Help with battery light, not charging and now high idle?

6 Jan 2011
Reaction score
United Kingdom
Ok, I am worried I have seriously messed something up. I'll start the story from the top so you know exactly what has happened..

I bought an active subwoofer for my car 2 days ago, installed it 2 nights ago. Checked several online guides of how to wire the amp using a wiring kit. The only thing I didn't follow exactly was the earth wire, I just attached it to the bolt for my rear seats with no cleaning or anything. (I assumed this would be fine as the bolt would make a clean connection within the car body)

So all installed and remote wire hooked up to my ash tray light that comes on with side lights/dipped. I havent yet used the sub as I'm waiting on a converter for my rear speakers, so no sound has been through it, it's just had power on. The day after the install I went to work (about 10/15 mins, I would've thought plenty of time to show signs of the battery dying) during my lunch I also drove around for an hour. It was only when I was on my way home at 5pm yesterday that I noticed any problems. Car started fine then 5 mins into my drive home I noticed the battery light was on but very faded but seemed to be flickering very slightly as I changed revs and I'm not sure but I think my headlights changed brightness, and then back to how they were. I immediately pulled the amp's remote switch out to turn it off. I then drove home and hoped it would be better in the morning.

Started my car this morning, and again it started fine but the battery light was doing the same thing. I went and got my breakfast and ate it in the car with the engine running. After about 15/20 mins of the engine being started my electrics stopped working. (well some of them) My radio died, the speedo, petrol gauge (These all run on a 20a fuse in the dash, had the same thing happen when that blew) the only difference this time was my headlights were affected, they looked like side lights. I kept my engine running and drove to work.

I got to work and pulled the earth cable from the back of my amp. (even though the remote power cable was already disconnected) I tried to restart my car and it wouldn't turn over, re-charged all day tested the battery and it was fully charged. The car started quicker than it ever has done but the battery light was still doing the same.

I got home (no problems) and checked connections on the battery, tightened up the bolts just in case I've been an idiot, everything seemed fine, I even disconnected the positive cable for the amp, same thing with the battery light when I turned it on.

I decided to go for a quick drive to see if the battery would charge and literally a minute after driving I noticed an unusually high idle when I put my clutch down. I did a U-turn and parked up at home just in case it was about to die.

I have read that re-charged batteries sometimes need to reset their idle, but I don't know why it didn't show any signs of that on the way home earlier today.

The last thing I did was restarted my car again to check the battery and it turned over, but only just so the battery isn't charging, and it's still idling high.

Sorry for the really long question, I just wanted you to know everything. My first guess would be it has something to do with the alternator, can a dodgy earth on an amp blow the alternator?

I need honest and preferably experienced opinions on what these symptoms are usually caused by.
Sponsored Links
Connect a voltmeter across the battery with the engine running, it should read 14 volts or a little over, if its much less your alternator is probably faulty.

Wouldn't have thought that the earth cable would have caused it unless you have wired the amp wrong. I would check the charge rate with a multi meter and see what you have. You should be getting about 13.8 to about 14.5 volts.
As above, a fully on charging light indicates not charging/broken belt. A faintly lit charging light can be harder to fault find.
Could be alternator, so if your not getting the above voltage on the voltmeter head for the alternator. Visually check around the alternator for bad connections or broken earth strap.

I doubt if it had anything to do with the wiring of the sub.
Sponsored Links
Thanks for the replies! I checked the voltage stood, and running today.

12v before I started it, 11v just after I started it and slowly dropping, then I left it a little while with the engine off and it was back up to 12v again?! I cannot understand.

I forgot to mention the first time round (stupidly) that the fan belt was slipping on a cold engine and since this has happened I hadn't noticed that.

I spent an hour under the car today, loosened the alternator bolts levered it to tighten the belt and tightened the bolts back up. Still doing the same thing, I thought maybe the belt had slackened off and was slipping without making any noise and not turning the pulley enough. Obviously not. I didn't check the cables while I was working on it, I don't know much about auto electronics, is anyone able to explain in any more detail what I need to look for regarding the earth on the alternator?

Just going back to voltage again, after I had done all this I left it running for a little while with my lights on and the battery seems to be losing 0.1v every couple minutes.

This is killing me I made plans 3 weeks ago to go somewhere tomorrow and then this happens! Has anyone got any suggestions? I've heard a local garage will test the alternator for me, will they do exactly the same as I did with the volt meter? I heard sometimes it can be a diode within the alternator?

Any help would be greatly appreciated!
Well, the alternator definitly isn't charging, so either its faulty you have no exciter voltage on it. Try connecting a wire from the + battery terminal to the small terminal on the alternator, if it brings it to life the energising current is missing, this could be due to a faulty bulb in the warning light or the wiring betwen that or the alternator, or posibly a faulty ignition switch, if that doesn't make it charge the alternator needs replacing.

I wouldn't beat yourself up - sounds like a nasty coincidence to me, wire in a sub - alternater fails, one of those things.

When you get the alternator replaced, find a decent earth for the amp - the rear light clusters connection to the bodywork is usually a good bet.

Also, why connect the remote lead to the side light circuit? Whats wrong with the dedicated amp remote or electric aerial terminals?
Thanks Peter, I'll try the wire from the battery this morning but I think it's gonna be a replacement :( Oh well, I've kinda come to terms with it now lol.

Thanks gman, the best thing, last night I noticed my clock was on 83666 talk about a coincidence! lol

Regarding the earth, I rememer a while back following one of my friends who had just installed an amp. I'm not sure where he earthed it but his rear lights were dipping as the bass was thumping. Wouldn't mine do this if I earthed it where the rear lights are earthed?

I don't have a amp remote connection on my head unit, hell I don't even have a sub out on my head unit, had to buy a converter for the high output in the rear speakers lol. My aerial isn't electric either, just a small one on the roof of the car. With those out of the question I thought where can I wire it up, I also wanted another switch so I could turn it off even with the car running and the key ignition cable for the head unit is harder to get at to wire another switch in than the side lights so I decided on the side lights.

Thanks for your help on this everyone, I feel a bit better now. Got a lift arranged to where I was supposed to be going and an alternator isn't to expensive for my car so happy motoring! :)
If you have found a good earth (bodywork where the rear lights are is fine), and the lights dip with the bass, it's usually down to a poor +12V connection.
This should be direct (fused) from the battery, if you still have lights dipping, you should consider a power cap - say 1 farad.
Just thought I'd let you all know I've been working on my car today, was going to take the alternator out and test the output but when I moved it from behind the engine the positive cable snapped off. Looks like it was literally hanging on by a shard of metal lol.

I re-terminated the cable and hooked it back up. I started the car up for a few seconds, I'm missing a bolt on the belt tension rail so it's slipping, turned it off before I got a chance to test the voltage across the battery but the battery light is completely off on the dash now, so fingers crossed! Will sort the tension rail out tomorrow morning.

Just wanna say thanks for all your advice and suggestions. I don't know much about mechanics but you guys helped me through it! When I have any problems in the future, I will come here first! :D
Sponsored Links