LED lights and Alternators do not play well together!

[warrenperkinson]

This post is less of a question, and more of a sharing of my learnings for today... the sub-title could in fact be 'Oh, what an idiot I am?????'.

Recently, I replaced all my dashboard lights in my 1971 series IIa with LED bulbs. What a difference it makes! I can highly recommend this relatively cheap upgrade (see the E10 bulbs at www.LEDlight.com) - I can actually see what speed I'm doing at night now... awesome.

But then my alternator crapped out....

Bugger.

Never mind... I thought that this would be a good time to upgrade my 15ACR alternator to a 17ACR alternator and I bought a new one (not an original Lucas one, just a knock off). I thought that the greater amperage output of the 17ACR would be better now that I had upgraded to a nice big 720CCA battery (because I have a battery powered winch).

But the new alternator didn't charge either... nothing, nada, zip...

Many hours and a lot of colourful language later I realised that replacing the 'charge' light in my dashboard with an LED bulb was the wrong thing to do! An LED bulb does not draw enough amps, and so the alternator thinks that it doesn't need to do anything because it appears to the alternator that there is no draw on the battery.

WHAT A DUFUS!

So, here I am, sharing my embarresing (and somewhat expensive) learning experience with the world in the hope that someone else may learn from my painful experiences and not do the same dumb thing themselves. At the very least, I hope I've given you a laugh (at my expense)!

By all means replace your dashboard lights with LED - it make a great improvement. JUST DON'T REPLACE THE CHARGE LAMP!

[o2batsea]

Absolute hogwash. Alternators don't care what kind of bulb is in the warning lamp circuit. Nor is a battery sized by it's CCA rating; that number is relatively meaningless.

[warrenperkinson]

Alternators don't care what kind of bulb is in the warning lamp circuit.

My alternator cares. Quite a lot. Unlike my ex-wife...

[mearstrae]

I agree with o2, there must be something else going on. The "Charge Light" should actually be called a "Discharge Light", it comes on when the alternator is trying to bring things up to normal, it should not be lit most of the time. It can show that something isn't right in the circuit, something like a loose drive belt will turn it on very quickly and keep it on. Battery storage capacity doesn't affect the charging circuit, I've seen a 850CCA battery charged by a 35amp Lucas Alt. with no troubles.

'95 R.R.C. Lwb (Gone...)
'76 Series III Hybrid 109
'70 Rover 3500S

[warrenperkinson]

It's interesting that this has sparked some debate.

I know for certain that it's the charge warning lamp that caused the problem. I know this because I simply swapped out the LED with the original filament bulb and the alternator started charging beautifully without making any other change.

Some internet research also indicates that an to fit an LED bulb as a warning lamp will require the addition of a resistor into the circuit of something in the vicinity of 48ohms. A LED bulb is too efficient and does not draw enough.

But, whatever side you take on this one, I'm happy because my alternator works nicely!

[SafeAirOne]

AFAIK, the charge warning lamp circuit needs to permit electricity to flow in 2 directions. Polarity doesn't matter with an incandescent bulb. LED's only only work in 1 direction. That direction is dependent on how the LED was installed.

Whether or not this affects alternator output is dependent on the alternator design, I imagine. I see no proof one way or another in this thread about the impact of an LED vs. incandescent bulb other than the empirical evidence that the OP provides (which works for me). Either the alternator DOES work with an LED (vs incandescent) or it DOESN'T.

Electricity behaves in a known, predictable manner based on facts that have been established for more than a century.

In light of the fact that the OP's alternator DOESN'T work with an LED charge warning lamp installed but DOES work with an incandescent bulb installed, I ask that you please support the claim that the OP's observation that his alternator DOESN'T work with an LED but DOES work with an incandescent bulb is "absolute hogwash".

[SafeAirOne]

This link here provides information on the theory of operation of the Lucas 17ACR alternator.

Pay particular attention to the passage in paragraph 14 that reads " VT3 however does not conduct because there is no voltage supply to its collector via the field winding. When the ignition switch turned on VT3 conducts through the field winding and warning lamp lighting the latter to full brightness providing sufficient field current (2 to 3 Amps) for the alternator to start generating when the engine is started."

I don't think you can put 2-3 amps through most LEDs without frying them.

This website here says "Most LEDs can pass 20 milliamps (that's .02 amps) continuously without damage..."

The 2-3 amps supplied through the charge warning lamp circuit, required before the 17ACR alternator can start producing electrical output, equals 2000-3000 milliamps; Well beyond what can possibly flow through most LEDs. Well beyond.

[leafsprung]

Kudos to mark for the leg work. Not an issue I have run into but nice to put that info in your back pocket!

This link here provides information on the theory of operation of the Lucas 17ACR alternator.

Pay particular attention to the passage in paragraph 14 that reads " VT3 however does not conduct because there is no voltage supply to its collector via the field winding. When the ignition switch turned on VT3 conducts through the field winding and warning lamp lighting the latter to full brightness providing sufficient field current (2 to 3 Amps) for the alternator to start generating when the engine is started."

I don't think you can put 2-3 amps through most LEDs without frying them.

This website here says "Most LEDs can pass 20 milliamps (that's .02 amps) continuously without damage..."

The 2-3 amps supplied through the charge warning lamp circuit, required before the 17ACR alternator can start producing electrical output, equals 2000-3000 milliamps; Well beyond what can possibly flow through most LEDs. Well beyond.

[Fraserb]

JUST DON'T REPLACE THE CHARGE LAMP!

I concour with you warren.
I had the same issue a few months, nobody seemed to think the LED was the problem and I went thru replacing the alternator etc and still getting no charge till I thought to go back to the one little change I had made.... old incandescent bulb back in and hey presto, working again! but still was told the LED wouldn't have caused it and must have just been coincidence.

just put all new aftermarket gauges in (can hear the groans now... but goes with the GPS speedo!!!*) and LED warning lamps, my previous LED issues were not even on my mind. button it all up and kick it in the guts, was all good. did notice the ignition light stayed on for the first drive, so got an expensive resistor and put that in. still no charge. reinstalled an incandescent bulb and is all good again.

talked to a few people that should be knowledgeable about it and they just mutter that it should be Ok.
but its not.


Now have a bulb holder hanging out under the dash.... luckily I cocked something else up so needs new dashboard anyway....

*GPS speedo is great! bit more precise than the 20MPH indication the old gauge gave.

[warrenperkinson]

I concour with you warren.

Phew, glad I'm not the only one to have gone through that...

...and I do the GPS speedo thing too, but I just balance my iPhone on the dash with an app that does it.

I like the original dials, and if needed I can always fall back on the '...but officer, my speedo said I was only doing somewhere between 20 and 60 MPH....' line. That's assuming I manage to get the old girl going fast enough to be pulled over.