About Knock Sensors

[johnrmclean]

My 1994 454 Mercruiser has a knock sensor on it. What do they do and how do they operate?

 

[alldodge]

If the module senses a knock it will retard timing, being a 1994 and a knock sensor means you should have a TB5. Once the knock is gone it will advance timing

 

[johnrmclean]

does disconnecting the knock sensor have any effect on performance or the main ignition module?

 

[Pete104]

Yes! It needs to see something. Also they want 87 octane fuel.

 

[Faztbullet]

the t-bolt V:
Another module piggy-backs onto the ICM. It is called ?knock module? After the engine is up to temp, the knock module will advance the timing and look for the RPM to increase. Then it does it again and again until it hears engine knock and backs off the timing. This is done using a knock sensor on the starboard side of the engine. This timing is the most efficient way to run the engine for power and MPG.

 

[alldodge]

does disconnecting the knock sensor have any effect on performance or the main ignition module?

No

On your TB ignition it has no effect if it is disconnected

 

[thumpar]

No

On your TB ignition it has no effect if it is disconnected

Except if you get crappy gas or the timing gets out of whack and the thing starts to knock. That could burn up the engine.

 

[Pete104]

Why would anyone run disconnected?

 

[alldodge]

Why would anyone run disconnected?

IMO you don't run it disconnected unless for a quick test. My comment was just what happens if you do disconnect it on a TB ignition.

If it was a MEFI-3 or higher then the ECM checks to make sure the sensor is working when the boat is first throttled up. If it does not detect the sensor knock then a code 44 is set and timing is retarded thereby reducing power

 

[hemi rt]

If you run it disconnected the ECM will see it's not there and go into limp mode, it sets the timing at a predetermined position and will limit the maximum RPM to help prevent damage but still allow the engine to provide some power to get you moving back to a repair shop/dock. Works the same as in a vehicle.

 

[Pete104]

IMO you don't run it disconnected unless for a quick test. My comment was just what happens if you do disconnect it on a TB ignition.

If it was a MEFI-3 or higher then the ECM checks to make sure the sensor is working when the boat is first throttled up. If it does not detect the sensor knock then a code 44 is set and timing is retarded thereby reducing power

I was responding to the OP question. And stick to that response.
What could you possibly test by disconnecting a knock sensor?

 

[achris]

My 1994 454 Mercruiser has a knock sensor on it. What do they do and how do they operate?

A knock sensor is basically a piezo crystal cut to resonate at the same frequency as an engine knock (detonation). When the crystal resonates it produces a voltage, exactly the same as the transducer on your depth sounder. That voltage is feed to the ignition module which will retard the timing until the knocking stops. The function of the knock sensor is to allow the ignition module to retard timing to protect the engine from damaging effects of poor fuel, and/or advance the timing for the best possible performance and fuel economy.

HTH,

Chris......

 

[johnrmclean]

thanks