reference sensor flywheel sensor crank sensor
 

This post is for searches. (wait, aren't they all? :))
Sensors are swappable - same part. Cheaper when ordered as a BMW part. Takes a special bolt shown here.

http://forums.pelicanparts.com/uploa...1319490901.jpg
http://forums.pelicanparts.com/uploa...1319491155.jpg

Reference / speed flywheel sensor mounting bolts. 6mm special bolt.
Here's some pics of the special bolts for holding down the sensors.
#1 is the correct bolt
#2 is the bolt from the pivot point of the top half of the back seats, which is a workable substitute.
#3 is a regular head 6mm bolt.
The reason they have that thick shoulder (looks like a cylinder spacer) under the head is only to stand the head of the bolt off from the sensor. The sensor has two "walls" very close to its hole as seen in the pic above.

You can see in the second pic that the head of a normal bolt becomes totally recessed in the sensor and you can't get a socket on the head to tighten it down. This is why you can't use a regular bolt of the same length.
To remove the sensors I use a 1/4" drive ratchet with a 14" extention. I use a wand magnet to catch the bolts. It's super important that you stuff/cover the sight hole in your clutch bell housing so hardware doesn't fall down there.
You can undo the 10mm (head) bolt that holds the bracket for the sensor wire connectors. That will give you more room back there. When you try to get your hand in there, you'll have to push the heater control valve out of the way. If you have to, remove the small hose clamp/water hose that goes to the J-pipe at the rear of your head. This is the return for the heater core. If you do this, you'll need to bleed any air out of the cooling system later- no bigee.
With the hardware out, spin the sensor some and lift enough to get two fingers behind it like you're pulling back a bow(and arrow) string. Sometimes they come out easy, other times hard. Use a penetrant and keep spinning back and forth and spraying it. There are at least 3 types of sensor brackets that Porsche used to hold the sensors and the worst kind is the one with the extra long steel sleeve in the BG position.
As I said earlier, the sensors are interchangeable, but make sure you get the correct sensor attached to the correct plug on the wiring harness. If you hook up the sensor functioning in the BG position to the plug for DG, then engine won't fire at all.

Very helpful summary. Thank you, from a site learner.

Maybe this thread should also mention the fact that if you remove the bracket, you will also need to check and adjust the sensor clearance, as per the Clarks Garage description:

Speed and Reference Sensors - Checking, Replacement, and Adjustment

CG also has some useful troubleshooting procedures.

Also, people with starting troubles who are wondering about their sensors should type 'tach bounce' into the search engine at this site!

Thanks again.

Regarding that tach bounce and my recent replacement of a fouled reference sensor...I never saw a difference in tach bounce before/after replacement. Perhaps the tach bounce is only affected with a speed sensor failure??

Don't mean to stray from Timmay!'s bolt post - pictures and notes are spot on for removing the sensor.