slipping belt

[Deanxm]

Evening,

I had a squeeky belt on the xm so yesterday i tightened it up a bit although it didnt seem loose. Started it and the ek......ek......ek had stopped, i got in this evening and when i started it she was slow to turn over and while cranking the tacho shot up to 7k RPM started fine but the squeeky aux belt was back, obviously the alternator was under some strain with the cold weather but when i put the rear window demist on the fan slowed noticably and when i pressed the brakes the added load of the brake lights brought the charging warning light on dimly, if i put the A/C on it got a little brighter but it had to be dark to see it.
I assumed dead/dying alternator but when i got the ammeter out it was showing 14.7v whilst running? is it the alternator or is it a problem elswhere?
The battery is a brand new bosch super duper 6 year warrantee thing and the terminals are brand new bronze jobbies, already had the problems with the lead clamps.

your thoughts are most welcome

D

[DickieG]

Worn crank pulley allowing the belt to slip was my first thought until I read of the alternator output voltage which then lead me to think of a regulator issue (over charging/additional load/belt/crank slip).

[Deanxm]

I think im going to have to make sure i dont have that two part crank pulley fitted and then clean the main connections up and go from there.

D

[MikeT]

I know you said it's a new battery, Dean but with those symptoms, I'd be suspicious of it and get it tested all the same.

[xantia_v6]

14.7 volts it rather high. If sustained, it will be straining the battery, alternator and belt.

The noise though could well be due to a worn belt or pulley.

[CitroJim]

Certainly sounds as if the alternator is overcharging. A good test of this is to see if the battery and alternator gets very hot. I once had a problem with the regulator on one of my old R4s. First I knew was when I smelt battery acid in the cabin.

The battery was found to be seething and steaming and the dynamo was red-hot (well, not quite red but jolly hot...).

The battery seemed to survive but died completely, not a volt to be had, after a few days.

14.4V is the absolute maximum you should see..

Check the battery a few minutes after a run and it should read around 13.8V. After an overnight rest this will fall slightly to about 12.7V.

[addo]

Mine's reading about 14.2-3 at idle with a DMM.

I'm figuring this is on the high side, but - correct me if wrong - diode failures always drop the voltage.

[CitroJim]

Mine's reading about 14.2-3 at idle with a DMM.

I'm figuring this is on the high side, but - correct me if wrong - diode failures always drop the voltage.

Yes, If they go open-circuit Adam. The alternator is a three-phase machine and diodes failing in the rectifier stack normally has the effect of taking a phase out and reducing it's output. Generally though the regulator will try to compensate and this will tend to overload the remaining phase windings/rectifiers and eventually lead to an avalanche failure.

14.2-3V is fine. No worries there at all.

If the car starts on the button and all is otherwise content, it's good...

[Peter.N.]

What is the condition of the belt Dean? They can go very hard and slippery, especially in this weather. As Jim says, 14.7 volts is to high, make your lights nice and bright but won't do the battery any good. Might be an idea to check it with another meter though - just in case.

Peter

[myglaren]

14.4V is the absolute maximum you should see..

Check the battery a few minutes after a run and it should read around 13.8V. After an overnight rest this will fall slightly to about 12.7V.

What about the effects of low temperatures on those figures.^

My car is very slow to fire lately which I am thinking is probably the battery although once it fires I am not experiencing any of the typical low voltage nonsense that C5s seem to enjoy, so the problem may be a slow starter motor.

I would go and check now but I have left both my DVMs at my daughter's and her place is out in the sticks and practically unreachable due to the white stuff.

I have no objections to replacing the battery but not if it isn't faulty. I do suspect it is the original though so eight years old.

Can't seem to find a direct replacement, the only info on the battery is :"12V L3 450/720A"

There is what appears to be a serial number:
P48287011 0478 955023

Which is probably no help at all.

If the car will start I may go and get it tested

[CitroJim]

What about the effects of low temperatures on those figures.^

The regulator should compensate. Even the old dynamo regulators had some degree of temperature compensation.

[myglaren]

What about the effects of low temperatures on those figures.^

The regulator should compensate. Even the old dynamo regulators had some degree of temperature compensation.

I meant the voltage across the battery terminals with the ignition off and the car having stood overnight. What should we expect to see once the surface charge is wiped out by using the headlights for a minute or so?

I did let the battery voltage drop a bit a couple of years ago, using the headlights to see what I was doing fixing daughter's Mini one dark rainy night.
On startup I got all the usual warnings, i.e. "your car is buggered" that all cleared by the time I drove home ({1/4 mile - next street )

[CitroJim]

Sorry Steve, I misunderstood

Yes, as temoperature falls, the off-charge voltage will drop considerably. under 12V on a really cold day....

With the "moderns" getting too clever for their own good now and being critical of battery voltage, all sorts of funnies are bound to happen in the cold...