Help me identify this! - 986 Forum - The Community for Porsche Boxster & Cayman Owners

njbray · 10-15-2016, 03:28 PM

Soo.... I put in a new battery couple of days ago. Today my battery light comes on. Have it checked and it is the Voltage Regulator gone bad. Seems like an easy job to replace.
I am looking at engine & alternator and I discover a black sensor or something that is bolted to the engine block below the alternator - But it looks like the blue & green wires from it have been cut or nibbled away. (I recently had signs of Mr Ratty making a home on top of engine block!)
Anybody know what this thing is or does?

Thanks & sorry for the crappy photos.

thstone · 10-15-2016, 03:34 PM

Knock sensor.

See this link: http://986forum.com/forums/general-discussions/54756-knock-sensor-help.html

thstone · 10-15-2016, 03:44 PM

The knock sensor detects knock or (more correctly) pre-detonation in the cylinders by sensing the harsh vibrations. The ECU will then retard the ignition timing to prevent engine damage. Retarding the timing usually results in a loss of power so the engine can feel weak. The ECU will set the check engine light (CEL) if the sensor or wiring fails.

njbray · 10-15-2016, 04:04 PM

Thanks for the quick replies and info.
It looks like the wires have been cut really close to the sensor. I am thinking that maybe the knock sensor on the other cylinders is hooked up and this one is deliberately left disconnected. Maybe the motor only needs one.
I have no engine check lights & car runs great.
When I pull the alternator out next week I will look closer.

njbray · 10-16-2016, 12:52 PM

The battery light went off this morning - runs fine.
Will keep an eye on it.

njbray · 10-17-2016, 05:43 PM

So it turns out I have a bad alternator - easy.
Just it pulled it out and I can now see what Mr Rat Bastard has been up too.

Apart from covering the engine block in poop & peepee he has also eaten through the wires to the knock sensor! See pic. I think I will unbolt the sensor (is it OK to do that?) trim the wires and solder it back together.
I never noticed any effect on performance - maybe I will gain some when done.

Off to order a new recon. Bosch alternator.

dghii · 10-17-2016, 07:22 PM

sure your Alt is bad? Sure its not the voltage regulator? The VR is a $50 part and bolts to the back.

njbray · 10-18-2016, 11:41 AM

I had it tested and came back as alternator bad.
I have changed a Voltage regulator before on a BMW - then the alternator went out 2 months later - so now I just do the whole thing.

njbray · 10-21-2016, 12:43 PM

Car is now back together. I had to solder new wires to the pins in the knock sensor plug.
put in new (recon) Bosch alternator. All working great.
I have heard of at least 3 instances of rats eating this sensor wire (911's & Boxsters) - I guess it is in a very convenient place for them.
I vacuumed out all the crap off the motor - Looking at the pine tree debris mixed with the chewed wire bits I think this could have been like this for years.
I am told I should have more power now....Yay! :dance:

If you have rats around you might wanna check this.