Electrical help PLEASE.

Sean said:

yer i know, but when someone mentioned attaching bulbs across the battery to try and lower the voltage (!!!!!!!!!!!) i went and hid behind the sofa in case the battery exploded

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Sean said:

my head hurts

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fanatic said:

pm me your number if you want
what happens is the ignition feeds the battery light+.
without engine running the alternator -
when engine running you have ++ and light goes out.

so if you run a 12 volt feed to the battery light terminal on alternator- it will give alternator regulator the reference voltage and charge
Therefore any 12 volt feed (fused in case it rubs out ) will get alternator working correctly

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classic kev said:

Sorry boss you lost me a bit there I'm pretty sure it is over charging, DVM shows too many volts across battery and from Alternator out to batt neg. Are you suggesting that I can take off the ignition/charge light wire (I think that's the small brown/yellow wire) and run a flying lead from that to the battery with a fuse in the way? Should be able to do that - can I be stooopid and ask what size fuse might be suitable?

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Sean said:

be careful kids (really)

messing with stuff like this kills or maims people for life

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Datatek said:

On the P38 the ignition warning light is fed from the BECM and in turn the BECM provides the + 12 volts to the alternator, this is so the BECM can flag up an alternator fault. The +12V feed is what starts the alternator charging (excitation voltage). The diode pack converts the 3 phase AC current produced by the alternator (AC = Alternating current) to DC for the cars electrical system. However, there is from memory one diode in the diode pack that is part of the excitation circuit which also affects the regulator. You may need to change the diode pack. It could also just be a faulty connection within the alternator.

Bottom line is that there is no regulation of the alternator output. Possible reasons for this are:-
1) replacement regulator NBG.
2) diode pack faulty
3) bad earth connection for the regulator within the alternator.

Hope that helps

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classic kev said:

Could be a bad earth but not really able to make a detailed test where I am. I think diodes are O.K. as charge light not illuminated when engine running or off but hear what you say. Will let you all know how I get on tomorrow. Ta muchly.

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Datatek said:

If the charge light is not lighting with the engine not running, on a normal car, that would mean something with the BECM involved it doesnt mean much

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classic kev said:

Luckily no BECM on a '91 Classic but then again, you don't get a helpful message to say your alternator is playing up and you're cooking your battery.

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Sean said:

I'm glad someone found a way to mention that didn't upset the elpful chap

just to confirm, you've not replaced the diode pack/rectifier yet ?

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classic kev said:

Luckily no BECM on a '91 Classic but then again, you don't get a helpful message to say your alternator is playing up and you're cooking your battery.

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fanatic said:

. fact there is no battery light =no reference voltage=overcharging.

Sorry, not so. The battery light provides the excitation voltage to start the alternator charging. No battery light = no excitation voltage = no charging, except that sometimes there is enough residual magnetism in the armature to kick the alternator off. When the alternator is running & charging the battery light wire serves no purpose
Overvoltage can only be caused by the regulator not doing it's job which on some alternators can be affected by the diode pack.

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Datatek said:

fanatic said:

. fact there is no battery light =no reference voltage=overcharging.

Sorry, not so. The battery light provides the excitation voltage to start the alternator charging. No battery light = no excitation voltage = no charging, except that sometimes there is enough residual magnetism in the armature to kick the alternator off. When the alternator is running & charging the battery light wire serves no purpose
Overvoltage can only be caused by the regulator not doing it's job which on some alternators can be affected by the diode pack.

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At last!
Someone who realises that the 12v is an exciter voltage, not a reference voltage! And that incorrect rectification can affect the regulator performance!
raise:
putting 12v straight from the battery to the exciter circuit is, in my opinion, plain bloody stupid, if not dangerous. St minimum it needs a switch. It is best to check if you have 12v on the exciter wire with the ign turned on. If not, check the warning bulb. If that has gone pop then no exciter volts.

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