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| Crank Position (G4) and Engine Speed Sensor (G28) Info - Function , Location and PNs |
| Posted by: UrS4boy (137) on 2009-11-28 04:12:57 | Share | Report |
Our 20vt turbo (3B, AAN, ABY and ADU) engines have a Motronic ECU that keeps track of the position and speed of the crankshaft in order to make decisions about fuel injector and spark timing, etc. To do this the ECU uses a Crank Position Sensor (G4) and an Engine Speed sensor (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 13 rpm (Manuals say 23 rpm but Paul N. of S2Central.net has confirmed 13 rpm) (i.e. the cranking speed and above). The G28 engine speed sensor is the trigger for firing the J17 fuel pump relay which also opens power up to a number of devices. The G4 crank sensor and the G40 Cam Position Sensor are needed by the ECU to determine exactly where the engine TDC is. This is because on any four stroke engine, the crankshaft turns twice as fast as the camshaft - so with G4 alone on the cranks at 62 deg btdc it's impossible to know if the engine is on the compression or exhaust stroke. The purpose of G40 is to identify the camshaft position and therefore determine the phase of the engine so that ecu has all the info it needs to start initiating coil and injector activation. (Reference Paul N. at S2central.net) After the engine is running, the ECU can run on G4 alone if the G40 fails - however, the ECU will retard the timing signifiantly if the G40 fails while running. While the fuel pump relay is still energized with a failed G40, the ECU will not fire the ignition or the injectors because it needs both the G40 and the G4 crank sensor signals to do so. Here is a crude sketch I did to show the relationship between the camshaft and the crankshaft, showing how the camshaft rotates at half the speed of the crankshaft, implying the need for signals from *both* the G40 Camshaft position sensor and the G4 Crankshaft position sensor:
Here is a schematic of the ECU wiring:
CLICK: AAN Motronic ECU device list and T55 connector pin-outs (with wire colours) and hyperlinks
Here is what they look like from the inside:
Photo courtesy of Jerry Scott Here is what they look like off the car:
Here is some info about checking the clearances and spacing, etc:
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 very 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. Here is the PN listing diagram for 7A, NM, 3B and ABY (all the same G4s and G28s) (Items 33 and 34):
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:
And here are the wiring colours under the black G4 and gray G28 harness connectors:
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 G4 and G28 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):
Here a post that describes the removal, cleaning and electrical connection maintenance for the G4 and G28 Click here 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 G4 crank position sensor is located when the sensor is removed, as shown in the lower right corner of the photo below, or through the timing hole in the bell housing):
Unlabeled photo courtesy of speedydtp247 Recently, a forum member had an engine non-start situation that developed very suddenly. He confirmed that he had power at the white firewall coil connections and that he had fuel pressure while cranking (therefore the G28 engine speed sensor was working) but there was no spark. Eventually, suggestions were made to check for the G4 Crank Position Pin. He looked and found that the pin was missing. More info is shown in this (LINK to DETAILED G4 PIN INFO):
The G28 Engine Speed Sensor on the edge of transmission, above the flywheel ring gear, sends its signal to T55/49 on the ECU in a Grey wire. The ECU uses and processes this signal appropriately to control fuel, timig and boost. It also sends a signal to the G5 tachometer in the instrument cluster. This G5 signal leaves the ECU on pin T55/40 (trace [79]) as a violet/black wire, goes through a T6a connector (as Pin T6a/1) and emerges as a grey wire (trace [79]). This grey wire enters the instrument cluster on trace [174] and instrument cluster pin T26a/12. Testing the G28 sender: Set DMM to resistance (2K ohms). Connect the leads to pins 1 and 2 (1 is on the left with the connector bump on top, 3 is on the right). Must be approximately 1000 ohms. If no, replace RPM sender. If Yes, connect the DMM between pins 1 and 3 and then 2 and 3, both must be infinite ohms (no continuity). Again, failed G28 = no start because the G28 signal is initially used by the ECU to trigger the fuel pump relay. If you have tachometer problems on your UrS, make sure that you have continuity between the G28 and the T55/49 ECU pin and between the T555/40 ECU pin, the T6a/1 connection and the T26a/12. If those show up good, you might have problem on the instrument cluster printed circuit board (cold joint/cracked trace that is often fixable). BTW, the [578] destination on the [80] trace is the Automatic Climate Control head, i.e. the reason you can read the RPM on Channel 28 of the Climate Control Diagnostics
Thanks to those who posted the diagrams and photos used in this post. Thread:
- Crank Position (G4) and Engine Speed Sensor (G28) Info - Function , Location and PNs
  - UrS4boy 2009-11-28 04:12:57 (191520 views) |
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