G28 sensor issues.

[UrS4boy]

The ecu needs G28 Engine Speed, G40 Cam Position Sensor and the G4 Crank Position sensor to start. Perhaps it's an earth issue as when strange things happen, that's often the culprit. We'll check the ECU earth wire and engine earth strap.
BTW - This engine has not been started since was rebuilt, so there is another unknown to factor in.

Oh how I know that the AAN (And 3B, ABY and ADU) ECUs need good signals from all three of the G40, G4 and G28 group to start. I have explaining this for years:

G40 info: http://forums.quattroworld.com/s4s6/msgs/22580.phtml

G4/G28 info: http://forums.quattroworld.com/s4s6/msgs/20986.phtml (Check for a missing G4 reference pin on the flywheel: http://forums.quattroworld.com/s4s6/msgs/51259.phtml

Grounds are absolutely essential to the ECU since always takes things to ground, rather than actually providing them directly with power. Case in point is the J17 fuel pump relay. The AAN ECU grounds were on the intake manifold in the C4 chassis.

J17 FP relay info: http://forums.quattroworld.com/s4s6/msgs/22583.phtml

[RichLV]

G28 sensor issues.

The most annoying thing in a 20vT, starting troubleshooting & misfires. I recently changed all 3 of those sensors, no starting issues from then.

Thing with the G4 and G28 is as the sensors get older, they emit weaker magnetic field and need to be closer to the timing pin & flywheel teeth. We sorted a similar issue by shortening the gap between the sensor and tooth.

You can swap the G4 & G28 to check whether it's a sensor fault (at flywheel & connector rack)


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[Stefen]

Here is a read out of the G28 and G4. Seem to be working ok, but this is the first time I've used a scope, so don't know if the signals look right. Should the G28 be a square wave?



G4 sample raw data (Volts)

-2.3330
-3.2194
-2.6553
-0.8018
1.6158
3.3081
3.6465
2.1557
-0.4956
-2.5747
-3.3000
-2.5505
-0.5601
1.9139
3.5498
3.6465
1.9381

G28 sample raw Data at spike (Volts)
0.9630
-1.0919
-1.2128
-1.3498
-1.4707
-1.5432
-1.5916
-1.5674
-1.4546
-1.2612
-0.9630
-0.6004
-0.2055
0.2055
0.5762
0.8824
1.1161
1.2773
1.3740
1.4304
1.4385
1.4062
1.3579
1.3095
1.2048
1.1242
1.0275
0.9227
0.8099
0.7212
0.6568
0.5681

Now, these voltages seem quite low, I wonder what voltages I should be seeing from the ECU and in this signal wire?
I also should point out that the scope cost £40 which is very much bottom of the range.

[UrS4boy]

G28 is basically just counting teeth on the flywheel, i.e. 135 teeth/minute = 1 revolution per minute. Doubt that it matters to the ECU (ECU will have been designed for the signal as you see it).

[twoqu]

Might be some useful info here:

http://www.s2forum.com/forum/showthread.php?t=30947&highlight=motronic+signals

[UrS4boy]

Might be some useful info here:
http://www.s2forum.com/forum/showthr...tronic+signals

I think the key part of that thread is this odf attachment.

http://www.s2forum.com/forum/attachment.php?attachmentid=28754&d=1287737550

[twoqu]

As RichLV has said there may be an issue with the distance betwen the sensor and the ring gear now you have changed the flywheel. Not an expert but I have seen an amplitude of 2.5V mentioned. Also you would expect a more constant waveform as the only variable would be the distance betwen the teeth and the sensor. Perhaps its not particularly concentric.
There is a distance between the sensor and the teeth mentioned somewhere. It is possible to machine the mounting block to move the sensor a bit closer if thats the issue.

[UrS4boy]

There is a distance between the sensor and the teeth mentioned somewhere. It is possible to machine the mounting block to move the sensor a bit closer if thats the issue.

"Somewhere" = for one example, my G4/G28 post, linked earlier in this thread:

G4/G28 info:

http://forums.quattroworld.com/s4s6/msgs/20986.phtml

Which contains this diagram from the AAN manual pdf.

[twoqu]

Looks like the dimension is between 0.5mm and 1.25mm between the tooth and bottom of the rpm sensor. Obviously thats easier to measure with the engine out but you might be able to work it out by measuring the components depth vs the depth to the tooth.

[twoqu]

VEMS users sometimes have issues I believe so that might be another source to mine!

[Stefen]

Firstly, thanks for the very useful info. It's just what I needed.

It's been a while since I worked at this engine, so I'm just getting up to speed now.

Comparing figures, my measurements seem to be showing the right waveform, but voltage is a little low. Yes, the amplitude of G28 does vary a little which suggests that it might not have the same clearance the whole way around.

We checked the clearance with feeler gauges (gearbox out) and it was around 0.75 IIRC so that is okay.

I remembered about pin 3 on the ECU (Fuel Pump Relay signal) being activated when signals are received from G28, but I'd forgotten that this pin was earthed at that stage. Having remembered now, we connected up a light to it and sure enough it lit up. Progress.

So given that is earthing, can be now be sure that G28 and G4 are okay?

The G40 (cam sensor) is the next port of call presumably as no spark is present?

[RichLV]

No spark? G40 is the first stop.

[twoqu]

Well if you have 2 chanel you can verify the G4 blip appears within the hall sensor window!
Essentially the ecu needs to see the engine spinning for the fuel (G28) and the G4 and G40 for sparks!
Again it could be its not seing the blip on the flywheel pin if it is also futher away or not in the correct position. What is the source of the flywheel out of interest?

[UrS4boy]

G4, G28 and G40 info

So given that is earthing, can be now be sure that G28 and G4 are okay? The G40 (cam sensor) is the next port of call presumably as no spark is present?

And I repeat:

G40 info: http://forums.quattroworld.com/s4s6/msgs/22580.phtml

G4/G28 info: http://forums.quattroworld.com/s4s6/msgs/20986.phtml (Check for a missing G4 reference pin on the flywheel: http://forums.quattroworld.com/s4s6/msgs/51259.phtml

[Stefen]

Okay, I'm sure of the next step now. Apologies, my head is fried as we have a newborn in the house.

The G40 plug:

Pin 1 - Will have to plug the scope in on Friday and check.
Pin 2 - Checked at 4.88V
Pin 3 - Earthed okay

Scope has 8 channels so will plug G4, G28 and G40 in together.