Alternator - when should it be charging?

[greg.harvey]

Hi all,

My Elan battery is not charging when idling. It charges while rolling along, but if sat a lights or ticking through a town in top geart it starts losing charge. Is this normal, or should I check the alternator? Latest mechanic wasn't sure, as he hadn't worked on an Elan before, but seemed to think it really ought to be neutral, not negative, while hot and idling.

Cheers,

G

[Tonyw]

Your alternator should charge at idle, if it is not you have either a faulty alternator or regulator.

TonyW

[bcmc33]

Tony's right - it's the regulator, and nothing to do with being an Elan.

It will be an easy thing to check - take it to your nearest auto electrical specialist.


Brian Clarke

[greg.harvey]

Thanks guys! I'll get it checked out.

G

[andyhodg]

\i have the same problem with my + 2. I have put it down to the alternator being the wrong diameter. Maybe the old dynamo pulley? I haven't got rpund to changing it yet. There is no regulator on cars equipped with an alternator so I don't think that could be your problem.

I am really not sure if the pulley is the correct size or not. May be somebody with a correctly functioning alternator could let me know the diameter of their pulley.

All the best Andy

[bcmc33]

The regulator is part of the electronics at the rear of the alternator.

The diameter of the pulley should be about 50 mm at the pitch of the belt.

If you're using the old dynamo pulley it will have an affect on output at tick-over.

I hope you're not using the old dynamo belt, as well.



Brian Clarke

[Jason1]

Greg

I agree you should get it checked or check it yourself, but I would not be surpised if it turned out to be ok. Alternator output is relative to speed, up to a point the faster the alternator turns the greater the output. At idle the pully is not turning very fast so very little charging is going on, if you then have lots of electrical load the battery can discharge.

Mr Bosch puts it:

"The current output of the alternator varies as a function of engine speed. The alternator can only supply a certain percentage of its rated current at idle if the current consumed by the loads is greater than the current output by the alternator, e.g., at idle, the battery will discharge and the voltage in the vehicle electrical system will drop."


1. I would connect a voltmeter to the battery with the engine off = take a reading. If you have under 11 volts you have a dead cell so replace the battery.

2. Start the engine = the alterator should charge now and bring the voltage over 12 volts. You will be surprised that how little over 12 volts the gauge will go, but the alternator only has to raise the voltage over the battery voltage to charge it. Note the reading.

3. Now with the engine running turn on the headlights = the voltage will drop, now increase the revs and note if the volts creaps over 12 volts. A good alternator will bring the voltage back over 12 volts.

Cars with misfires stall as the alternator is working so hard to increase the output it stop the engine, so check you are not running too slow at idle.

4. To check a bad battery you can connect a voltmeter to the fully charged battery and crank the engine. A good fully charged battery will read at least 8 volts.

Hope this helps

Jason

[greg.harvey]

Thanks Jason.

With the engine idling, the meter in the cockpit reads a voltage about 11 volts, so if it is correct then this is not strong enough. When on the motorway/driving at normal speed it reads 13 volts.

That said, I've never flattened the battery in 4 years of use, so it could be the meter in the cockpit I guess?

Cheers,

G

[tonycharente]

Hello G,
It's altogether possible that it's your dash gauge that is under-reading - I've certainly had that on a car. You can buy a dirt cheap multi-meter for a just few pounds, set it on the "up to 20 Volts" range, connect it across the battery and (1) switch on the ignition and then (2) run the engine, in both cases comparing the multi-meter reading with your dash gauge.... Then you'll know.
Tony

[bcmc33]

Lots of good advice being given. One of the best buys I've made was a digital merter from eBay - remarkable accuracy for a few pounds.

http://cgi.ebay.co.uk/DIGITAL-LCD-MULTI ... dZViewItem

My alternator charges at tick-over, but I must admit I've never checked it at tick-over wth the headlights on.

Brian Clarke