Alternator not charging

[churchy]

Car stopped charging while out on a run, I managed to get home but when I checked voltage across battery without car running and with the car running was the same, 12.35 volts.

To lazy to check wiring I ordered a new alternator, fast forward 3 days and in the garage with the new alternator fitted and checking voltage across battery to find exactly the same readings.

I walked back into the kitchen because everything looks better with a cup of tea, 15 minutes later when I started checking wiring. I found a connector at the alternator end burned out as the wire had come loose. Great I thought easy but being the person I am and 3 years ago I completely rewired the car with a gbs wiring harness but that harness does not cover the alternator wiring, so I thought lets rewire the alternator wiring and be done with it.

Now I'm running a pinto engine with a bosch alternator. On the bosch are 3 connectors two large and 1 small. One large connector went straight to the battery live the other large connector went to my kill switch, so when the key is connected it goes straight to the battery. The little connector goes to my warning battery charging light but I haven't connected this yet as I'm waiting for a 86 ohm resistor to put in line before it goes to the warning lights as l have smith flight dials and this is what they say, this light wasn't connected before but as I am rewiring I wanted the light to work.

All excited turn the key on stared the car checked the battery and went to make a cuppa as things always look better after a cuppa.

Still 12.56 volts with car running and the voltage drops to 12.34 volts with lights on so not charging. After a lot of swearing I check the two large connectors when unplugged from alternator, I have continually across both  to the battery and also 12 volts with battery connected, the alternator has a good earth back to the battery as I  checked continually. Last chance I put the old alternator back on with the same reading the voltage is the same with car running as not.

Sorry for the long description but my question is have I made a school boy error or are both alternators  faulty. Any help gratefully received as I'm all tea out. 

Cheers andy

[dandan62]

I am no great expert but I'm pretty sure the alternator relies on the warning light circuit to excite it to start charging. You probably need to connect yours up. Worth a Google or confirmation from another source. Good luck.

[alanrichey]

Yes, that’s correct, the light is essential.

[churchy]

Thank you gentlemen, I shall wait until I have wired that ignition light and let you know how I get on. Very much appreciate your input.

Cheers andy

[Jonty Wild]

Any idea why my light stays on for ages after the car has started? It doesn't seem to be causing any problems.

[IanS]

Jonty this could be the brushes not making good contact due to either sticking in their guides or just being worn to end of life. As they warm up they get better contact.

[phaeton]

You are saying you are using a resistor in the circuit, have you tried just using a 5W bulb, it could be that your circuit is not drawing enough current until the revs increase or the voltage drops.

Caveat I am not an expert or even claim to know much about it, But as I understand it the alternator dumbly measures the voltage on the direct from battery wire, it compares that with the voltage on the wire that goes via the Exciter bulb it then uses these values as to whether it needs to charge or not. If the Exciter wire is not connected, the bulb blown, or it's not drawing enough current it can't do that, so doesn't charge.

When we used to use Acewell dash's we used to either hide a 5W bulb behind the dash painted black, or add a resistor the value of which now escapes me, but I did think it bigger than 86ohm.

[Sparepart]

I have recently been investigating this subject due to having destroyed an alternator trying to change the brushes,.... we don't talk about this. However, here is what I think I believe is relevent to this thread. The connections on the alternator are a group of three spade terminals in a line, two large and one smaller. The two large terminals are connected together, that is they are essentially one terminal, check this out with your meter. On my car they are both connected to the same heavy duty cable going directly to the positive terminal on the battery. The small terminal is connected to the alternator regulator circuit, the circuit that controls the current feed to the field coils which regulates the output from the alternator (on the big terminals). When the alternator is generating power, it uses the generated power to feed the field coils under control of the regulator. So it's a chicken and egg situation. If the alternator is not generating power it can't engergize the field coils. This is the situation when the engine is first starting up. A small current is needed by the regulator in order to slightly energize the field coils to generate a tiny current that is then fed back into the coils etc. until the process builds up to the large current output produced by the alternator. This small current of course comes from the battery. It is convenient to use the resistance of the filament in an incandesant light bulb to allow only a small current to the regulator because as soon as the alternator starts to generate a greater current at 12 volts, no current will flow to/from the battery to the regulator and the light bulb will go out. We call this bulb the "ignition light". On the Sierra donor, this bulb is rated at 2.2 watt. At 12 volts this means that the current flowing to the regulator at startup is 183 milliamps, and implies that the bulb provides a resistance of 66 ohms. It's important not to feed too much current to the regulator, it might cause harm, so any lower resistance in this circuit should be avoided. Higher resistance is safe as long as enough current flows to get enough initial field in the field coils. Replacing the Ignition Light with an LED or any high resistance device/circuit usually prevents enough current reaching the regulator, accordingly a resistance can be introduced IN PARALLEL, remember the formula 1/R = 1/r1 + 1/r2, such that the total resistance is reduced towards the nominal above.

[phaeton]

17 minutes ago, Sparepart said:

However, here is what I think I believe is relevent to this thread.

Why is there no 'Like' option for this post?

[Darren Knowles]

Needs 12 v via batt light as others say to excite the alternator try reving it to 5-6 k mine used to kick in at that , ive now wired in batt light 

[Sparepart]

BTW. Any old type of resistor won't do. It must be able to operate without overheating and burning out. The 2.2 watt ignition light gets quite warm, eventually can burn out and stop the alternator charging. It is not expensive to buy a resistor that will more than cope with the power. I attach photo of an example below, only costs around 3 quids and should last until the end of the universe. You would need one at whatever ohms you are after of course not necessarily the one shown.

Edited May 14 by Sparepart
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