Alternator fault.

[dougal9887]

Can anyone identify and explain this fault please?
I switched off the ignition key, the engine kept running and the charge warning light showed red.
The reason the engine kept on running was that there was 4.5v on the charge light/field power wire (white) at the key on position which was enough to keep the ECU relay open and hence the engine running.
The fog lights are on this circuit. If they are switched on this voltage disappears and hence the engine stops.
The alternator is new and has behaved normally for a few hours running.
So my question is, what is the fault that would leave a voltage on this wire at key off?
(Swapping back to the old alternator, and normal operation, proved that the voltage wasn't from elsewhere)
Dougal.
Edit:
Now that I'm thinking about it - the alternator 12v are on the alternator side of the warning light and the battery 12v on the ignition switch side of the warning light. When the ignition side voltage is taken away by key off, the resistance at the warning light bulb must be sufficient to prevent any current flowing from the alternator side and it follows that the current from the alternator on this wire must be very small to be stopped by the bulb and that therefore this fault is excessive current on the alternator warning light wire allowing this small voltage to be seen on the ignition side. Or is that a load of rubbish?
I realise that if the key on was powering an ignition coil this fault would not be noticed since the voltage is so small.

[billwill]

You could try changing the wattage of the ign light.

Or put a diode in the lead from the IGN light to the IGN/F terminal of the Alternator, arranged so that current can only flow FROM the IGN light TO the Alternator.

The overall circuit of an alternator was shown recently in a nearby thread.


lotus-electrical-f38/alternator-conversion-wiring-t39043.html

[bob_rich]

Hi Dougal
I had exactly this when I fitted an ECU to my +2S.

I found that the back feed from alternator to the ignition warning light was enough to keep the ECU relay held on.
I tested both the bulb current that can flow without the bulb visibly glowing and the current that would keep the relay held were consistent with the problem. I fixed mine by making up an assembly with a rectifier diode such as a IN4001 and a 470 ohms 6 watt wire wound resistor wired in parallel and then this assembly placed in series with the ignition warning lamp feed to the alternator. Diode cathode towards the alternator control. I assembled mine with a male and female bullet and secured the whole assembly in heat shrink sleeving. Have attached a picture. Problem completely solved.

Hope this helps best of luck

Bob

[dougal9887]

Thanks for the replies and ideas.
For a voltage to be present on the IGNITION ON terminal at key off, the 12v from the alternator warning lamp wire must be finding a ground of some sort. The resistance on the white battery side wire to ground is 34 ohms so no wonder a voltage is seen and 4.5v seems consistent.
So is this likely to be the combined resistance of the gauges to ground that is being measured? (and the path for the current?) or could it be a leak to ground in the voltage stabilizer?
Another thought - would a suitable resistor placed in the supply wire to the relay, causing this circuit to require more than 4.5v to operate, solve the problem?
I thought I might try a 5 watt bulb in series to the relay to start with?

[billwill]

Bob, why did you put a resistor across the diode?

[dougal9887]

Bob,
Does that arrangement then allow full battery power to the alternator field for start up via the diode, but a reduced alternator current by-passing the diode via the resistor to the warning light? And thereby insufficient current to hold the relay open.
Dougal

[bob_rich]

Hi Again

Bill. My experience ( retired electronics engineer who has seen more damaged semiconductors that I would care to remember!) is that anything with brushes ( alternator slip rings in this instance) and inductors (alternator field winding and coil winding on the relay) can have a risk of producing reverse voltage spikes that could damage the diode. So the 470 ohm resistor meant the current that tries to flow back through to latch the relay would when multiplied by 470 ohms, this voltage was less than the reverse voltage rating of the chosen diode, but a current well below the measured holding current of the relay. It is probably over the top but I had the diode and resistor handy. A diode alone may be OK but pick a higher reverse voltage rating to be safe

Dougal the added diode forward voltage drop around 0.6 of a volt will permit the ignition light to work fine and once the alternator is spinning it will generate enough voltage to back off the volts across the warning light bulb and extinguish it. The small current through the ignition warning lamp is usually more that enough to kick start the alternator regulator into life. If you add a larger wattage bulb it may make the problem worse - - the issue is the cold resistance of most bulbs ( irrespective of power) is much lower that their hot resistance by a factor of around 1/6th to 1/10th

I have attached some notes from my rebuild diary on the problem for your interest / assistance

Hope this helps

Best of luck

Bob

[rgh0]

Bob

I admire real expertise. I don't know a lot about electronics engineering compared to other areas in mechanical or chemical engineering but I know someone who knows his stuff

cheers
Rohan

[miked]

It was standard practice in the standby diesel generator industry to fit a diode and fixed resistor (no lamp) to excite the alternator. Otherwise the fuel relay would hold and keep the electronic governor powered and the set running. Yes cold tungsten resistance of lamp can confuse.

Mike

[dougal9887]

Bob and Bill,
Thanks so much for your help on this one. I did think that I'd identified the problem correctly but had no idea of a suitable solution.
And what a great forum - what would be my chances of running into an electronics engineer with a Plus 2, who had EFI and the same relay problem? Millions to one!
I picked up the recommended components at Maplins today for the princely sum of 48p. So I'll put the new alternator back in together with the mod. and let you know how I get on. I think the reason that the old alternator, 17ACR, doesn't cause this problem is that it is not or barely charging at idle. The new Montego 70amp alternator has a healthy output at idle.
Dougal.

[dougal9887]

Bob,
Brilliant mod. Soldered it up and installed it when I got home.
Works perfectly.
Many thanks.
Dougal.

[bob_rich]

Hi Dougal

Glad it all worked out OK. I think a lot of modern car EFI systems use diodes now to ensure that relays drop out in a controlled manner and order when the engine switched off.

All the best

Bob