Power TCU trough Ignition position -Battery Drain- Solved
 

Well, it seems that the day has come for our 2005 x5 3.0i to start draining the battery because of a somehow faulty TCU unit. The bluetooth module (Fuse 74) seems to keep waking up the car from sleep mode every 10/15 minutes or so, drawing a 0.5Ampere for a few minutes. Result = dead battery after a night or two in cold days.

Long story short: Besides this power drain, the bluetooth module is working great and I love the quality of the microphone and everything else. I really would like to keep it in working condition.

The solution that came to my mind is to power the TCU trough a power source that is directly bond to the ignition switch position. Logic: Key turned = TCU powered, Key removed = TCU without power.

I could cut the original power supply to fuse 74 and steal the power from the rear lights, but I guess the LCM would notice a too high power drain. Maybe some of you have a smarter idea or approach on how to power this unit (obviously always securing the circuit with the proper fuse).

The usual fix is to simply pull the fuse for a day or so then plug it back in. For whatever reason that seems to usually remedy the problem.

Maybe approach the problem at a different angle. Use the same power feed but install a relay to open the existing circuit when the key is off. Not sure though what effect this would have on the TCU operation, it might need to have power all the time or maybe need power before the key is turned on.

I can not imagine that it will cure the problem permanently, or have you experienced it? I took the battery out of the car to recharege it - this should be equal to pull the fuse, and only after reattaching the battery I looked for the drain and found it. That tells me that the problem is always there or, at least, coming back when it wants.
I will now use the car without fuse 74 in the coming days, to confirm the culprit 100%.

This seems to be a nice strategy in fact - the relay would be piloted by the rear light supply, right?

Prior i will obviously check if the system works when the tcu is powered on later...

There must be other circuits you could use, but yes if you want to use rear lights then using a small relay (and a capacitor and diode because the light circuits are PWM) you might be able to get away with not triggering a warning from the LCM. Does the power draw come from the TCU itself or the fact that it wakes up the car?

Fuse 74 responsible for:

• Telematics Control Unit (TCU) or Universal Electronic Charging & hands-free unit (ULF)
• Voice Input
• Eject Box
• Compensator

I cured mine by removing it and putting it in a box in the house. Hasn't bothered anything since :)



I tried replacing it at first with a unit off ebay which worked for a week or so and it was back draining the battery again.

Went to an aftermarket radio and ditched the tcu was the final solution. That said your idea of making it key switch would be a good solution.

I have experienced it. Both times after an alternator failure. Disconnected the TCU and reconnected later. Problem solved. If it doesn't, pulling fuse 74 will confirm it is actually the TCU. If you pull the fuse and leave it a couple days and the battery isn't drained, there you go. If it doesn't, then it's something else. Other culprit is the NAV if you have it or the FSU.

Thanks a lot to all for the help.


I actually don't think the faulty part is the TCU (even if it's the one which drains the power). The fact that you replaced yours and had the same problem a few days later somehow confirms this thinking.
In my case the module keeps waking up every 5-10 minutes ...draining for 3-4 minutes and then goes to sleep again. I really would like to understand what is causing this but....it's so time consuming that I really tend towards simply cutting the power supply trough the ignition switch position.