Passat 3BG 2003, trunk won't unlock, KSG flooded

Hello fellow diesel mechanics,

"For the past 1.5 years, I've also been working on diesel engines occasionally, not just my Beetle. And my diesel is currently giving me some headaches. The vehicle is a VW Passat 3BG, 1.9 TDI, 130 hp, Highline (estate/wagon), manufactured in 2003, with almost all comfort features." It was purchased used, and the cruise control system was added later.

The following events occurred over the past 6 months:

In June 2022, after heavy rainfall, I discovered about 10 cm (approximately 4 inches) of water in the footwells on both the driver's side front and rear. The car was drained, and the drain passages at the front of the water tank were cleared.

In August 2022, during my road trip to Scandinavia, the rear left door closed and became stuck. No way to unlock the lock.

In December 2022, the rear left door will be completely disassembled, the lock will be broken out, and a replacement lock will be installed. This works perfectly.

December 2022 - 2 days after replacing the lock: the tailgate can only be opened unreliably. After some research, I discovered that the comfort control unit (KSG) 1C0 959 799B is located in the driver's footwell and is likely waterlogged. Until yesterday, I hadn't had time to fix it, but the trunk could still be opened by repeatedly pressing the open and close buttons or by firmly kicking the floor on the driver's side.

Yesterday, we removed the covers and other components and pulled the KSG (Comfort Control Unit) out from under the mat. Water was leaking out of the KSG, which apparently had been trapped inside. We opened the KSG and let it dry. This is what the plugs looked like...

Okay, so we cleaned the connectors on both sides, and then we observed the following:

PIN number 2 was completely rusted. PIN No. 6 was peculiar: an orange/brown cable was coming from the KSG (likely a control system or similar), which then entered a separate heat shrink tubing where it split into 5 cables of the same color, all of which were neatly separated from each other.

I then did some research and found the following information about the KSG online:

Application of a 1C0 comfort control unit for VAG, e.g., the 1C0 959 799B / 1C0959799B H01.

15-pin connector (the mating connector is, for example, 1J0962615).

2 - Interior monitoring.
3 - DWA Siren
4 -
5 - Head contact.
6 -
7 - R47: Antenna cable for central locking and anti-theft system - shielding.
8 -
9 - Interior monitoring.
10 - R47: Antenna cable for central locking and anti-theft system - antenna.
11 - DWA Horn.
12 - Terminal 30 - 12V
13 - Terminal 30 - 12V
14 - Right turn signal.
15 - Turn signal, left.


23-pin connector (the mating connector is, for example, 1J0962623).

1 - Door contact.
2 - Rear hatch opening.
3 - Rear hatch unlocking.
4 -
5 - Terminal 15
6 - CAN-LOW
7 - Rear hatch contact, level 2.
8 - Close the sunroof.
9 - CAN-HIGH
10 - Rear hatch contact, level 1.
11 - Diagnosis - K-Line
12 - Sunroof release.
13 - Lock cylinder contact.
14 - Instrument lighting.
15 -
16 - Mirror heating.
17 -
18 - Rear hatch unlocking.
19 -
20 - Interior light, terminal 30, -12V.
21 - Interior light, terminal 31, ground.
22 - Terminal 30 - 12V
23 - Terminal 31 - Ground

"In my opinion, the error was then obvious, and we simply soldered the cable for the tailgate opening (PIN 2) directly to the control unit because we didn't have the equipment to re-crimp those connectors. Well, that's the end of that; it probably didn't work. Then we thought: maybe it's missing the CAN LOW signal, so we soldered the 5 cables from the engine side to the single cable from the control unit." Still nothing.

Then we noticed that pin 3 was free, but there was a cable connected to pin 4 instead. So, we disconnected the cable from pin 4 and soldered it to pin 3. Still nothing.

So, it appears that all the problems we could perceive have been fixed, and yet the trunk still won't open. Can someone please help me before I lose my patience and set the car on fire?

Best regards,
Alex.

PS: Just as a note: my OBD2 device, when used with the CarScanner app, never displayed any error messages.

Hi.

You won't achieve much, if anything, with an OBD 2 scanner here.

The most helpful thing would be to post a complete VCDS scan here.

My approach would be to back up the original coding from the control unit while it's still possible, then source a suitable replacement part based on the part number, and finally obtain the connectors with the remaining wiring.

Since these Passat models have reached the end of their lifespan and are often used as parts cars or donor vehicles, replacement parts are available for very cheap.

You can either completely replace the connector, cutting each wire on the old connector in a healthy section and then soldering them, or you can individually replace only the affected sections and then remove them from the connector.

Okay, so let's put everything together, connect the control unit, and then, assuming there aren't any other hidden problems, it should all work.

It's not uncommon for the buttons and actuators in the tailgate to fail after over 20 years.

Hi Rüdi,

The KSG itself appears to be in good condition, at least for the moment. Unfortunately, I currently don't have the ability to perform a VCDS auto-scan. Spending almost 500€ on something like that is still a bit too expensive for me right now. I'll see if I can find someone who knows someone who has access to it and can take a look for me.

Is there a way to determine if the problem lies with the actuator, the button, or the control system?

Best regards,
Alex.

Zar wrote:

Is there a way to determine if the actuator, button, or control system is at fault?

Sure, simply applying direct power to the cable leading to the actuator will immediately tell you if the actuator is still working. If that doesn't fix it, a broken cable, especially in the area where the body meets the trunk lid, should also be considered.

If it's still working, a measurement at the input and output of the control system (KSG) will tell you whether the button or the KSG itself is the cause.

Regards,
Guste.

Hello there...

My name is Andre, and I'm brand new to this forum.
I'm experiencing the exact same problem, except in my case, the fuel door and two doors also won't open.
The water level was probably about 8 cm high, but the KSG (presumably a specific component or area) was dry.

But you can also check the tailgate button with a multimeter to see if it has continuity.
But if the button feels strange when you press it, meaning it doesn't click or engage properly, I would say...
Then the small button inside got stuck... this has happened to me twice already. I took the button apart, disassembled it, cleaned it, put a little grease on it, and reassembled everything, and it worked again.

Greetings.

Hi everyone!

Currently, the weather is pretty awful – I'm not really motivated to lie outside in the car and work on things when it's snowing.

Next, I'm going to test whether the KSG actually dispenses anything when I press the button.

How do you measure the tailgate button when the tailgate is closed and won't open?

Greetings.

Hello fellow diesel mechanics,

Following my last message in this post, we applied power to the stepper motor and found that it was working without any problems. Therefore, it became clear that the problem lay with the control unit board (KSG).

Our makeshift solution was a red button that we attached to the dashboard, which provided 12V directly from the battery to the actuator motor.



The only drawback is that you have to pull something simultaneously while pressing the button because the release mechanism on the back doesn't disengage.

As you probably know, a good temporary fix can last a lifetime – in this case, the button served us well from February 12, 2023, to August 19, 2026. However, since we are currently planning for children, and my wife rightly pointed out that it's impossible to get the stroller out of the trunk alone, the KSG (likely referring to a specific part or system) probably needed to be repaired/replaced.

I found someone in Germany on a classifieds website who was selling parts from a Passat that happened to be the exact model and trim level as mine, and I bought the KSG (Klima Servicegerät) plus wiring harness from them (the wiring harness was also damaged). My original KSG had the color code 07X, but unfortunately, the color code couldn't be read from the replacement KSG because he had already removed it.

At first glance, the KSG looked very similar – only one component on the circuit board was different. There are two types of connectors: a 15-pin and a 23-pin connector. The wiring on the 15-pin connector of the replacement cable harness was different. Fortunately, my 15-pin connector was fine, so I only had to replace the 23-pin connector.

So, yesterday I cut off the 23-pin connector, my red button, and our other attempts at making connections, and then soldered the new connector using solder joints.

Just a heads-up: this is a really tricky job. Since you can't really get the old wiring harness out of the car, you end up sitting on the ground next to the car for three hours in a cross-legged position, twisting wires and soldering.

But the good news is:
After soldering the connections and replacing the KSG (control unit), everything is working again. The tailgate can now be opened using the button on the driver's door and the button on the tailgate, and I was also able to reprogram the key.

The era of the red button is coming to an end!