71 Elan Plus 2S Tachometer Pegs at the flip of a switch!

[billwill]

Alternators have a link between the power output and the 'exciter' input. Which is what raises the voltage on the alternator side of the ignition bulb. If that link is missing I think you would get the symptoms you describe.

Often the link is through some diods, inside the alternator, so if those have 'blown' you get that symptom (and probably poor charging.)

Also the wattage of the bulb is critical (I seem to remember it should be a 3-watt bulb). So if you changed the bulb for a new one, that might be the problem.

https://www.picoauto.com/library/traini ... alternator

[mjbeanie]

Thanks Bill for the suggestions.

I checked, the bulb is a new and proper 3W. I am going to press on, but stay aware of anything suspicious. It stays dimly lit at idle. The car has nearing operational condition, now that is has been refurbished and reassembled. That dash is now installed forever, I hope.

Considering there are no other "anomalies"- I think I did pretty well rewiring the whole flippin' car, front to back with one small indication system hiccup. I am a retired "OCD" mechanical engineer but I think I can live with this...

(And it is easily remedied by bulb removal...lol)

[billwill]

Oh no it aint!

If you remove the ignition bulb in a properly functional alternator system, the rotor won't get excited & it probably won't generate any electricity .


PS: I forgot to say above that the bulb must be a filament bulb not an LED.

[Andy8421]

Thanks Bill for the suggestions.

It stays dimly lit at idle. The car has nearing operational condition, now that is has been refurbished and reassembled.

Just to confirm your symptoms:

Light comes on brightly when ignition switched on
Light glows dimly at tickover
Light goes out fully when revs pick up
Battery voltage with the engine running at above 1500rpm is between 13.8V and 14V

If so, you don't have much to worry about. If your tick over is relatively low, and the car has just had a run, it is possible that the alternator can't quite manage the 13.6V to match the battery float voltage and the light will come on dimly.

Less likely, it could be the symptom of a blown diode in the rectifier pack. You could try monitoring the alternator output voltage with lights, wipers and heater blower on and the engine above 1500 rpm. If the voltage is much below 13.8V, then suspect a blown diode.

[mjbeanie]

Hi Andy,

Everything is stated correctly below, however the light doesn't fully extinguish. I feel I may get a more accurate representation with the car operating on the ground. I am getting close to running my first test ride on my street. All other systems are go. Just a bit more brake and suspension work left. It has been a disassembled shell on stands for a year now, and the engine has only been operated during assembly, for testing and troubleshooting purposes.

The only disturbing fact that nags at me is that this new observation began the moment I fixed the tach.
(The indicator light was 100% perfect in its operation. )

Yet there are no wiring changes or missteps that I can find. Still wondering if this tach fix is influencing things along with a residual current somehow to the darn light. (Since they share a common wire to the voltage stabilizer.)

Thanks for the continued help-I owe you guys a beer..