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MG Midget and Sprite Technical - Lucas Altenator Question

Hi gang

I have a '78 1500.

Just a question about Lucas altenator output.

I noticed over the past few weeks that my battery didn't seem to be as strong as normal, so I installed a voltmeter under the dash to monitor the altenator output.

And yes, the output was drifting down (13.0 dropping to 12.5 volts) & I noticed it wouldn't start charging until the engine was revved to 2500-3000 rpm, then it would charge until I shut it off...re-crank & the same pattern.

I replaced the altenator with a reman unit & now have a good, strong charge of 14.0-14.5 volts.

But, the same pattern with the replacement: it won't charge until revved, then a steady charge of 14+ volts until shut off.

Is it normal to have to rev the engine to start the charge on these altenators or did I get a reman unit with a similar problem?

I never noticed it before, because I had no meter to monitor it.

Any thoughts or ideas?

Thanks,
Dave
Dave Rhine ('78 1500)

Alts should start to charge from below 1K rpm (on rev-counter).

Do you have anything that might be using current?

Check alt belt tension and engine/battery earths; solenoid posts and so on...

A
Anthony Cutler

Anthony...thanks for the quick response!

At least, now I know it's not normal...

Altenator belt's new & tight, earth's have all been removed & cleaned, I even added an extra one from the engine to the chassis awhile back, solenoid's new & so are the cables & terminals.

I'm on vacation this week, so I've got time to work on my little car.

I think I'll do a key off drain test, although the battery stays charged, that's not a problem.
And check to make sure I have no + to - shorts on any circuits.

I have noticed (since installing the meter) that when I press the brake pedal the meter drops about 1.5 V, the same with the turn signals, the headlights drop the meter around 1.0 V, but I do have the headlights relayed.

Could I have a lighting circuit problem, but I don't see how that would cause no charge at intial start up.

A bit confused, but that's normal!

Dave
Dave Rhine ('78 1500)

How is the ignition light performing?

I ask this because if the lamp is going out when the car is started then the alternator is working correctly. If so you are maybe seeing the effect of the discharge on the battery on starting that will take a little time to overcome and revving the engine would naturally reduce this time.
Bob Turbo Midget England

Did some tests: no drain or shorts I could find, I did discover that I apparently pulled the meter + lead from my lighting supply circuit...the cause of the newly installed meter drop, so that problem's solved!

I verified altenator charge by connecting a VOM across the battery posts & no appreciable drop with lights on.

So back to the original question, why would it take revving the engine to initiate altenator charge?

Once it starts charging, it will stay charging @ 14+ volts even @ 800rpm.
Dave Rhine ('78 1500)

Good question Bob & bobo me hasn't a clue!

The light hasn't worked since I bought the car last Summer, I know, I should trace that problem down too!

I've let it idle on intial start up for upwards of 5 minutes & it still won't charge until revved up.

Just odd that the original & now the replacement do the same thing.

I did verify that the altenator does supply a negative to the light with the key on, then the negative goes away as soon as the altenator starts spinning, hence, the light, if working would go off.

I'm thinking maybe the bulb's just burned out, I suppose I should verify there's no problem in the ignition light circuit.

Thanks
Dave
Dave Rhine ('78 1500)

you need 12v through the warning light to power the alternator. It energises the stator coils, so that is probably why the alternator doesn't charge until later

The stator coils provide the magnetic field that the rotor turns through to provide electrickery

Lucas alternators are supposed to be "self-exciting" at about 3000 rpm (measured on the alternator pulley not engine bottom pulley) which seems to be when charging starts to happen on your car

fit a new bulb and then you should get the complete circuit and charging should begin at lower pulley revs
bill sdgpm

Thanks Bill, that sounds logical.

I didn't know I had this problem until I installed the aftermarket volmeter in the cockpit.

I'll try that!
Dave Rhine ('78 1500)

I agree with Bill. You can check the wire that feeds the alt. to see what the voltage is when the key is on or the engine is idling and not charging.
tomshobby

Bill

You're my hero!

Electrickery indeed!

I swapped the bulb with the known good one from my high beam indicator & she charges as soon as the engine starts running.

The older I'm getting, the thicker my head's getting, how simple & obvious...right in front of me & I chose to ignore it & make it complicated.

I need to adhere more to the KISS principle!

Well I guess it's time to sit in the corner for awhile with my dunce cap on!

Thank you muchly!

Dave :)
Dave Rhine ('78 1500)

oh no I wanted to say that!!!

Never mind should keep close to a computor and get in quick... :)

Pleased it was an easy fix Dave.
Robert (Bob) Midget Turbo

I also have a similar observation, on start up ign lamp stays illuminated and voltage shows 12v until car is revved to 2200rpm, the lamp extinguishes and voltage needle pops up to around 13.8v. from then on it's fine.

its been like that since I fitted 4 yrs ago. its not a ACR but a later one off a mini from a scrapper.

Bill it's been a while since i was "self exciting"

dan
d cusworth

Bob

You alluded to the ignition light & had me thinking, so all's not lost.

And yes, somtimes you've got to be quick on the keyboard on this forum...not like me, I'm a 2 finger pecker!

Thanks again Bill, Bob & Tom...ya'll were all over it!

Dave ;)
Dave Rhine ('78 1500)

but now Dave needs to fix the high beam lamp after having to bust it to fix it?

:-)

glad we helped




Dan.....

ditto
bill sdgpm

Busted, but fixed, I'm a happy boy!

:)
Dave Rhine ('78 1500)

This thread was discussed on 03/06/2010

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