car keeps running with keys out
Monkeybasher - 4/3/06 at 01:26 PM
I feel kinda stupid, but for the last 2,000miles since I built my car and SVA'd it my alternator has never worked and I never realised.
. Shows
how little load there is on these cars. I have only charged the battery once. I thought it was low due the car sitting unused.
Anyway I now know why after a quick look at the wiring diagrams. I left the charge warning light out which is in series with the alternator, Doh. So
I thought I'm not fussed about this light so I'll short the plug where the lights meant to be. started my car and great 14v+ at the
battery. Alternators working. Then I tried to to turn the car off at the ignition and the car kept running.
I then assumed that it must need some sort of resistance so fitted a lamp and the lamp stayed on all the time and the alternator didn't work.
Finally I started the car with a short in place and then swapped the short to a lamp while the car ran. The car worked as it should 14v at the battery
and the car turning off with the ignition and the light coming on briefly as the engine died.
What am I doing wrong should the lamp be an LED?
Cheers
Steve
omega 24 v6 - 4/3/06 at 01:41 PM
The lamp needs to be in the ign side of the circuit and not a permanent supply. When the alternator is charging there is a 12 volt feed in this
circuit which will be enough to keep the ign circuit going. Therefore you need it to be broken by the ign switch being turned off.
JoelP - 4/3/06 at 01:57 PM
i believe the current through the bulb energises the windings, or somesuch technical mumbo jumbo, hence its essential. If your bulb breaks, the alt
wont work but theres also then no warning bulb to tell you! 
Monkeybasher - 4/3/06 at 02:01 PM
quote:
Originally posted by omega 24 v6
The lamp needs to be in the ign side of the circuit and not a permanent supply. When the alternator is charging there is a 12 volt feed in this
circuit which will be enough to keep the ign circuit going. Therefore you need it to be broken by the ign switch being turned off.
The lamp is on the switched side. If I have a light in place then its off until I turn the ignition on, it then remains on while the engine runs(with
alt not working) then goes out when I turn ignition off.
Cheers
Steve
MikeRJ - 4/3/06 at 02:10 PM
You may have killed the alternator by running 12v straight into the field without the current limiting lamp.
Monkeybasher - 4/3/06 at 02:32 PM
quote:
Originally posted by MikeRJ
You may have killed the alternator by running 12v straight into the field without the current limiting lamp.
I didnt realise the lamp was there for any other reason than a warning, does it have to be a specific wattage, voltage, etc?
I'm pretty sure the alternator still works. If I touch 12v to the alternator field while the car is running it immediatly puts extra load on
the engine and the voltage jumps to 14v, it then self excites and keeps running at 14v.
I think I will run a new switched wire and see how I get on, I am relying on the Tiger wiring loom being ok just now.
Cheers
Steve
omega 24 v6 - 4/3/06 at 02:35 PM
quote:
You may have killed the alternator by running 12v straight into the field without the current limiting lamp.
This is highly probably If you can try and meter across the battery terminals with the engine running and alternator/warning lamp connected
you should get around 14 volts. if it's only 12 then your alternators more than likely fubared.
flak monkey - 4/3/06 at 03:01 PM
The alternator 'feed' from the alternator is actually a switched earth according to both the sierra and escort wiring diagrams i have. You
need a switched live feed (originally black/yellow in the sierra/escort) to one side of the bulb, the other is connected to the warning light wire
from the alternator (blue on both sierra and escort).
Cheers,
David
[Edited on 4/3/06 by flak monkey]
britishtrident - 4/3/06 at 05:11 PM
Suspect you have the big terminals on the alternator wired through the ignition switch -- they should go straight to the battery or the battery side
of the main starter conections..
as others have said the ignition warning light goes to the switched side of the ignition switch.
[Edited on 4/3/06 by britishtrident]
[Edited on 4/3/06 by britishtrident]
mark chandler - 4/3/06 at 05:22 PM
It sounds to me like the dash light, although correctly connected (on the small terminal) is holding the field coils on.
The simple fix is to insert a diode in-line with the bulb and not worry about fiddling with lots of wiring.
Regards Mark
Bob C - 4/3/06 at 08:49 PM
my understanding is this:
the alternator effectively has 2 sets of diodes in it - a big set connected to the battery terminal, and a little set which take over supplying the
field controller as soon as the engine is spinning.
Normally as soon as the little diodes are supplying the field controller the dash light goes out as this is no longer supplying this circuit.
When you put a short where the bulb should be, you now supply the whole "ignition switched" electrics (including the ignition system)
through the little diodes.
It sounds to me as though they have survived the experience & if you stick a bulb in circuit everything will be OK
cheers
Bob