Alternator pulley too small?

[rogerowen]

Daughter No 1's battery is going flat about every six weeks. Moggie is in daily use (30 mile a day +), and battery is recent. I'm wondering if the pulley is too small. Ignition light stays on till about 1200 RPM, Amp-meter registers only a very small amount of charge. I'm rubbish with electrics - could there be an electrical fault? How to measure Alternator Out-Put?[frame][/frame]

[daveyl]

I believe making the pulley bigger will make it worse. If you have an electric test meter set to 'volts' you should get a reading of more than 14v across the + and - terminals of the battery, with the engine running.

Actually, just tested my 'modern'.
12.8v across the Battery terminals before engine started, and 14.4v engine running.

And then 'Morris'
13v across the Battery terminals before engine started, and 14.3v engine running at 2000rpm

I had to increase the revs (Morris) because it was under 13v at idle. My Morris has an alternator like yours.

[rogerowen]

Thanks for the tip, will give it a try as soon as this blessed rain stops!

Cheers,

Roger

[katy]

Is the belt tight enough? Looks loose in the pic.

[rogerowen]

I'll notch it up a little and see if that works - always worried about over-doing it and totalling a water pump. Did that once on my lovely TR2 (still have funny dreams about It)!

Cheers,

Roger

[bmcecosse]

The pulley is fine - possibly the alternator is failing - maybe lost a phase or two. To test - run the engine at decent revs with the headlights + fan on and see if it manages to balance the load. Yes - worth nipping up the fan belt i you can - and tell your daughter to 'manage' the load when in traffic - ie headlights off and careful use of wipers........

[mike.perry]

I believe that the fan belt needs to be tighter with an alternator than a dynamo as the alternator is more prone to squealing, also alternator belts are normally toothed on the inside to fit around the tighter curve on the alternator pulley.

[bmcecosse]

The 'toothing' leads to early failure....avoid.

[daveyl]

As Roy has said 'managing' the load in traffic is a good tip. I have a 'volts' gauge fitted to mine (a lot of folk don't like them but it is part of a triple gauge cluster) and the biggest draw on power is the heater fan.

With the lights and wipers on it still comes in at 13.5v but as soon as the fan is on it dips down below 12v unless the revs are up high.

[rogerowen]

Now had a chance to measure volts at the battery and I'm getting 12.6 at tickover and only 13.4 at revs. Is there a tweak somewhere or is this likely to be a fault in the alternator? Any ideas anyone?

[bmcecosse]

Not the best reading -what do you get at revs with headlights and heater fan running?? As I suggested earlier - it may have lost a phase or two..

[rogerowen]

Daughter's out and about in Marmaduke most of the time, so I only get fleeting chances of 'Dad can you fix this - NOW!' But as soon as I have it in my grasp - I'll try the suggested load test. If it is a phase down - does that mean a new alternator?

[lambrettalad]

yes cheaper to buy than repair if you go to auto jumbles

[bmcecosse]

Unless you can 'manage' the electrical load - then yes. An alternator with one phase down will still better a dynamo, but if it has lost 2 phases then it will certainly struggle in winter, but may scrape by in summer.......

[rogerowen]

Thanks Roy, I'll get a letter to Santa!

Cheers,

Roger

[rayofleamington]

The alternator should produce more output, so needs more torque.
The extra torque AND a smaller pulley increases the belt load, so in turn it needs the belt to be tighter.

however - beware overtightening the belt. it is well known to cause the bottom pulley to fail (most common with the pressed & riveted pulleys, then the spot welded ones, less likely with the fully cast ones, and I've not seen one go with a damper/pulley)

[rogerowen]

Thanks Ray, will give that a try before hunting for a new alternator.

Cheers,

Roger

[bmcecosse]

No harm getting a second hand spare in stock - but give us the loaded test results (with a good tight belt) before rushing out to buy a new one.