Yesterday my '99 A6 threw yet another engine light. This time it was P1338 (Camshaft Position Sensor (G40): Open or Short to Plus)

So out came my trusty multimeter to check the voltage in the wiring harness at the cam position sensor. The result confused me a bit. It's supposed to be 9 to 14.5 volts. What I was getting was -10.38 volts... (note the minus sign)

How does one end up with negative voltage?

 

Are you absolutely sure you didn't have the test probe cables plugged into the meter backwards? I've had that happen enough times over the years...
Other posiblity is the ground point you had the meter negative tied to really had a positive voltage on it - remember, the meter floats and just reads the difference in voltage between two points. If you had the - (black) probe on +13V and the +(red) at +2.5V you'll read -10.5V.
Jim

 

The test procedure said to check the voltage at pins 1 (positive) and 3 (ground). I had the red probe in pin 1 and the black probe in pin 3. And I'm 100% sure I had the probes in the right holes on the meter. I even checked with a second meter to be sure I wasn't imagining things.

I'll have to go back tomorrow with my own meter (which I trust more) and check it out again. The car's still at the shop now.

 

Sounds right, but it's pretty suspicious that you measured the right voltage but reverse polarity. Possibly got the pins backwards? There's nothing in the car that I know of (outside the stereo) that would produce a negative voltage with respect to true ground. Maybe try measuring each of the pins to the negative battery terminal or the engine block to be sure.
Jim

 

It's all a moot point now. I went back to check the sensor again, and the voltage tested fine with my own meter. So I went to start the car, and it backfired violently and jumped timing. Now the valves are busted and the heads need to come off.

God I hate this car. It's been nothing but problems.

 

Sorry to hear that...
Jim