Trailer control module issue

[Homerlovesbeer]

Hi all,

The X5 I bought had someone do a dodgy wiring for a reverse camera where they didn't go through the rear tailgate grommet so water ended up in the battery tray and the Trailer Control Module caught fire and melted the connector.

I cut off the burnt connector and bought another TCM plus connector and painfully soldered all the wires together.

After connecting the TCM and turning the car on I get the dreaded fast indicators after a few seconds and then the "check trailer lights" error message.

I've painfully traced all wires and they have continuity and are in the correct wiring order.

With the TCM module NOT installed my lights work normally.

Something I've noticed is if the TCM is installed, all the lights work on the car EXCEPT the reverse lights. I have not hooked up a trailer however I have tested all the trailer coupling pins and all are powering on as they should. Even the reverse pin for the trailer receives a 12V signal with the car in reverse.


If I disconnect the TCM module the reverse lights on the car now work as normal.

I really have no idea what's going on. It's like the Light Control Module stops sending a reverse signal to the car lights with the TCM connected yet is happy when it isn't connected?

I've confirmed that no power is being sent to the car reverse lights with the TCM installed.

Ideas? I'm running out of them and spent days diagnosing using wiring diagrams from ISTA.

EDIT: Chatgpt reckons the module needs coding to the car even though it's itentical with the same part number as the old burnt out item?

[Bdc101]

The AHM (trailer module) should not need coding on any E53s, to my knowledge. (This is a great example of why we don't use ChatGPT to diagnose car problems, if we didn't already know that...)


If you get the check lights message as soon as you turn the car on with the AHM plugged in, it sounds like the AHM is shorting something out that is not shorted out when it is disconnected. That makes me think it's an issue with the AHM or the AHM wiring. But the car reverse lights come from the LCM if I recall correctly, so that seems odd that the LCM would stop illuminating the reverse lights when the AHM is connected. It makes me think something is shorted (or perhaps was shorted and now is broken) due to the original shorting and melting issue. Someone else might chime in here as I am not an expert on the LCM other.


You might consider just using some splice connectors to splice into the tail lights, and wiring up your trailer module that way. You'll get the "check lights" message but at least everything will work. My car was wired up this way when I bought it and I towed for four or five seasons with it like that, before I spent a bunch of money on a wiring harness and AHM module.

[80stech]

+1 I think I would be looking for a short (or maybe even a poor ground) in the wiring starting with the harness going to the trailer. I am assuming you don't have a trailer connected ?? Are you really sure that you have all the wires matched up correctly?

[Homerlovesbeer]

Thanks fellas,

Yep I'm sure every wire is correct. The weird thing it sounds like the Trailer Control Module thinks there is a trailer connected and is throwing the fault because there isn't any load on the circuit from bulbs.

It's also possible that the reverse lights are supposed to not come on with a trailer connected?

I checked pin 13 which is the Canbus to the LCM and I get 12v with the TCM disconnected and 10.8 with it connected.

Strange.

The second hand TCM I bought could also be faulty I suppose.

Appreciate the help!

[80stech]

Yes, I was actually going to mention that (I actually started typing then erased it) that maybe the reverse lights don't come on with the trailer or that it might be a coded option but I think you should really focus on the "fast lights" and code, not so much on the reverse light. Trying to check a bus data line signal with a digital volt meter is just going to add to the confusion as well. I don't know if Bdc101 is 100% sure that the module doesn't need to be coded and I have no experience with that.

Double and triple check the basics and your work and try to get a wiring diagram.

[80stech]

Have a good look at the connectors, if anyone has been prodding or not using the right tools or maybe there is some oxidation, especially if the connectors got hot, it could a problem.

[Bdc101]

For what it's worth, I installed a junkyard AHM into my 2003 and it worked with no coding.

[Homerlovesbeer]

I've checked the wiring with ISTA diagrams and things seem to be as they should.

I've checked the 7 pin trailer socket and all connectors are doing what they are supposed to (left indicator gives 12V at the correct pin etc etc).

I've tested continuity from the trailer control module to the trailer plug and all tests as it should with the correct wires going to the correct pins.

It's like the Trailer Control Module is defaulted to trailer connected?

I just don't know where to go to from here. I've spend hours and hours on this and all pins go to the right location.

The only other thing I can think of is the amount of soldered joins is creating some resistance to think it is connected? I soldered properly each wire and shrink wrapped each of the individual cables (all 15 of them).

[Homerlovesbeer]

Originally Posted by Bdc101

For what it's worth, I installed a junkyard AHM into my 2003 and it worked with no coding.

I was thinking that would be the case (unfortunately for me)

[80stech]

If any of the wires where burnt or corroded (or you didn't have solder hot enough, rosin flux core?) the solder will not connect electrically even it seems to hold mechanically. Did you replace both sides of the burnt connectors ? Can't say enough to "check the grounds"