limp mode but no error codes

[20Vcqdriver]

When it goes into limp mode, if you switch the ignition off and back on does it regain power?

Yes! Did it tonight, tripped out on dual carriageway as I passed 4500rpm in 3rd, Switched off and restarted on the move and back to having boost again..

[4WheelDrifter]

Check your knock sensors and wiring.

[20Vcqdriver]

These were new last year. Heard they must not be overtightened is this correct. How do you test?

[denniselbows]

the knock sensor bolts should be torqued to 20nm i would say only way to check them is to slacken them off and retorque them.

[S2central.net]

20Nm is correct... Only electrical test is a resistance check. You *should* be able to drive with them disconnected. That will raise fault codes and retards timing by a good amount but I don't think it goes into limp mode with disconnected knock sensors.

If you are getting false levels of knock detection - it would explain things as first of all the ECU starts to retard timing on detection of knock - it does this a few times to try and cure it (usually bad fuel or extremely hot air) but if that fails then it turns off active boost control ==> limp mode.

I hope its this - I'd be pouring petrol on the back seats soon if its not found soon :-)

[20Vcqdriver]

One thing has occured to me. When I bought the new knock sensors 18months ago during the upgrade, I had to buy two of the green plugged version as this was the only one available without having to go through a spanish inquisition on car reg number, chassis number etc. Difficult when car used to be a KV 10V! Question is which position should the green plug in the loom go to and is it critical, would it cause this issue if sensors were the wrong way round, including affecting how quickly the power comes in (seems to decide what to do for a second before letting me have full power after going WOT as boost rises and then takes off as though it goes to full boost and then provides the correct fuel and timing to match 0.5 secs later....)

[S2central.net]

Ahhhhah.... They are critical - different sensitivity on them.

For 3B
BROWN one 077 905 377A (930mm long) - tracks to ECU #11 which is G61 for cyls 1-2-3
Green one is 054 905 377A (740mm long) - tracks to ECU #29 which is G66 for cyls 4 & 5

Checked this across the family album twice and over three other 3B tech resources here.

[20Vcqdriver]

Ahhhhah.... They are critical - different sensitivity on them.

For 3B
BROWN one 077 905 377A (930mm long) - tracks to ECU #11 which is G61 for cyls 1-2-3
Green one is 054 905 377A (740mm long) - tracks to ECU #29 which is G66 for cyls 4 & 5

Checked this across the family album twice and over three other 3B tech resources here.

Was told that other than the length of the leads there was no difference and could be used. Is this not the case?! Anyone else running with the 2 green plugged knock sensors?

[S2central.net]

It is more than cable length... Don't you think Audi would have made the same sensor with long enough cable for both locations if that was the case ?

One sensor picks up knock on three cyls whilst the other listens on the other two so they have different sensitivity AFAIK.

Them not being torqued correctly will not help though... All the books say this is important.

[20Vcqdriver]

It is more than cable length... Don't you think Audi would have made the same sensor with long enough cable for both locations if that was the case ?

One sensor picks up knock on three cyls whilst the other listens on the other two so they have different sensitivity AFAIK.

Them not being torqued correctly will not help though... All the books say this is important.

Perhaps I'm being sceptical but if Audi see an opportunity to charge more for a slightly longer cabled sensor with a different coloured plug then they'll jump at the chance....
Will retorque and check positions, after that will try and source a new knock sensor (brown one) though going on my last experience I'll need someones reg number and chassis code to get it....

[denniselbows]

i run 2 green knock sensors wish out any faults . Can't see this is your problem.

[S2central.net]

Perhaps I'm being sceptical but if Audi see an opportunity to charge more for a slightly longer cabled sensor with a different coloured plug then they'll jump at the chance....
Will retorque and check positions, after that will try and source a new knock sensor (brown one) though going on my last experience I'll need someones reg number and chassis code to get it....

NO chassis number required - just ask for the BROWN knock part number I posted above.

[20Vcqdriver]

Just looking at the drawing and there appears to be a resister accross T3x1/T3x2 and T3y1 /T3y2. What value should this be just in case it has become disconnected.
Any other tests I can do?

[S2central.net]

Sorry Dave - what drawing ?

[20Vcqdriver]

Sorted at last

At last I can enjoy the homefry to the full after checking the knock sensors, one was loose. Tightened to 20nm and off down the road, let the engine oil warm up and booted it. Back is the quick spool up and gone is limp mode. What a difference a knock sensor makes. Just clocked a 2.5 sec 50-70mph in 3rd which is plenty quick enough for me.....at the moment:mischeif: With any luck the 2.75" exhaust in July will get me a bit more over this 7A 2.25" system.
Thanks to everyone for the help and advice. One to remember for the future.