Mercury 500 50hp, 1976

[gregg_a_g]

So I have an old Starcraft aluminum boat with a Mercury 500, 50hp electric start (1976 according to the S/N). It was getting a bit warm, so I replaced the water impeller (toast) and got it all back together. Of course, when I was so excited to finally try it out, the key broke off in the ignition. I was able to get it out okay. I removed the Mercontrol and found the wires a little crisp - okay, but starting to deteriorate. So I got a new ignition switch, a Marpac 7-1150 (like a Sierra MP41000). I think I got it installed correctly, and according to "Chris1956" -
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B terminal gets red wire
I terminal gets white wire
C terminal gets grey wire
M terminal gets salmon wire
M terminal gets black wire
S terminal gets yellow wire.

The brown wire connects directly to the sender pole of the tachometer.

If you are using the original MerControl, make sure the keyswitch fits in the control. Originally it was a tight fit with the 5 post switch.
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So when I switch the key to "run", I can push in on the key and hear the choke solenoid engage. I used a voltmeter and see 12V at the coil. I turn the key to "start", and the engine turns over nicely - but never actually starts. It doesn't sound like it's getting any spark (no popping at all). I don't quite understand how the "salmon wire" is used, but I have it connected to one of the M terminals (ground?). Is this a "mercury switch"? How does this work? Is it some kind of "kill" circuit? Any hints on troubleshooting?

I have the wiring diagram, but don't know where to look for the "mercury switch" or how it's supposed to work.

 

[racerone]

Well----12 volts from the battery cranks the starter.---But 12 volts from battery is not used to make spark for the plugs.----Work carefully before big $$$ mistakes are made.

 

[Chris1956]

The mercury (small m) switch grounds the ign circuit, killing the spark, when the motor tilts up unexpectedly, like when you hit something at speed. it is located near the switchbox, and looks like a automotive condenser, but with a plastic shell.

If your motor is a 1976, it will have a pretty standard ADI ignition. The stator will feed power to the switchbox. When the trigger indicates that it is time to fire the spark plugs, the switchbox will pulse the ignition coils.

Check the condition of the engine wiring harness. Look for missing insulation. Other than that, a service manual can be a friend to you, for just this problem.

 

[gregg_a_g]

Thanks for the info Chris. Are there any "downloadable" manuals? Or do I need to look on eBay/Amazon/craigslist and find a used one?
I was looking for the first "troubleshooting" steps. I guess one of the first
steps is to use a spark plug tester and see if the engine is generating a spark at all.

 

[gregg_a_g]

Well, I got a spark plug checker (just the cheapie from HF), but I can verify all of the spark plugs are sparking. There's gas in the carburetor, but it's just not hitting at all. Usually if you have air, at least a bit of gas and a spark you'll at least get it hitting a couple of times.

I think 90-75512 is the factory manual for 1975-1978. It looks like there are a couple on eBay, so I think I'll get one before they're all gone!

 

[CaptnKingfisher]

Salmon wire is part of the Killswitch. M to m usually grounds the coil preventing spark from my understanding. But if you're getting spark and fuel and you had compression before I'm sure you've got it now.. and your Killswitch is not engaged if you've got spark.

Few thoughts.. Do you keep a trickle charger on your battery? Try a voltage drop test.. battery voltage should not drop below 9v while cranking. Test this at starter selenoid where your yellow wire is connected. If it's below 9 volts there while cranking, do the same test but on battery. If this is good then you have too much resistance in the circuit somewhere and need to check your connections and wire condition.

Test voltage at battery, then at switch with key in "run" position. How much degradation do you have in volts?