16 replies to this topic

[Neon trifle]

My battery light came on the other day then after checking the voltage at the battery (11.5 with engine running) I changed the alternator for a new one. However the problem is the light still remains on....could this be that the battery is dud? Do I need a healthy battery giving out >12v to exite the alternator fields? thus turning out the battery light.
I have but my voltmeter accros the NEW alternator terminal (small wire) and earth and get around 2 volts and less than that accros the main wire. Tried the voltmeter at the battery with engine running and get around 11v.
Thanks in advance for your help

[minislapper]

With the engine running you should be getting around 14v !

[fikus01]

the light going out is a sign the alternator has been switched on!! ideally you should get 13.6volts or thereabouts to keep the charging going ok!! sometimes giving the alternatos a little tap with the happer can bring it back to life but not always and not for very long if it does!!

[Bounce]

clean the termination on the small wire, thats the one that switches the alternator on and turns the light out. if youve had a new alternator id investigate that wire

[Neon trifle]

Thanks
Just been in the garage..........put voltmeter on battery light wires behind dash and get 12.5v on white wire and 2-3v on brown /yellow one, I cannot explain this volt drop from one side of bulb to the other? The brown/yellow wire goes to the alternator and I get the same reading there (2-3v). Perhaps this is just a flat battery? As I mentioned earlier if I dont get 12v at alt will it then not exite? or maybe some kind of short to earth on the wire between bulb and alternator.
Any thoughts?
cheers

[Bounce]

If it was a short to earth youd get 0V as there would be no potential difference between the wire and earth, seein as they are connected. Its probably a high resistance somewhere in the wire, check along the wire as far as you can.

[Neon trifle]

Thanks bounce i will check..........but how am I getting 12.5v one side of bulb and 2.5 on the other? I am checking voltage at conector block that plugs into dash. Sorry to be a PIA but I need the car again in morning so will have to work on through night....yawn!

[fikus01]

if the wire is ok it sounds like the electrical switch in the alternator has given up or didnt work in the first place!!!!

can u check the resistance of the wire that goes from the dash loom to the alternator? brown yellow wire!!

seriously have u tried giving it a tap with a hammer!! tried and tested AA method of getting 1 to work!!

[Neon trifle]

Do you mean checking the continuity of the br/ye wire? or by disconecting at alternator and checking that it is earthing a little somewhere?
Thanks

[web45]

tap it with a hammer it shold work

[fikus01]

both

[Bounce]

its a new alternator guys. shouldnt need a tap!

You did tighten the belt up properly didnt you?

[GraemeC]

New alternators don't always necessarily work, especially as most of them are re-con units. I've just sent a new one back that had a loose regulator pack and hence had broken about 3 wires internally.

It is difficult to measure voltage onthe lamp due to the way the circuit works. The alternator effectively puts a voltage on the circuit to try and balance the voltage from the ignition switch on either side of the bulb, hence turning it off. If the light is coming on then chances are the circuit integrity is OK but the alternator is struggling to excite properly or give out sufficient power.

Best thing to do is swap the alternator out with a KNOWN good one and see what happens. My money is on your new one being duff.

[Neon trifle]

Thanks, Belt is at correct tension.
Tapped with hammer without any joy.
Looks like I need to get new alternator...........trouble is they are in short supply for an MPi and the firm I got this from sold me there last one and they were £40 quid cheaper than anyone else at £70. Halfords want £240 so its an expensive experiment.


The only other thing I can think of is just before problems started I had had battery out and connected wrong way round (what a dummy lol) but it was only a quick splash.

[miniboo]

hmm well that could be your problem.

cheaper to try a new battery then.

but you could hav also fried the alternator