PorscheFanatic202

I have been chasing an airbag issue for a long time now. I replaced the airbag control module thinking that the logic was messed up, but the other one I got replicated the same issue.

The problem is, I trip hex code 30 (Short Circuit in Warning Lamp). I am using 1.2W bulbs for all the indicator lamps in the dash. The way the issue comes is that the seatbelt light comes on, but not the airbag light or the central warning lamp (and I think i know why this happens in this order).

The central warning lamp is only triggered by certain lights. When these certain lights get a positive signal, that same positive signal travels over a diode to the central warning lamp. The airbag light will trip the central warning, but not the seatbelt light.

The seatbelt light and the airbag light are supposed to come on together (for some unknown reason due to the mystery circuit inside the "gong" relay: This has been extensively discussed on this forum). However, I get the seatbelt light before the airbag light.

Here is my hypothesis: The system starts in the reset state (no error codes). Then the airbag warning lamp wire (and all connections) shorts to a ground somewhere. This causes the airbag control module to detect a large current and throw a "short circuit" code. It tries to trigger the airbag light, but since the circuit is grounded the lamp will not turn on nor will the central warning lamp since it needs to see a "+" signal as well. However, the seatbelt light is on since the airbag light is trying to be on. After a while, the short circuit goes away and the warning lamp circuit isn't grounded anymore and the airbag lamp and central warning lamp come on.

I'm fairly certain this is the case and I have done the following. I have looked for a short circuit in the warning lamp wire. I went as far as pulling the entire flexible circuit board off the back of the instrument cluster and inspecting the pins on each end of the connectors on the wiring harness. Most of the wiring harness is hidden behind the HVAC unit and I cannot inspect it fully, but I cannot find any ground fault (short circuit) in this wire.

Has anybody had experience with fault 30 "short circuit in warning lamp"?

PorscheFanatic202

bump

968workaholic

Did you try no air bag bulb and see if you get the open circuit code?

PorscheFanatic202

I just realized this got a reply. What is the intent of this?

PaulD_944S2

Is the airbag light in the instrument panel the correct one and does not have a shorted filament?

PorscheFanatic202

I assume that since the light bulb works, it isn't shorted out. Can someone explain to me why the seatbelt and airbag light are illuminated simultaneously? I cannot find any connection within the wiring diagram, but the mystery "gong" relay box has the connections but no internal wiring diagram published.

PaulD_944S2

If you have access to the Porsche shop manuals, Vol 3 Section 68, Diagnosis, Faults 30 & 31, (and in the "Checking airbag system operation, Note) a thorough reading will tell you that Porsche designed it that way for the U.S. market; i.e. both the Airbag and Seatbelt lights will turn on when there is a fault in the airbag system.

My guess is that Porsche was concerned that an open lamp failure in the Airbag system indicator would be a personal safety issue so having both indicator lights come on offered a form of cheap redundancy.

PorscheFanatic202

I glossed over that and missed it. I'll read that section again. Right now the temporary fix is putting tape over the central warning lamp so I can see the road at night without squinting.

PaulD_944S2

Haha, tape! Standard fix for "Check Engine" light!

I wish I could help you more, but I'm afraid that long distance troubleshooting is very difficult at best. I think you understand the system well, but locating the source of the problem is always challenging. You just need to think about the design and take it a step at a time.

BTW, have you tried to reset the airbag system? Since you have been able to read the set code, you may not have a true fault, it just may be a stored code. There is a procedure on Clark's Garage that I followed a while back, but also discovered that in playing around with the system I was able to reset it by clearing the code(s).

PorscheFanatic202

Yes, I have reset it many times. Back to finding clues and research. Thanks for your responses!

PorscheFanatic202

I will be digging today. I am going to simply connect the warning lamp wire directly to a separate lamp and reset the system to see if it is caused by the instrument cluster. If this fixes it, i'll assume that the cluster is shorted somewhere and i'll just keep the lamp disconnected. aka remove the bulb.

PorscheFanatic202

Gathered some data yesterday. Made my own data logger using an arduino. I measured the voltage across the airbag bulb so I can see when it is on. I suspected that since fault 30 (short in airbag lamp) keeps tripping, that I would see a spike associated with the airbag light coming on. I can definitely see some voltage spikes, but with no associated warning light. Unsurprisingly, I wasn't able to get the warning light to come on at all. Here are the results after two runs. Anybody have some insight? I'm not opposed to pulling the light bulb, but if I keep getting an actual fault, the central warning and seatbelt light will illuminate as well even if I pull the airbag bulb. I don't want to pull all three.

PaulD_944S2

Wow, you are certainly going to a lot of effort to fix this, great!
Some more ideas, you've already covered quite a few:

1. With the ACU disconnected, see if you can light the bulb by applying 12 VDC to the circuit. Put a 5-10 ohm resistor in series with the power supply, or use a current limited power supply to about 1/2 amp, so you don't cause any wiring damage.

2. Try ohming out the warning light circuit path from the ACU (airbag control unit) to the instrument panel for shorts. You may need to pull the IP and remove the bulb so that it doesn't "short"out the path.

3. With the IP out, trace out the air bag lamp circuit to see if there are any shorts.

4. Check the light bulb and and socket base. Maybe swap it with another IP bulb or two.

I don't see any mention of whether the air bag IP indicator comes on when you first turn the key to start the engine. Does it?

Worst case, see if anyone around you has an air bag car you could swap the IP with. If you want to ship it to me, I would be happy to test it in a couple of 944s that have airbags and let you know what it does.