Alternator light on again

[derm.1785]

The dreaded alternator light is on again after I thought it was fixed. Internal regulator system and I have bridged the lamp with a resistor which seems to still have resistance but the light won't go out. What voltage should I be getting at the blue wire into the lamp holder when ignition is on and should the voltage rise a little with revs, and what voltage should it be ???, can I just bridge the lamp temporarily to see if it will charge the battery or would this blow the alternator ???

[NeunElf]

You didn't mention what specific car you have--that might help. Also, I'm guessing you mean the alternator light stays on when the engine's running.

But, as far as I know, the alternator lamp glowing indicates that the battery voltage is higher than the alternator output voltage. Changing the resistance at the alternator light won't do much. Eliminating the resistance by bridging the alternator light with a piece of wire might damage the alternator. Shorting the blue wire to ground will damage things.

My shop manual doesn't say much about how the voltage regulator works and the wiring diagrams has some symbols I can't follow. But, the shop manual's advice sounds good to me:

3. Start engine.
If the control light does not go despite
increased engine speed, or when it lights
up in certain rpm ranges, then one or
several diodes [in the alternator] may be
defective.

[derm.1785]

Could be but it's a brand new alternator, the light was on before I fitted the resistor to the lamp ( based on previous good advise from guys on here ), the light went out only for about 10 minutes and then came on again. Can't quite figure out what the problem is, like I mentioned before I don't know what kind of voltage I should be getting at the blue wire across the Rev range. 1971E BTW

[NeunElf]

About all I know comes from the shop manual and the wiring diagram:

1 = "Starter"
2 = "Generator"
3 = "Regulator"

The blue wire connects to the D+ terminal on the regulator (which also connects to the D+ terminal on Alternator). The diagram's about all I know about the internals of these particular alternators. It shows separate diode sets for the D+ and B+ terminals so I can't guarantee the voltage is the same but there's a good chance it should be about 12 to 14 with the alternator working properly.

I don't think the light bulb plays a big role in regulating voltage but I have one crazy idea:

21 = "large instrument cluster"

If the other side of the alternator bulb were grounded you'd have the light on whenever the alternator produces voltage. But, the gauges wouldn't work if the blue/yellow wire were also connected to ground--it supplies power to run the gauges.



My electrical engineering classes were a long time ago but I still think you have an alternator or voltage regulator problem. Could it be that something's causing your battery to temporarily disconnect from the alternator-regulator? That could repeatedly damage the alternator.

[derm.1785]

Does anyone out there know what the voltage should read at the blue wire to the alternator bulb at idle and when revving , I don't know of a way to test the alternator while still in the car ?

[304065]

Derm, go edit your profile to make absolutely clear which car we're talking about, so the right advice can be given. . . now 71E, I used to own the very car.

Here's how the "blue wire" charging circuit works. Power comes out of the battery and goes to the ignition switch then through a "blue wire" to one terminal of the bulb holder for the warning light, then across the filament in the bulb and back to the engine electrical console, where it connects to the voltage regulator at the D+61 pin, then the "blue wire" goes through the 14 pin connector into the engine harness and over to the D+/61 terminal on the back of the alternator, where it encounters the "trio" diodes which are reverse-biased so no current can flow to ground.

Inside the voltage regulator, the D+/61 from the bulb and the DF (dynamo field, or the positive brush for the rotor) are connected together, so that current flows through the blue wire into the rotor and to ground, completing the circuit, so the warning light bulb glows.

When you start the engine, the current in the rotor induces a voltage in the stator windings, which is rectified by both the main diodes (for DC power to the battery) and by the "trio" diodes for a voltage output to the D+/61 terminal on the back of the alternator.

When the voltage on the D+/61 terminal on the back of the alternator is EQUAL to the voltage at the battery (on the other side of the bulb) then no current can flow, and the bulb goes OUT.

If the bulb is still lit, that means that the alternator field is still being powered by the battery, and not by the alternator itself, which means you aren't charging.

Make sense? The tricky part is understanding why the bulb goes out.

Now, why did the Factory in 1982 recommend the use of a resistor in PARALLEL with the bulb contacts, and later go from a 2W bulb to a 4W bulb? It's to increase the amount of current flowing in the blue wire at low RPM, so that the DF would have more current, so the alternator would start charging at a lower speed. This is called "Zero-ampere RPM" and adding the resistor in parallel lowers the resistance in the circuit (formula is R1xR2 divided by R1+R2, so if the bulb is 72 ohms and the resistor is 91 ohms, then the parallel resistance is 40 ohms, so practically speaking the power bumps up to 3.6 watts) You get the idea.

Now, you have an "internal regulator" alternator which I don't recommend for a whole host of reasons, but it is what it is. I assume you didn't remove the original wiring for the external regulator. So I want you to do exactly the following.

1. What is battery voltage measured at battery terminals with the engine off?

2. What is battery voltage measured at battery terminals with engine running?

Now, with the engine not running but the key in the ON position, measure the voltage between the D+ pin of the voltage regulator socket. I know you don't have the voltage regulator there anymore, that's ok, I want the voltage between the blue wire pin of that socket and the engine case.

You can also put a jumper wire between D+ of the socket and the engine case, when you do that you are grounding the circuit which should cause the bulb to glow brightly. If it doesn't there is a defect in your blue wire forward of there.

[derm.1785]

304065, massive thank you for taking the time to reply to my post. After going through your list of what and where to measure I had noticed that I have no voltage at the blue wire at the terminal to the D+ at the regulator . This I thought strange so I did the bridge wire that you suggested, whilst doing this I noticed that the two earth wires connected to the regulator housing were a little molten/out of shape. I cut these back to investigate further and found that the blue wire and one earth had fused. This is obviously why the light had been off previously and then suddenly came back on. The only issue I have now is that after running the car for a few minutes the brown wires in question are very hot, nearly melting through their protective layer. I can only assume that I have a poor earth or maybe need to upgrade my earth to the rear cluster.