vas
Well-Known Member
right, evening all, another odd one with the solution this time for a change 
both my engine's VDO viewline voltmeters are displaying whatever they feel like which is v.often WAY above right values.
Checked with a voltmeter on the batteries as well as on the alternators AND using the v.helpful Victron BMV700, I was sure that my alternators were fine, however I didn't like the fact that usually on the stbrd engine 5-10min after firing up needle would shoot up to the pin 32V, it was an uneasy feeling.
So decided to have a go at checking them up and sorting them.
Opening them up wasn't particularly difficult, once you figure it out (it's always the case!)
OK, mine are the older ones so two male spades on the back for + and - plus a smallish hole for the low voltage warning lamp and a larger hole for the push fit lighting.
Unscrew the bezel
Prise (carefully!) out the rubber gasket will usually come up with the lens.
Undo a nut (the only one at the back!) don't undo it fully, turn gauge upside down and press on the bolt and nut to release tension and loosen the whole assembly out of the casing.
Prise out the needle (take a pic beforehand in order to not mess up too much when pushing it in again)
With a T6X70 torx screwdriver undo the two tiny black screws holding the besel.
OK, now you have the lot apart.
Notice that you have:
a coil nicely wrapped up in a metal "cup",
a 215Ω 2% resistor (probably 2W or more) which is getting bloody hot
a 220Ω trimmer
and a diode
Following my 700+ h of engine use living on a nice and rather warm lower helm, the heat from the resistor messes the graphite on the trim. As a result, values of the trim change randomly and a lot. On the bench at home with a clean adjustable powersupply, it would move from 24V cold to 25 then drop and play a bit and eventually shoot up to 29-30... All within the first 10mins from cold. Then it would stay there for a while but occasionally shoot down or whatever. Not nice.
Solution:
simple, remove the 220Ω trim and replace with new, you need to adjust it to a value of around 70Ω (or thereabouts).
Once done, put it back together, and use the oddly placed hole on the top through the threads to drive a V.SMALL screwdriver and fine tune it.
It's not more than a half hour job (he says...), OK make it one hour.
BTW, if you want to confirm that this is the issue you're having (and assuming you HAVE checked with a decent digital voltmeter that the values that reach the gauge are right) is to place said tiny screwdriver down this hole (before taking it apart!) and simply pressing the trimmer (ok, you don't know what you're pressing yet, just trust me. If by lightly pressing there, values jump about, you have a buggered trimmer.
hope it helps someone with VDO gauges, and really surprised none has reported such a problem before. OK, I searched, honest, now wait for FARSCO to come up with a dozen threads describing the problem and the solution
cheers
V.
both my engine's VDO viewline voltmeters are displaying whatever they feel like which is v.often WAY above right values.
Checked with a voltmeter on the batteries as well as on the alternators AND using the v.helpful Victron BMV700, I was sure that my alternators were fine, however I didn't like the fact that usually on the stbrd engine 5-10min after firing up needle would shoot up to the pin 32V, it was an uneasy feeling.
So decided to have a go at checking them up and sorting them.
Opening them up wasn't particularly difficult, once you figure it out (it's always the case!)
OK, mine are the older ones so two male spades on the back for + and - plus a smallish hole for the low voltage warning lamp and a larger hole for the push fit lighting.
Unscrew the bezel
Prise (carefully!) out the rubber gasket will usually come up with the lens.
Undo a nut (the only one at the back!) don't undo it fully, turn gauge upside down and press on the bolt and nut to release tension and loosen the whole assembly out of the casing.
Prise out the needle (take a pic beforehand in order to not mess up too much when pushing it in again)
With a T6X70 torx screwdriver undo the two tiny black screws holding the besel.
OK, now you have the lot apart.
Notice that you have:
a coil nicely wrapped up in a metal "cup",
a 215Ω 2% resistor (probably 2W or more) which is getting bloody hot
a 220Ω trimmer
and a diode
Following my 700+ h of engine use living on a nice and rather warm lower helm, the heat from the resistor messes the graphite on the trim. As a result, values of the trim change randomly and a lot. On the bench at home with a clean adjustable powersupply, it would move from 24V cold to 25 then drop and play a bit and eventually shoot up to 29-30... All within the first 10mins from cold. Then it would stay there for a while but occasionally shoot down or whatever. Not nice.
Solution:
simple, remove the 220Ω trim and replace with new, you need to adjust it to a value of around 70Ω (or thereabouts).
Once done, put it back together, and use the oddly placed hole on the top through the threads to drive a V.SMALL screwdriver and fine tune it.
It's not more than a half hour job (he says...), OK make it one hour.
BTW, if you want to confirm that this is the issue you're having (and assuming you HAVE checked with a decent digital voltmeter that the values that reach the gauge are right) is to place said tiny screwdriver down this hole (before taking it apart!) and simply pressing the trimmer (ok, you don't know what you're pressing yet, just trust me. If by lightly pressing there, values jump about, you have a buggered trimmer.
hope it helps someone with VDO gauges, and really surprised none has reported such a problem before. OK, I searched, honest, now wait for FARSCO to come up with a dozen threads describing the problem and the solution
cheers
V.
