Question for Ray ref alternators

[Lazy-Ferret]

I know when you look at the majority of instrument clusters, the warning bulb for the alternator is usually a different "special" bulb, compared to the other bulbs in the cluster. I also know that the ignition voltage is fed via this "special" bulb, and used to excite the windings, so the alternator can start to produce an output.

Now what I do not understand though is, what is actually special about the bulb, and what impact changing the value of the bulb would have, e.g, does lowering the resistance of this bulb possibly increase or decrease the alternator output.

The specific reason for the post is, after reading various bits of info on the web, I connected a relay between the bulb feed of the alternator, and ground, in order to create an "engine running" signal, for other devices on the car.

Since then, I have realised that my battery is discharging over only a few days of none use, and the alternator does not seem to be able to cope with things like it used to, e.g. first thing in the morning, with the glowplugs still pulsing on and off, plus the wipers and dipped lights, there is not enough output to bring the voltage up to 12v until the glowplugs stop pulsing.

I got to thinking, and have realised that the way the relay has been wired means it could be exciting the alternator coils permanently, and of course is also in parallel to the dash bulb, so will also have affected the effective value of that feed.

So I now realise I need to change how the relay is connected. But the questions I have are, will having a voltage connected to the alternator 24/7 for a few months have actually damaged the alternator, and also, what should I do, if anything, to sort out the effects of putting a relay in parallel to the warning bulb, in order to keep the supply to the alternator correct?

 

[rustic]

I could answer that, but since you are waiting for Ray, I'll wait for his reply lol.
:lol





But basically, you can't do what you are doing, as you are killing the power to the field winding, so the alternator won't provide a charge.:doh
Rustic

 

[macabethiel]

Charging light

My understanding is that the charging light only illuminates when the path between charge output and live is sufficient for it to illuminate as the alternator when not putting out sufficient amps acts as an earth. (Due to voltage difference a bit like a wonky tail light bulb can result in indicator light causing a side lamp to illuminate)

Once the alternator has a output greater than the battery voltage the lamp goes out. Messing with the bulb whattage will alter the amount of current required to illuminate it. Putting a relay in the circuit in parallel will lead to a drain - thats my thoughts for what they are worth.

 

[rustic]

This is a typical alternator schematic.
The feed to the field coils comes through a bulb, usually 2.2 watts, the value is not critical, the field coil, ie alternator, when rotated produces an output, there are three diodes that then feed a supply to the field coil to keep that feed going, so the ignition lamp now has 12 volts on each side, so extinguishes.
If you place a load ie relay, or any other load, on the other side of the bulb, ie alternator side, that is connected to ground on the other side, will make the field coils become close to 0 volts, so the alternator can't charge.
You might be better placing the relay in parallel with the indicator bulb, however, this might give a back E.M.F. pulse, ie spike, than can damage the voltage regulator, so the detail in this will depend on the type of alternator fitted.
What is it you are trying to do? there could be better ways to do it.
Rustic

 

[kitchenman]

I had a similar arrangement on my Mav before I fitted the second alternator, the wiring went a bit like this
I connected another wire to the alternator small terminal, ran this to my relay coil terminal, the other relay coil terminal went to earth, this worked great on my Partner van also on the Maverick to charge my leisure/second battery.

 

[Lazy-Ferret]

Cheers for the responses guys, it does work, and has worked on cars I have had in the past... It is a very common mod, and you will find it all over the web for things like split chargers and caravan feeds.

Basically, it means that what ever is connected via the relay contact can not be switched on until the engine is started and up to speed, which stops issues with current draw before you start the engine, and things being turned on and off as you turn the ignition key via the many modes. It is far more reliable than the voltage sensing ones, which have a habit of hunting, switching on and off. The last one I had would see the car voltage rise up to the voltage where it would switch on the caravan, then the current draw of the caravan would cause the voltage to drop, which would drop out the relay. This would go on for a while causing all sorts of issues with the electronic devices in the caravan.

I have also seen it done via the oil pressure switch, but this can be a bit harder to get to, and on my T2, the oil light goes out while it is being cranked, even if it should fail to start, so is not as reliable.

I had a similar arrangement on my Mav before I fitted the second alternator, the wiring went a bit like this
I connected another wire to the alternator small terminal, ran this to my relay coil terminal, the other relay coil terminal went to earth, this worked great on my Partner van also on the Maverick to charge my leisure/second battery.

It's the most commonly recommended way, and I have used it many times in the past. The problem is, sometimes things have a habit of working fine, even though you should not really do it that way, but people get away with it. After the problems I have been having with the alternator output, I started thinking about it, and realised it started playing up not long after I added the relay. I then got to thinking about what you were really doing when you add the relay, hence the reason for my question.


Thanks for the bulb wattage Rustic, any idea why it is that specific size, and is it the same for all alternators, or is that the bulb size specifically for the T2?

It was explained to me, that the light basically is only there to get things started, then, as mentioned, if the voltage on the alternator gets too high, feeds back through the bulb illuminating it, and also, if the voltage on the alternator field drops too low, again, it illuminates, so the Bulb serves 3 uses. I just wondered what detrimental effect the relay would be having being put in parallel to the bulb, but thinking about it again, I realise it is not in parallel, since the bulb is 12v one side, the relay is ground the other side.