Hi people,
Im currently trying to replace my old 1.7 crossflow with a Saab 2.0 turbo (b204l) and trying to get on with the electrickery side of things.
Now, I've decided to make a loom from scratch (original sensors & ecu though) and as elecrics arn't my strong spot I'm struggling a little.

I decided to make the most basic loom I can to get the engine started to begin with, and here's my question (finally)..
Is a charging light needed for the battery to actually receive a charge from the alternator as I'm half sure I read a few times that the bulb is actually needed for some reason about it kicking off the charging process and if bypassed with a wire it ruins the circuit?
I dont understand how a bulb being in a circuit is any different (for flow of electricity) than just having a straight wire??
Sorry to ramble but its late, i'm drunk and I'm sure none of that made sense...
Cheers, Kev

i have an issue with mine, the light doesnt go out when the engine is running. I have been told that there are several different alternator systems and it depends on which type you have as to whether it needs the bulb in the circuit.

Mine still doesnt work though so i may be talking out my a**e

Can't see that the lamp serves any purpose other than it tells "IF" the alternator is charging, so really it's quite handy to have one. Cheers Ray

I think that the bulb allows current to flow to produce a magnetic field for the alternator to begin operation. Once the alternator is producing power it is self sustaining.

I have seen some motors in the past that had a similar system, but they would still start without it, as long as they'd been run recently enough that there was still residual magnetism in the cores.

the builb is there to show charging or not and if you didnt have it it would be like dead shorting the wire down to earth .ignition on engine not running you have 12v to the builb and earths out through the ault. with the engine runnung the terminal on the ault becomes the same as the ault output ie 14.5v you then get voltage to both sides of the builb wich turns the builb out

hope this helps you mate

You need the bulb.
As well as indicating when the alternator is making juice the flow through the bulb pre-excites the circuit to create the current. No flow through the bulb= no charging.

Which is why the bulb blowing is seriously bad news because you not only lose the ability to see that you're charging but you lose the ability to charge at all.

You can miss out the bulb a straight wire because bulbs have a pretty low resistance but some charging circuits won't work without a seriously low resistance. On my ST charge circuit I had to omit the charge bulb in the end and just have zero resistance- even the lowest resistance bulb I could find was enough to prevent the alternator coils working....

my main concern about not having a bulb was that if the alternator belt snaps while on track, or the road for that matter, i have no water pump either!

not sure what else your belt drives

Blimey, that was fast!
Bulb it is then, I had to do it properly sometime so I guess it may as well be from the start!
Cheers all
Still cant quite get my head around quite why it works though so I'll put it down to magic
Kev

When I did my dash I used LEDs for warning lights,the charging light LED didnt work to well.I had to rev the engine quite hard to get it to go out,and was told you needed a proper bulb to excite the circuit.

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Originally posted by triumphdave
When I did my dash I used LEDs for warning lights,the charging light LED didnt work to well.I had to rev the engine quite hard to get it to go out,and was told you needed a proper bulb to excite the circuit.

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