1979 Mercury 70 EL Thunderbolt Kill Switch Install

[Cromagnon]

My dad just gave me his boat and I need to install a kill switch. It has a 70 EL Mercury Thunderbolt with a MercControl Throttle. Can anyone give me direction as to how to start this project? Thanks.

 

[Chris1956]

First you need to see if it is ADI ignition or battery powered CDI. I expect it will be ADI, but you need to look. If you have 3 or 4 coils, it is ADI. A single coil and distributor is battery powered CDI.

The rest of these instructions are for ADI ignitions. I can advise on battery powered CDI if necessary.

If ADI, you need a kill switch that connects when it is tripped. That is you want to ground a wire on the switchbox to kill spark.

A 1979 motor could have an orange or salmon colored killer wire, or it could be a black/yellow wire. 1979 was kind of the changeover year for that. I expect it will be one of those colors.

Whatever color wire you have, it must be connected to the killswitch, and the other wire grounded.

You can run a new wire from the switchbox and ground to the killswitch, or you can tap into the MerControl. It is likely easiest to take a pair of wires and put ring terminals on one end and connect it to the killer wire on the switchbox (orange or salmon or black/yellow), and a ground, and run it back thru the gunwale to the killer switch.

 

[Cromagnon]

First you need to see if it is ADI ignition or battery powered CDI. I expect it will be ADI, but you need to look. If you have 3 or 4 coils, it is ADI. A single coil and distributor is battery powered CDI.

The rest of these instructions are for ADI ignitions. I can advise on battery powered CDI if necessary.

If ADI, you need a kill switch that connects when it is tripped. That is you want to ground a wire on the switchbox to kill spark.

A 1979 motor could have an orange or salmon colored killer wire, or it could be a black/yellow wire. 1979 was kind of the changeover year for that. I expect it will be one of those colors.

Whatever color wire you have, it must be connected to the killswitch, and the other wire grounded.

You can run a new wire from the switchbox and ground to the killswitch, or you can tap into the MerControl. It is likely easiest to take a pair of wires and put ring terminals on one end and connect it to the killer wire on the switchbox (orange or salmon or black/yellow), and a ground, and run it back thru the gunwale to the killer switch.

Chris, it has 3 coils so it is an ADI. Thank you for the timely and detailed response. I’m on vacation now and will be trying to install soon. I might have another question once I get started. Thanks again!

 

[Cromagnon]

What size wires would you recommend to run from switch box to kill switch?

 

[Chris1956]

It does not need to be very thick. 16 or 18ga probably works fine.

 

[Cromagnon]

So I need to buy a switch that is normally open and closes to kill the motor? See attached image of one I’m considering buying.

 

[Scott Danforth]

Wires should already be to the helm. Check the wiring schematic in the factory manual

 

[Chris1956]

Yes you need a switch that closes when tripped. That looks to be the correct switch, however, I would hope they would explicitly tell you it's function.

 

[Cromagnon]

Thanks. Learning the language of marine wiring.

 

[Cromagnon]

Is this the switchbox and Do you know which 2 wires I need to attach my kill switch wires to? Thanks.

 

[Chris1956]

That is the switchbox, look for a yellow/black wire. That will be the ign killer wire. It needs to ground to kill the ignition.

 

[Scott Danforth]

there will be a yellow and black wire on the MOB switch.

if your MOB switch has two purple wires, then its for an I/O