[Adam_F30]

Hello all. Can the ECU be reset by disconnecting the positive and negative battery cables and touching them together for ~30 seconds? Other makes/models seem to respond to this procedure but can't find any post on the forums saying "yes, this is a good/fast way of resetting the ECU without frying anything." The thought behind this is that touching the cables together will create a short and drain the remaining power stored in capacitors within the car. My concern with this is, it's a BMW; nothing is easy on these cars.

The reason I want to reset is because my AC/heat aren't working suddenly after unplugging my battery a few times to install LED lights and BMS stage 1. I've done the obvious, unplug and replug originals to see if the new parts caused this. I'm convinced the numerous unplugging of the battery created several fault codes which resulted in the AC/heat system not having enough power and sending fault codes to run at reduced power. The fan blower works but will not go to full power. All modes of the damper work as well. Before taking the car into the shop, wanted to give this one last try. I've already left the negative terminal disconnected for an hour and problem still exists. I've unplugged the negative terminal of the battery just now and plant to leave it overnight to see if that does anything.

Thanks in advance for the help.

[M-terkait]

I disconnected the battery like 1000x times during my ownership while fiddling with jb4 and other mods, never had an issue with my AC works flawlessly during the 50*C weather. I think there is something wrong with AC, not the battery.


maybe try removing the stage one tuner before anything else? although I doubt it causes any problem.

[Adam_F30]

Thanks for the input. I'll try unplugging the tuner and leaving the negative terminal off overnight also to see if that fixes anything.

My assumption is that nothing is wrong with the AC because the heat is also not working. Does that sound valid or am I off-base?

[madmonkey]

If you still have this issue OP it could just be your battery is marginal charge. When your battery is old, it may not have a high enough charge and when that happens, our cars start to turn off non essential accessories in order to make sure our cars still start and run. Check you battery voltage and try reconditioning it with a good battery tender.

[CayenneTurboS]

Hey everyone,
my wife has an x6M and I want it to be a hero and change the ignition coils my self. I unplugged the negative before beginning work and about 2 hours later, all ignition coils have been changed. I plugged the negative back and turned the car on. well, all the lights came on, steering wheel, abs, 4x4, pretty much every light on the dash and the car dropped on the passenger side and stayed high on the other side, doesn't help that the car is not on a level ground, it's in my driveway parked on top of snow. But anyways, I heard that it's probably the computer has to be reprogrammed or reset. the positive and negative are really hard to reach so I connected an electrical wire from the negative to the positive away from the battery of course, and I left it all night. Next morning, I connected the positive and then the negative, started the car and same thing even my radio's setting was still good, time and radio station, makes me believe that the computer did not reset.....

any idea what else I can do to reset the computer.

the dealership is 4.5 hours away from my city and no mechanic in town will touch it.

[Billfitz]

Disconnecting the negative terminal doesn't cause any problems. It may take a few minutes for everything to come back to normal, but the worst thing that can happen is that you have to reset the clock. If you have the clock set to automatic it will do that by itself too, it just takes a minute or two. There's no such thing as coincidence, so if you did some changes to the car and the car doesn't work right afterwards the changes are probably the reason why.

If you still have this issue OP it could just be your battery is marginal charge.

Agreed.

[328iX]

I'd just like to add, it is well known in BMWs that poor battery health can cause a myriad of random/weird electrical issues.