UrS4boy

Our 20vt turbo (3B, AAN, ABY and ADU) engines have an ECU that keeps track of the position and speed of the crankshaft in order to make decisions about fuel injector timing, etc. To do this the ECU uses a Crank Position Sensor (G4) and an Engine Speed sensro (G28). Without good signals from these sensors, the engine will not start (the ECU can't tell where the pistons are) or it will stop running after starting.
Regardless of whether it is powered or not, the crank position sensor only generates a signal when the engine is running above 23 rpm (above the cranking speed).

Here is what they look like from the inside:


Photo courtesy of Jerry Scott

The part numbers are:
G4 (Item 14): 034 905 381 igntion timing sender (black)
G28 (Item 15): 034 905 381A engine speed sensor (grey)

The G4 and G28 are both common across the Audi 80, 100, 200, UrS, RS2 range from 1985 MC to the 1997 UrS AAN and 1995 RS2 ADU and everything in between, i.e. good ones might be availble in wrecking yards.

The G4 and G28 sensors connect to the ECU engine harness on a connector rack on the firewall. Here is the call out for an AAN:



You will get a code on the engine speed sensor as failed if you retrieve the codes with the engine off, i.e. false positive.

These sensors can fail but it is not common. More likely to be the cam position sensor or fuel pump problems before these if you have a starting/running issue. You can change them yourself (easier than the knock sensors to get at). That said, they are magnetics and there have been cases of the sensors picking up metallic debris that causes them to malfunction. Therefore, you *might* want to remove and clean them if you are having strange starting or running issues. Here is an example of an engine speed sensor that had accumulated metal debris (the tip is magnetic):



Testing a G4 is something like this: "Test the resistance between between pins 1 and 2 of the black connector end of the G4. (With the bump on the middle of connector up, the pins are, left to right, 1 ,2 and 3.) Between 1 and 2 you should have "approximately 1000 ohms".
If NO = replace, If YES - test between pins 1 and 3 and then 2 and 3. Both should be infinite ohms, i.e. "open", If NO = replace. If the G4 is fubar'd the engine will not start. If the engine is running when the G4 fails, it will keep running, but never start again (until you replace the G4).

The other thing that you *might* want to check when you are having strange starting issues is the presence of the timing pin (actually an elaborately shaped, square-ish bit) is still on the flywheel, as shown at the 8 o'clock position in the photo below (should be visible through the hole where the crank position sensor is located when the sensor is removed) (LINK to DETAILED G4 PIN INFO)



HTH