Mercruiser 170hp 470 no spark

[MartyMartin]

Bought a boat with the 170hp 4 cylinder that ran great when I looked at it. Brought it home and no spark!
Found that if I removed a wire from the negative side of the coil I had spark again. The schematic shows that wire going to the tachometer and when disconnected the tach doesn't work. but I don't think the problem is in the tach itself.could it also likely be in the switch that interrupts the ignition when changing gears? That wire seems to short the coil when the key is turned to the start position.

 

[Scott06]

Bought a boat with the 170hp 4 cylinder that ran great when I looked at it. Brought it home and no spark!
Found that if I removed a wire from the negative side of the coil I had spark again. The schematic shows that wire going to the tachometer and when disconnected the tach doesn't work. but I don't think the problem is in the tach itself.could it also likely be in the switch that interrupts the ignition when changing gears? That wire seems to short the coil when the key is turned to the start position.

Not uncommon that a bad tach or wire with shirt to ground will kill the spark
yes shift interrupt could also kill the spark but that would not be the tach wire . Disconnect wires at shift interrupt switch and test.
shift interrupt is only to go from in gear to neutral, should never trip on land or going into gear unless problem with shift cable

 

[MartyMartin]

So...once the boat is running I can re-connect that wire going to the negative side of the coil and the boat keeps running well and the tach works. I am going to see if that wire is being energized with 12V when the key is in the start position...Will report back

 

[MartyMartin]

So...once the boat is running I can re-connect that wire going to the negative side of the coil and the boat keeps running well and the tach works. I am going to see if that wire is being energized with 12V when the key is in the start position...Will report back

Crazy, The problem seems to have resolved itself. It starts fine now with the wire reconnected. I didn't see any voltage on that wire when cranking so I reconnected it and all is well. I don't trust that it will stay well now though.

 

[nola mike]

Crazy, The problem seems to have resolved itself. It starts fine now with the wire reconnected. I didn't see any voltage on that wire when cranking so I reconnected it and all is well. I don't trust that it will stay well now though.

Tap on the tach and wiggle the wire when running and see if it hesitate/stalls

 

[Scott Danforth]

did you service the points?

 

[MartyMartin]

Tap on the tach and wiggle the wire when running and see if it hesitate/stalls

Well...After shifting between forward and reverse (Reverse isn't working) The issue is back. Looks like I need shift cable adjustment and work on the interrupt switch setup. I am looking for good info on adjustment/repair.

 

[MartyMartin]

did you service the points?

Has Solid State conversion and runs great except for the intermittent issue

 

[nola mike]

Well...After shifting between forward and reverse (Reverse isn't working) The issue is back. Looks like I need shift cable adjustment and work on the interrupt switch setup. I am looking for good info on adjustment/repair.

So it's the shift interrupt?

 

[ROY WILLIAMS]

Well...After shifting between forward and reverse (Reverse isn't working) The issue is back. Looks like I need shift cable adjustment and work on the interrupt switch setup. I am looking for good info on adjustment/repair.

the shifting interrupt cable switch !!!! the adjustment switch and the switch rusted/lube/cable ...
my MCM470 years ago 1978 ... 2024 good engine ..
yes the lube cables 470 engine ..

 

[Scott06]

Well...After shifting between forward and reverse (Reverse isn't working) The issue is back. Looks like I need shift cable adjustment and work on the interrupt switch setup. I am looking for good info on adjustment/repair.

Most likely you are looking at replacing the cable .when It wears and drags tripping the interrupt is a common symptom