Only getting spark when stop button pressed

[motox25]

I'm having a very strange issue on a 93 GTX I'm working on, when the starter button is pressed I've determined the spark plugs are not firing, but if you tap the stop button while cranking the plugs will fire, the engine will start and run fine. What's even more strange is once it is running the stop button does what it is supposed to and shuts the engine down. I'm at a loss on this one :banghead:

 

[motox25]

I think I might have found my problem, I pulled the electrical box and found a ring loop terminal loose inside the box. The terminal has 2 black wires going to it, 1 going to the coil and the other going to the electronic control board. Where does this attach?

 

[LouDoo]

I'm not sure what a ground ring loop terminal is, can you post a picture?

Lou

 

[motox25]

Here is a picture of the loose terminal, 1 wire goes to the coil and 1 goes to the electrical control box, both are black.

 

[LouDoo]

The picture didn't work, but if it's a black wire it goes to ground.

Lou

 

[motox25]

Great, thanks Lou. And the ground for the electrical box looks to be going to the mag housing, is that correct?

 

[motox25]

Didn't fix it, after reconnecting the loose ground wire I am still having the issue.

 

[motox25]

I think it might be the electronic controller / speed limiter. I have tried to rule out any switches and bad grounds by attaching a good ground wire to the location I've highlighted in red on the wiring diagram. When I do this the engine cranks without the lanyard attached but still won't spark unless I break and reattach the circuit.

 

[Coastiejoe]

It pretty much has to be the brain as the ONLY time the start button provides power to anything is when it is pressed.

In the old days when this happened to a vehicle, I would run a relay to the coil and it solved the issue. So, though I don't know for sure, I believe it is an internal component that would supply power to the coil(s).


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[motox25]

You've got me thinking, the only time the coil is not getting power from the brain is when it is starting, it gets power when it is running. Could the start circuit be the issue? I didn't think it was because the starter is cranking.

 

[howie099]

You sure all the ends are plugged in to the correctly in the electrical box? It is seems like crossed wire, stop switch, or bad MPEM. Everything goes through the MPEM but haven't heard of this issue with the MPEM. The stop button kills it unless your stop button is messed up. A stop button is cheaper than the MPEM to try. it sound like the the stop button is doing the reverse giving you spark when you press it to start instead on taking away the spark.

 

[Coastiejoe]

You've got me thinking, the only time the coil is not getting power from the brain is when it is starting, it gets power when it is running. Could the start circuit be the issue? I didn't think it was because the starter is cranking.

I would look at the specific schematic and see what feeds the coil both during cranking, (to confirm it gets it only when the start button is pressed) and once it is started. What feeds the coil?

 

[Coastiejoe]

It looks like there are 15 amp fuses on the positive side of the solenoid that feed the Speed Limiter as well as the Rectifier. Check these to make sure they are good.

 

[motox25]

The coil only gets power from the MPEM during start and run and the MPEM gets its power through the solenoid. All fuses are good as well. It almost has to be MPEM.