|Vehicle details:||Audi TT Quattro|
|Symptom:||CAN Bus error,
Intermittent ESP light
|Author:||Mark Banks | Holmer Green Service Centre|
A 2001 Audi TT Quattro arrived in the workshop with a complaint from the customer of an intermittent ESP light. At first the fault was not obvious at all. We had heard that the ABS modules could be a common failure but I had to be sure before fitting. A quick scan of the fault codes revealed CAN Bus comms errors: not a lot of assistance, but as usual a place to start.
The fact that the fault was intermittent didn’t help either. If the fault was present, the ESP light would go out on initial start up but then come back on as soon as the car was driven down the road.
When the ESP warning light was on, the bus line signals looked like this:
See how the signals go from opposing each other to pointing the same way but still trying to maintain the voltage differential (almost).
After a good visual inspection of the wiring, following the twisted pair as much as possible, and the high-to-low bus resistance checks, I made sure the fault was present. Then, thinking that it was transmission-related as the car had to be driven to produce the fault, I scoped the bus lines with the ABS module diconnected. Unfortunately the scrambled signal was still present. I reconnected the ABS and took a logical step along the transmission to the Haldex module on the rear diff. (Remember, this is the four-wheel-drive Quattro version of the TT). Disconnecting the module and scoping the twisted pair finally displayed a good set of bus signals.
With ignition on and the Haldex diff controller unplugged, the bus line signals were fine.
I stripped down the old module to check the circuit board with a magnifying glass and could not see any obvious water ingress, dry joints or meltdown problems. Ah, don’t you just love electronics!
A new Haldex controller (about £600/$950/€880) cured the problem!
March 26 2014
Haldex diff controller can be rapid. Just need to replace Signal Line Choke (B82790 T513 0215) with a wire. It’s hard to do but it can be done.
March 25 2014
Problem is solved by replacing joint coils (B82790 T513N 0215) by normal wire. After that communication back and i measurement block in VCDS nr 125 has sill 1 for ECU i ABS.
April 16 2011
Perfect illustration of how to fault trace in a logical order. It has helped a great deal. Well done and keep up the good work
October 23 2009
Bretto regarding no start….....If the bus is pulled to ground on either High speed terminal 6/14 or low speed bus terminal 1 it will not start….check resistance to earth on terminal 1 with batt disconected. if you have zero resistance its gone to ground on one of the modules. As I said before locate each module and disconnect. Watch the multimeter as you go through each module if the reading changes bingo fault found. Bus is not smoke and mirrors. Remember you need three things for a module to communicate 1=Power 2= Ground 3= Communication.
If its a short to voltage on the module you will see battery voltage on 6/14/ or 1 Remember let the bus go to sleep before testing at DLC depending on the brand it can take up to 3/4 mins before it says night night Good luck
October 02 2009
Charlie - great reply, just had vehicle towed in with CAN fault codes/no start will try your reply. Keep up the good work all
September 23 2009
Good example, now I understand the importantance of an oscillscope.
September 21 2009
wonderful work…it’s always good when we have techs that share this kind of info.
September 19 2009
Not surprised. Simple diagnosis. Terminal 6/14 at DLC should be 60 ohms then scope each terminal to view can high/low if theres noise disconnect each module till it goes…well done
September 18 2009
Excellent breakdown of route taken & superb pictures shame it isnt all this easy !!! lol