Variable voltmeter readings

[Oldromola]

I am experiencing an unusual (well, in my experience it's unusual) variation of readings on my voltmeter in that the readings are going DOWN at higher revs. At low revs the reading is normal at about 14 volts, but this drops to less than 13 volts at 3000 revs. These readings are during the day without the lights or wipers on.

Has anyone an explanation? If the worse comes to the worse I have a reconditioned alternator in the attic!

[stustag]

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Hi Robert

I would do the normal checks with a multimeter to determin if it is the battery or the alternator?

http://stagownersclub.mywowbb.com/forum1/761.html

Stuart

[KOY 23]

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You need to check the battery voltage with a multimeter to see if the voltmeter is correct.

Is the heater pipe to the screen vent on properly? I had a case years ago where the volts dropped as car warmed up, it turned out to be the heater blasting onto the console mounted voltmeter, which it did not like.

John.

[Oldromola]

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Thank you Stuart for the link. As I don't think the condition is terminal just yet I am hoping somebody will know the likely cause of what is happening before I go to the lengths of testing as suggested in your case.

Thank you also John for your reply. I don't think the heater is the culprit, as the volts go back to normal every time the revs drop. The battery voltage is fine according to my multimeter.

[KOY 23]

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If the actual voltage is correct, then there must be a problem with the voltmeter or connections, I would check them and the earth for the voltmeter and the rev counter.
John

[Oldromola]

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Good thinking John - the earth to the rev counter can be a devil.

[Oldromola]

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The problem is now resolved, so what caused it? Checking the battery and alternator charge with a multimeter is all very well when the car is stationary, but I have yet to work out a method for doing so at 60 mph.

I checked the various connections as best I could and that included taking off the main earth, thoroughly cleaning all round and then refitting. It is practically impossible to check the block terminal on the alternator without removing it, so in the end I reluctantly decided to go for it. Thank you Kevin Garrod for your method.It worked just fine, and is easier than going beneath the car which is what I have done previously.

Well - out came a rather sorry looking alternator. Not only was one fan blade missing, but a total of three had gone walkies! The alternator was new in the autumn of 2006, but the fan and pulley were the original. The lack of three fan blades must have caused some increase in heat, and I am wondering whether this was why the voltage was dropping at higher speeds. In went one of my spare alternators,back on went the hot water pipe clip, back on went the power steering pump and back on went the battery terminal. And.......nothing. Just a click from the starter motor and not even a glimmer from the ignition warning light.

So.......yes, everything came off again and out came the alternator. At this stage one of the wires going into the block terminal simply fell out, thus explaining all the nothingness. Carefully ensuring all the leads (there are 3 of them) were as firmly connected as possible the alternator was re-fitted as before. This time BINGO,I was in business! I have since travelled only about 20 miles but the voltmeter is rock steady at not far off 14 volts whether or not the car is crawling or flying.

It is now looking a strong possibility that I was suffering from both an over-heated alternator together with a weak connectioninto the terminal, so I shallfit a new fan and take the offending alternator to be tested.With a bit of luck it will still be ok, and can go in the attic alongside my 'emergency Stag parts' collection for a future occasion.