Audi A3 8V 2.0 TDI: Intermittent 5V, Limp Mode

Hello,

"My Audi A3 engine with code CRLB occasionally misfires under load and then frequently enters limp mode. Using an oscilloscope, I observed that the 5V supply, for example, to the crankshaft sensor, drops out for approximately 10ms at that moment. By externally powering various sensors with voltage, I have already ruled out the following sensors: crankshaft, camshaft, manifold absolute pressure (MAP), exhaust gas recirculation (AGR), and knock sensor."

Im Fehlerspeicher steht auch was von Referenzenspannung 2/B. Welche Sensoren liegen da noch drauf? Über den Schaltplan bekommt man das meiner Meinung nach nicht raus. Who can help?

Best regards,

Daniel

Hello,

"There's still a wiring diagram attached, but I'd have to guess now to figure out how everything is connected and powered."

Note: The 5V is coming from the ECU. Either the ECU is faulty (which is less likely), or you have a loose connection at the ECU connector (also check the pins for damage), or there is a damaged wiring harness. An interrupted 5V line is not critical, but be careful, as the insulation may be worn away, which could cause a short circuit to ground.

Adding even more power to it is suboptimal.

Best regards, Rainer.

Thanks, Rainer. However, the wiring diagram doesn't even specify what 5V, signal, or ground are, let alone which cables are connected to a 5V line inside the control unit. According to the diagram, all the sensors are individually powered by the control unit. How can I obtain this information?

Hello,

That's annoying. I need to check the connections and cable colors for a well-known sensor, like the mass airflow sensor (MAF).

https://community.dieselschrauber.org/en/viewtopic.php?t=3347

"It might be worth checking the exact type of mass airflow sensor (MAF) installed using a VAG parts number and verifying that the connector pinout is up to date..."

Best regards, Rainer.

"Unfortunately, the cable colors are always different. What 5V devices are actually available? I could also just test them all. Or I could short the 5V line to ground and see what doesn't work?"

Hi,

"I wouldn't necessarily call it a short circuit." However, it's easy to find because the 5V comes from the ECU, and there are pinout diagrams available.
The VCDS Autoscan shows which ECU is installed (at least the part number, which is also printed on the control unit), and that allows you to determine the pinout (which is documented in various chip tuning tools).

Sure, here is the translation of the text from German to English:

"LG" is an abbreviation for "Liebe Grüße," which means "Best regards" or "Best wishes" in English.

The part numbers are 04L 907 309 R or 04L 906 026. Unfortunately, I can't find anything about them online.

Hello Daniel,

Unfortunately, I don't have the pinout information for the 04L 907 309 R either. As far as I know, in the EA288 engine, in addition to the KW, NW, MAP, AGR, and DK sensors connected to the 5V reference, you typically also find the rail pressure sensor, the differential pressure sensor from the DPF, the position sensor for the turbocharger wastegate, and the throttle pedal (the two sensors in the pedal have separate power supplies). I can't be certain which ones are connected to reference 2/B, but I would definitely check the differential pressure sensor; it's often overlooked, and the wiring is located near the exhaust manifold, so it can sometimes get chafed.

Here's how I would proceed: Leave the oscilloscope connected to the 5V line, start the motor, and then connect the remaining sensors one by one (without short-circuiting them). When the voltage drop disappears, you've found the circuit in question. While doing this, gently wiggle the wiring harness, especially where it runs along the motor and transmission, and at the mounting clips.

If the car is driven mostly for short distances, I would still check the DPF and EGR, as these are common issues with the 8V 2.0 TDI. Here is a useful overview of maintenance and typical weaknesses of the 8V: Audi A3 8V Maintenance and Weaknesses

I'm curious to see what it turned out to be in the end.

Best regards,

"The 5V cable near the intercooler was chafed through by the differential pressure sensor and was intermittently shorting to ground."