Battery connected backwards, now won't start. 2005 Cooper S
 

I haven't used the car in a while and went to start it up this morning. Battery was dead due to inactivity.Disconnected the battery terminals and used jump leads to connect terminals to an exterior battery but inadvertently connected to the wrong terminals on the exterior battery for just a second. Battery connected backwards. Now, car will not start. The engine turns, but I don't hear the fuel pump kicking in and the temperature gauge is stuck on high. Checked all fuses in footwell, and fusebox in the engine compartment including the larger fuses on the underside of that fusebox. All seem OK. Could it be that the regulator is blown and polarity could be reversed on the alternator? Would that also cause the fuel pump to stop working?

Would start I guess with the ignition off disconnecting the battery and letting the car fully power down. This means letting whatever residual power that remains dissipate too.

You can press on the brake pedal to expedite this or just let the car sit with the battery disconnected for a few minutes.

Carefully and following proper procedure reconnect the battery. Then see if things are better.

If not might be worthwhile to check relays too. At least the fuel pump relay. In fact you probably would want to check relays while the car is powered down.

Beyond that I would then query the car electronics for any codes even if the CEL is not on. Any codes will be proprietary codes so you will need a MINI specific scan tool that is capable of reading proprietary codes and hopefully being able to provide some detail as to what they might mean.

Or arrange to get the car to a reputable shop/dealer service department. Chances are high your car is not the first car that has been in for this.

Hi there, Have just done the exact same thing. Furious with myself. Did you manage to fix it? If yes, please can you give me some pointers? Many thanks, Sean

Hi Sean. After dismantling a number of items, including the fusebox in the motor compartment, I discovered that a small yellow 5 amp fuse was indeed blown, on that same fuse box. I manually removed many fuses and inspected each, and just one little fuse turned out to be the problem. Is your fuel pump priming when turning the ignition on? What lights do you see light up on the dash when turning ignition on?

Roger, you absolute legend, just been out there and... it was exactly that fuse! A very happy car... and an even happier owner! Thank you... You've made my Sunday 😃

Hi Sean, happy to hear its solved! To be honest, I was super relieved when I had found the broken fuse and replaced it and all was well. The sound of the engine starting up was music to the ears. I was imagining having to tow the car to a garage and having the ECU swapped and reprogrammed. That's what many people were replying in posts elsewhere. Happy motoring!! :)