Plodding on with faulty alternator

aroominyork

This is a technical, not financial issue, but rather than signing up for a new forum I'm trying my luck here.

Mercedes B Class, 2008. The alternator warning light on the dash came on this morning. Local garage (honest folk) said it's the alternator only charging at 11 somethingorothers. They can replace it on Friday (and it might involve dropping the engine, whatever that means (I think I can guess) and it'll take between 2 and 5 hours depending on that).

In the meantime they suggested I don't drive it and they clearly didn't want to give advice which might lead to me blaming them if it conks out away from home. So my question is whether I can safely drive it a mile or two or could it suddenly stop without warning any minute? Are there any warning signs of imminent stopping? And if it starts, is it likely to keep running if I do not stop and restart the engine?

tr7phil

If you only need to drive a short distance I'd charge the battery up and risk it, you'll probably be OK.  It's not a hard job to change the alternator yourself, usually only two bolts and the plug.

aroominyork

By "charge it up", do you mean run it while stationary with my foot on the gas?

wongataa

aroominyork said:

By "charge it up", do you mean run it while stationary with my foot on the gas?

No, use a car battery charger plugged into the mains.  The alternator is failing and therefore can't charge the battery so using the engine won't work.

AdrianC

aroominyork said:

Mercedes B Class, 2008. The alternator warning light on the dash came on this morning. Local garage (honest folk) said it's the alternator only charging at 11 somethingorothers.

Volts.
Or, rather, it's not charging.

The 12v battery in your car is nominally 12v - the actual voltage varies according to the state of charge.


Run the battery flat, it shortens its life.
Run the battery REALLY flat, it shortens it a lot.

They can replace it on Friday (and it might involve dropping the engine, whatever that means (I think I can guess) and it'll take between 2 and 5 hours depending on that).

It means exactly what it says...

If they can do it through the gap available, great.
If not, then they'll have to dismantle a lot more to lower the entire engine/gearbox/suspension subframe to get at it.

In the meantime they suggested I don't drive it and they clearly didn't want to give advice which might lead to me blaming them if it conks out away from home. So my question is whether I can safely drive it a mile or two or could it suddenly stop without warning any minute? Are there any warning signs of imminent stopping? And if it starts, is it likely to keep running if I do not stop and restart the engine?

Ish.

Starting the engine is definitely the single biggest draw on the battery, which is not being recharged by the alternator. BUT... there are a myriad of computers consuming power all the time. When the voltage starts to drop too far, you're going to get a lot of oddities going on - and quite probably something as simple as braking (brake lights...) or indicating will cause the voltage to dip far enough for the computers to have a little sulk and cut the engine. And, of course, you won't be able to restart.

daveyjp

The car will run, but you are relying on the battery to supply power, not the alternator and as stated once the battery voltage drops the car may shut down and you are stranded.

My dad once managed to complete a driving lesson with a pupil with no alternator after the belt snapped, but this was an old school mechanical pump diesel which didn't need much power to keep the engine running.

BTW OP, your garage are correct.  Changing the alternator on B class is a pain as the subframe and engine need dropping,

Ganga

If you only want to go a mile or two ,walk! it will do you good and no chance of the car breaking down in the middle of no where and costing you a lot of money.

marlot

I did 100 miles with a failing alternator one Christmas.  It was a bit nerve-wracking but was fine.  I bump-started the car, turned off everything electrical I could and was ready to call out a breakdown service.

But that was starting with a fully charged battery.  Which yours won't be unless you have a battery charger.

jimjames

Unknown to me I had a failing alternator on my car a few years back. I did a 15 mile journey home, 5 miles in the car stopped on the motorway without warning and complete loss of all power along with every light on dash flashing up. Fortunately I was able to glide it to the hard shoulder but it wasn't something I'd have chosen.

AdrianC

If we're on "failing alternator" stories, then... the voltage started to drop about 150 miles from home, with rain possible, and cutting it fine on being back before dark...

...at the end of a 3,000 mile driving holiday.

A set of five quid brushes was all that was needed, a five minute job.

mobileron

Find a local auto electrician ask him to refurb it.save u loads of cash.  ebay ones from £70.00 full refurbish and warranty.