73 Merc 20 HP Remote Problems

So I have a strange problem. I have a 73' Merc200 20 HP and when I have it connected to my remote controls it will turn over but will not fire. I have no spark in either plug. But when i take it off remote and pull start it, it fires right up and runs awesome. I've checked continuity in my external and internal wiring harnesses and I can't find anything wrong. I've tested the key switch and everything seems to work as its supposed to. So i'm at a loss here. I replaced the switch box because after testing all the coils and the trigger everything under the flywheel tested good. All the electronics in the motor are fine and work like they are supposed to. Somewhere between the motor and my control box there is in issue and I'm hoping someone can help. Thanks!

-Andrew

Here is the wiring diagram.

Sounds like you have a wiring issue in the kill circuit of your key switch so that it's grounding the ignition and not allowing it to produce a spark. Just for testing purposes, try disconnecting the orange wire from the back of your ign switch and see if it will run. Be advised, you'll probably have to disconnect the fuel line and run it out of fuel after you disconnect that wire in order to kill it.

Thank you JMichael! I will try that when I get home from work today.

Well I tried what you said Mike and still no spark. So I took the key switch off the remote and hard wired it right to the internal wiring harness plug on the motor. When the key switch is in the on position there is no spark, when you turn it like you are going to start it then I get spark. So it's got to be a bad key switch. So I'll get a new one and hopefully that will solve the problem. Thanks for the suggestions!

You gotta do what you think is right but I personally don't believe it's the switch. The reason for that, is that there is only 1 wire from the switch that connects to or should affect the ignition and that is the orange one for the kill function. All the other wires go to the starter solenoid, the elec choke if you have one, the tach is you have one etc. No other wires go to the ignition unless it's wired wrong. :lol: Let us know what you find out.

If you feel like trying one more thing I do have something in mind. Look at the schematic you posted. See down by the switch box where it says NOTE on the schematic? Disconnect the orange wire at that point and see what happens. I'm thinking you might have a short in your wire harness.

Well you were absolutely correct. I disconnected the keyswitch completely and just plugged the external harness in and guess what? Still no spark. So I examined the harness and about 2 feet from the plug at the engine the external insulation was broken and all the wires inside were cracked and corroded. Once I cut the harness off from that point I got my spark back.

The joys of 30+ year old wiring. I think I am going to try and splice the wiring and make my own harness back to the control box.

The problem is the key switch. It pretty much fell apart as I was taking the wires off. So can I use a new replacement switch in place of buying an OEM one? I've noticed, and correct me if I am wrong, that this switch combines the magneto and ignition circuits. When the engine is off, key switched on, and battery connected there is 12v at the orange ignition/kill wire. When the engine is running there is 35-36 volts on that wire. So is the magneto sending current back to the battery through that wire?

Again I really appreciate the help.

I use to own a 75 model of that same engine and had the same problem with the harness insulation falling apart. I also tried to "rebuild" the harness but I found the wire was too corroded to be able to get solder to stick for splicing purposes. You'll be fine with another switch as long as it is the correct one for your setup. Are you saying that with the orange wire disconnected, there is 12v present at the lug where the orange wire connects to the switch? Since it comes out of the enclosed switch box, I really don't have any idea what voltage reading you would get from the orange wire with the motor running.

When you switch the key "ON" there is 12V to the orange wire. When you start the motor and it is running, there is 36V on the orange wire. I was just wondering if that wire was how the stator was sending voltage back to the battery to charge it. Most keyed ignitions have a separate circuit for charging the battery. This one seems to combine the ignition circuit with the charging circuit. Unless I am completely wrong.

I could be mistaken but I'm pretty sure there shouldn't be any voltage on that orange wire unless the motor is running. The switch should ground that wire when the switch is in the off position and there's no reason to apply voltage to the orange wire since it's used to ground the ignition voltage and kill the engine. So feeding 12v in to the orange wire makes no sense. It sounds like you have the switch wired wrong or it's got a short in the switch. Wired wrong being the more likely problem in my mind. + I don't believe the orange wire has anything to do with the charging circuit.

See thats the weird part. You can't engage the starter solenoid without putting 12v to the orange wire. I've tried just putting power to the starter through the solenoid and the solenoid will not work without power to orange wire first. This is quite the confusing little system.

You've got something wired wrong. I know for a fact that it doesn't require 12v to the orange wire for the electric starter to work if things are working properly. I know this because when you have the motor set up for tiller steer/control (what I converted mine to), the orange wire is connected to the kill switch which does nothing but ground the orange wire when the button is pressed. And the electric starter still works when you hit the start button.

I'd start taking a good look at your orange wire routing. Look for any breaks or kinks in the wire. Disconnect the wiring harness and do a continuity check of the control side of the orange wire to ground. That wire should have infinite resistance with the key in the run position and very low resistance (<2 ohms) in the off position. If you are getting a resistance value in the run position, then this wire is your problem. You will need to re-run a new wire to replace it.

Make sure that your fingers aren't touching the wires or probes, as the continuity through your body will actually give a resistance reading.

Silly question, are you looking at an orange wire or a faded/discolored red wire??? The red wire goes to your solenoid and would need 12V to engage it.

Your wiring should match one of these two. Not sure when the difference happens




Good luck!

kofkorn said:

Silly question, are you looking at an orange wire or a faded/discolored red wire??? The red wire goes to your solenoid and would need 12V to engage it.

Good luck!

Click to expand...

Very good point and that would explain a lot.

Thank you all so much for the advice and help. My set up is the second choice there. Here's what is happening. My key switch no longer works and I've had to cut my external harness almost completely off to get rid of the bad wiring inside. So I've removed the key switch and wiring problems.

So what I did to test the internal wiring harness was run individual wires to the internal harness plug contacts that the external harness plugs into. Then I connected the main red and black wires to the battery terminals and touched the yellow wire (from the starter solenoid) to the battery, as if the key was turning to start the motor and nothing happened. I then touched the orange wire to the battery and the yellow wire to the battery and the solenoid fired and the starter turned over. That's why I'm so confused as to why power would need to run through the Orange wire and into the switch box, cause it shouldn't have to. The Orange wire comes straight from the ignition switch box (which I've replaced) and then is split between the kill Button on the motor and the internal harness that goes to the remote. I've checked that wire and everything is normal. No signs of wear and it works properly. When the engine is running, if you connect that wire to ground it kills the motor.

At this point I would just run the motor as a tiller but I don't have a tiller handle so it's either find a tiller handle or figure out the electrical problems.

Your interior and exterior wiring harness should be the same age. If your exterior harness is shorted and falling apart, the internal harness is probably in the same shape. Have you checked for shorts in the internal harness? As the schematic shows, the yellow and black wires are the only 2 wires that have anything to do with activating the starter solenoid. So if doing anything to the orange wire is affecting what the starter solenoid does, you've still got wiring issues.

Well I'm not exactly sure what I was doing but like you all have been saying, you definitely don't need to put power to that orange wire to make the starter run. I plan on making a new wiring harness and just ditching the old one. I've got a new ignition switch on the way so hopefully it will all come together nicely. We took it out today and the motor runs awesome. Super easy to pull start and with the orange wire its easy to kill. Just one problem, I have myself another spun prop. I had it repaired last season and apparently not repaired well. I'm not a big fan of the prop design on these mercs. I'd much rather just have a shear pin like the old johnson/evinrudes. Anyhow, thank you all for the help, I really appreciate it. Onto the next problem, haha