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Author Topic:   Engine harness and connector on 1994 Mercury 225
Frank O posted 07-11-2008 09:09 PM ET (US)   Profile for Frank O   Send Email to Frank O  
Recently I've been spending considerable time troubleshooting the kill circuit on my 1994 Mercury 3.0L Offshore 225-HP carb 2-stroke outboard. This was causing an intermittent fault which prevented starting the engine at times.

After going through the kill circuit component by component -- it consists of the ignition switch, the kill switch, the remote harness from the console to the outboard, the engine harness within the outboard, a shift interrupt switch and diode, and connections to the engine's six ignition modules -- I concluded that the fault was in a segment of the engine harness (the harness within the outboard). This harness has an eight-wire female connector on one end which plugs into the remote harness running to the console. The other end of the engine harness terminates in various connectors systems within the outboard.

I found that one relatively short length of wire (the black-yellow wire for the kill circuit) running from the eight-wire connector was not as isolated from ground as it should be. Normally an ohmmeter should measure infinite resistance between this wire and ground when the wire is disconnected from everything else. However, I found that the resistance between this wire and ground measured about 6 megohms, apparently providing enough of a current path to shut down the engine's ignition modules at inappropriate times.

The engine harness consists of plastic-coated stranded wires bundled together with electrical tape. Although it's possible that insulation on some part of the black-yellow wire in this harness became frayed, I think that by far the most likely failure point is in the eight-wire female connector. This didn't appear 100% pristine to my eye, and I wouldn't be surprised if subtle salt intrusion over the years created current paths between the female sockets in the connector plug. (By contrast, the wires in the remote harness (the harness between the outboard and console) -- which terminate in an eight-wire male connector -- appear to have excellent isolation from each other.)

Interestingly, it appears that a replacement engine harness is available from Mercury, but the price is a drawback -- about $530. Is there any rehabilitation I could do on the existing plug? Or does anyone know if it's possible just to buy a new plug and splice it onto the eight wires involved?

Thanks for any thoughts. I'd also be interested to know if this kind of problem is at all common.

jimh posted 07-11-2008 09:47 PM ET (US)     Profile for jimh  Send Email to jimh     
Frank--I don't have any first hand knowledge about the KILL circuit in the Mercury motor, but, if it follows what is often done on other motors, the voltage present on the KILL circuit can be quite high, much higher than the usual 12-volts and on the order of 300-volts. This high voltage may be jumping through the insulation on the wire in your harness and grounding, and such a short to ground would have unpredictable effects on the engine.

I suggest that you temporarily abandon the KILL circuit wire in the harness. Instead, just run a new wire outside of the harness, and wire it in place of the conductor in the harness. In this way you will be able to test to see if the harness is the source of the problem. I would just bypass all conductors, running a wire from the engine ignition module right to the kill switch.

Frank O posted 07-11-2008 09:57 PM ET (US)     Profile for Frank O  Send Email to Frank O     
Jim -- Indeed, this is what I did today. Bypassing the wire in the harness, I ran a temporary wire from the kill circuit in the engine to the ignition/kill switch, and the engine started and stopped perfectly. (Without the wire, of course, there is no way to stop the engine, short of running out of fuel.)

Although one approach might be to make this bypass wire permanent -- zip-tie it to the remote harness, and run it through the tunnel to the console -- it would be much more convenient in the long run if I could rehabilitate or replace the plug on the end of the engine harness. I wonder, for example, if there is any kind of solvent (H2O?) I could soak the engine harness plug in to remove what I assume is probably a lot of salt buildup? I seem to see various third-party sources around on the Internet for harness-type products -- I wonder if it's likely I can find the eight-conductor plug by itself anywhere? Mercury just seems to list the entire harness at $530 on their parts list.

jimh posted 07-11-2008 11:17 PM ET (US)     Profile for jimh  Send Email to jimh     
Frank--Good work.

If you see visual evidence of oxidation or ingress of water in the connector, then, by all means, try to flush it out. I would proceed as follows:

--Switch the primary battery disconnect to OFF;

--Separate the wiring harness connectors;

--Spray an electrical contact cleaner onto the pins and sockets of the connectors;

--Blow the connector pins and contacts dry with compressed air (of moderate pressure).

--Re-check for any visual evidence of corrosion or deposits remaining;

--Repeat until connector is clean and dry.

For an electrical contact cleaner there are many practical products. I'd say try:

--WD-40 (which is suspected to contain kerosine)
--CRC-56 (similar to WD-40 but ingredients unknown to me)
--Specialized electronic cleaners such as DeoxIT

Pure (or nearly pure) isopropyl alcohol is a good rinsing agent it you used a less sophisticated cleaner product. But check the bottle ingredients. Drug stores sell alcohol in a 1:1 solution with water; that is okay for your shoulder but not for electrical contacts. An alcohol rinse should remove any residue from the cleaner, which is good--unless you used a specialty cleaner that claims its residue is helpful.

This procedure may cure the problem if the flash-over point was in the connector. If the flash-over point was in the wiring of the harness, this will not resolve it.

If you see signs of the flash-over point such as arching or burning in the connector, you should try to remove the carbonized material, Using a very small screwdriver as a scraping tool, try to remove any of the connector body which has been burned or carbonized. Clean the area as above. Let the area dry completely. You can suppress any tendency to arc by painting this area of connector body with a material called corona dope.

Some sources for corona dope or red insulating varnish:


I don't know about the Mercury connectors, but generally I have seen that most of the connectors used in outboard motors have a rubberized body and use pins and sockets that are inserted into the body. If that is the case, you may not need an entirely new connector. You may be able to just replace a bad pin or socket, or just replace a bad wire and re-insert it into the connector body with a new pin or socket attached.

gss036 posted 07-11-2008 11:35 PM ET (US)     Profile for gss036  Send Email to gss036     
Frank, had that happen to my Merc 200 hp about 3-4 years ago. You could actually see where it was arcing across the pins in the connector. Rather than putting out the big $$ for a new harness, I just had the plugs cut off and hard wired the wires. Like my wrench said, you probably will have to have a new harness when changing motors anyway. Got by that since then, but then blew the engine this past March, so he was right on.
If you have isolated it to that one wire, I see nothing wrong with just running a permenant seperate wire for the kill switch.
Frank O posted 07-13-2008 09:44 AM ET (US)     Profile for Frank O  Send Email to Frank O     
Thanks much for the suggestions. I'll follow jimh's procedures to attempt to rehabilitate the plug connector on the engine harness. If that doesn't work, I'll probably make the bypass wire permanent and route it through the tunnel under the deck. At this point I'm not real enthusiastic on making it a more complicated project by installing a new plug. Like gss036, I may be repowering within a couple of years anyway.

jimh posted 07-13-2008 11:05 AM ET (US)     Profile for jimh  Send Email to jimh     
Frank--If that connector body is rubber, I don't know how well the corona dope will work. It may be worth a try, as the bottle of corona dope is $6.

An alternative, and possibly better solution, is to run your own separate wire as suggested. Fasten the new wire to the existing harness. Install a one-pole connector for the new wire at the harness connector. In this way you will be able to disconnect the motor if needed.

I think these KILL circuits take a rather brute-force approach to killing the spark. They just short out the capacitor in the capacitor discharge ignition. That capacitor typically has several hundred volts on it, which accounts for the arcing in the connector. A high voltage like that will jump to ground with a little moisture to help establish a path.

Frank O posted 07-27-2008 12:05 PM ET (US)     Profile for Frank O  Send Email to Frank O     
Update -- Yesterday I went to the boat to work on the engine harness plug:

I was hoping that the spot where the grounding was taking place was in the kill-circuit wire that leads out of this connector, which would have made for an easy fix. However, I removed this particular wire from the harness, and it's still grounding even with the wire completely isolated -- so the problem must be in the plug.

I thoroughly cleaned the plug and female pin sockets, using first QD Marine Electronics Cleaner and then DeoxIT. I don't see any oxidation or damage around the pin sockets. This cleaning, however, didn't fix it. The ground path must be on the side of the plug where the wires enter.

Unfortunately I'm not seeing any way to open up the plug body short of snipping at it with wirecutters -- it appears to be molded shut, I'm sure because of the marine environment it was designed to face.

But I have two questions:

1) Does anyone knows of a good way I'm not seeing to get into the side of the plug where the wires connect?

2) Are these eight-conductor plugs available somewhere out there? Mercury seems to just sell the entire engine harness assembly for $520.

If I could find a compatible new plug I might be game for splicing it in -- although making my existing juryrigged kill-circuit wire permanent by routing it through the tunnel under the deck is looking like an increasingly attractive option.

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