WAJetboating
Member
There is a black box under the storage bin on the driver's side (under the windshield). It's barely labeled with a Mercury part number and has a few wires going into it. It is formally called a "Mercury Throttle Guardian" and its purpose is to prevent the engine from being revved above idle when the boat is in neutral. It is a VERY poorly documented "feature" that doesn't appear on the schematics anywhere.
On these boats, "neutral" is defined as the FNR lever being centered. As I'm sure you're aware, the impeller is spinning whenever the engine is engaged so the Throttle Guardian is really almost useless. Also on the C2K, there is a mechanical interlock between the FNR lever and the throttle lever that prevents the latter from moving unless the former is in F or R so the Throttle Guardian is redundant.
In addition to power and ground, the Throttle Guardian has a tach wire (input), a wire from the FNR lever (input), and black-with-yellow wire (output). This latter wire connects to the other black-with-yellow wires that ultimately go to the coil packs for the spark plugs.
The Throttle Guardian works as follows: When the FNR lever is in neutral AND the tach signal is above ~2500 RPM, it intermittently grounds the black-with-yellow wire. Grounding that wire kills spark on all cylinders, but since it does so intermittently you get a very roughly running engine. Once the Throttle Guardian is triggered, it doesn't stop this intermittent behavior until the RPM's are brought back to idle.
On my boat, this module turned out to be failing intermittently. I'd be idling and start to ramp up the engine, and suddenly get this horrible roughness that felt exactly like I was hitting a rev limiter - except that it happened around 2500 RPM's and wouldn't go away unless I backed off all the way to idle. Worse, it didn't always go away... sometimes I had to kill the engine to fix it, and sometimes I had to restart the engine multiple times before it reset itself.
My oscilloscope confirmed that something was grounding the coil pack kill circuit. Since it felt exactly like an RPM limiter, I ended up buying a new RPM limiter (which is back on the engine, ~$150) and basically disconnecting everything on the entire black-with-yellow circuit (which includes all sorts of things including the keyswitch and killswitch) before getting to this mystery box that nobody knew about. Lots of research later I figured out what it was. So I installed a quick-disconnect and waited.
The next time the engine behaved like that, I kept it running and dove under the windshield to disconnect the black-with-yellow wire going into the Throttle Guardian. The engine INSTANTLY started running properly again. I touched the connectors back together and the engine went right back to stuttering. Separating them restored proper operation. It was 100% repeatable. Needless to say, I left the black-with-yellow wire disconnected and everything was perfect from then on.
Since the module is redundant to the mechanical interlock in the FNR/throttle assembly, I never replaced it. It's still there, still powered up, and probably still making improper decisions about what my engine should be doing - but its output is disconnected so it can't hurt anything anymore.
I still have my original, now known to be perfectly good, RPM limiter. The $150 replacement that I didn't need is still installed on the engine. Anyone need an RPM limiter? {grrrrrrr}
So: If this sounds like what you're experiencing, you can cut the module's black-with-yellow wire and install a connector. Next time it happens, disconnect the connector and see if the problem goes away. This one wire will tell you whether your Throttle Guardian is starting to fail, and you won't have to uselessly spend $150 on a new rev limiter and waste lots of hours figuring out something that doesn't appear ANYWHERE in the schematics or documentation {growl}.
Report back and let us know what you find!
On these boats, "neutral" is defined as the FNR lever being centered. As I'm sure you're aware, the impeller is spinning whenever the engine is engaged so the Throttle Guardian is really almost useless. Also on the C2K, there is a mechanical interlock between the FNR lever and the throttle lever that prevents the latter from moving unless the former is in F or R so the Throttle Guardian is redundant.
In addition to power and ground, the Throttle Guardian has a tach wire (input), a wire from the FNR lever (input), and black-with-yellow wire (output). This latter wire connects to the other black-with-yellow wires that ultimately go to the coil packs for the spark plugs.
The Throttle Guardian works as follows: When the FNR lever is in neutral AND the tach signal is above ~2500 RPM, it intermittently grounds the black-with-yellow wire. Grounding that wire kills spark on all cylinders, but since it does so intermittently you get a very roughly running engine. Once the Throttle Guardian is triggered, it doesn't stop this intermittent behavior until the RPM's are brought back to idle.
On my boat, this module turned out to be failing intermittently. I'd be idling and start to ramp up the engine, and suddenly get this horrible roughness that felt exactly like I was hitting a rev limiter - except that it happened around 2500 RPM's and wouldn't go away unless I backed off all the way to idle. Worse, it didn't always go away... sometimes I had to kill the engine to fix it, and sometimes I had to restart the engine multiple times before it reset itself.
My oscilloscope confirmed that something was grounding the coil pack kill circuit. Since it felt exactly like an RPM limiter, I ended up buying a new RPM limiter (which is back on the engine, ~$150) and basically disconnecting everything on the entire black-with-yellow circuit (which includes all sorts of things including the keyswitch and killswitch) before getting to this mystery box that nobody knew about. Lots of research later I figured out what it was. So I installed a quick-disconnect and waited.
The next time the engine behaved like that, I kept it running and dove under the windshield to disconnect the black-with-yellow wire going into the Throttle Guardian. The engine INSTANTLY started running properly again. I touched the connectors back together and the engine went right back to stuttering. Separating them restored proper operation. It was 100% repeatable. Needless to say, I left the black-with-yellow wire disconnected and everything was perfect from then on.
Since the module is redundant to the mechanical interlock in the FNR/throttle assembly, I never replaced it. It's still there, still powered up, and probably still making improper decisions about what my engine should be doing - but its output is disconnected so it can't hurt anything anymore.
I still have my original, now known to be perfectly good, RPM limiter. The $150 replacement that I didn't need is still installed on the engine. Anyone need an RPM limiter? {grrrrrrr}
So: If this sounds like what you're experiencing, you can cut the module's black-with-yellow wire and install a connector. Next time it happens, disconnect the connector and see if the problem goes away. This one wire will tell you whether your Throttle Guardian is starting to fail, and you won't have to uselessly spend $150 on a new rev limiter and waste lots of hours figuring out something that doesn't appear ANYWHERE in the schematics or documentation {growl}.
Report back and let us know what you find!
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