No rpm

[trj]

Hello guys!

After not getting any respons from this forum, I finally got it from another.
I post it here before, and it's conserning the rpm guage not working, as well as the starter sticks for ca: 10 sec after start ups when warm engine.

Now I found another guy with the same issue on another forum. The crankshaft sensor is changed.
The only thing we found out so far, is that there is no rpm signal detected in the diagnostic instrument. It only happens with warm engine, and if you restart it's gone, but sometimes you have do it several times.

Where from is the engine speed picked up from. It must be som kind of bad connection, or electric component that are effected by temp.

Help me out here guys, I'm fed up with this problem now.

/Robban

[Jim Haseltine]

Engine speed (RPM) sensor G28 is mounted in the bottom of the gearbox bellhousing close to the bellhousing/oil intake module joint. It's an electro-magnetic sensor which 'reads' the starter ring gear teeth on the drive plate. If your starter sticks on, some of the teeth on the ring gear may be damaged which could cause the sensor some problems. One of my previous cars had similar RPM system - sometimes it would start and run perfectly, other times it would throw the CEL and misfire. Could get it to run ok again if I stopped and restarted the engine. Unfortunately I only discovered that the cause was a badly chipped tooth on the flywheel ring gear when rebuilding the engine after the misfires melted a piston.

[trj]

Thank's Jim!

But this only happens when the engine is warm. Can it still be the flywheel then?

[Jim Haseltine]

Possibly but I doubt it, more likely it's the sensor playing up when it's warm. I'd not be surprised if you found a coating of metal dust on the tip of the sensor as such things love to attract metallic debris. I'd clean it up and see if it made any difference.

[trj]

Maybe I didn`t mention, but I changed the sensor a year ago with same result. No dust on the sensor.
Is this sensor the only input for engine speed?

[IanH755]

I'd be looking at the wiring by now if changing the sensor hasn't fixed it. Sadly, physically checking the wiring is a massive pain to do and is quite labour intensive as you have to open up various looms between the ECU and sensor to see if there's any damage. With it being mostly effected by heat its probably a couple of chafed wires which occasionally short-out when hot or similar, as a guess.

[dandan760]

Engine speed (RPM) sensor G28 is mounted in the bottom of the gearbox bellhousing close to the bellhousing/oil intake module joint. It's an electro-magnetic sensor which 'reads' the starter ring gear teeth on the drive plate. If your starter sticks on, some of the teeth on the ring gear may be damaged which could cause the sensor some problems. One of my previous cars had similar RPM system - sometimes it would start and run perfectly, other times it would throw the CEL and misfire. Could get it to run ok again if I stopped and restarted the engine. Unfortunately I only discovered that the cause was a badly chipped tooth on the flywheel ring gear when rebuilding the engine after the misfires melted a piston.

Hi mate, So I’m the other guy with this issue but it only happens once in a blue moon, is this G28 speed sensor separate to the crank sensor by any chance? I’d assume there are 2 separate sensors being as the engine still starts with no rpm shown?

[Jim Haseltine]

I don't think there's a dedicated cranks sensor - at least I've never found one. However, actually, there are 5 sensors. The engine speed sensor (G28) which either reads the ring teeth or a marker on the drive plate (I thought it was the ring teeth but having read a bit more I may be wrong on that) then there are 4 other sensors which individually read the positions of the camshafts. I think that the ecus combine those signals (ecu1 for cylinders 1-5, ecu2 for 6-10) to work out that the engine is spinning and trigger the ignition.
The more I think about it, the more I'm of the opinion that if the engine starts and runs but the tach isn't showing anything then the problem is between the ecus and the instruments.

[trj]

Thank`s for input guys!
But Jim, dont forget that the starter always stays on when this happens. And this is not a mechanical problem in the starter, couse the relay to the starter is on.
So it feels like the ECU don`t get the feedback signal that the engine is running, and there for don`t shut off the starter relay. The starter always shuts off after
a certain amount of time. I hav not measured it, it`s just an appreciation. But it`s a security limit to protect the starter, for shure. So, with this said, do you still
think it`s a comunication problem between the ECU and instrument, Jim?

[Jim Haseltine]

What I've managed to trace from the circuit schematics so far:
Starter relay 2 is energised by starter relay 1 being switched by a signal from pin 74 of the 94-pin connector on ecu1.
The signal from the RPM sensor goes to a connection point in the engine compartment loom where it is split to both ecus, arriving at pin 36 of the 60-pin connectors on each of the ecus.
As yet I've no idea how the rev counter gets a signal - I've yet to find it mentioned in the schematics but then that's no surprise because as well as diagrams which cover items of equipment specific to the RS6 (like two ecus, 10 injectors and 10 coils with ecu1 controlling 1-5 and ecu2 operating 6-10) there are several hundred (or possibly thousands) of other individual diagrams covering every A6 model in which you have to locate pieces of general equipment. At the moment I'm chasing something called the 'control unit in dash panel insert' which appears all over the place.

[trj]

OK, thank`s a lot so far Jim!

The first thing I think is to find the connection point. If there is oxide on the connections, the resisistance increases when the engine bay heats up, and CAN change the
values from the hall sensor into the ECU. I will also try to look in my files after diagrams.

Post here Jim, or better my messenger: Robert Johansen. I have contact with another guy on FB with the same problem.