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VW/Mercedes Trailer Module J345 Explanation and Repair Procedure

By on April 29, 2019
automobile electronics repair












In this article we will see the concept of a modern CAN bus trailer module. How it is build up, how was it repaired, also how to connect on the bench and with pinout description, which are  was made by me. I found no pinout description or any usable repair/debugging data on the Net for this little computer.

car electronics repair

This module is used most in VW and Mercedes vehicles from maybe 2008 and up. The module serves for cars which are equipped with towbar for trailers.

The trailer module monitors and is powering the lights on the trailer like break light, turn light etc.  Without his tool in the car the trailer light won’t work and an error light would blink on the dash board of the car if the trailer light wiring would be connected in parallel to the car back light.

The communication between the trailer detection module which is in the original VAG documentation labeled as J345 module is communicating through the CAN bus network. Some technicians talk about this module like a simple “stupid” device what is not necessary in a car, but let’s check out a few simple tasks what actually a trailer module can do and how.

The trailer module is connected through the CAN network to the:

– ABS control unit

– Dash board

– Light switch

– Anti theft system

Ok, somebody would say, what the hack does the trailer module do with the ABS module?! Modern cars are intelligent beast, when you connect a trailer to the car and you have a trailer module like this one from this text, the ABS module will know does a trailer is on the car and the braking parameters should be corrected due to the extra load behind of the car.

So to say, the ABS module will use another setup when you press the brake pedal until when you drive your vehicle with a trailer on it.

car abs module schematic

This picture was taken from the original VW documentation

The anti theft system or so called IMMO system will monitor your connected trailer until you drinking a coffee with your friends and left the car on some parking with the trailer on it.

If somebody try to disconnect the trailer from the car until the car is locked and the immo system is armed, it will immediate recognize an unauthorized trailer disconnection and run the alarm. These are just few of the advantage of a trailer module like this one.

Let’s see the module build up:

J345 trailer detector module

The J345 trailer detector module

Block descriptions:

Block 1:

This is the complete power stage of the control unit.

It is built up from 3 main parts.

  1. A linear 5V voltage regulator
  2. U5021M – Digital Window Watchdog Timer
  3. ATA6025 – Watchdog IC

Block 2:

Is the main cpu unit which is a Motorola 4L52H uC.

Block 3:

Is the circuit which connect the J345 to the CAN network.

The TJA1054 CAN transceiver IC has the capability to communicate with other computers on the CAN network and also to communicate with the appropriated diagnostic tool.

Block 4:

This block is actually the power output stage to the trailer connector.

It is built up on some sort of Smart High-Side Power Switch IC’s.

I didn’t found any datasheet for the 66010 labeled IC’s, just some China manufacturer

selling them on Aliexpress and similar web sites. So I suppose that IC’s are real/original China product what is a bit hard to believe, because of the car brand VW and Mercedes…

Here is the pin out of the connector:

Smart High-Side Power Switch IC’s

+12V – Power +12V input

GND – Ground

Output – Is the +12V output to the lamp on the trailer CAN-L/CAN-H – Is the CAN communication. You can do several tests on the bench.

Checking the voltage regulation part:

To check the voltage stabilizer IC you have to power on the unit with your bench power supply.

Connect the car diagnostic tool to the CAN pin of the J345 unit and try to connect to the ecu of the car. You will get an error message from your diagnostic tool because you can actually not connect to the J345 without the car onboard supply unit. That’s ok, but you have to send any CAN message to the CAN transceiver so they will wake up the main power regulator.

When you tried to connect with your diagnostic tool, measure the input and output voltage on the

3 pin voltage regulator. It should be an input voltage = to battery voltage/bench power source voltage. The output voltage should be 5V.

After a while the power block will go to sleep mode because no more CAN messages are travelling through the CAN network.

Checking the CAN protocol part:

In this test you will check the J345 unit does it react if a CAN message is travelling through the CAN network. A bad CAN network circuit in the BLOCK 3 could result in a failure like no light on the trailer even the light switch is on or a DTC (Diagnostic Trouble Code) set for a no CAN communication between the J345 and the onboard supply control unit.

This test you can perform the same way like in the previous test above, where we test the voltage regulator. If the voltage regulator will not wake up when you try to connect with your diagnostic tool then you probably have a faulty CAN communication, maybe the TJA1054 CAN transceiver IC is bad and need to replace.

Checking the output stage:

In this section you can check for output voltage which is going to the trailer light’s.

Check the 66010 IC’s like this:

Take out from the pcb the desired IC.

Connect +12v to the middle pin.

Check the voltage between the two side pins and ground.

You should have no reading.

Anything over 0V is an indication to a bad 66010 IC.

My case why I got this unit for repairing was no trailer light on the trailer.

All the output IC’s was bad because of a car accident where the wires was broken and in short because of the accident. The protection stage in the IC’s didn’t react fast enough to disconnect the power and save the outputs. Maybe because it is a poor China product, I don’t know. After replacing all the 66010 IC’s the unit was on the road again.

I hope you enjoy this tutorial and will save lot of repair time.

Christian Robert Adzic

This article was prepared for you by Christian Robert Adzic from Novi Knezevac-Serbia.

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  1. Mark

    April 29, 2019 at 9:05 pm

    Hey Christian,
    A well written and detailed article!
    So many systems are on a network and make our jobs more interesting. I have seen problems because technicians don't understand that even a trailer is on a network. We can't just think of a trailer as something that just gets towed behind a vehicle anymore - they become an integral part of the vehicle when connected.
    Thanks again for the article - I'll be storing this information for reference.

    • peter

      March 19, 2022 at 5:10 am

      I prefer the good old days when you just hook up some wires to the taillights of your car, and the car didn't noticed anything because it didn't had any computer crap. And the only thing that could fail was a fuse when you hooked up a trailer with too many lights on the standard fuse. And all it took was just a fuse with a higher amperage. And to operate a power consuming device over a simple switch, you just needed to add a simple mechanical magnet contact switch.
      Life and cars were soooooo much simpler back in the days. And i like computers, like in my smartphone and PC, but not in cars!

  2. Ghashaan

    April 29, 2019 at 9:20 pm

    It's a great article.
    Your presentation of ideas is super-easy to grasp. Even a newbie like me could snatch a large portion out of your tutorial.

    Sadly, these kind of car trailers don't exist in the Maldives. Even if they do (somewhere in the country), I don't think anyone would consider it for repair as there won't be any technicians available.

    I enjoyed your article a lot.

    Waiting for the next one!

    • Christian Adzic

      April 30, 2019 at 5:16 am

      Thanks for supporting my article.
      When I would be in Maldives I would never care about such of devices, I would be as much as possible somewhere diving into the Ocean. 🙂

  3. Albert van Bemmelen

    April 29, 2019 at 10:36 pm

    Great article with a lot of useful information for VW, Mercedes and Trailer owners! Why parts are available but no datasheets exist keeps us in the dark many times! Good thing though that you still were able to check if the 66010's were defect or not and how to test them. You must have ordered new 66010 parts and probably just checked those out later on to compare them with the bad ones.

  4. Yogesh Panchal

    April 30, 2019 at 12:12 am

    Good Job! Christian Robert

  5. Parasuraman Subramanian

    April 30, 2019 at 12:19 am

    Great article taking us insight! Very professional, indeed!

  6. Ian Corry

    April 30, 2019 at 7:11 am

    Excellent article Christian,
    The CAN bus system is something that many vehicle technicians do not fully understand. I spent many years in the automotive electrical business and watched it evolve in leaps and bounds. I remember when we were asked by a main dealer as to what could be the problem with a new vehicle that would not start. It transpired that the buyer had asked for his car radio system to be fitted instead of the factory fitted radio. No one gave thought that the car radio also doubled as the CAN bus gateway for all of the vehicle electronic control units. The result was a complete failure in all the cars electronics!

    • Christian Adzic

      May 1, 2019 at 12:28 am

      Thanks for supporting my article.
      Yes, many times the owner of the car just buy some fancy car radio and the seller of the radio also give him or sell to him an adapter for connecting the car radio to the vehicle original cable/connector. And here goes hill down.
      Mostly the cheap adapters from PRC are shorting out the CAN-bus system and voalla! You got the BINGO! 🙂

  7. Robert Calk Jr.

    April 30, 2019 at 4:49 pm

    Good job, Chris! Thanks for the article.

  8. kouaplack augustin david

    May 4, 2019 at 7:01 am

    great job and very nice too

  9. David

    November 11, 2019 at 5:20 am

    Hi Christian,
    Your ECU repair article is very impressive.
    May I ask if you have repaired any RST (Roof Soft Top) controller for Mercedes benz? I just got 3 pieces to fix. However, I am lack of technical information for these stuffs. Maybe you are able to help me out a bit.
    I am located in Austria, not far away from Graz.
    If you need more information or pictures, please let me know.

  10. Chris

    March 30, 2021 at 2:08 pm

    Hi David, sorry cos I'm so late with my answer but it is hard to check every replay and many times I miss several questions.

    But however, I never repaired that RST controller, but I don't think it is very difficult to repair.
    I have many friends in Wienna, and I also speak German language.
    So, if you interested to stay in touch pls ask mr. Young to share with you my email address and you can contact me...

    Otherwise thank you cos you support my article.

    Keep care of covid19!


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