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I'm trying to understand what the VVT values coming from the ECU are about.

Correct me if I am wrong: As far as I can tell, there are only 2 halls sensors (one on each inlet cam). Exhaust timing is fixed. The Vagcom will give the position (in cam degrees) of the cams from these sensors. The only other timing sensor is the crank position sensor.

To increase torque, the dephaser pushes the inlet cams into a position about 22° out at certain rev ranges. I am pretty sure there is a just an on/off adjustment. Either the cams are at 22°(approx) or they are at 0°. Confirm or deny?

Now, the crux of my question is this: when I am looking at the measuring blocks for the timing, I've got group 90 which is giving me the amount of dephasing going on for both inlet cams in degrees (camshaft angle). When I rev the engine, after 1300rpm, the dephasers kick in (I can hear them) and my angles go from 0° to about 22°. So far so good. Thing is, when I log group 93, which is supposed to be the phase positions of the Hall senders, I get +3°, even when the dephasers are activated. This is within specs, incidentally.

How does this 3° phase position relate to the 0° of the VVT (presumably coming from the hall senders) and why does it stay at 3°even when the inlet cams are 22° out?

If there is only one sensor per cam, where is this other information coming from?

I don't have 100% answer however I would expect this to reflect the difference between what the sensors are reporting and what the ecu is expecting.

So I think that the ECU is using the crank position sensor and the cam sensors to determine that there is an "error" of 3 degrees.
The reason that it doesn't change is that it doesn't really matter what position the cam's are (as reported by the cam sensors) - they will always be 3 degrees off from what the ECU determines to be ideal based on the position of the crank.

Yes, I was thinking that there would be some theoretic fudging going on, but I would really like to know what and how much. If your dephasers are borked then you won't get the right offset, so I wonder what values are valid in this case. I'm tempted to unplug the dephasers to see what that does to the values.

Not so much as fudging... The ECU knows the exact crank angle and based on that knows what the intake valve advance should be.

The key is if the cam measuring block is "demanded" or "actual" readings. Unplugging the dephasers should give you a good idea. If the value still reads 22 then you know it's just an output variable indicating what the ecu is trying to do not whats actually happening. Try doing one only so you can see a difference.

If the values remain at 0 then it's reading direct from the sensors... now again this could be one of two things in that it could be fudged. For example, there are other sensors I've seen logged that just say "if it moves by X then it's fine"... So what might happen is that the hall sensor could read 15 degrees but if that's over the tolerance then the ecu might report it as 22 (<beep> but it can happen with some ecu's).

I'm reasonably sure the 3 degrees is simply the absolute error in the physical timing of the valves which is why it doesn't change based on what the valves are actually doing.

adsgreen wrote: So what might happen is that the hall sensor could read 15 degrees but if that's over the tolerance then the ecu might report it as 22 (<beep> but it can happen with some ecu's).

I don't believe this is the case, because this is the measuring block to check that the VVT is working correctly. It can be that the timing isn't varied enough (bad dephaser) and it doesn't reach the acceptable range (16-25°) so I do believe that the ECU is reporting a genuine value here. I.E. Cam timing wrt crank timing.

adsgreen wrote: I'm reasonably sure the 3 degrees is simply the absolute error in the physical timing of the valves which is why it doesn't change based on what the valves are actually doing.

Could well be. I would suggest that this is the fudged value then.

If I unplug the dephasers, I wonder if the ECU will even try to dephase. Might have to put a resister in there or something to fool the ECU that they are still plugged in.

adsgreen wrote: I'm reasonably sure the 3 degrees is simply the absolute error in the physical timing of the valves which is why it doesn't change based on what the valves are actually doing.

Could well be. I would suggest that this is the fudged value then.

If I unplug the dephasers, I wonder if the ECU will even try to dephase. Might have to put a resister in there or something to fool the ECU that they are still plugged in.[/quote]


Why fudged? The hall sensors are looking like they are reporting a delta change from closed (0 degree). So 22 degree indicated would mean it's 22 degree from it's "closed" base line.
I wouldn't think the cam control mech is that advanced that it can vary it that much.
The 3 degrees could be based on the position of the crank sensor relative to the hall sensor reading for it's base line. So if the crank sensor reads say 0 degrees and the hall sensor is reading 22 degrees but the timing of the hall sensor is slightly out of phase with the expected position of the crank then this would be the 3 degrees error. Not so much as fudged but derived from the other values and the timing.
Not 100% sure - just basing on similar systems

I see what you mean, and it makes sense. I think that in this case my dephasing values being close to zero are misleading - on most RS6 (or RS4 B5) the values are generally quite far from zero. I think that it's just in my case that the timing belt was replaced correctly. I'll try and find an example.

Have a look at this RS4 reading. Maybe you are right, maybe the the -10 and 0 are the absolute readings. Not sure where the -1 is coming from though.

Bump

When I get some time, I'll trace back the measuring block parameter to an internal ECU variable & see where that takes me. Keep you posted

Cool. That's what I'm interested in.

Did you get a chance to look?

Sorry mate, not had a chance yet. Will try and peek tonight for you.

sweegie wrote:Sorry mate, not had a chance yet. Will try and peek tonight for you.

Am I the only one that found that comment quite amusing in a geeky way?

You're just taking a poke.