it and check the whole ground bus supplying them. I bet there's a serious resistance someplace between them and the negative battery post.

[jhill0014]

Sorry if this seems amateur, but what do you mean by, "bus"?
When you say resistance, does that mean a short? A disconnected ground wire?

[VStar650CL]

No worries, learning about anything has to start somewhere and we all started at zero. There's no shame in that.

Resistance is something in between a short (something connected which shouldn't be) and an open (something disconnected which should be connected). Think of it as a restriction in the electric flow, It's a partial connection which lets some electricity through but not all of it. In the case of ground wires, it's usually from corrosion or fatigued copper in the wire strands. Resistances always generate heat somewhere in the circuit. I won't go into the reasons here, but in the case of coils with transistor drivers, that heat always manifests in the transistors and coils, not at the actual spot of high resistance.

In electrical terms, a "bus" is any collection of wires that connect to a common point. The two wire sets connecting all the OBD devices together, for example, form a CAN bus which all terminates at the OBD port. The ground bus on your car is the web of wires, chassis parts, and engine parts which all connect together electrically to terminate at the negative battery post. Somewhere in the path which starts at your coils and distributor and ends at the post, there's very likely a crummy connector or some other electrical restriction which is causing your ignition parts to heat up.

[jhill0014]

ok After doing some looking around, I think I found the short. I am going to replace the 2 modules, replace the 2 coils, and repair the short. It is supposed to be attached to the distributor, the negative. The positive is attached to the distributor correctly. But not the negative. Thanks for your help so far and I will keep you posted.

[jhill0014]

Hey Vstar. While I'm waiting on parts, quick question.

Does that truck HAVE TO HAVE the ignition modulators? I have seen pics of this motor with the coils only going into the distributor, no modulators.

[jhill0014]

Those round ignition coils do not appear to have a connector for a module. I'm confused.

[VStar650CL]

Not sure what you mean by a modulator. The PTU in your distributor can be called a spark module, if that's what you mean. Some other Nissans from that period use a CAS for spark timing instead of a mag pickup and have PTU's located outside the distributor. You can also call those modules. It's not the correct lexicon for a Nissan, but most people will know what you're talking about.

Here' a quick primer on how spark works. All electronic spark systems share three things in common, a sensor to know when to fire, a driver to ground and then release the coil, and the coil itself. When the sensor (in your case the mag pickup) tells the driver (in your case the PTU) to fire the coil, the PTU first grounds the coil for a short period to charge it, then releases the ground to fire the plug. All coils "kick" at high voltage when they're released, that's called back-EMF and it's what produces your spark. There are various ways to accomplish those things, but they all use the same principles and all employ variations of the same three components.

[jhill0014]

I'm sorry. I used the wrong term. NOT MODULATOR.

The discovered that the ground wire coming off the back of the distributor was hanging by a thread. Could this be the resistance you mentioned earlier? The problem that's frying those ignition components?

[jhill0014]

Can you help with this issue? The attached pic is the exact module that was on the truck when I got it, the one that burned up. In the picture is the Intake side module, HOWEVER, Amazon says that it does not fit my truck. I'm confused. I hope you don't mind all the questions.

[jhill0014]

Can you help with this issue? The attached pic is the exact module that was on the truck when I got it, the one that burned up. In the picture is the Intake side module, HOWEVER, Amazon says that it does not fit my truck. I'm confused. I hope you don't mind all the questions.

[VStar650CL]

The discovered that the ground wire coming off the back of the distributor was hanging by a thread. Could this be the resistance you mentioned earlier? The problem that's frying those ignition components?

Yes.

[VStar650CL]

module.pngCan you help with this issue? The attached pic is the exact module that was on the truck when I got it, the one that burned up. In the picture is the Intake side module, HOWEVER, Amazon says that it does not fit my truck. I'm confused. I hope you don't mind all the questions.

Nissan coils and PTU's are pretty simple-minded stuff, IIRC the intake and exhaust on the Z24's only differed in having a 4th wire (feedback I think) on the intake side. Chances are if the connectors look right then it will work, even though it might be spec'd for a different model.

[jhill0014]

Apparently this is a new product that replaces the old style. Can you look at the module and tell me if this is a UNIVERSAL module that will attach to BOTH the INTAKE side and the EXHAUSE side?

https://www.zoro.com/ntk-ignition-contr ... /G7427111/

[VStar650CL]

Can't say for sure from those pics, but it certainly looks like you could tweak that subharness to work the exhaust side even if it isn't designed for that. I don't think your ignition wiring will be any different from an '89, so you can use the diagram on PDF page 369 (EF&EC-154) here if you need to do any mixing and matching:
https://www.nicoclub.com/service-manual ... k_1989.pdf

[jhill0014]

VSTAR,

Ok. I replaced all the plugs on both sides of the block, I replaced the two melted modules, exhaust side and intake side. I replaced the two coils as well. I also fixed the ground wire that goes to the side of the distributor; (I thought this was the problem.) The truck started right up. HOwever, After a couple of minutes letting it run, the intake coil and module began to really heat up. Not taking a chance, I turned the truck off. Still think there is a wire problem between the intake coil and the negative battery terminal?

[VStar650CL]

Could be, although coils and PTU's do get warm in normal operation. Back-probe those ground wires with a safety pin or T-pin and measure voltage drop to the negative battery post (the post, not the lug). If they read under 50 millivolts (0.05V) then the grounds aren't an issue. If they read higher than that, trace back along the ground circuit until the spot where the reading drops, the resistance in the circuit will be just upstream (toward the engine) from there.

[jhill0014]

VStar,

Thanks for the reply. But, to be honest, I didn't understand much of what you said. I really appreciate your assistance so I want to send you a video of this mess I'm trying to get thru. Stay tuned and thanks again.

[VStar650CL]

Okay, let me be a bit more specific. Back-probing is how you check voltage on a circuit while it's still connected, you slide a pin in beside the wire from the back side of the connector to make contact with the metal pin. Put your voltmeter on the lowest scale (usually 200 millivolts) and with the engine running, measure from the negative battery post to the ground terminals at the coil and distributor. If either one reads higher than 50 (on a millivolt scale, 0.05 on a volt scale) then something isn't making good contact. If you then follow the ground wire back toward the battery post, at some point you'll come to a spot where the reading gets lower. Your bad connection will be in between that spot and the last spot you measured.