alternator not charging until i rev the engine above 3k

[youngprodigys]

as in the title. dont know how this developed but im hoping somebody has had the same problem.

when i turn the engine on the alternator light illuminates and doesnt go out unless i rev the car above 3k - after doing which it starts to charge the battery.

a brand new alternator has NOT cured this.
tried 3 different batterys and that has NOT cured this.

has anyone else had this probelm - perhaps with toothed belts etc etc.

[hayesey]

checked the condition of the wirng to the alternator?

[youngprodigys]

brand new mate. got a really good wire going from the alternator to the starter motor and a big thick cable going from the starter to the battery in the boot. which has caused me no issues up until now

[youngprodigys]

nope. brand new and is making a good earth, checked using multimeter

[youngprodigys]

well i dont think my old alternator was faulty, i then got a spare from my mate which i know works and tried that - failed! so i bought a new one anyway to be on the safe side. so it just cannot be the alternator.
i have tried an old trick of using a bulb on the blue wire and the 12v from the battery - same result you have to rev it.
what im going to do now is remove the earth side of the equation. im about to run a bloody big thick wire from the alternator casing straight to the battery negative terminal....fingers crossed this is it!

[youngprodigys]

SOLVED

Cause of the fault was a VERY small short circuit on the instrument cluster. still to locate which wire it is  (theres 32) but i have bypassed all electronic on the car and started up the engine with only the bare minimum wiring.
this included running my own earth sense bulb for the alternator direct from the battery.
the short was only a very small short (less then 0.013ohms) so it was not a high enough short to blow the fuse- which when removed the short disappears.

its incredible that such a small short has caused such a massive problem. the car now runs on idle with no problems when it is revved!!!!


If required i will write up a guide on how i located the short so others can follow it. (i got an electronics guy at work to run me through fault finding in circuits)

[Andy]

0.013 ohms is a pretty decent short-circuit! Fingers crossed it's easy to fix the clocks - or wiring loom if that's where the fault turns out to be.

[youngprodigys]

sorry i was on the wrong scale i think its 0.000013 it was undetectable on 2k, 20k, 200k AND 2M only on the 20M scale did i get 0.13 mate - it was bloody tiny didnt even make a spark connecting the battery.

[youngprodigys]

shud say 13ohm. I'm a spaz

[youngprodigys]

it was a short mate i located it to the Audi TT dash instrument cluster. it has a short circuit on the computer which sits in the middle of the screen. normally this would not pos a problem for the TT as it has a larger capacity battery and a larger alternator and the drain is negligble but because mine isnt so highly powered the drain on the circuit is high. and because this same wire powers the alternator light bulb - it makes the car fail.
i has the car charging again using a bulb from the battery and have isolated the short circuit during start up using a relay.

apparently this is a common problem with the TT's normally resolved by removing 2 fuses and leaving the car to reset. if left alone this short circuit eventually fucks your fuel gauge and engine temp. my bypass has corrected this.

BLOODY CARS!

[youngprodigys]

lol - my bad! im sure there is a picture of it in my other thread - the "help please" one