Rear parking sensor sounding false warnings when the car is running

I have tried multiple things to sort out the issue but to no avail.

BHPian greenhorn recently shared this with other enthusiasts.

Hi all,

I’m facing this rather weird issue. My recent aftermarket parking sensor has started to act up randomly – mostly when the engine is running and at idle RPM.

I’ll let my FNG sort it out, but figured I’ll pick your brains too.

Steps tried so far

  • Replaced with a brand new parking unit, still happening.
  • Replaced the reverse switch in the gearbox which provides the signal for the reverse light that’s powering the sensor.
  • One thing we were able to spot was that the issue goes away when I rev the engine.
  • I’m using uprated T15 LED bulbs in the reverse light, but this sensor was working for several months without issues with the same bulb.
  • My theory is that the voltage regulator on the alternator is falling. Going by past history, my alternator is due for an overhaul anyway (lasts 3-4 years between overhauls) plus at low rpm tachometer keeps dying.

You can see the voltages in the video. They look low to me. But the weird thing is that the sensor works fine at 11.6v with the engine off, but goes kaput at 12.5 with the engine on. I’m getting a lot of flickering at idle too with my bulbs.

Worst case I’ll just put in a direct line from the battery with a relay triggered by the reverse light signal, but what’s wrong really?

I’m planning to take a multimeter and check the voltages at the sensor.

Here’s what BHPian audioholic had to say about the matter:

Install the sensors in another car and check. I think might be something to do with ultrasonic interference from some component in the car or the ECM. Checking the voltage at the sensor won’t reveal anything since the sensor will be driven at its own voltage and not necessarily 12v. You would do better with an oscilloscope to check the reflections on the sensor if you really want to debug the sensor. Since you say the issue goes away when you rev the engine, I really think it’s the injectors which are driven by a frequency that causes interference on your reverse sensor.

Here’s what BHPian Jeroen had to say about the matter:

Check all the wires for chaffing. Strange problems such as these are often caused due to poor grounding/mass. So check and clean all grounds, connectors and so on.

Here’s what BHPian CannedShroud had to say about the matter:

When the car’s engine turns on, it switches from a 12v system to a 14v system. Not exactly… But if you check the voltage through the OBD port, you’ll find the same findings. Maybe the step-down driver on your cam has fried?

Check out BHPian comments for more insights and information.

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