Airbag error DTC 1739

[yerbito]

Hi Everyone,

I'm currently restoring a 2003 Cayenne.

One of the issues I had was that the airbag control module couldn't be reached. After fixing that, DTCs 1738 and 1739 came up. I unplugged and cleaned the crash sensor connectors, as they had water inside. The faults went away for about a month.

While waiting for the MOT (still no appointment), I kept starting the engine once or twice a week. But last week, the airbag light came back on. I connected PIWIS and again got DTC 1739 – open circuit on the front right crash sensor (the one under the right headlight).

Since it had worked fine for a while, I pulled the connector and cleaned it again – but no luck. I remembered a scrapyard that had a similar car with a minor roof dent on the driver’s side and intact airbags. I grabbed the crash sensor from that side and swapped it in – still nothing.

Then, when cleaning the connector again, I noticed the blue wire wasn’t as firm as the other one. I gave it a gentle pull and it snapped – completely corroded, which makes sense given how much water had come out of that connector.

So I went back to the scrapyard and grabbed the connector with a bit of wire, cut and soldered it in. I know soldering isn't the best method in airbag wiring, but I wasn’t sure how much of the harness would need replacing, and the solder job is solid. I tried both sensors again – still no success.

I checked continuity from both wires of the right crash sensor to the airbag module (pin 82) and to the ground point nearby – both have continuity.

Then I started measuring the resistance of the sensors (plugged into a spare connector). With the multimeter hooked up correctly (– to brown, + to the other wire), I got no reading at all. If I reversed polarity, I got 0.5kΩ. What surprised me was that I got the same reading at the airbag module plug (pin 82 to ground), with the sensor connected – same results, both ways.

I then measured the left crash sensor (which isn’t causing any issues – yet). It showed 2.1kΩ directly at the sensor with correct polarity, and 0.5kΩ reversed. At the airbag module connector, I got ~5kΩ. The other four crash sensors showed the same ~5kΩ at the module.

I mentioned it to a friend who’s an electronics tech. He said that although 5kΩ seems high, it’s probably correct since 5 sensors measure that and don’t throw any faults. We concluded that both the original sensor and the scrapyard one were likely bad, based on their different readings.

So I ordered another one off eBay from a scrapyard, and it arrived today. It measured the same as the other two at the sensor output, but once plugged in, the airbag module showed 2.5kΩ. Closer to 5kΩ, but the fault remains.

At this point, I’m a bit lost – and I need to get this sorted for the MOT.

Over the last few weeks I’ve also been considering keeping the car for some light off-road use, and in that case I’d probably want to remove all the crash sensors and replace them with resistors or dummy loads to avoid faults. I’d also disconnect the airbags to prevent any unexpected deployment on bumps or jumps.

So, I have a few questions:

• How can I really tell if a crash sensor is good?

• Is it possible to emulate these sensors with a resistor? If so, what value should I use?

• As far as I understand, the crash sensors are connected directly to the airbag module and to ground, right? So continuity should be enough?

Thanks in advance – even just for reading this far!

[yerbito]

Nobody can help me? 😞

[Loren]

You need a Porsche specific diagnostic too (like PIWIS or Durametric) to test these circuits.

If the whole car has been subject to water damage (like from flooding) then you may way more corrosion problems than you wish to tackle.