Battery light going on and off

[horsemanuk]

I'm hoping that my alternator isn't on its way out but I'm not sure wether it's the battery or alternator at fault. The light blinked on and off yesterday just before I got to work. On my way home it was on whenever I accelerated but seemed to go off if the engine was just idling. I bought some dionised water and to the battery up, it was fairly dry. Since the water diluted the electrolyte the charge want enough to start the car after topping up so I had to get some jump. Overnight I gave it about six his charging. This morning, it started fine but the light is behaving the sane - on when revving the engine, off when idling.

Can anyone suggest how to check which is at fault?

[MrDoodles]

To be fair, if you've let the battery run dry, it could be either or both! :(

To save a lot of messing about, I would find a local Auto Electrician and get him to check both, to save you guessing which is wrong! ;)

[okenora]

The alternator is likely not charging, the light illuminates when alternator output is lower than battery voltage. If it was solely the battery at fault then when the car is running the light would not light.

Check the voltage with a multimeter (or the obd) when the light is on to confirm this, chances are it is the brushes in the alternator starting to fail but it might be a loose connection that's breaking when the car is revved or on a real off chance the belt slipping.

[horsemanuk]

I had a bit of a play with it.

The OBD "19" then "9" shows two numbers. The one in the top-left is 9.0 and the one in the bottom-right hovered around 111-113. If the top-left is voltage then it's well low.

It did a single start of the engine without any problems. When I turned it off and tried to turn it back on there wasn't enough charge in the battery.

Interestingly, when it was running I noticed that the light is off below about 1,100 rpm. It was at EXACTLY 1,100+ rpm the light comes on every time I revved the car.

[horsemanuk]

Oh, I also borrowed my neighbours car charger (Argos special) and the LEDs that show how much charge the battery has showed FULL immediately?! Clearly, that's not right.

[SD1too]

The OBD "19" then "9" shows ... around 111-113.
That will be 11.1 to 11.3 volts. Was the engine running at the time?

Oh, I also borrowed my neighbours car charger (Argos special) and the LEDs that show how much charge the battery has showed FULL immediately?!
It's said that this happens when a battery is 'sulphated'. Connect the charger again, but this time ignore the lights and leave it connected for 24 hours. You might be lucky and save the battery, but it sounds as if it may be failing due to continued undercharging. If your answer to my first question is yes, it would point to the alternator.

Simon

[horsemanuk]

Yeah, that was with the car running. In fact, I've just remember that the AA came out to recode a key to my car and he couldn't do it. He said that the voltage was low which meant that his thingymajig couldn't connect to the car. Possibly the alternator has been on the way out for a while.

I may or may not need a new battery but it certainly sounds like a need a new alternator. I know that possible the bushes could be replaced or the regulator but I need the car to get to work so I'll have to go with whatever is quickest.

Thanks for your help! :)

[planenut]

Is there any chance of borrowing an alternator or battery to see if things alter?

[SD1too]

Yeah, that was with the car running.
Sounds very much like a faulty alternator then. Give your battery a chance by connecting the charger now, and leave it connected until Monday morning.

Simon

[okenora]

the further info of only 11.3 volts while running proves the alternator is not charging, most likely brushes have failed/worn. The battery charger problem indicates the battery may well be past it's sell by date too. As said, ignore the chargers lights and leave it on charge for 24 hours, but budget for a new battery as you will be lucky to recover it to anywhere near full health.

Replacing the alternator brushes leaving the alternator on the car is not a hard job and relatively inexpensive too, from £10-30 depending on what option you choose.

[breakfastinsmethwick]

Is the drive belt tension correct?

[horsemanuk]

I'm guessing that the bushes aren't likely to be available at a normal motor factors shop? I need the car running asap as I have to travel 28 miles (each way) to work so I can't be without a car for more than a day or two. Waiting for postage will take too long, which means that a new alternator (£113 +vat) is going to be quicker, if more expensive.

[horsemanuk]

Is the drive belt tension correct?

How can I check? It's tight but how tight is tight enough?

In fact, a loose belt is the only reason I can think of why the voltage drops after a certain RPM, when the belt starts slipping as the pulleys spin up?

[okenora]

worn brushes can also do that. at low speed they manage to maintain contact but as the speed increases they lose contact, triggering the battery light. A loose belt would be making some sort of noise.

Worst case scenario for parts is a 100 mile drive (borrow car) or train to DMGRS in Southport to collect a regulator, sure he will do his best to help you

[breakfastinsmethwick]

See private message

[mrufff]

Pm sent horsemanuk
Maybe helpful

[horsemanuk]

Hmm. I've taken the regulator off and it doesn't appear (to me) that the brushes look worn enough to cause an issue. My regulator also doesn't look like the one in the 'how to'. I don't have those two little paddle connectors protruding out the bottom. Instead, I've got a third larger connector. There are pictures on my other thread:

http://www.the75andztclub.co.uk/forum/showthread.php?t=172302

This seems to imply the regulator isn't at fault. What else could it be? The alternator itself?

I'm tempted to put it all back together again and see if just reseating everything is enough to sort the problem out?