Out of the blue one recent evening I got into my SLK320 right after dinner, the car still warm from the ride home. Crankity crankity crank, no start. About an hour later I tried again, and guess what it started right up as usual.
After some time spent searching the web I decided most likely my Crankshaft Position Sensor was giving out. So I replaced it, and so far, no more problems.
There are several write-ups that helped me do the replacement, but I thought I could add a little something to the pool of knowledge out there. Here are my notes.
Some tools that came in handy:
* A small flashlight so you can see in the deep recesses of the engine compartment. Absolutely necessary even on the sunniest days
* An E8 socket. May be hard to find. Got mine from Harbor Freight. Magnetic would be nice. Even harder to find.
* A socket extension, 6-8" long. Typically 1/4" drive size
* A small 1/4" ratchet driver.
* A magnetic parts finder/extractor. This looks like a small telescoping radio antenna with a powerful magnet on the tip.
Remove the engine cover/air cleaner assembly to give a little more room to work. I usually then place a rag over the exposed MAF sensor's mouth to keep stuff from falling into the intake
Removing the connector:
The connector atop the sensor is L-shaped, bent upward. It has a clip that locks onto a tang on the topside of the sensor. To remove the connector you must squeeze that clip so that it pivots outward and away from the tang on the sensor. The spot to squeeze the clip is on the inside of the connector's "elbow". It is completely hidden from view, you must feel for it. When squeezing, it may help to push inward and wiggle a bit, to free up that clip; then gently pull the connector off the sensor. It should not take much force to pull off; if it resists, you might not have the clip pivoted off the tang enough.
Once the connector is off the sensor, it's time to prepare for surgery. The sensor is located in the bell housing, between the engine block and a shield/bracket. Some people have removed that shield to get better access. I tried. It requires removing two of the bolts that hold the transmission to the engine, and they are torqued on pretty tight, as well as difficult to reach. So instead I pried the shield about a half inch away from engine, which gives enough room to work.
Next stuff a small rag in the space below the sensor, to catch various parts and prevent them from falling to an even more inaccessible spot. I even placed a magnet on a string, just below the sensor, so that if the bolt did fall, it would get grabbed by the magnet.
Luckily I never dropped the bolt, but it sure would be easy to do.
Removing the bolt:
The sensor is held in place by one E8 bolt, located on the bottom side of the sensor. There is no gasket or sealer for this sensor.
Using the E8 socket and extension and ratchet, begin unscrewing the bolt. The threading on the bolt is a little more that 1/2" so somewhere along the line you should take the ratchet off the extension and finish off unscrewing by turning the extension with your fingers until the bolt makes that clicking feeling like it's finally out; then turn the other way 1/4 turn to barely tighten the bolt enough so that when you pull off the socket the bolt stays put. If you have a magnetic socket you can omit this step and just extract the bolt.
Now extend your parts extractor to about 9" and gently insert it until the magnetic tip attaches itself to the top of the bolt. Then carefully twist the extractor and pull out the bolt.
Removing the Sensor:
Just pull it straight out. No twisting or prying should be needed.
Installing the new sensor:
Test fit the new sensor. If it doesn't slide in easily, you may need to file or scrape off some mold-flash and put a drop of oil on it until it slides right in easily. This becomes important later.
After several unsuccessful attempts to poke the bolt through the sensor and into the bell housing, I came up with a trick that worked easily. With the sensor out of the car, stick the bolt through its hole in the sensor. Then put the socket and extension onto the bolt. Finally wrap a rubber band around the connector side of the sensor and the socket, so that the socket can be turned but won't drop off. See picture
Feed this whole assembly down into the engine. When the sensor is properly situated in its hole in the bell housing, carefully begin tightening the bolt by turning the socket extension. When it's hand-tight, finish off with the ratchet until the bolt is torqued in well. Then pull out the socket and extension. You may need some pliers or a wire hook to grab the rubber band if it's left behind.
Finally just push the connector onto the sensor until it's securely seated.
Be sure to remove any rags, magnets, etc., from the working area. Replace the air filter/engine cover.
Start the engine. There is no adjustment or calibration needed; the engine should just spring to life, not run rough, and not throw codes.
Celebrate with the beverage of your choice.