1976 Mercury 850 Thunderbolt ign, wont stop running with ign switch.

[David84AET]

Cant get it to shut down, and need a wiring diagram. Allready replaced ignition switch, and I'm getting a ground signal to the orange wire on the control box with the key in off, ground goes away when on. Once started it wont die, unless choked out. Bypassed the harness (orange) wire as it is tucked away and innacessible. Anyone else ever have this problem? Any ideas, I'm running out of things to check here!

Thanks,
David

 

[David84AET]

Re: 1976 Mercury 850 Thunderbolt ign, wont stop running with ign switch.

Oh, BTW Serial # is 4367955

 

[dwco5051]

Re: 1976 Mercury 850 Thunderbolt ign, wont stop running with ign switch.

Go to the switch box and with a jumper ground the terminal the orange wire goes to. If that kills the motor than the problem is more than likely an open circuit in the orange wire some place from the switch box to the ignition switch or in the black wire in the harness from the switch to ground.

Dirty contacts in the plug from the control unit to the motor??

 

[David84AET]

Re: 1976 Mercury 850 Thunderbolt ign, wont stop running with ign switch.

Yes, I forgot to mention that if I jumper wire the control box from orange wire to ground, it kills. There is resistance in the ground wire, and no ground to control box grounds. I really need the correct diagram, but cant find it. I replaced the stator and control box a few years back, replaced alot of rotten wire, and everything was fine then. It fires RIGHT up, just a flick of the key. Anyone know where I can get the diagram? Maxrules doesn't seem to have it????

David

 

[Sunfish12]

Re: 1976 Mercury 850 Thunderbolt ign, wont stop running with ign switch.

When you say you bypassed the orange wire did you disconnect it?

 

[achris]

Re: 1976 Mercury 850 Thunderbolt ign, wont stop running with ign switch.

Since you asked so nicely

 

[achris]

Re: 1976 Mercury 850 Thunderbolt ign, wont stop running with ign switch.

There is no ground to the control box case. the ground runs to the back of the ignition switch.

 

[David84AET]

Re: 1976 Mercury 850 Thunderbolt ign, wont stop running with ign switch.

Think, I think,,,, it's figured out,,, the connector to the engine ( harness from controls ) is not providing a ground from the key switch in off position. Must have got broke while routing it through. Was getting ground signal from ign. to orange wire, but no continuity through harness end on ground wire from key switch. Very clean connect, so no problem there. Good news is I have another connector from an old control. All this time I was troubleshooting the orange wire. Time for splicing in the AM. Post update as soon as she quits runnin"! Sorry to those who have the opposite problem,,,,,,
Thanks all,
David

 

[David84AET]

Re: 1976 Mercury 850 Thunderbolt ign, wont stop running with ign switch.

OK, update, still won't stop running :-( I found broken wires in the main harness from control box to engine, so I replaced it with a brand new OEM one. The ignition switch wiring in the control box was all rotten, so I replaced it with a good looking used ignition swicth, complete. Carefully soldelerd and shrink tubed all wires to correct color corresponding wires. Same problem. Should there be resistance between the orange wire and battery ground, or engine block with the key in the off position? Where exactly does the ignition switch get a ground from anyways? If I ground the orange wire, it dies right out, wont turn off with key. HELP!!

Thanks, David

 

[David84AET]

Re: 1976 Mercury 850 Thunderbolt ign, wont stop running with ign switch.

SOLVED! The starter was wired incorrectly. The control harness ground was wired to the top lug on the starter, and there was no ground wire from the starter to the cowl. I Just moved the harness wire to the cowl, and then provided a jumper lead from the top lug on the starter, to another cowl bolt. Voila! Whats weired, is that is how it was on the donor boat this engine came from, and it WORKED FINE. I would not have noticed, except for examining/comparing with a parts dog I had got for the lower unit. The only really good thing that came from this, is I learned a bit, I have a new OEM harness, and a much better ignition switch. Thanks guys for all the help!
David