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Tan, MerCruiser Certified Technician
Category: Boat
Satisfied Customers: 10966
Experience:  40 yrs. experience with complete engine repairs both inboard/outboard and MerCruiser Certified Technician.
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I have a ignition problem with my 86 supra comp, The

Customer Question

I have a ignition problem with my 86 supra comp
JA: Sometimes things that you think will be really complicated end up being easy to fix. The Marine Mechanic I'm going to connect you with knows all the tricks and shortcuts. Tell me a bit more about what's going on so the Marine Mechanic can help you best.
Customer: The ignition fuse blows when i connect the negative wire from the the ignition switch. Motor will just turn over and not start
JA: Is there anything else important you think the Marine Mechanic should know?
Customer: no
Submitted: 1 year ago.
Category: Boat
Expert:  Jason replied 1 year ago.

Hello my name is ***** ***** am I speaking with?

What is the make and model of the engine in the boat?

Customer: replied 1 year ago.
My name is***** have a 86 Supra comp with a 351 pcm motor
Expert:  Tan replied 1 year ago.

Hello Joe. My name is Tan.

If you are connecting a negative wire to the key switch that is why you are blowing the fuse.

There is no negative to be connected to the key switch. When you connect a negative to a positive circuit you are creating a direct short.

What was the initial problem.

Customer: replied 1 year ago.
Motor will not start. Has gas to the carb. Has spark. There are three terminals on the ignition switch the wires are red, white, and black. Where does the black wire go?
Expert:  Tan replied 1 year ago.

The wires should be red, yellow and purple. The red is battery power the yellow is start circuit and the purple is ignition. Since your wires vary from marine standard you will have to trace the wires and see where they go. If the black is supposed to be the purple it should go to the I terminal of the gauges then to the ignition coil on the engine.

Black is typically negative and shouldn't be connected to the key switch. So you need to find where the black goes.

If it is ground that would explain the fuse blowing.

Customer: replied 1 year ago.
I have red coming from the fuse block. White/red going to the motor. The black wire?
Customer: replied 1 year ago.
I just got the boat back from having the stringers and deck replaced. I'm wondering if they disconnected the black wire in the process and connected it the the negative terminal.
Expert:  Tan replied 1 year ago.

All I can tell you is what it should be. I can't see what you have. The only way to know is to track the wire down and see where it goes.

The white/red is more likely yellow/red which is the start circuit.

Do you have a volt meter?

Customer: replied 1 year ago.
I do. I'm a hvac tech not a mechanic I will admit. Without a schematic I'm totally lost on this. The red/yellow is in the wire harness going to the engine. The black wire has to go somewhere on the instrument panel I just have no idea where.
Expert:  Tan replied 1 year ago.

Ok the black has to be a ground. Do this go to the engine and look at the ignition coil positive terminal what color is the wire?

Here is a diagram of a typical instrument panel. You probably don't have the neutral safety switch in the control it will be on the transmission. So the yellow/red will go straight to the engine bypassing the control. Look at the key switch and you will see how it should be wired. Due to the age of the boat I'm sure allot has been done and your boat may vary from this diagram. But this is BIA standard.

Customer: replied 1 year ago.
I have red and purple on the positive terminal of the coil. The red wire is going to the distributor purple is going to the ballast resistor.
Expert:  Tan replied 1 year ago.

See the ignition wire is purple that is what it should be on the ignition switch. Locate the wiring harness ( at dash) look at the wires coming out of the harness do you have a purple wire?

Customer: replied 1 year ago.
there is. I have a schematic for the motor. It shows red, red/yellow, and purple coming off of the ignition switch. I just have no idea where the purple wire goes.
Expert:  Tan replied 1 year ago.

The purple(ignition) wire goes to the I terminal of the ignition switch. Red to B terminal yellow/red to S terminal purple to I terminal.

As you can see there is no black wire to the key switch.

Customer: replied 1 year ago.
The purple wire is connected to the switch. Where does it go from there is my question. Thanks
Expert:  Tan replied 1 year ago.

It goes to the gauges and then the engine. That is the 12 volt ignition wire to power the engine and gauges. Where it physically goes on your boat I can't say as I am not there to trace the wire.

Customer: replied 1 year ago.
would this wire being disconnected prevent the motor from starting?
Expert:  Tan replied 1 year ago.

Absolutely that wire supplies voltage to the ignition coil. With it disconnected the engine will have no spark.

Customer: replied 1 year ago.
Ok I figured it out. Thanks
Expert:  Tan replied 1 year ago.

Your welcome.....Please don't forget to positively rate my help as that is the only way I get credit for helping you.

Thank you........Tan

Post back if you need further assistance.

Customer: replied 1 year ago.
Will do thanks
Expert:  Tan replied 1 year ago.

Your welcome...To rate just click on the stars at the top of the page 3 or more please.

Customer: replied 1 year ago.
Actually now I think I have a carb issue. Whin I turn the motor it tries to fire but I think too much gas is going in the primary. I can see it dripping. I would think that would be flooding it out. There is no float adjustment on their Holley carb. Had it rebuilt last summer. The last time it ran it backfired and quit and hasn't ran since
Expert:  Tan replied 1 year ago.

Flooding is a stuck inlet needle allowing fuel to continually flow through carb. The carb has to come off and be cleaned and the inlet needle replaced. The engine sitting causes all kinds of problems with carbs.

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