Bent Reluctor Wheel: symptoms and cure

Several years ago, an engine came into the shop that had a VERY bent #4 rod. It looked like it should have broken.

Anyway, the rest of the rebuild was fairly normal. We replaced the damaged rod and trashed the piston. Balanced the new set against the #3 piston and rod and slapped it back together.

We could not get the engine to time. It was way up or way down off the timing graph. When you raised the engine above idle, it would smooth out and seem to run ok. The I.Q. was all over the place at idle. Idle balance numbers were way off. For a couple of days, we were stumped. No part we changed made any difference.

With the broken parts of the engine strewn before me, I sat like Hamlet, with Yorick the dead piston in hand..

Act 5, Scene 1:

Alas, poor Piston! I knew it well Randy(as I spoke to my workmate):
an engine of infinite spirit, of most excellent quality:
it hath borne me down the road a thousand times; and now, how
abhorred in my imagination it is! my bile rims at
it. Here hung those piston rings that I have revved I know
not how oft. Where be your gibes now? your
gambols? your tune? your flashes of merriment,
that were wont to set the engine a-roar?

So, while examining the piston thus, I realized an oddity. The bottom of the piston was damaged at the skirt. Quickly,I assembled the piston and rod and clamped it onto the #4 journal of an available crankshaft. I'm sure you would have done the same...

The piston skirt with the rod so bent would not pass the reluctor ring without bending it. The reluctor wheel is the steel dish that is read by the engine speed sensor on an ALH or AHU engine. The reluctor wheel has 4 cutouts on the wheel set 90 degrees apart. The 1/4" gaps in the wheel are read by the Hall's sensor and the information sent to the ECU. Along with several other inputs, the ECU uses the speed sensor information in the idle compensation block. This can be read in the VCDS, engine module, block 13. This block will tell you how much an injector is adding or subrtacting fuel in relation to the other pistons.

At idle, if a piston is determined to be slow by the timing in the engine speed sensor, more fuel is added to speed it up, or conversely, fuel is removed to slow it down. That is the only compensation that the ECU has to vary rotation speed between each piston, with the goal to make idle rotation as smooth as possible. However, this is ONLY an idle compensation. Once the engine gets above 1500 rpm, each cylinder is fueled as determined by the injection pump. There is no fueling compensation above idle. It's complicated and gets worse...

If then, the reluctor wheel is bent more than about 1/8", one of the timing marks cannot be read by the engine speed sensor. The ECU gets a read that makes it appear it took a LONG time for the next mark to come up, so the ECU makes the #1 injector go to maximum fueling to speed that cylinder up. The whole model for idle compensation is then screwed up.

I dropped the oil pan and with a screwdriver, I straightened out the reluctor wheel. The car tuned and the idle compensation module worked correctly again. Problem solved.

This problem can happen if the crank is dropped. The conditions of the error read will change if another segment of the reluctor wheel is bent, but it will make an error read. The best thing to do is make sure on any assembly to check the reluctor wheel is spinning with little variation.

Once installed the easiest way to determine if there is a reluctor read error is with a oscilloscope. There should be 4 distinct spikes of equal height and distance. The other way to check is by removing the engine speed sensor and measuring the distance to the reluctor wheel. It should not vary in depth more than a 1/16". The plate should not swing outside of the window laterally. Gaps in the plate should be equal sized. You will need a mirror to see it.

Awesome Post.

Thanks for the heads up Frank.

And oh so poetic! A true dramatic read, thanks Frank!

Had a 2.0 FSI gasser with a bent reluctor wheel. Only around a left hand turn the vehicle would hit a rev limiter at 2300 rpm. Sounded like a neon with a two step on it lol. All sorts of weirdness can happen if that thing is ever so slightly tweaked.

There were two recent episodes of goofy idle balance reads that prompted this post.

One was only affecting the idle balance by showing the #1 and #2 piston +1.8 and -1.8. When the injectors were reversed, the readings did not follow the injector, but stayed with the cylinder. That should make it a dead giveaway what's up. The other hint was that the previous owner, a VW tech, replaced the turbo. That should be a warning bell, as that is one way to create the hydro lock necessary to bend the wheel.

The other car was more interesting. The engine speed sensor was removed and the reluctor wheel inspected using a mirror. A piece of the reluctor wheel was torn out when the #4 cylinder's rod bent. When the fault was finally discovered, the owner, and being a very good welder, he decided to restore a 1/2" piece of of metal to the wheel. That would save the aggravation of removing the tranny and dropping the crankshaft to replace the reluctor wheel.

Unfortunately, after welding in the missing piece, the car ran WORSE! As it turns out, the magnetic qualities of the metal use to replace the missing metal were not the same. The hall's sensor made a worse mess of things. After replacing the reluctor ring, the car returned to normal.

Here is a cell phone photo of an Ocilloscope trace showing one bad sensor slot on a TDI.

I have read (Bentley?) that the sensor wheel must be replaced, along with new bolts, if it is removed.
The holes in the plate and the bolts deform to hold the plate securely against the severe power pulses of the diesel engines. If you try to reuse them there is a little slop and the slop gets worse the longer it is run, resulting in inaccurate position sensing, and poor engine operation.
Let's hope this one can be bent back into shape.

Electrical issues can make these symptoms, too.

http://forums.tdiclub.com/showthread.php?t=461147

Franko6 said:

One was only affecting the idle balance by showing the #1 and #2 piston +1.8 and -1.8. When the injectors were reversed, the readings did not follow the injector, but stayed with the cylinder.

Click to expand...

1.8 is interesting because there is a guy here struggling with an issue where all four cylinders read +/- 1.8 in block 13.

Thanks for bringing this thread back.

Sort of resolved mine with a pull-up resistor. Link in post #8 is updated.