Charging system mystery? Solution needed!

[ElvisWedgely]

I start up the S2 and the battery light stays on, volts read very low and alternator not charging. By shorting the circuit between the two wires on the back of the alternator whilst the engine is running the alternator begins to charge, the battery light on dash goes out, the volts go up and all is back to normal until the engine is switched off again. Then its back to square one.
It's obvious that the alternator needs to be excited by the thinner live wire on the back of the alternator to start the charging, and this for some reason is not happening, even though power supply to the thinner wire on the back of alternator is live when the ignition is switched on. I know it's not a faulty alternator because I have had the alternator professionally tested. We have done various tests and we cannot find a break in any of the wiring. Someone suggested that the ignition light bulb could be the wrong wattage and this I have not eliminated yet, but I will replace the bulb to see if it makes a difference. Other than what I have mentioned, has anyone else had a similar problem and how did they solve it?. I've never come across anything like this before. suggestions/solutions/advice most welcome.

Tony. TCB.

[Oldred_V8S]

ElvisWedgely said:

I start up the S2 and the battery light stays on, volts read very low and alternator not charging. By shorting the circuit between the two wires on the back of the alternator whilst the engine is running the alternator begins to charge, the battery light on dash goes out, the volts go up and all is back to normal until the engine is switched off again. Then its back to square one.
It's obvious that the alternator needs to be excited by the thinner live wire on the back of the alternator to start the charging, and this for some reason is not happening, even though power supply to the thinner wire on the back of alternator is live when the ignition is switched on. I know it's not a faulty alternator because I have had the alternator professionally tested. We have done various tests and we cannot find a break in any of the wiring. Someone suggested that the ignition light bulb could be the wrong wattage and this I have not eliminated yet, but I will replace the bulb to see if it makes a difference. Other than what I have mentioned, has anyone else had a similar problem and how did they solve it?. I've never come across anything like this before. suggestions/solutions/advice most welcome.

Tony. TCB.

The thinner wire is live with 12v but is a poor connection or dodgy wire allowing sufficient current to flow? You can get 12v flowing through your fingers but not enough current to be of any use.

[BountyHunter]

Isnt it an internal diode that tells the alternator to switch over to charging ?
eg thats why it doesnt charge the battery, until its 1st hit a certain voltage - eg why it doesnt turn off the red light till it hits around 2000rpm for the 1st time.

[GreenV8S]

The alternator needs a small current supply to power the field coils until it generates enough electricity to be self sustaining. Usually this comes from the battery via the ignition switch and the no-charge warning lamp - the lamp acts as a current limiter. You said you have a voltage there so it's obviously got some connection, but if there's a poor contact or you have the wrong lamp rating then it might not supply enough current to get the alternator to start generating.

In order to start, the alternator needs to have a combination of enough magnetic field and enough rotational speed. It's not uncommon for some engines to need to be revved slightly in order for the alternator to start up, even when the excitation circuit was working properly. My S2 needed to be revved pretty hard in fact - around 1700 rpm would usually do it. The V8S needs far less and usually starts due to the slight rev flare when the engine catches, but even so will sometimes not come in until the engine has been revved a bit.

So - have you tried revving the engine to 1500 rpm or so and see whether that's enough to get the alternator started?

[mikel003003]

I think it is standard they all need a throttle blip to turn the light out and start the alternator charging

[Gerald-TVR]

Also if you replace the 2.2w warning light with a 5w that will give a large enough load to extinguish the warning light without opening the throttle.

Do you have a wiring diagram, if not send me your email address and i can send a PDF of wiring.

[Barkychoc]

Note the charge light is usually a higher wattage bulb than the others so if it has been replaced or swapped with one of the others that may be your problem.

[ElvisWedgely]

Barkychoc said:

Note the charge light is usually a higher wattage bulb than the others so if it has been replaced or swapped with one of the others that may be your problem.

Do you know what the wattage of the ignition light should be and what type of bulb it's meant to take?, as I've only just got the car and the previous owner might have messed about with it. I do have a wiring diagram and we have checked all the wires the best we can, and the alternator has been checked at an alternator recon shop and now we're down to the bulb theory. Thanks for all the information so far gentlemen, it's been a great help and any other ideas would be welcome. It's probably something very simple but we are stuck at the moment. The answer may lie with the type of ignition bulb and it's wattage.

Tony. TCB.

[Barkychoc]

I have a sierra haynes manual at home will look later or someone feel free to beat me to it.

[Barkychoc]

I had almost given up trying to find something in the various manuals I have, only ref I can find is in a Vauxhall Chevette manual which says instrument bulbs 1.2 watts except alternator lamp which is 3 watts.

Hope this helps

(S steering rack is based on chevette rack btw)

Chris

[ElvisWedgely]

EUREKA....Problem solved. Previous owner must have taken a bulb from the unused dash lights and fitted it for the ignition bulb and of course 2.5 watt is not enough to excite the alternator into action. I thought I'd try a higher wattage bulb but I couldn't find one to fit the holder, so I went to the local spares shop where I am well known for unusual experiments on the car and bought a bulb holder and a 5 watt bulb, spade clips and wires, and made up a new ignition light. Plugged it in to the ignition light wires and hey presto. Charging system back to normal. I have a bulb hanging under the dash at the moment and looks a little untidy but who cares, at least it all works. I'll do the tidying up later. Hours of work to rectify such a small and simple problem.

I want to thank everyone here who have given me ideas and shared my problem. All is working now except the rev counter but there is already a thread running on that so I should be able to solve that next.

Tony. TCB.

[swimmer27]

[quote=ElvisWedgely]EUREKA....Problem solved.

Well done.
Reminds we when my fuse for the engine cooling fan kept blowing, I checked all connections, earths etc etc and even turned the fan by hand to make sure it was running free etc then one day I could smell burning, fan motor was burning!!

[techbotics]

Does the lamp cover etc come forward to get at bulbs or Ido you get at it from back of dash ?

Thanks D

[ElvisWedgely]

techbotics said:

Does the lamp cover etc come forward to get at bulbs or Ido you get at it from back of dash ?

Thanks D

There is a little panel that comes off on the drivers side near your left leg that holds the cigarette lighter. Once you pull this panel off or push it down sideways, you can access the bulbs from the back fairly easily. The only problem is I couldn't the same bulb as a 5 watt or any other watt other than 2.5 so I've had to do some modifications at the back on the existing bulb holder to take a bigger bulb.

Tony. TCB.

[techbotics]

Ok. Thanks will take a look

Cheers D S3