[urq] Finally re-assembled engine and car...won't start

Wolff wolff at turboquattro.com
Mon Nov 1 10:55:26 EST 2004

The TDC sensor is in the top of the bell hosing. It reads a pin on the
flywheel. It is a much more accurate way to measure the position of the
crank than the hall sensor. However, being a 4 stroke motor, the pin passes
the sensor twice for every revolution of the cam. The hall sensor is used to
determine the correct pass of the pin to use for ignition timing. I have
heard that this comparison only happens during the starting of the motor and
once running, only the flywheel pin is used. However, on a 3B I know from
experience that the ECU does check the hall sensor postion after the motor
is running as well and will cut boost if there is a significant difference
in relative position.
----- Original Message ----- 
From: "Thatcher Hubbard" <thatcher.hubbard at gmail.com>
To: <urq at audifans.com>
Sent: Monday, November 01, 2004 7:19 AM
Subject: Re: [urq] Finally re-assembled engine and car...won't start

> Okay, I'm glad to know that it's likely not that my coil has failed.
> I know about the Hall sensor window and stuff, but where is the TDC
> sensor?  I didn't even know these cars had one, I guess I thought
> that's what the Hall sensor was for.

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