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Dynamo testing


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Hi.  Had a charging issue on Vitesse since I got it (8 years ago) that has pointed to the dynamo, as related by the basic test (according to various publications) of removing the two cables and bridging the two dynamo connections with a wire and measuring voltage at idle, around 700 rpm. This has checked out at between 5/6 volts. Apparently it should be around 16/18 volts.

Volts will rise to 16/18 figure at about 1500 rpm.

Thing is, it's doing these same figures on three different dynamos (my original, and a recon, that I bought as a private sale and looks like it has been internally renewed, clean cotton and all new connections, brushes etc), lastly a dynamo donated by a club member, that was apparently working, though struggled in winter with city driving at night, with wipers and heater on (she replaced with an alternator). Al the brushes are in the wear spec and not sticking.

Have dismantled all three and done the internal resistance tests, ie, field coils, adjacent commuter segments and all have checked out ok.

Charge light has always gone out after idle with all  three and have made sure dynamo's are earthed when testing by jump lead from dynamo to battery negative.

Have all the dynamos failed and it's a coincidence there all checking out with the same results?, or am I overlooking something?.

Thanks a lot for any ideas,

Dave 

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Biggest problem with a dynamo Dave is that it can't charge fast enough. So it depends on the driving you do, short miles everything on = flat battery. Summer motoring nothing on.... all ok. If you charged your battery every night you would be ok, but it would be simpler just to fit an alternator. It is also completely reversable if you wanted to go back to dynamo, but i doubt you ever would.

Tony.

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I certainly think all the dynamos are ok as long as their output volts rose smoothly and quickly during the testing.

My Vitesse still has its dynamo and it gives no issue at all including running a 10A radiator fan but as Tony says I never take it out just for a short run...

Could you have a regulator problem?

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Hmm... it's "well known" that dynamos don't charge enough and alternators are better, but it's not as clear-cut as all that. A dynamo in good condition with a correctly set control box will keep your battery properly charged, unless you really only do short journeys with everything on (when an alternator will also struggle!) If you have a weak battery and don't want to take it off the car to charge, a dynamo will quite often recover it better than an alternator, as long as you aren't loading it up beyond the rating.

700RPM sounds too low for that test and it's a fairly crude test anyway. If you've got three dynamos all showing the same results then I'd suspect those are the "normal" results and your charging problem lies elsewhere, probably the control box. Sure, replacing the whole system with an alternator would "fix" that too but it's not the only possible solution. And if your problem is actually a failing battery then an alternator won't make a jot of difference.

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Thanks for your replies. Car has a amp meter from when I bought it and always shows a big negative charge, say when the headlights etc are switched on, even when running on the motorway at say 3000 rpm, so I assume not a slow engine running thing.

Has always been like this, even with different (good) batteries.

Could be worth looking at control box and even trying a new one, about £20 (if there any good?).

 

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cant that be the way the ammeter is connected? It can be in the supply to the cars systems or to the battery with the first always showing a current flow one way and the other both ways but tending to zero when driving.

If you had the battery discharging that much all the time I dont think the car would restart...

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Posted (edited)

Thanks.  The amp meter shows around neutral when driving, as soon as anything is switched on, then moves to the negative, regardless of rpm.  I always assumed it must the dynamos at fault, as they all checked out with low voltage at low rpm. Could it be the amp meter wired up wrong maybe?.

Not great at electrics, though could maybe figure out how to disconnect amp meter, though would it have replaced the standard cable from control box to solenoid (I think this is how it would be).  

I always charge the battery after use, as varies from a bit low, to very low if have been using lights/wipers.

  

 

Edited by Dave Clasper
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well neither way of connecting the ammeter is wrong as some people want to see what current is going in and out of the battery while others want to know the total system load.

Does your meter ever show positive, after first starting for example when a large current should be flowing into the battery to charge it?

The ignition warning light is the other way of seeing if the dynamo is producing a higher voltage than the battery and therefore charging it - if it comes on at low revs, especially when you start turning stuff on, then you know youve got a problem....

If all this checks out ok then I think maybe your battery is on its way out😟

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Thanks a lot.

I don't recall the amp meter ever showing a pos reading?, (so maybe not good), though will keep an eye out.

The ignition warning light, as far as I have observed, has always been normal and only comes on at idle, regardless of anything being switched on, (so maybe good), though will keep an eye out.

I think I will have to have another read up on the details on charging, to see if I can fathom these basics.

Cheers, Dave

    

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As Scrappy says nowadays a voltmeter is a better choice because the installation is much easier and the reading easier to understand: voltage dropping = not charging sufficiently.

An ammeter has to have large cables connected to it to cope with the maximum currents that can flow and these are generally not fused so if theres a short to earth in either the instrument or wiring to it you have a big problem...

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36 minutes ago, glang said:

the reading easier to understand: voltage dropping = not charging sufficiently.

Not sure I agree with that. A properly installed ammeter (which Dave's sounds like it isn't) will show +ve=charging, -ve=discharging, which is about as easy to interpret as possible. No need to watch for things changing or (potentially very slow reacting) needles "dropping".

The case you could make for a voltmeter being "easier to understand" would be for the ones with coloured bands, where green=all good, red=problem. After all, some discharging occasionally on a car with a dynamo isn't a problem as such.

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58 minutes ago, RobPearce said:

Not sure I agree with that. A properly installed ammeter (which Dave's sounds like it isn't) will show +ve=charging, -ve=discharging, which is about as easy to interpret as possible. No need to watch for things changing or (potentially very slow reacting) needles "dropping".

Its just that in my experience the 50mm ammeters fitted are pretty crude and inaccurate as theyre made to read up to 50A positive and negative so a little trickle in or out is hard to be sure of. Therefore your battery can be slowly discharging without indication and the first you know is that it hasnt got the guts for a restart.

Voltmeters by their nature are easier to manufacture to read accurately so a change is easier to detect especially as you say using one with coloured bands...

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Thanks folks

If the Amp meter is wired incorrectly, could this be causing the charging issue?.

If so, I can have a good look at it and describe how it's wired in. Would that be ok for some feedback from your selves?.

I'm aware of the safety issues of amp meters, so will look at getting rid, though I thought at first it could be handy to establish if this may be causing the charging issue?.

Cheers, Dave  

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I dont think youve got a charging issue Dave or your ignition light would be coming on. To confirm this measure the voltage at the battery at tickover with everything turned on...

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Thanks Glang. Will try that. Have looked after connections/earths, so should/could be ok and wiring appears good condition and not generally messed about with.

Just don't understand some of the basics of the travel of electricity, with some of the components (mainly charging I think).

Cheers, Dave  

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As I say Dave if you have a meter set it to measure up to 20v DC and check the battery volts after youve fully charged it (a new battery will read over 13v). Then start the car and measure again. The reading should be a little higher than the previous one although if rising you may have to wait a little a while as it charges and reaches its stable value.

A car battery is made up of lots of lead plates connected in series (to give the required voltage) AND then connected in parallel (to supply the large current needed). This means that any deterioration of the plates can still leave it with a reasonable voltage reading but it cant produce the full current it should. Theres no way to test this 'capacity' without specialised beefy high current equipment so you can think the batterys good when it isnt. Im not sure if even those green condition indicators on modern batteries can really show this...

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Thanks Glang. Will do that next time I'm using the car, though even when a new battery was fitted some time back, I seem to remember it was showing around 12.5 volts for this test.

This low reading can be other issues and not just the battery, I thought?.

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Just to add (I can't seem to edit above), Every time I use the car, the battery will need a various level (depending on the driving conditions) of charge.

Once charged, even if left for a while, it cranks the engine super fast and for a good while if needed.  

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Just seen that Glang sorry, but eight years of problems i would have sold the car by now, or at the very least just fitted an alternator as it can easily be put back to standard should anyone want to.

Tony. 

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2 hours ago, poppyman said:
2 hours ago, poppyman said:

but eight years of problems

 

Thanks, a "problem" rather than "problems".  Ltd time, so has been taken up with gearbox rebuild, quite a few other repairs and ongoing maintenance.

At a stage with a bit more free time now.

Cheers, Dave 

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