My 1990 5.0 LX/V8 Mercruiser with Thunderbolt IV ign. only starts when I run a hot wire from + battery post to + coil. I have unhooked the tach wires from the back of the tach itself with no change. I do know that the two wires at the very bottom of the shift interrupt terminal block can be wiggled slightly and are oily. Could this be my problem? I can not get tightening screw off to clean and tighten without taken whole assembly off and working on a bench. Unsure how to perform this process.
Yes that can cause a no start and spark situationwhat the switch does is ground out the coil from sparkingI would check with the key on if you have 12V to the coil, if you do then unplug that switch sand see if it sparks then
Could my coil be bad? Am I bypassing the coil by running the hot lead? the boat ran fine last fall now will only run with the hot lead to the coil. The grn/wht wire from the interrupt switch runs to the back of the outside of the distributor, I did unhook that with no luck. As stated in my original question I am unfamilar with how to disconnect the switch and need help in that area as well.
yes coil could be baduse an ohm meter, it should be under 1 ohm using the positive and negative posts
I had the local parts store put an ohm meter on it and yes it was less.
copy and paste this and do these testshttp://www.marinemechanic.com/site/page251.html
I have this chart already, the hot lead tells me all these parts work except the coil and the coil was tested. Yes it still could be coil and I will check that out. Lets do this........if it is not the coil and all other ignition parts are working (because the engine does run) what then? as stated I need help with the shift interuppt switch I do believe.
post the serial of the motor and then i can post some charts
Engine #OC770809 with 4 barrel carb. ,
Eng #OC770809 with carb
ok give me a little bit, i had to run away from the shop
also you never replied to me telling you I unhooked (or so I thought) the interrupt switch at the back of the distributor? shouldn't this have taken it out of the loop. Take your time, I have lots as I am working.
Yes it's out of the loop
so it is the green and white wire that runs to the back of the distributor?
you can check that wire by touching it to ground, the coil should fire, if it does its not the distributor sensor
Relist: Answer quality.this expert keeps talking in circles, he is asking me to check things we already know work. Can I get a different answer please, without having to go back through everything?
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Without the hot wire from the battery + to the coil + connected, do you have voltage at the coil when the key is in the run position?
Hi James, I am working on this old school without gauges, I do not have an answer to your question only I did take coil to parts store and it passed ohms test, I will try for a voltmeter tonight when I get home. Other than tach wire and interrupt switch what else blocks coil voltage?
Not getting there in the first place blocks voltage. Nothing else. It doesn't matter what parts are keeping your plugs from firing, If you have an internally resisted coil, with the key on, you will have 12 volts at the coil + post.
The very fact that you are hot wiring the motor and it runs tells me your ignition switch is bad, or your safety lanyard has been pulled, or the safety lanyard switch is bad or the purple wire that connects all of these things together is cut or broken. .
This has nothing to do with your coil or anything else. You are just not turning on the ignition because the power to do that isn't making it to the coil from the key switch.
You can make a test light to test the coil +. Just find any 12 volt light bulb and connect one wire to the outside of the metal part of the bulb and one wire to the center post on the bottom of the bulb.
Then with the key in the run position, hot wire removed, touch one wire from your test light to bare metal of the engine block and the other to the coil+ terminal. (the one with the purple wire on it) If your light doesn't light, The problem is one of the things I outlined above. if it does, but dimly, Remove the coil and read what is printed on the side of it. If it says internally resisted coil, you have the wrong coil on the motor.
Dim light + internally resisted coil = wrong coil
No light = bad key switch, lanyard (kill switch), or broken purple wire.
ok, the coil is the same one as last fall and boat ran flawless, boat was winterized, covered and put away. Went to start this year and nothing. I have read in other posts about bad tach or bad interrupt switch. Boat does turn over from key switch just wont start without hot wire?
Exactly my point.
Your coil is not getting power when you turn the key to run. If it were, you wouldn't need the hot wire.
And because hooking the hot wire makes the motor run, There is nothing wrong with the motor or any part of it. You are just not getting power to the coil.
That means only one of the things below can possibly be the problem with your motor.
The key switch is not turning power to the purple wire on, when the key is turned to "run".
The Safety Lanyard has been pulled.
The safety lanyard switch is bad.
The purple wire that runs between the key switch and the coil + is cut, broken, or disconnected.
You need to check each of these things.
Running that hot wire would not make the motor run if the tach or the interrupt switch were killing the spark. Both of those things kill spark by grounding the coil - not stopping power to the coil +
got it and thanks.
Please don't forget to come back with a rating
James my apologies, we had 2 conversations going and I must have messed up my last question. Can you explain how I would bypass key switch and safety switch for a test? I did check lanyard it is there and hooked up? your best guess at this happening over storage months my problem?
To see if it is the key switch all you need to do is turn the key to run and check the purple wire for power.
Do you have any gauges? If the key switch is bad, None of the gauges will turn on when you turn the key to run.
To check the safety lanyard kill switch, put a jumper across the two wires connected to it, then try to start the motor. This will only work if the key switch has passed, so test that first.
I will look at the gauges did not notice if working or not. And you are saying turn key to the ON position or the START position? Apologies for being naive boat mechanics not my thing.
When you turn the key to On position you can check the gauges, But On or Start, doesn't matter, the purple wire is supposed to be hot in both key positions
Awesome and to check purple wire I literally have to follow it from the switch to the + coil? is this difficult? any suggestions on doing this James?
No, turn the key to "run/on" you need to check it to see if there is power on the purple wire. If you have no testing equipment, take a piece of wire or a paper clip bent into a U and briefly touch one end to the connector that the purple wire is attached to and the other to the connector the yellow w/red wire connected to it. If the starter kicks in, the key switch is good If not, replace the key switch
and if key switch checks out good, the as you previously stated: The purple wire that runs between the key switch and the coil + is cut, broken, or disconnected. Then I need to trace this wire provided I have jumped out safety switch correct? is that difficult and how?
Yes, and it can be a pain all right, but what you can do is check the big plug on the motor. If the key switch and lanyard switch both have power then unplug that big rubber plug on the motor and check the contact that is on the boat side of the plug number 5 is the contact for the purple wire. See the diagram below for the routing of the purple on the motor. e
could I run a jumper from purple side of key switch to + of coil for a test of the purple wire?
Yes, but only if the key switch has checked out good. Hook it to the coil + on one end and the "I" terminal on the key switch.
You will still need to test the lanyard switch though
yes. Thanks James for your knowledge and patience. If I am in a bind on the weekend or have yet another question, is there a way to get you? or do I start over with someone else?
Just save a link to this question, you can come back here any time to continue with this issue. It will come straight to me.
If you have a new issue, just put "this is for James" in the title and I will get it. But anything even remotely related to this issue is a follow up as far as I am concerned, those are free, so save a link to this question for those.
12 years Certified marine mechanic, performance boat builder
You are most welcome