Voltage Loss

[Joe87]

Hi chaps,
Have had this problem for a while, posted on the old forum but still not got it sorted.
I get a low voltage when the engine is running, usually around 12-12.5 volts - I understand it should be about 13.6v. The voltage also drops when I switch things on, such as headlights - I've got the voltage down to about 10v before...Have tried another battery so don't think it's that, and pretty sure it's not a voltage drain anywhere. When I put a fully charged battery in I get around 13.6v for a while which gradually drops.

So could it be either the alternator or voltage regulator?

Any ideas much appreciated.

[Nate]

is this the voltage on the dash gauge?

measure the voltage from the big red wire right on the back of the alternator (+ve) and the case of the alternator (-ve) this should (hopefully) be 13 and a bit volts. assuming it is, measure again at the battery. this will probably be less - 12ish. now measure between the case of the alternator and the battery -ve. this should show 0.0V, but if the main engine earth strap is past it, you may see 2 or 3 volts. this can be down to a bad connection where it bolts to the gearbox mount. unbolt it, clean everything up with some find emery cloth or similar and remeasure. if it now 0V measure battery voltage which should now be the same as alternator voltage. if there is no change then try replacing the earth strap (universal ones available from all good motor factors for about a fiver. you will need 2 - mount to chassis leg and another to go to the battery.

assuming that all tested ok, measure between the big red one on the back of the alternator and battery +ve. if there is a voltage showing here, you'll need to replace (or supplement) the main alternator to battery cable.

obviously these tests should be done with the engine running, just be careful of rotating things!

[Joe87]

Nate, thanks for the response. Finally got round to trying what you suggested, voltage across the alternator was well over 13v, same with the battery. Voltage drop between the battery -ve and alternator case was about 0.05v, not sure if this is large enough to worry about? Tried turning lots of stuff on and both voltages dropped to just below 13v, and the voltage between the battery and alternator went up to about 0.15v. Strangely the volt meter inside the car shows a much lower voltage than my voltmeter does.

Any clue? Cheers

[Funky Diver]

Incorrectly fitted gauge is my guess.

If you have enough wire try hooking the gauge up to as close to the battery on a positive live and a good clean earth.

If you're still getting spurious readings turning stuff on loads whilst comparing directly to the output on your meter at the same points... then the gauge is knackered. If it's staying steady, then it's wired up incorrectly.

for arguments sake (and I'm keeping this VERY basic so anyone with electrical theory I know about the relationship V, I and R)...

12 volts are available to use. If a light uses 3 volts in it's operation, that is going to leave 9 volts in the rest of the circuit. If your meter is attached here, say, then when you turn the lights on it is going to read 9 volts. This is what appears to be happening in your wiring.

[Nate]

more or less what he said. gauge fault.

[Joe87]

I'm in the process of putting in a new volt meter at the moment so I'll see if this one works any better...

Cheers.