4.2 ameter oscillation

Talk about the E-Type Series 1

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Brucey
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#1 4.2 ameter oscillation

Post by Brucey » Sat Nov 10, 2018 5:36 pm

S.1 4.2 owned for quite a while. Noticed that the ammeter swung wildly to full charge, stayed there, then proceeded to oscillate outside of normal parameters only returning to 'normal' swing after approx. 5 miles of driving. No concerns, just a head scratcher. THEN - I fitted a new battery [aged, old unit not holding charge for very long] and upon start up the ammeter dives immediately to full charge, then proceeds to oscillate across both scales. Upon driving off this oscillation, with a definite bias towards full charge, continues intermittently, with normal readings. Very odd!

I have had numerous S.1s and other '60s Jaguars and have never experienced this before - any ideas? :scratchheadyellow:

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cactusman
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#2 Re: 4.2 ameter oscillation

Post by cactusman » Sat Nov 10, 2018 6:00 pm

Hard to know but if the charge is intermittent the needle will swing wildly as it is not damped. Check connections and if they are ok suspect the regulator that sits under the heater box.
Julian the E-type man
1962 FHC
1966 MGB....fab little car too

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keithmac
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#3 Re: 4.2 ameter oscillation

Post by keithmac » Sat Nov 10, 2018 10:27 pm

I have exactly the same problem. It can be stopped sometimes by turning on the headlights. Next time I start up the meter starts oscillating again. Especially at idle.
With all the other issues I am dealing with on this car it has been too far down my list to attend to.
Regards
Keith
Early 1967 S1 OTS, 1952 MG TD

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JagWaugh
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#4 Re: 4.2 ameter oscillation

Post by JagWaugh » Sun Nov 11, 2018 8:50 am

Check the brushes. If they're stuck, or worn enough that they aren't making contact consistently then you'll get a jumping needle.

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Brucey
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#5 Re: 4.2 ameter oscillation

Post by Brucey » Sun Nov 11, 2018 4:02 pm

Thanks guys. The particularly strange thing is why it has become so pronounced since fitting a new battery.

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MarekH
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#6 Re: 4.2 ameter oscillation

Post by MarekH » Sun Nov 11, 2018 6:49 pm

Dear Brucey,

If the ammeter needle is undamped and oscillating wildly, it is because there are large current pulses on the line or it is resonating at particular frequencies/currents. You have reported that this is worse with a new battery and others reported it is less so when the headlamps are turned on.

Now lets consider what is going on.

If, for argument's sake, your alternator is set to 14v and your old battery normally reads 12.5v, then the battery gets charged whenever the rectified alternator voltage is higher than battery voltage. Contrary to what you might think, the alternator doesn't put out a smooth 14v. What actually happens is that each winding on the alternator puts out a sine wave going from -14v to 0v, up to +14v, then down to 0v and further down to -14v, repeated. This is then rectified to become a 0 to +14v to 0v to +14v. (I have ignored any voltage losses as current passes through the diodes here.) This means that it spends quite a lot of time below 12.5v. When this is happening, that alternator winding is not powering the battery - it is doing nothing! When the rectified alternator voltage is briefly above 12.5v, current flows and the battery is charged.

This means that although you may using electrical energy is a nice even measured way, the recharging of the battery is happening in pulses. It also follows that the new battery is being hammered by shorter, higher current pulses than the old battery, as the duty cycle is lower. So if per cycle, the alternator spends 10% of its time above 12.5v and you are using a steady 5amps, the alternator is delivering short 50amp pulses into the battery to recharge it.

If this sounds like bad news, then yes it is. The good news is that there is more than one alternator winding and that the alternator is spinning very fast. If the alternator windings are all well connected by their respective brushes and they overlap nicely, then pulses will overlap and current delivery is smoother. The battery condition is also important as it is the only thing acting to damp out the electrical mismatch between arrival of energy and its consumption further downstream.

kind regards
Marek

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keithmac
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#7 Re: 4.2 ameter oscillation

Post by keithmac » Sun Nov 11, 2018 8:59 pm

Thanks Marek,
So does this mean my alternator is failing?
Early 1967 S1 OTS, 1952 MG TD

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MarekH
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#8 Re: 4.2 ameter oscillation

Post by MarekH » Sun Nov 11, 2018 10:47 pm

I've no idea whether your "alternator is failing". I've simply outlined the two most likely mechanisms by which the ammeter needle can be made to move that way. The advice to check the condition of the brushes is a sensible next step. If they don't conduct nicely or the commutator is worn, then that'll need fixing.

kind regards
Marek

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