Motor only fires up when key is turned to off

[Deus_Vult]

Hey guys, I'm really stumped on this one. My 1967 Mercury 500 will spin and spin and spin when the key is turned to crank, but it won't start. However, once I turn the key is turned to off, the motor fires up for a second then quits. Do you have any ideas on what the problem might be? Thanks.

 

[racerone]

Load test battery.-----Inspect battery cables.-----Inspect starter motor.

 

[Scott Danforth]

did you replace the key switch at any point?

your motor needs an outboard key switch (magneto ignition)....shorts to ground to kill, open to run

sounds like you have a powered ignition key switch.... open to kill, connect +B to ign to run

 

[Deus_Vult]

did you replace the key switch at any point?

No, I haven't replaced the key switch. As for the other information you gave on the key switch, I'm afraid I'm not quite sure what you mean.

To add some context, I just finished rebuilding the motor and was attempting to start it yesterday and it did the same thing...until after fiddling with some wires it started working. I thought maybe it was fixed until I tried to start it today and the same thing happened again. I was doing some other reading elsewhere about similar issues and came to the conclusion that maybe my problem had something to do with the coil (does that sound right?)

 

[merc850]

Identify the ignition system from these 2 diagrams.

 

[Deus_Vult]

Identify the ignition system from these 2 diagrams.

Neither one of those are the same as my ignition system, but I just found this picture that is the same.

 

[merc850]

That system uses a trigger that is located in the distributor and one sign that it's bad is the problem you are having. Are all the wires in good condition no bad insulation or copper? Remove the mercury switch wire from the switchbox and see if it will start. Remove the distributor cap and look at the contacts and the rotor tips for corrosion DO NOT TRY TO REMOVE THE ROTOR it's molded onto the shaft. Check to see if you have 10v or more at the red and white wires on the 4 terminal side of the SB while cranking motor, check to see if the distributor has a ground strap from the case to the block. To test the trigger follow the steps in the pdf.

 

[merc850]

You can also have a friend hold onto the spark plug wire while cranking.Ha Ha trick or treat! Recheck the timing (should have mentioned that first).

 

[Deus_Vult]

That system uses a trigger that is located in the distributor and one sign that it's bad is the problem you are having. Are all the wires in good condition no bad insulation or copper?

I re-wired 90% of the wires a couple months ago because they were falling apart. The rest look fine. I'll check the trigger though. What am I looking for to see if it's bad?

Remove the mercury switch wire from the switchbox and see if it will start.

Sadly, there doesn't appear to be a mercury switch in the motor.

Check to see if you have 10v or more at the red and white wires on the 4 terminal side of the SB while cranking motor, check to see if the distributor has a ground strap from the case to the block.

There is a ground from the distributor to the crankcase. I'll have to test the wires once everything is put back in again.

Recheck the timing

Already did, timing is perfect.

Thanks for your replies.

 

[merc850]

The merc switch is a safety device not needed for regular operation. The pdf will tell you what will happen. When you get it running don't rev the crap out of it on the driveway - keep the revs about half throttle.

 

[Deus_Vult]

That system uses a trigger that is located in the distributor and one sign that it's bad is the problem you are having.

I ended up taking the cap off of the distributor and can now see the trigger. Are there any signs I should look for that indicate whether or not it's bad?

The merc switch is a safety device not needed for regular operation. The pdf will tell you what will happen. When you get it running don't rev the crap out of it on the driveway - keep the revs about half throttle.

Thanks for the clarification. For some reason, my motor doesn't have this safety switch in it, or at least it isn't where it should be.

 

[merc850]

You can't see the problem because it's usually in the electronic circuitry that's why you have to use the pdf troubleshooting guide.

 

[Deus_Vult]

You can't see the problem because it's usually in the electronic circuitry that's why you have to use the pdf troubleshooting guide.

Alright I'll do that thanks for your help.

 

[merc850]

The only mechanical problem is the trigger disk might not be turning with the shaft, you could check that.

 

[Deus_Vult]

It's been a little while since I've followed up, but I think I may have narrowed down the problem to a wiring issue somewhere between the starter solenoid and the ignition switch or battery.

For context, I ended up putting the motor back together yesterday and tried turning it over again, but this time the starter just spun slowly and then stopped. Then the solenoid would just click. I tested the starter and the solenoid and both are operating fine, so I'm thinking that there most be some faulty wires that are restricting current flow (I do know that there are some shoddy areas in the positive wire coming from the battery, whoever did the wiring on this boat before I bought it didn't do a good job).

I'm planning on testing the wires tomorrow, maybe tonight if it isn't too cold.

Thanks to everyone for their help and advice, I appreciate it.

 

[Deus_Vult]

I ended up replacing the positive battery wire, yet this still hasn't fixed the problem.

I just finished testing the white and brown wires according to the CDI PDF and it seems that the brown wire stays at 0.5 volts when cranking until the key is turned off, at which point the voltage jumps to a much higher number for a brief moment.

I hope this means I found the issue, but in the PDF it is stated that if the voltage on the brown wire is under 9 volts during cranking, the "pack" is most likely the problem. Now in my case, would this be the switchbox, the coil, or something else? I'm not sure what the pack is. This might be a dumb question, but I just want to make sure before I take anything out.

 

[merc850]

The 1st page of the pdf is for testing the CDI replacement switchbox it's the second page that tells you how to troubleshoot the trigger, the trigger wires are on the port side of the engine.

 

[Deus_Vult]

Well I wish I could say it's a happy ending, but it isn't.

I ended up testing the distributor, the coil, and the switchbox and they all worked fine. After this and some further testing, I found that the spark plugs only seemed to work when the ignition switch hit a certain position somewhere between the run and crank positions. So I took apart the MerControl box and found that the wires at the ignition switch terminals and the ignition switch terminals themselves were in bad shape, so I replaced that section of wires (not the entire harness) and the ignition switch. Now the system won't even spark on some cylinders.

I guess I'm at a loss here. My only guesses at this point are that it could be the rest of the wiring harness, the battery, the plugs themselves, or maybe possibilty the trigger still, even though I thought I ruled that out already.

Sadly, this is it for now, I can't work on it anymore until next spring because it's too cold and there's snow here now, so I have to winterize the motor within the next couple days. Any last suggestions would be much appreciated. Thanks for everybody's help. I may continue this thread come springtime.

 

[Faztbullet]

Replace trigger

 

[Deus_Vult]

After extensive testing, I found out that the trigger was indeed the problem after all...lol.

I miraculously was able to find a replacement online, and sure enough that fixed it. Thanks everybody!