Alternator issues

[LoTex]

The alternator conversion I installed in 2019, has stopped charging the battery on my Elan Sprint. I'll take her out for the usual 30 minute drive and when I get back it won't turn over. This has happened several times now. The battery is new, and shows a 95% charge when I check it. In order to get the car started I have to put it on the charger for a short while and then it will work.

I don't think the alternator itself is bad- I initially found a loose ground wire from the alternator but having reattached the wire, the battery is still discharging. A multimeter shows continuity between the alternator and the ground wire at the voltage regulator. I also noticed yesterday when I got back from my drive that the fuel gauge was not reading correctly, but once the battery was recharged it read correctly.

Any thoughts on what may be causing this would be greatly appreciated!

[snowyelan]

Does your ignition light bulb in the tachometer light up when you turn the key to the run position BEFORE you start the engine? The alternator needs to see continuity thru the bulb element. If the bulb is burned out the alternator won't function.

[LoTex]

Scott,

Thanks for the suggestion- the red ignition light does go on in the run position.

Steve

[Craven]

If you can measure voltage, check at the battery terminals stress at the battery terminals the alternator needs to be supplying a voltage with the engine running at least 2000 rpm, close to 13.8 volts. You should see the battery volts rise from tickover to 2000, if you have a helper the ignition light should extinguish as the engine speed increases.

[LoTex]

Thanks for the suggestion! I'll give it a try this weekend.

[billwill]

Scott,

Thanks for the suggestion- the red ignition light does go on in the run position.

Steve

Ah, but does the ignition light go out after the engine is running at a reasonable speed?

You haven't put an LED bulb in that position have you? This is the one place where it NEEDS a filament bulb?

[Slowtus]

https://www.howacarworks.com/ignition-s ... battery%20.

Covers most of what you need.

[billwill]

https://www.howacarworks.com/ignition-system/troubleshooting-the-ignition-warning-light#:~:text=The%20ignition%20warning%20light%2C%20sometimes,is%20not%20charging%20the%20battery%20.

Covers most of what you need.

That article was written (I suspect) before LED bulbs became common and does not warn you to avoid an LED bulb in this location, but otherwise seems a good article.

[Andy8421]

As Bill points out, alternators don't generally 'self excite' and pick themselves up by their own bootstraps. Dynamos do, as there is enough residual magnetism in the stator so that on startup the little output of the dynamo rotor is enough to help the stator become energised which then boosts the rotor output which helps the stator and so on until the dynamo is running at steady state.

You can sometimes get an alternator to boot up without an ignition warning light by giving the engine a really good blip of the throttle, as there can be just enough residual magnetism in the rotor to get it running, but generally an alternator relies on the current flowing down the ignition warning light circuit to give the rotor its initial magnetism.

Designing rotating electrical equipment is a dark art, and I don't know why alternator stators don't have a little residual magnetism built in to avoid this issue - perhaps even a little magnetism is enough to cause over voltage output in a no-load situation.

edit: Should have added that the problem with using an LED as an ignition warning light is it doesn't allow enough current to flow into the rotor, whereas a filament bulb does.

[Craven]

Not all alternators use the warning light as a source of initial excitation current, Bosch for instance use a resistor feed for the very reason of warning light failure.

[LoTex]

Thanks for all the ideas and the ignition light tutorial, very useful!

Having tried all the suggestions I still haven't sorted out the problem. The ignition light is no longer on while the engine is running, in fact now it doesn't come on at all. I've checked the wiring as best I can and my best guess is that there is a problem with the alternator itself, although it only has about 1,000 miles on it. She stills starts and runs well but the battery is still discharging so I'm not too keen on taking her out for a drive, even with my trusty jump starter in the boot.

Might be time to take her to my local mechanic...

Steve

[mbell]

Do places like AutoZone have alternator testers?

Maybe remove it and take it somewhere to be tested. Then can be sure if alternator or the wiring in the issue.