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1975 280z electrical issue


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Good evening all,

 

I've posted on other forums hoping for an answer to my current situation, but no one has had a similar issue or can give me a good direction to go with. Here it goes:

 

I've got a 1975 280z, and the ignition coil is firing with the key in the "on" position. The coil sparks every few seconds with the key in the "on" position, and when I start the engine it runs very rich ( I have a wideband AFR, and the ratio at idle is between 11~12). I know that the ignition coil should not be firing unless the engine is running, and because of confusion on the other forums I should clarify that by "firing/sparking" I mean that the ignition coil's lead to the distributor cap discharges every few seconds, until I return the key to the "accessory" or "off" position. I discharges with enough power to be audible, and (as I found out the hard way) with enough juice to shock you. The distributor, cap, rotor, coil, wires, and plugs on my vehicle are all new/rebuilt. It's the dual-pickup version (non-California) for the dizzy. This issue popped-up recently, it was running fine and this started out of nowhere.

 

I've followed the tests in the Fuel Injection bible for our cars, and the test for the "ignition coil trigger input" fails. Rather than battery voltage, I get a widely fluctuating voltage, probably because the coil is firing when it isn't supposed to be. All other tests check out fine, including air/coolant temp readings which I know can cause the car to run rich.

 

I'm just hoping someone has had a similar issue, and knows a fix-action. Thanks for any advice, and for taking the time to read!

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BlueZ31, I'm just pulling this outta my...er...well...  Have you checked the air gap between the reluctor and magnetic pickup.  If there are wires in the distributor check to make sure they aren't making contact with anything.  Just some ideas.

-Phil 

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He said it happens with the engine not running, so no spinning of distributor parts.  Weird problem.

 

For the coil to discharge the primary coil circuit has to be broken, after being energized.  It could happen either on the supply side or the ground side (ignition module side).  I think that even the early ignition modules had a safety circuit designed in that would cut the power after a certain amount of time to save the coil from overheating.  Not sure if happens slowly to avoid a spark or quickly, giving one spark.  Maybe it's failing and not cutting off completely.

 

Anyway, (Edit - Maybe ) it's your ignition module.  You could determine if it's power supply or ignition module grounding by disconnecting the power to the coil positive and running a jumper directly to the battery positive.  Turn the key on so the module is powered and see if it still sparks.  If it doesn't spark anymore, (maybe) there's something cutting power on the supply side, maybe the ignition relay.  If it does still spark, (maybe) there's something happening on the grounding side.

 

Could also probably be the coil heating up and opening circuit.  Try disconnecting both post wires on the coil and wiring directly to battery and ground.  Don't leave it that way too long, it will get hot.  If it still sparks, it's the coil.  This might actually be the most likely cause.

Edited by NewZed
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  • 2 weeks later...

So I've swapped out the old transistor ignition unit for an E12-13 module from a '76, and it stopped the coil from firing in the "on" position. The engine is still running very rich, however. I've gone through and double-checked the water/air temp sensor outputs, the AFM readings, throttle position, etc. and everything is correct per the Fuel Injection bible. Plugs are clean, wires/cap/coil is in working order as well. Not sure where to start looking next, maybe fuel system? The pump and pressure regulator are both new.

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Idles rich, or drives rich?  Rich as defined by an AFR number?  "Correct" means that resistance matches temperature as shown in the chart?  Or it means "continuity"?  Testing was done at the ECU connector or the component?  The temperature sensors don't "output" anything.

 

Numbers are your friend when working on the EFI system.  Numbers make a big difference on the forums.  Without numbers you're just part of the herd.

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Resistance readings are correct at the ECU connector for the various sensors, per the Bosch fuel injection manual. I've got a wideband AFR gauge, and it's hovering around 12~13 at idle. Voltages to the coil, ECU and injectors are all correct as well. I know that exact numbers can be helpful on forums, but given that everything I've checked so far matched the tech manual, I thought it unnecessary to post the readings.

 

I think I might have a short somewhere in the wiring in the rear half of the vehicle, because I had one several months ago that a local body shop caused when they removed some exterior trim and spot-welded the holes left by the trim. I fixed that one short (it caused the taillight fuse to blow) because it was immediately noticeable. This current issue with the car running rich cropped up around the same time, so I may need to delve into the wiring with my multimeter.

Edited by BlueZ31
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I've got a wideband AFR gauge, and it's hovering around 12~13 at idle

Good luck.  You've only defined one "problem" and it doesn't seem like big deal.  Idle AFR is a bit rich, but everything else is correct..

 

Here's a decent reference.  It's Megasquirt and you don't have the tuneability, but since you're only worried about idle you could open up the idle air adjustment screw on the AFM.  Described in the Engine Fuel chapter of the FSM. 

 

"Batch fire engines, however, have issues at idle with intake pulse reversion sending some of the fuel into an adjacent cylinder. So, if your engine is batch fire, you'll need to add a little more fuel so all the cylinders can stay at 14.7:1 or richer. Batch fire engines typically idle best in the mid 13's, so if you're leaner than that, put in some more fuel and see if that stabilizes the idle.:"

 

http://www.diyautotune.com/tech_articles/megasquirt_idle_tuning.html

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