Well, I've noticed a *lot* of hits coming from google lately in the access log for
this site. Quite a few folks seem to be looking for info on Volkswagen camshaft position
sensor problems. It's no surprise, really. Lots of people have problems relating to
this sensor, and the online forums are full of posts by people looking for answers.
My own experience with this sensor is on ABA engines, but as far as I know all of the
VW 2.0 liter 8 valve Motronic engines use this sensor.
So, where is the camshaft position sensor and what does it do? The sensor is physically
located within the body of the ignition distributor. It's electrical connector is on
the outside of the distributor. The sensor feeds a signal back to the ECU (onboard computer).
The name "camshaft position sensor" is kind of misleading. The computer uses the
signal to determine the position of the camshaft; however, it does so on the assumption that
the camshaft/intermediate shaft/crankshaft timing is 100% correct. To elaborate, the ignition
distributor is directly driven by the intermediate shaft. The intermediate shaft is driven
by the timing belt, which also drives the camshaft and the crankshaft. This means that
the computer can calculate the position of the camshaft by knowing the position of the
ignition distributor. The camshaft position sensor is actually a hall sensor (hall sender)
that sends a signal once per revolution of the distributor. One revolution of the distributor
equals one revolution of the intermediate shaft, which equals one revolution of the camshaft.
So that's why it's called the camshaft position sensor when it actually lives in the
distributor.
So, why do so many people have problems with this critter? So many people have problems
because they have timing belts replaced by people who don't appreciate how much the whole
system relies on the timing between the camshaft, intermediate shaft, and
crankshaft being absolutely correct. The ECU has enough input from other sensors on the
vehicle to tell if the intermediate shaft is off by as little as one tooth. The DTC (Diagnostic
Trouble Code, or Fault Code) that usually plagues VW owners is "Implausible Signal". That
means that the ECU has detected that the signal from the camshaft position sensor cannot
possibly be correct, based on other feedback from the engine. If your timing is off by a tooth,
then this is the message you will get.
So, is that it? My timing is just off? Well, not necessarily. If you have the "Implausible
Signal" code stored, then it's probably your timing. However, you may have a different message
stating that the signal is either open or shorted to ground. If you have such a message, then
timing is not the problem (at least, timing isn't the *only* problem). This is the message
you will see if the sensor is faulty (or disconnected); however, it could also indicate a wiring
problem. Don't replace the sensor until you have thoroughly checked the wiring between the
sensor and the ECU. Using a multimeter, check for open circuits, and also check for continuity
between the signal wire and ground (there shouldn't be any!).
I think it's my timing! What should I do? If you had the timing belt installed by someone else, then
hopefully it was a VW specialist who will readily admit their mistake and correct it. If you took
it to an ordinary shop. then you'll likely have a hard time with that... and they'll probably even
try to charge you so they can replace the sensor that is almost certainly still fine. If you're the
one who did the timing, then re-do it until you get it right. I rarely get it right on the first
try. so don't feel too bad. You must follow the procedure in the Bentley manual exactly as they
say. And make sure you are following the procedure for the correct engine. On many of the newer
VW engines, such as the ABA engines, the marks on the v-belt pulleys are no longer used for
timing. For these engines, the mark on the camshaft sprocket gets aligned with a mark on the
inner portion of the plastic timing belt cover, the crankshaft gets aligned so that the #1 cylinder
is at TDC (top-dead-center) (which can be done by lining up the TDC mark on the flywheel with
the "pointer" mark on the inside of the timing hole in the transmission bellhousing), and the
intermediate shaft must be positioned so that the ignition rotor points directly to the mark
on the distributor housing. Consult the Bentley manual for details, and pictures. *DO NOT*
attempt any of this unless you know what you are doing. Seriously, you can damage the engine
internals (valves/pistons) otherwise. I take no responsibility for any damage you cause. Many,
many people have this problem after having a timing belt replaced. If you are going to have the
belt replaced, I highly recommend you have it done by a VW/Audi specialist. It will be more
expensive, but they'll be far more likely to get it right.
Yikes, that's a lot of info! Yes, it is - but that's not all of it. I recently bought an A3
Golf with this problem, only it was caused by something a little different. The previous owner
had swapped around all of the plug wires so that the firing order was still correct; however, the
plug wire for the #1 cylinder was no longer plugged into it's correct location. Basically,
the wire positioning was "rotated" a little bit. The ECU *requires* that the plug wires be installed
in their correct locations (#1 wire gets plugged in closest to the timing mark on the distributor
body). Otherwise, the ECU will think that one cylinder is firing when another is firing. The
car will still run, but you will never get rid of that code and the check engine light will
stay on. So, if you've recently replaced your plug wires (or had someone else do it), make
sure to check that they are installed correctly.
I apologize for this long and tedious post, but it's my hope that somebody will find it useful.
I will try to periodically do a post like this, where I address issues that I can see people
are searching for.
