No Spark anymore

[chewy51]

Thanks in advance for any help provided. I have read through the forum posts on "no spark" and found quite a few suggestions.

Short story. Rebuilding 1996 Mercruiser 4.3. Everything was going as planned, until I went to fire it up. I have it on blocks with the + battery terminal to starter, negative to ground engine. I use the plug to jump 5-6 6-7. Engine started right out of the gate(yeah). The throttle was stuck open however and she revved up quite a bit. Needless to say I starting putting wires to get her to shut down. First the battery terminal next the jumper form 6-7 when I realized the battery wasn't needed anymore.

That was the last time she started over two days ago. For almost a day I couldn't get her to turn over. One of the leads on the emergency shutoff switch came disconnected. It took me two days to find this. Checked every cable with most of the focus on the starter over these two days.

So she now turns over, but she wont fire. I had been focused on the timing, but decided to make sure I was getting any spark. Turns out there is no spark.

What I have done.
1. Attached spark plug end of spark plug cable #1, held against block, while starting(no spark)
2. Attached spark plug to end of cable coming from coil to distributor, held to engine block while starting, no spark(should this work?)
3. Ran volt meter from negative battery terminal to each side of coil. Both sides 12volts. The forums say with key on, but since I am on blocks I assume jumping 5-6 on main plug will accomplishes this(?)
4. Checked distributor is turning = yes
5. Inspected leads inside of distributor cap. One of the leads is broken, not badly, but cracked. It will need to be replaced, but that wouldn't prevent other plugs from sparking. Would it?
6. Disconnected Shift Switch

I believe my problem lies with the coil, but I wanted to checked with the forum to make sure I haven't bumped another cable which is causing my issue. The coil does show 12v. I would appreciate direction on what else to check before I start purchasing parts. Also if any of my tests where invalid or inappropriate please comment.

Secondary question. The distributor has a black cable coming out the back of it. Round with a tiny hole(ground). Where does it attach? No screw that small on the block.

Thanks
David

 

[biggjimm]

That little black wire from the distributor doesn't by chance go to the coil - does it? I don't think there's a ground wire coming out of the distributor like that. Not that I've seen anyways. But on mine the coil - wire going to the distributor is black with that tiny little terminal on it. But if so it shouldn't have started the first time unless in your frenzy to shut down your run away motor you might have yanked it off of the coil??? I'd also say if it started right up then your timing isn't keeping it from starting now. Unless the distributor is loose that is. Good luck & welcome to the best place to learn about your boat. Jim.

If you post what model & serial # your engine is I can perhaps look up the wiring diagram in my book & tell you for sure what that black wire is for.

 

[alldodge]

Do you have a TB-4 or TB-5 ignition?

 

[Bt Doctur]

being that new should be electronic and the wire from the bottom of the dist body is used as an additional ground.
If you accidently disconnected the battery and it still ran the alt was putting out power. running wild like that it may have damaged the module. there are certain test procedures that must be done to verify it.
Do you have a plastic square block mounted on the starters battery lug, and does it have :
1 two wires, a red and an orange
2 only the orange one

 

[chewy51]

Yeah I don't know if it should go to the coil. The distributor-ground that it. could anybody confirm where there distributor ground is mounted on their 96 4.3 motor.

tb-4 ignition

No box on starter. Red and orange wire.

Thanks for the help.

 

[alldodge]

Here is the TB-4 troubleshooting chart

 

[Bt Doctur]

The last one I did had the dist ground wire going to one of the mounting bolts for the amplifier on the riser

 

[chewy51]

So I believe it is the ICM. The flowchart leads me there. Is there any information on how to the test the ICM or the ignition sensor with a multi-meter. I have found several people that have flow charted this and the sensor was bad but the chart says module. I would like to test both before shelling out 400$. Also has anybody successfully repaired an and ICM. Soldering and replacement of circuit is not outside of my wheelhouse.

Thanks
David

 

[achris]

..... First the battery terminal next the jumper from 6-7 when I realised the battery wasn't needed any more....

Yep, you pulled the battery off with the engine running. The alternator lost it's reference and shot the voltage sky-high. You have most likely fried both the ICM module and the sensor inside the distributor.

You can simulate the sensor with a couple of resistors, if you have an electronic bent. Same the other way. Actually quite easy the other way. With the power on (12V on the positive of the coil) see if you have 12v at the red/white on the sensor. If so, turn the engine slowly with the meter measuring the green/white of the sensor. It should swing between 5v and 0v as the window across the sensor opens and closes. Or, just turn the engine so the window is open, then use the end of a 6" steel rule to simulate the window opening and closing... If you have no 12v coming from the ICM, then just link a wire from the coil posiitve (that does have 12v on it) to the sensor red/white.... Then do the test ^^^...

If the sensor shows good, then you know the ICM is faulty for sure, especially if no 12v coming out.. If the sensor is faulty, to test the ICM you need to simulate a 0 to 5v signal on the green/white terminal..... If you need help with that, just post back.

Chris.......

 

[chewy51]

Is a Ignition Module off a Thunderbolt IV V8 interchangeable with at Thunderbolt IV V6.

 

[Bondo]

Nope,... They're even different between different V8s,...

Ya need the same advance curve,....

 

[chewy51]

Thanks for you help it was the ECM. Damn expensive I went with the Delco Voyager full electronics kit. Better to replace everything(distributor, cab, coil, ecm) for half the price at its age than one part.