Ok, here's the issue (well, one of the many issues on my 2003 Cooper)

If I sit at a light, with my headlights on, after a little while my lights will dim, as if something is sucking down voltage. They come back up almost immediately, but it's pretty annoying. I don't hear anything turning on, so I'm not sure if something is that I haven't noticed or something like that.

Also, if I give it a little gas while stepping on the brake, sometimes it stops the dimming. Also also, sometimes when I'm sitting at lunch with the car running, and the headlights off, my radio will shut off for a second like it has lost voltage.

So, my thoughts:

- Slipping belt. I haven't take a really close look at the belt yet, but I'm basing this off previous experience with previous cars (not minis)
- Bad battery. The battery is pretty old, I'm not sure exactly how old, but it's the easiest thing to check. If my battery is pulling down the charging system, it could account for these issues.
- Bad alternator/voltage regulator. Past experience with other cars.

Now, in addition to this I have a coolant leak coming from some where. I'm going to pressurize the system this weekend and see if I can figure out where. But one of my thoughts is antifreeze getting on the belt. Sure, it's kinda "out" there, but it is a thought going through my head.

Also going through my head is that my power steering seems pretty noisy, lots of squealing when I'm turning. But I know the fan is working on it because I've seen it working.... I don't think that's an issue at all, just kinda bringing it up since it is "engine" related.

I do have my check engine light on, and that's for engine temperature too cool. So I have a new thermostat to replace it with in case mine is stuck open.

So, lots of info here, and I'm thinking if it gets beyond me, I'll take it to my local mechanic (trust worthy and has worked on a few of my cars previously when it got beyond my time Vs. effort scale). But, if it's going to be an easy fix (replace belt, replace thermostat, tighten hoses), I'd much rather do it myself and save the money.

 

Regarding your coolant leak, there's a couple standard spots. Mine was leaking because the gasket was bad. It's pretty easy to spot and there's an excellent write up here on the forum with a bunch of helpful tips.

And why not replace the battery if its very old, it might fix it and you probably need one anyway.

 

How many miles on the car?
The dimming light imo is most likely from the alternator or the voltage regulator...things get old, and wear...sounds like when the car drops to idle, either the voltage regulator cant adjust right or the alternor brushes are worn...resulting in lower output at low rpms...or a combination of both.
I'd live with it till it gets too bad...a failing battery with a bad cell CAN sometimes suck lots of power....but a simple loadtest on a bench will tell you more.
The ps pumps were changed a bit in 2005 to make them more quiet...so run it till it fails, the new one will be better.....
belt...should be changed regularly..about every 2 years with a pulley...can cause lots of $$ dammage if it snaps.....

 

The car has 160k on it.

I'm not 100% sure what the last person did to it maintenance wise, I'm still looking at things on it.

I'm going to run some tests on it today. Pressurize the coolant system and load test the battery and see what I see there. I also want to look at the belt and maybe run it a while so I can try and see if anything is slipping. Maybe run a meter on the alternator while its running and see how the out put looks.

Today or tomorrow. Yard work first, then mini work.

 

Changing the thermostat will likly (50% chance) fix a coolant leak...the gaskets tend to go on them. Then look at the coolant tank on the S, they tend to leak along the seam...they ARE pressurized.
The belt on the min runs everything, including the waterpump...off a set a gears on the bottom of the Sc....so the belt has lots of tension...
And getting the mini bleed after dooing cooling system work can be a pain...just be sure to use BOTH bleed bleed locations. Folks tend to use the ONE by the rad only, then get an airbubble, and overheat. Good luck!!

 

They make some uv dye for the water system along with a black light at night you can easliy spot a leak. I got 195k on my 04 s and i had to replace alternator at around 165 or so. My radiator fan went out the same time (brother hooked new battery up backwards dont know if it was all related or not) my power steering went out years ago i ran over a stick or something that broke that little fan. I didnt know it just melted something and stayed on all the time running or not. Drain battery every i stopped. Finally went out. And i got used to driving it without it before my pump came in. Helped my biceps in the process and people dont ask to borrow my car any more so i left it off.

 

I ended up replacing the radiator cap that stopped a lot of the leaking not 100%, but a lot. I tested the cap and it didn't allow the system to pressurize.

When I pressurized the system I didn't really see any other leaks, but I still see some under the car after I park for a bit. But it's a whole lot less.

I think I may be replacing the alternator soon. I think that may be the cause of my troubles. And while I'm doing that, I'll replace the belt.

 

Something I noticed. If I sit still, sometimes the voltage drops so low my radio and trip reset. Sometimes just my radio resets and not the trip. If in sitting still and more the steering wheel back and forth, the lights dim quite a bit.

I'm thinking belt and alternator. Although I'll probably replace the battery for good measure.