Altenator and Ignition light

[Captaindp1]

Hi Everyone,

I've been working on the speedometer's warning lamps of my 1974 XKE S3 and encountered a series of challenging issues. Would really appreciate any advice or insights you all might have.

Replacing Bulbs and Discovering More Issues:
While I was replacing the bulbs for the fuel and high beam indicators in the speedometer, I noticed that the glass of the ignition light bulb was broken. After removing the remnants and fitting a new bulb, I observed something odd: the bulb stayed lit continuously. This led me to realize that the alternator wasn't charging. Which has not been a problem until I changed the light bulbs.
Alternator Troubles:
In an attempt to fix this, I replaced the old alternator with a refurbished Butec model I found on eBay. Unfortunately, this didn't solve the problem – the ignition light remained on, and the alternator still wasn't charging.
Further Diagnostics:
I checked all connections and fuses, and suspecting a faulty regulator, I went ahead and replaced that too. However, the issue persisted. Then, something intriguing happened. When I removed the ignition light bulb, the alternator started to charge. But upon reinserting the bulb, while the charging continued, the voltage became erratic. It spiked to 16V, then settled around 13V, and fluctuated with the RPMs between 13V-16V. I confirmed these readings with a multimeter at the battery.
This situation has me stumped. Driving the car with 16V will boil the battery and is not how it should be. Has anyone here encountered something similar or have any suggestions on what could be causing these issues? Any help or guidance would be greatly appreciated!

Looking forward to your responses.

Thanks!

[MarekH]

Dear Anon,

Bulbs blow because too much current passes through them. It's happened twice, so you need to work out how you can draw such a large current. Current drawn is inversely proportional to the resistance in a circuit. Logic says you will find a short circuit to earth somewhere.

The bulb lights when there is a voltage across it. Unlike most circuits, which start at the battery positive and end at the battery negative, the filament bulb in the alternator charging circuit is fed by the battery on one side and by alternator diodes on the other side. This means it has to be insulated from the chassis, as that is NOT part of its circuit. You also have to use a standard filament bulb, as it is not obvious which voltage will be higher, and when. For example, when you first turn the key, the bulb has one side at "12v" from the battery, but the other side has zero voltage, as the engine is not running, so the bulb lights as its circuit completes through the alternator. When running correctly, the battery side of the bulb is charged up by the alternator on one side of the bulb and the alternator outputs that same voltage to the other side of the bulb - the bulb extinguishes because there is no voltage difference across it. (If the bulb lights during normal running, it is likely because one of the alternator diodes is failing and voltage output via one set of diodes along the first leg is different from voltage output from another set of diodes to the other side of the bulb along the other leg. Since you don't know which way around this might be, it is inappropriate to use LED bulbs in this circuit, since they are directional.)

The alternator has only two states of being. It either produces full output voltage (when its field circuit is connected) or it produces zero volts (when its field circuit is not connected). It is the job of the regulator to keep turning the alternator field circuit on and off so the average voltage is 14 to 14.4v. Since F+ is connected to 12v, If you cannot control alternator voltage, then it suggests F- is being permanently grounded. (It should be grounded through the regulator, which will be alternately completing and breaking the field circuit so the alternator B+ never has zero output, nor full output, but holds an average of 14 to 14.4v.)

Since you have a new regulator and a new alternator, you probably have a wiring/connection problem such that you are not regulating the alternator's output.

The word "circuit" also means that your alternator has to be properly connected to chassis ground and battery negative. The car has a frame ground strap and the engine has a grounding strap for these very reasons.

The manual has diagnostic tests for the electrical equipment.

kind regards
Marek

[Captaindp1]

Dear Marek,

Thank you for your detailed and insightful explanation regarding the electrical issues I've been experiencing. Your analysis of how excess current leads to bulbs blowing and the intricacies of the alternator's charging circuit is incredibly helpful.

Based on your advice, I'll focus on investigating a potential short circuit to earth, as well as ensuring the correct type of bulb is used for the circuit. Your explanation about how the alternator operates, with its two states and the role of the regulator in managing the field circuit, provides a clear direction for my troubleshooting efforts.

Given the new regulator and alternator, I agree that a wiring or connection problem seems likely. I'll also ensure that the alternator is properly connected to the chassis ground and battery negative, as you've highlighted the importance of these connections.

I plan to work on the cables as per your suggestions and will conduct the diagnostic tests outlined in the manual. I will get back to you with the results of these efforts.

Thanks once again for your invaluable assistance.

Kind regards