1975 mercury 500 50hp outboard

[Midwesthunter]

So first off let me introduce myself and say thanks in advance for the help. My name is Kyle and I just recently bought a 16' tracker with a 1975 mercury outboard. I rewired from the controls back to the motor. I do not have keys for the motor yet as I have a new ignition on order, but when I hooked the leads to the battery the motor spun over. Could this be as simple as the ignition cylinder being stuck in the start position?

 

[CharlieB]

Re: 1975 mercury 500 50hp outboard

Wires crossed or the starter relay has the contacts burnt/welded together.

Leave the battery disconnected, take an Ohm meter and check across the two large terminals of the starter relay, if you get any reading other than zero, replace the relay.

Next test Ohms between the Positive battery cable and the starter relay trigger wire, usually a small yellow wire with a thin red line. If any reading other than zero then you have a wire crossed.

One of these should be your problem.

 

[Midwesthunter]

Re: 1975 mercury 500 50hp outboard

Wires crossed or the starter relay has the contacts burnt/welded together.

Leave the battery disconnected, take an Ohm meter and check across the two large terminals of the starter relay, if you get any reading other than zero, replace the relay.

Next test Ohms between the Positive battery cable and the starter relay trigger wire, usually a small yellow wire with a thin red line. If any reading other than zero then you have a wire crossed.

One of these should be your problem.

By relay are you referring to the started solenoid? And for the second part am I still leaving the battery unhooked? Thanks!

 

[CharlieB]

Re: 1975 mercury 500 50hp outboard

Correct on both.

If you do not have a meter handy, disconnect that relay/solenoid trigger wire, the small yellow w/red line then connect the battery terminals. If the solenoid is shorted the starter will turn, if it is crossed wires then the trigger wire may be hot but disconnected the solenoid will not trigger until you re-connect the trigger wire.

 

[Midwesthunter]

Re: 1975 mercury 500 50hp outboard

Thanks charlie! My other question is i have a wire coming out of the front of the bottom cowl assembly that runs to the console that is a momentary push-button switch. I assume that this is the kill switch, could anyone shed some light on this? I have a wiring diagram for this motor that i have followed to a "T" but no where on the diagram is this included. Thanks again for all the help

 

[CharlieB]

Re: 1975 mercury 500 50hp outboard

Most likely the choke. Look at the wire colors, compare to the wire colors on the choke solenoid.

 

[Midwesthunter]

Re: 1975 mercury 500 50hp outboard

So I was able to get the motor turning over with the key, only problem is I have no spark. Not sure which wire to check first to make sure I have power going into/out of the ignition any advice? I'll add as we'll it did kick over twice before I lost spark. Have not replaced plugs yet, but they look ok

 

[CharlieB]

Re: 1975 mercury 500 50hp outboard

The wires going to the push button switch, are they contained within the wiring harness or do they look like someone added them? Other than the key switch and the choke button there is no need for any other push-button switch.

The distributor Thunderbolt electronic ignition did use a green wire from the switchbox to the coil, but this should NOT be used with a switch to ground and kill the motor.

Follow the wires back, see what they are connected to.

Troubleshooting a 'no spark' the best method is spelled out in the CDI Ignition Guide. Find your ignition and follow the simple tests.

ISSUU - CDI Electronics Practical Outboard Ignition Troubleshooting by CDI Electronics