Hi guys,

I ran into a problem with my 502 Mag swap. The engine starts up fine, but I cant shut it off!

I tried everything, switched off the batteries, pulled the engine harness plug, removed the key completely but nothing worked. To get it off I had to pull the coil wire!

So, where do I start to look? I tried to see if anything was wrong at the starter, but as far as I can see all the wires are in their correct place and not bridged.

So, could it be a faulty relay? The alternator wiring?

Any thoughts wold be very appreciated.


Regards,

Johan

it might be simple...if it stays really running, and not "dieseling", then it's probably a bad key switch. I've had that happen before.

Are you using a one wire alternator or 3 wire? I've seen them bleed power from the 3 wires voltage sense circuit and engergize the ignition. Try running the engine with the alternator wiring disconnected, run it on the battery only to see if that changes things...

You may have cooked the diodes in your alternator.

Thanks guys,

I kind of ruled out the ignition switch since it worked with the "old" 7.4 engine, but strange things can happen over winter.

I'll focus on the alternator, then. I actually have 4 or maybe 5 wires on my alternator, orange + black for power and ground, and red/purple and pink for control.

I will try without the alternator connected.


Thanks,

Johan

This will help you out also

http://www.boatfix.com/merc/Techbk/96/96HGD4.PDF

Do you have safety lanyards? Did you try them, they should work since that grounds the ignition, black wire with yellow tracer.

Probably either the alternator or the relay that turns on the ecm. I have seen them stick. You have 2 relays....one to turn on the ecm and one to turn on the fuel pump. The fuel pump relay is controlled by the ecm, so if the ecm stays on, so will the pump. Swap out the relays with each other and see if it stops.
Eddie

Hi again,

I looked this through again, and have some more data points; first of all I can turn off the engine with the key when I unplug the purple L2 wire. So, it starts up fine, volt meter reads 14V as usual, when I turn off the key the engine still runs and the volt meter pegs to max (16V). If I instead unplug the L2 wire, I get no charging (volt meter reads 12 V), but I can turn it off.

I tried to measure on the alternator, but I really don't know what to look for. I measure 1.6 V over the purple L2 wire when the engine isn't running. Is that correct or do that point towards a broken alternator?

I still have the replaced 7.4 MPI motor next to the boat. Could there be a missmatch between this approx 1996 502 Mag MPI and the old 2000 7.4 MPI? Should I try to switch alternators?

I could always buy a diode and mount on the L2 wire; or do I create another problem then? Doesn't the alternator create the right output then?

Please let me know your thoughts.


Regards,

Johan

Ok, so this problem resolved, and a new one appeared.

It turned out that the purple Exitation wire shouldnt go to L2, which apparently is the charge indicator light. And, the red sense wire was placed where the purple wire should be. So, it wasnt so strange it didnt work.

But now the charging meter show battery voltage, not charging voltage. I can see the voltage rise as the battery is charged.

Any thoughts?


Regards,

Johan