help no electrics on dash

[guesty90]

hi there i have a problem with my nova i was driving along today then all of a sudden the cd player went off then i noticed that the fuel gage and temp gage were not working when i rev the car the battery light comes on but the car still starts up any ideas

[Yorkie]

Im not a mechanic but with the dash lights and stuff i would put that to a blown fuse but then i cant explain the battery light coming on.

[Gaz1300SR]

Alternator sounds duff to me.

edit: could it be an earth strap. I am sure someone else will know.

[MK999]

earth, unfortunately the main one for the dash is right behind it, if it's a mk2 I'm pretty sure you can see it with just the clocks out, if it's a mk1 remove the dash and replace with a mk2 item first, then locate the earth just behind the clocks. If it's not that one gone try gearbox earth strap, battery, etc etc

[MK999]

Alternator sounds duff to me.

If the alternator was dead car would work pretty normally until it just spluttered to a halt, been there done that lol

Edit: Actually re reading the first post, everything is now working again? and your battery light comes on when you rev up?

[Yorkie]

Once you have left your car engine to cool, go back and feel the alternator, if its still warm then its draining your battery instead of holding the charge meaning it needs replacing ( learnt from experince ) But like Mk999 said it would work pretty normally untill it just splutterd to a halt ( also experinced )

[MK999]

Amanda, If the regulator on the alternator is dead and the alternator internals still work, it runs on AC rather than DC and varies between 12-13 volts and 20+ rather than a steady 14.6ish, that would give the battery light when revving symptom, and also probably as you correctly diagnosed, blow a couple of fuses as it spikes to 20 odd volts

[Yorkie]

Hmm yeah my knowledge in that area goes no further and that didnt make any sence :) All im going on about is my own experince from my ford fiesta using terms that were used to erxplain it to me by sam in a way it could possibly make sense lol But at least i diagnosed some thing right, hope for me yet :D

[MK999]

Did try and find something that explains it simply but it all mentions rectifier bridges etc without explaining them at all, so not much good

Basically though an alternator is a reverse motor so it puts out an alternating current (+V, -V, +V, -V however many times per second) This is due to the way a motor works with it's windings etc and the magnets inside it, if it didn't alternate it would just flip over 180 degrees and stop turning as the magnetic poles would be aligned north to south. The current in a motor is switched with brushes to keep it turning the same direction, and they swap the current at the same speed as the motor turns.

In an alternator you can't really do this as it's rather inefficient (and otherwise called a dynamo!) So it's done by some clever circuitry which is your regulator, which works on diodes so the current only ever flows in one direction +V, and an even clevererer diode called a zener diode that only lets +14.6V through.

So if the reverse motor bit is still working and creating voltage and your regulator goes boom you get a changing voltage, which alternates positive to minus.

[Yorkie]

i got the first bit :) ..... your so clever :)

[Yorkie]

i just tried repping you but i cant. Still very clever :D

[MK999]

Diode is an electric gate with a no entry sign on one side lol

[windfreek]

yeah that'd be a wheatstone bridge rectifier (or simply 4 diodes together)

as MK999 said, its a no entry sign for current, the output from an alternator likes to travel back and fourth down the wires about 60 times a second, the reason you have 4 diodes is a pair each controlls the +ve and the -ve you need for the battery

imagine you have a rollercoaster and each time it enters the station it alternates how ppl get on and off, sometimes ppl get off to the right and on to the left, but the next carriage ppl get off to the left and get on to the right,
so how do you stop ppl waiting to get on to the right trying to get on as ppl get off to the right, well that would be a one way gate, but then those ppl cant get on so you then put another one way gate in, with those gates on both sides you can control the flow of ppl, or in this case current, thats what a rectifier does, makes sure current only goes in the direction you want it to.

http://www.ibiblio.org/kuphaldt/socratic/output/animation_bridge_rectifier_fast.gif

you can test the rectifier on the alternator by disconnecting the output leads from the alternator and connecting a multimeter upto those connections on the alternator,

with your leads on one way round you will get a continuity reading, swap the probes over and if its working right, you wont get any continuity showing, if its goosed it will show it both ways or not at all, it could also be one of the primary windings in the alternator itself, i had to re-wind mine as the output was low due to the enamel burning thru on one of the windings and causing a short, (ask my wife,i never do anything the easy way lol)

edit* MK999 did you manage to get your elecy windows sorted from off your alarm?