This is an informational post about the Engine Speed Sensor. If you're having a starting problem or a ignition (spark) problem, please read.-----STORY - WHAT HAPPENED-----
So I was having a problem with my car not starting intermittently, and it progressed to my car dying while driving and then not starting. If I let the car sit for 30 minutes or a couple of hours, it would finally start, but would die in a few minutes. Sometimes it wouldn't die that soon, but it would eventually die. I narrowed it down to no spark. After researching on here, I decided to replace the Engine Speed Sensor (ESS). This post is just for searching purposes and to let everyone know what I went through and what fixed the problem.
Everything I read on the boards led me to believe the ESS was the prime suspect. Without the ESS, the car will crank and crank, but there will not be a spark.
I read in my Bentley that to check the sensor, measure the resistance, and it should be between 500 and 700 ohms. It doesn't tell you how to test, it just lists the specifications. There are 3 pins on the connector, and with it installed in the car, I couldn't get a reading on any two pins, but that didn't make sense, because the car would run.
I took the sensor out with a 5mm allen wrench and tried the test again. I got a reading of 860 ohms between the center pin and one side pin, so I thought I had confirmed a bad sensor. I ordered a new sensor for $80 ( I know you can get it cheaper, but I needed it next day, etc, etc...), and when it arrived, I checked the resistance -- 850 ohms. I really didn't know what to think, so I installed it anyway. Car has ran like a champ ever since I installed it -- no dying, no starting issues.
-----SENSOR INFORMATION-----
Here are some things I learned while working with this issue.
The ESS, or G28 sensor, is located on the front of the block below and to the right of the oil filter housing. It is black plastic and has a single black cable coming out of it and is held in place with a single 5mm Allen bolt. The black cable has a white 3-pin connector located (on a mkIII) just behind the cooling fans and to the left of the starter. There is another 3-pin connector right next to it that is brown. Don't confuse the two, the other is for the front knock sensor.
I don't know how to test the sensor. My bad sensor and the new sensor had the same resistance of 850 ohms which is above the listed spec.
How it works -- The sensor is installed in a position where it is almost touching the large gear on the crankshaft. The large gear has 2 missing teeth. By playing with the sensor and a continuity tester, you will notice that the sensor is actually a very strong magnet. When you bring it close to a metal object and away again, the ohms will drop to zero for a quick burst (it has continuity). So, this tells me that when that 2-tooth gap comes around, the sensor signals the ECU with a short blip of continuity.
-----OTHER STUFF-----
If you're having a no-spark issue, be sure you think about the ESS. It's VERY easy to replace, and is cheaper than other ignition system parts if you shop around...