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Wierd volt meter readings


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Fitted a VDO voltmeter a while back and come to conclusion it is reading about 1V below (probably need to upgrade wires), So running normal is around 13V, put on side lights drops to 12V, put on headlights and heater and it drops to about 11V. Have grown used to this and do not bother too much.

Last couple of days though it has been dropping much further and last night with just side and head lights on it dropped below 10V on the meter. As soon as I turn lights off ,not whilst driving:-), it comes straight back up.

I am pretty sure battery is ok since starts immediately, even with this morning's snow on ground. So does this imply alternator is not giving out the current that it ought to (in which case can I fix or is it new alternator) or could the voltage regulator be on the blink (in which case I presume swap in a new one)

It is disconcerting driving with volt meter below 10V since even allowing for my suspected 1V error it is still pretty low and wonder if lights will all go out.

Any thoughts or suggestions would be most helpful.

Thanks and best wishes

Malcy

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Try connecting the voltmeter directly to the battery terminal with a decent gauge wire (8A+ will do), and then try it all again. If you get the same results, then the alternator has had it, but if not, then the wiring in the dash is the likely problem - corroded terminals on switch gear connectors and in the fuse box are highly liklely to cause this trouble.

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Thanks. I had been coming to conclusion that it was probably a wiring issue. So run a +ve and -ve 8A wire from battery direct to voltmeter and presumably still take the basic 12v supply off ignition feed (working from memory). Will also put multi-meter direct on battery too (done this before) for comparative purposes).

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Leave the wiring as it is for the test - just move the voltmeter connection directly to the battery instead of (and not in addition to) the existing connection to the fuse box or ignition circuits, or leave that too and just use your multimeter since you have one and try the lights with the engine running. If the battery voltage drps, it's an alternator problem, but if the voltage at the battery stays steady, only then do you need to start poking around the dash wiring.

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Exactly how much current are we expecting the voltmeter to be drawing here? :blink:

Virtually nothing, but when chasing voltage drops, it seems a good idea to make sure that the voltmeter itself is getting a good feed with as little resistance as possible. It's only a temporary wire for the test, after all.

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forget the voltmeter, use a multimeter to test the alternator across the battery.

you may find your voltmeter connector are dodgy/damp/corroded or something.

Since he has one, then yes, use the multimeter rather than the voltmeter (I already said that above). I originally suggested the direct bypass because I didn't know he had the tool and because others may have a similar issue and not own a multimeter.

Regardless, while the voltmeter may be inaccurate, it's unlikely to be showing different readings based on lighting selections because it is faulty itself. If it is readinga voltage drop within the ignition circuits, then it's likely to be corroded terminals in the fuse box or on the ignition switch.

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Right. Now been out into howling wind and rain but have multi-meter readings off the battery posts - just in cabin LED light on. Engine off 12.48V. Tickover 13.2V. Tickover and sidelights on 12.8V. Tickover and dipped headlights on 12.25V. Tickover and full beam plus two spotlights 11.87V

Suggests to me that battery is OK since mostly short drives recently and ofen in dark. To be honest I am not sure whether this all says alternator is OK or not but it certainly suggests that there is some corrosion somewhere inbetween the battery and the volt meter. Anyone able to shine a more intelligent light upon these readings for me??

Thanks

Malcy

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Assuming the engine had enough rpm to excite the alternator coils (my 300Tdi is fine off the bat, but my 200 needs a small blip on the throttle to excite the coils before the alternator will work at idle), then it sounds like you have a bad alternator - only a depleted battery should suffer a voltage drop like that with the alterator producing a charge, but you don't report starting difficulties, so I have no reason to suspect battery issues.

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If I were to wire a voltmeter directly and permanently to the battery for the most accurate reading, would it significantly reduce the battery if the vehicle were regularly left for a week at a time as most weekend land rovers are ?

Mo

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If I were to wire a voltmeter directly and permanently to the battery for the most accurate reading, would it significantly reduce the battery if the vehicle were regularly left for a week at a time as most weekend land rovers are ?

Mo

wired my 2nd voltmeter to my 2nd [winch] batt, I used a 4 pin relay to turn the voltmeter off when ignition is off.

post-20-0-03968200-1360971216_thumb.jpg

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Thanks Ralph but I'd like to come straight off the battery. So how much power would a voltmeter draw ?

found this info sheet from VDO http://www.vdo-gauges.com/media/instructions/TU00-0755-0000002%20Cockpit%20Int'l%20Voltmeter%20Data%20Sheet.pdf

it details Current consumption: 67 mA = 16 V (without illumination)

63 mA = 32 V (without illumination)
without a switch in the circuit the voltmeter will be showing a continuos reading
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Right. Now been out into howling wind and rain but have multi-meter readings off the battery posts - just in cabin LED light on. Engine off 12.48V. Tickover 13.2V. Tickover and sidelights on 12.8V. Tickover and dipped headlights on 12.25V. Tickover and full beam plus two spotlights 11.87V

Suggests to me that battery is OK since mostly short drives recently and ofen in dark. To be honest I am not sure whether this all says alternator is OK or not but it certainly suggests that there is some corrosion somewhere inbetween the battery and the volt meter. Anyone able to shine a more intelligent light upon these readings for me??

Thanks

Malcy

Was/is it slow to start? I think your battery is ok, I'm pretty sure a good alternator should chuck out 13.8-14 volts. You could take it for a good run down the road and see how the readings compare then. Can anyone else shed light on what their alternator chucks out?! my landy is 100 miles away! sorry I'm not much help.

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Was/is it slow to start? I think your battery is ok, I'm pretty sure a good alternator should chuck out 13.8-14 volts. You could take it for a good run down the road and see how the readings compare then. Can anyone else shed light on what their alternator chucks out?! my landy is 100 miles away! sorry I'm not much help.

14.4volts at least with the engine at 2000rpm and no loads at all.

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as above, 14.4-14.6 with no loads, switch everything on, it will drop a volt, but you dont want it going below 12.75. as said this is with the engine spinning at 2000+rpm

so as i say in a previous thread i would do the test again with the engine spinning at 1500-2000rpm and see what readings you get.

HTH

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Thanks for all comments - was tied up with other things today as first Sunday for yonks when weather was good so got propshafts greased, oils all checked, dodgy dipstick sorted and "new" winch bumper tested ready (plus drilled some mecessary holes) then off for blasting and coating.

Will try and get time tomorrow to run tests with the engine running faster. Cannot recall make off top of my head but will try and post that too when I have new results. Engine starts pretty easily all the time.

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Test it with the multimeter across the battery and see what readings you get, then do another from the back of the alternator to the chassis and compare, if the readings come back different/lower across the battery, you could have earthing issues.

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