Speed Sensor Values

Hello colleagues,
My TDI (engine code ARL) is having problems again. The speedometer has stopped working, but the rest of the instrument cluster is still functioning. Surprisingly, there's no entry in the error memory, which is unusual since almost everything else is usually recorded there. The self-test of the instrument cluster using VAG COM works, and so does the speed test. I've already replaced the speed sensor, but it still doesn't work. I measured the connector with the ignition on, and one pin seems to be grounded, one pin provides about 2.5 volts, and the last one provides 9.5 volts. Does anyone have any ideas? The 9.5 volts seems a bit low to me; I would expect 12.6 volts (battery voltage). Does the sensor work with such a low voltage?
Best, Olli.

hi,

It should actually be exactly 10.0 volts. The speedometer and tachometer work with a defined and constant voltage. => See "voltage stabilizer" for more information.
The on-board electrical system (including the positive terminal and battery) provides voltage values that vary depending on the load, ranging from 11 to 15 volts. This is too large a range for an accurate display.

For example, a speedometer range of 0-240 (units) can be translated to a voltage range of 0-10 volts.
If we assume a speed of 200 km/h, that would be 8.3333V. However, if the range were 0-15 volts, that would correspond to 12.5V for 200 km/h. Similarly, for a range of 0-11 volts, it would be 9.1666V.

"Imagine your speedometer jumping erratically whenever you turn on the air conditioning or heated seats, because the defined operating value is changing."



I can't tell you specifically what's wrong with yours, but try installing a different combination (immobilizer needs to be adjusted!).

Best,
krissi

Hello,
"Then, the measured voltage of approximately 9.6 V is within the tolerance range. If the instrument cluster were defective, wouldn't the diagnostic test using VAG COM also fail?"

Hello,

I can't tell you if it's within the tolerance because I don't know how accurately you measured it.
In any case, the voltage stabilizer should have a stability of less than +/- 1% (I believe it was even only 0.5%).

It's just a matter of whether it's 10 volts or only 9.6V; the important thing is that it's always the same, regardless of how much load is currently being demanded. (Maybe do a test with the motor running and various consumers connected => I used a thin needle inserted through the cable to tap into the voltage of the + and - cables ==> the connection must remain in place).

It's possible that the functional test was flawless, but I've often experienced situations where it started working again after a change => I can't explain why.

You might have a cold solder joint in the combination unit or a bad connection in the plugs and cables between the sensor and the combination unit.

You can also test if your combination works by installing it somewhere else and trying it out.

I personally never completely rely on the readings from VAG-Com; my own experiences with my car and other vehicles have shown me something different.

However, VAG-Com is an excellent tool for narrowing down issues and analyzing errors, and that's something that can't be disputed.

Krissi.