FAQ
TL;DR: Typical sensor bias is 5 V with a signal of ±1 V (≈2 Vpp). “These 5 Volts provide a constant component for digital logic circuits.” [Elektroda, koszynek, post #18395018]
Why it matters: This FAQ helps Audi A6 C6 3.0 TDI BMK owners and techs quickly diagnose hard-start issues by validating sensor power, signal, wiring, and parts fitment.
Quick Facts
- Typical no‑load bias for the inductive path reported as ~5 V on this platform. [Elektroda, koszynek, post #18393930]
- Expected signal swing with bias: about 0.7–3.7 V around ~2.7 V mid-level. [Elektroda, koszynek, post #18395018]
- Crank and cam sensors shown as the same type in referenced program data. [Elektroda, -kris-, post #18395130]
- Best practice: test inductive sensors by resistance at ECU plug and with scope/LED, not by DC supply check. [Elektroda, Pawel wawa, post #16676446]
- Final fix from OP: incorrect flywheel installed caused symptoms despite “correct” readings. [Elektroda, koszynek, post #21438811]
What’s the correct supply for the crank/camshaft sensor on the Audi A6 C6 3.0 TDI BMK?
Users observed a typical ~5 V bias on the inductive sensor circuit without load. This bias allows clean digital interpretation of the AC signal generated by the sensor. With the sensor connected, the signal oscillates roughly between 0.7 V and 3.7 V around a ~2.7 V midpoint. That behavior aligns with CMOS logic thresholds. Use this as a functional reference, not an absolute spec sheet value. “5 V provides a constant component for digital logic circuits.” [Elektroda, koszynek, post #18395018]
Why is measuring DC power on an inductive (reluctance) sensor misleading?
A two‑pin reluctance sensor is an active element that generates voltage from a moving tone wheel and permanent magnet. It does not require external power to create the AC signal. DC checks alone can mislead diagnosis. Use resistance checks at the ECU and view waveform while cranking. “The induction sensor does not check the power supply voltage.” [Elektroda, Pawel wawa, post #16676446]
How should I test the crankshaft sensor correctly?
Use this three-step method: 1. Measure sensor resistance at the ECU connector to verify the harness. 2. Crank the engine and view the waveform with an oscilloscope or a test LED. 3. Compare cylinder sync and pattern consistency across several seconds of cranking. This avoids false conclusions from open-plug DC readings. [Elektroda, Pawel wawa, post #16676446]
I read 2.54 V on the plug—what does that mean?
A mid-level reading near 2.5–2.7 V suggests a biased input awaiting the AC signal. When the inductive sensor swings, the ECU sees roughly 0.7–3.7 V. That range crosses logic thresholds for reliable edge detection. Treat 2.54 V as a bias indicator, then validate the waveform while cranking. [Elektroda, koszynek, post #18395018]
Are both camshaft and crankshaft sensors the same type on this engine?
Referenced program data shown in the thread lists both sensors as the same type for the BMK engine. This aligns troubleshooting steps and expected test methods for each sensor. Always confirm by part number before replacing components. [Elektroda, -kris-, post #18395130]
Is a 12 V supply expected on the inductive sensor?
No. The thread’s final technical position shows a ~5 V bias used for signal conditioning and digital interpretation. The inductive element produces the AC component; the ECU bias sets a usable reference. Do not chase a missing 12 V on a two‑pin reluctance sensor. [Elektroda, koszynek, post #18395018]
Could stretched timing chains cause my hard starting instead of the sensor?
Yes. Early in the discussion, timing chain stretch was raised as a plausible cause when signals and power seem off. If patterns look irregular or phase correlation fails, inspect timing before replacing sensors. Chain and guide wear can desync cam/crank relationships. [Elektroda, paszczakq, post #16642145]
Where is the crankshaft sensor located on the BMK in this thread?
The thread does not provide a confirmed physical location. The requester asked where to find it, but no exact placement followed. Consult the factory manual or official parts catalog for location and access steps on your VIN. [Elektroda, adrian230492, post #18393525]
What waveform or amplitude should I see while cranking?
Expect about ±1 V around a DC bias when referenced to ECU ground, yielding roughly 2 V peak‑to‑peak. With the designed bias, the observable swing is around 0.7–3.7 V. Stable, periodic teeth and known gaps should be visible and repeat each revolution. [Elektroda, koszynek, post #18395018]
What current does the bias path use, and why does it matter?
The reported bias path uses approximately 20–40 mA. That current sets the mid-level and helps the ECU register clean transitions from the sensor’s AC output. A short or open will shift this current and disturb the bias, producing starting issues. [Elektroda, koszynek, post #18395018]
How do I check the harness without back-probing at the sensor?
Measure the sensor circuit resistance directly at the ECU connector. This validates both the sensor coil and the entire run of wiring in one step. It also reveals high resistance, opens, or shorts without disturbing the sensor plug. Then scope the signal while cranking. [Elektroda, Pawel wawa, post #16676446]
My readings look fine. What obscure fault should I consider?
Consider a mismatched or incorrect flywheel. The original poster later confirmed that all parameters appeared correct, yet a wrong flywheel caused the no‑start/hard‑start condition. Tooth count or indexing errors can defeat sync despite healthy sensors. [Elektroda, koszynek, post #21438811]
What is an inductive (reluctance) sensor in simple terms?
It is a two‑pin magnetic pickup with a coil and permanent magnet. A passing toothed wheel changes the magnetic field, inducing an AC voltage. It creates its own signal without needing external power to generate the alternating component. [Elektroda, lukashb, post #18394646]
Why can measuring on an unplugged connector mislead diagnosis?
An unplugged connector lets you see only the ECU’s bias or floating voltage, not the true loaded condition. Plugged-in measurements or back‑probing with the sensor connected reflect actual behavior. Always validate under load while cranking. [Elektroda, Pawel wawa, post #16676446]
What ultimately solved the original hard-start complaint here?
The car had an incorrect flywheel installed. Once identified, this explained the persistent starting problems despite sensors and readings appearing normal. Correcting the flywheel resolved the issue. [Elektroda, koszynek, post #21438811]
