2. Cleaning and de-corroding pins of connector on VSS.
3. Replacing connector and splicing into existing wiring.
4. Replacing VSS.
Result: After each "fix", the speedo works again for a while (from a few minutes to a week)then it's back to the needle stop again. This seems to be related to a disturbance of the wiring or a ground connection somewhere.
Today I belatedly checked the signal coming from the VSS by logging the vehicle speed on VAG-COM. Zero reading, suggesting the problem lies not with the instrument cluster but somewhere between the VSS and where the signal is picked up by the speedo.
However, I am intellectually challenged by the Audi wiring diagrams in the Bentley manual, and can't make any sense of where to start checking for signal continuity.
Art LeBrun wrote in a post in Sept 2000:
"G22 sends it's signal to the instrument cluster tp pin T32-28. The instrument cluster outputs the signal to the A27 bus from T32-3. The ECM receives the VSS signal from the A27 BUS at pin T80-20.
"I would check the voltage and ground to G22. Should be battery voltage and a solid ground. The output signal should vary (on/off) between around 0 Volts in the OFF to around 4 to 5 Volts in the ON (square wave on a scope)."
That was in relation to a VW Jetta, but presumably there are some commonalities.
Does anyone have any advice where these components are actually located? I'm a bit out of my depth here, and it would obviously save time and money if I was able to take the car to an auto electrician and point an index finger in a knowledgeable way at the various likely problem areas.
Grateful if anyone is able to provide any assistance as this has been going on for weeks. Driving through speed camera zones without a speedo is a stressful business...
Perhaps you can find it locally. This is the ONLY stuff that I have found that actually works. I also use it when I assemble computers and repair electronic gear.
If that does not fix the problem, I would go to the next step and look for a break in the wiring harness someplace near the VSS, particularly where the terminal ends are crimped onto the wires. This is VERY tedious work; so I tend to start with the easy stuff first. Good luck!
The Audi A4 Manual A401 is for North American-spec cars, but the wiring in that area might be close enough to apply to what you have. You might also check out a 3-day subscription on the web.
Since my last post, I tried a whole lot more fixes including duplicating both the wires coming out of the VSS connector - in the case of the signal wire, taking it all the way to the connector on the back of the instrument cluster.
By this time, nothing seemed to make much difference and the speedo had a mind of its own. Some days it would work, other days it told me I was travelling at 110 km per hour when the local constabulary had me clocked at 140. I'm going to be spending a bit of time in the Local Court explaining myself, I'm afraid (however, the dealer was very cooperative and gave me a letter outlining all the measures he (and I) had taken to resolve the problem. This might work...)
Then finally - a breakthrough!! I pulled out the replacement speedometer sender and found a little concave groove halfway down the shaft which wasn't there when I first fitted it. It exactly matched a groove on the original sender, signifying there was something inside the diff contacting the plastic shaft of the VSS. The only thing that could be was a loose drive ring, which presumably would sometimes provide a correct signal, sometimes show an underreading signal (it was unlikely ever to revolve at a speed greater than the drive shaft) and, when it slipped a little further out of range, fail to deliver a signal at all.
With this new knowledge, I authorised the dealer to pull it apart (the car is of course well out of warranty). Up to that point, we were all reluctant to do this as there was no error code other than "implausible signal", meaning the problem could have lain elsewhere. And it's not a cheap repair...
Of course, the dealer found the ring had slipped on the shaft. Luckily, it had not disintegrated and there were no mashed gear sets to replace.
The speedo is now operating perfectly. Let me tell you how traumatic it is driving with an instrument you can't rely on, in a country where the authorities are paranoid about speed. They have all Australians brainwashed into believing that every accident is solely the result of speeding, to which they have - ahem - "responded" by trying to buy and install the entire world's supply of speed cameras...
Best wishes to you and the forum members for a happy and safe New Year.