Intermittent Starting '04 A2 TDI 90

Hi, my normally splendid 2004 TDI 90 A2 occasionally fails to start. When it doesn't, the engine and glowplug lights come on and stay on and diagnostics indicates a fault with the (G28?) engine speed sensor, but could be the G40 hall sender that sits above it . However, when it does start, it runs fine and once, in a fit of self-healing, it started without showing any warning lights.

The 80ah battery is healthy and gives 12.7V stationary. I've had limited experience of a failed Hall effect sensor before and I think they either work or not once so I'm not sure that the fault really lies with the engine speed sensor, especially as the local dealer has changed two of these ever, so they are pretty reliable.

Before this happened, I had a flashing glowplug so the dealership changed a no 1 cylinder glowplug and shortly after, my problems began buit this may be coincidental. Incidentally the very helpful Audi RAC man got the car going with a squirt of 'easy start' up the air intake and I've used this method several times since.

Any pointers would be very welcome.

Hi David113, welcome to the A2oc.
Sorry I don't know exactly what's wrong with your car but the following info may help with the diagnosis.
As you know, G28 is the engine speed sensor. In addition to speed, this sensor detects the position of the crankshaft and the information is used to compute the exact point of injection and fuel quantity. I think this is the only sensor on the car which does not have a back-up or default setting so when it fails the engine won't run.

G40 is the hall sender. This identifies the exact camshaft position to determine which cylinder is on compression for starting the engine. If this sensor fails the signal from the G28 sensor is used as the back-up. It's likely the engine will need to turn over a few more times before the ECU works out which cylinder is ready to fire.

If it always fired up with a squirt of easy start it's a fair bet the G40 sensor is faulty.
However, you are getting a G28 related fault code with an engine which eventually starts, which is a bit of a contradiction - unless there is a bad connection or an intermittent fault on the G28 sensor

Cheers Spike

Spike,

Thanks very much for the quick repsonse - very helpful. First, I'll dismantle the block connectors and clean them up as they seem vulnerable to dirt etc. and see what happens, otherwise, off to the garage.

Cheers

David