Alt. relay or pulley problem???

[Nev62]

OK, here is the problem, at idle volt meter sits on around 12v. Turn something electrical on and watch the volt meter drop. At Hwy speed at night with lights and radio on, sits around 13 to 13.5v, turn the wipers on and it drop to around 12.2, put the demister on it it drops to around 11.8, put rear demister on and your down to around 11, indicators take you down to around 10, foot on brake takes you down to around 9.5 hi-beam and anything else takes you under 9v.

idleing around the bush at night and your luck to see much over 10v

If you keep the revs way up 2500rpm (this is a 6.5 diesel) plus and you might just get some charge - maybe

Which would be the most likely cause..

Alt - faulty
relay - faulty
pully - too small
earth - bad connection??????????????

[festy]

Alt brushes need replacing - or at least cleaning?
It could also be a dodgy regulator, but the voltage dropping with load makes me think bad contacts between the alternator's brushes and commutator.

[-Scott-]

I agree - start with brushes/regulator.

Charge at high revs suggests the AC/rectification side functions, but you're not getting enough field current to charge at lower revs. Field current is controlled by the regulator, and flows through the brushes. A problem with either could produce the results you're seeing.

Regulator may be easiest to replace first? Checking/cleaning the brushes would be cheapest.

Definitely get more advice on your pulleys - I don't think that's causing your problem now, but mismatched sizes won't help.

Good luck,

Scott

[DamTriton]

The farkin obvious, but where are you actually measuing the voltage from? Make sure the points of measurement are clean and that the batt earth to engine earth and chassis earth are good.

There is no way a set of windscreen wipers would/could EVER cause that amount of voltage drop on a half decent battery, and the fact that you haven't mentioned any problems with starting the vehicle seem to indicate more of a "measurement" problem than an"actual" problem.

[Nev62]

you haven't mentioned any problems with starting the vehicle

Sorry, yes after normal day time driving engine starts fine, nice and strong. After night driving and especilly in wet weather, hope to hell it starts after the first couple of cranks. Biggest voltage drop noticed with indicators on, followed by hi beam etc. Running dual starting batteries (its a GM 6.5 thing - one standard battery might give you one go and I'm not gonna fork out $700 for a hi energy battery)

I'll check out the bushes first, thanks guys.

[ausoops]

do your headlights dim significantly when you are getting less than 10V on your meter. if they aren't really dimming then you have ok voltage because any less than 10.5-11 volts you really start to notice a big drop in brightness. if this is the case the meter is picking up either a drop on a local circuit or possibly a rise in earth potential around its earth point.