Alternator late cut in fix.

[Griffinr]

I recently bought my first S3 and was surprised at the late cut in of the alternator (2500 RPM). A few searches of the forum's and a thumb through Steve Heaths book reveals that "they all do that" "its normal".
Well its not on mine now I've just fixed fixed it!! The alternator now cuts in as soon as the engine fires. My neighbours will be so pleased at 6:30 in the morning.

In the standard circuit, to initiate the charge of the alternator a small current is applied to its small terminal. This initiation circuit has the ingnition warning light in it and the amount of current passed depends on the size (wattage) of the bulb. The warning light circuit effectively earths through the alternator untill the alternator starts to charge then both ends of the circuit reach the same potential and the light goes out. The simplest fix is to put a higher wattage bulb in the warning light. As I could not easily access the bulb in the warning panel I added a second 2.2watt panel bulb to the circuit in parallel with the original. The additional bulb is wired between the small alternator terminal and a fused ignition swithed live.
If you want to test the theory first connect 2 wires to a small panel bulb, connect 1 end to the battery positive and the other to the alternator small terminal (bulb between the 2) Start the engine and check for alternator cut in. Good Luck.
Ps. Dont overdo the bulb size or you could damage the alternator if the ignition was left on for a long time without the engine running.

Edited by Griffinr on Monday 12th October 09:15

[hvdweerden]

Hi Rob,

Good find, seems like an easy fix. Question remains : why was it designed to go out above 2500 RPM ?

Br, Han

[johnnywgk]

If this works exellent. probably the best 1st post award goes to you.
Well done. Have to wait now for someone to come and burst our bubble lol.

[johnnywgk]

Edited by johnnywgk on Monday 12th October 11:26

[Russ35]

There is a reason why it does this, a Ford engine specialist did tell but I cannot remember why. It is not just a TVR thing, it also did it on the Sierra/Granada V6 cars.

[tvrgit]

Russ35 said:

There is a reason why it does this, a Ford engine specialist did tell but I cannot remember why. It is not just a TVR thing, it also did it on the Sierra/Granada V6 cars.

Not on my Granada 2.9 it didn't! I did 140,000 miles in that thing and it never did this once!

I'd be interested to know why it does it - although I thought that the OP about TVR inadvertently using a different bulb rating made a lot of sense (it is a TVR after all...)

[Danlor]

Rob,
Aren't we halving the current in the original bulb by putting the bulb in parallel?
So should we be putting a slightly smaller wattage bulb in the alternator circuit?
Have to have a play....

[Griffinr]

Danlor said:

Rob,
Aren't we halving the current in the original bulb by putting the bulb in parallel?
So should we be putting a slightly smaller wattage bulb in the alternator circuit?
Have to have a play....

No, it would if you put it in series. The bigger the wattage bulb the more current it draws so the bigger the magnetic field induced in the alternator and the lower the cut in RPM. The bulb is acting as a resistor as too much current would damage the alternator electronics.
It only needs 1 to 2 watts extra in the bulb (or additional bulb).
Anyway it does work, got it from an ex-Lucas engineer, it was apparently used on early Morgans that had the alternator driven off the gearbox.

Rob

[techbotics]

Do I have to put a bulb in or would some other passive component do the trick.. a short circuit with resister perhaps.?

I can totally believe the TVR bulb story..

Damian

[Griffinr]

techbotics said:

Do I have to put a bulb in or would some other passive component do the trick.. a short circuit with resister perhaps.?

I can totally believe the TVR bulb story..

Damian

A resister would work but no idea what size or type. Bulb is much simpler. (I can understand bulbs, I'm a mechanical engineer not electrical!!)

[Bobhon]

As a fellow mechanical engineer, can I add that the formula you need is Ivy Watts. Or in real terms, I(current in amps) * V(voltage in volts) = W(power in watts).
So if you wanted to replace a 12 watt bulb with an equal resistor then it would need to draw 12watts/12volts = 1 Amp
Then you need to use the V(voltage in volts) = I(current in amps) * R (resistance in ohms) formula.
So to draw 1 amp from a 12volt system you would need 12volts/1amp = 12ohms.

Working out the real numbers for a 2 Watt bulb gives you...

2watts/12volts=1/6amp (working in fractions here, remember them?)
12volts/1/6amp=72 ohms ( you get extra marks for showing your workings)

Hope that helps

[techbotics]

Good stuff..

Cheers

D

[Barkychoc]

And the resistor needs to be at least 2 watt power rating or things get a little warm

[clarenceboddiger]

Now you tell me


http://www.pistonheads.com/gassing/topic.asp?h=0&a...
Ok so now I'll recon the old one and fit it. Good post.

Edited by clarenceboddiger on Saturday 17th October 07:59

Edited by clarenceboddiger on Saturday 17th October 08:00