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MG MGB Technical - ignition light glowing

read the archives and nothing really fits...is there a simple wiring diagram that an idiot can follow.my ignition is dim at idle but glows brighter as engine rpm increases and more load is added. battery stays charged even at night. took these readings with a digital voltmeter, measurements across the battery
engine off nothing on =12.25 v all on =11.40v
all on is headlights,wipers,fan, radio and brake lights
engine at 1500 rpm all off = 13.90 all on = 13.13
i connected the voltmeter between the large brown wire on the alternator and the positive battery terminal
all off = 0.00 v added headlights = +0.04 added fan = +0.05, added brake lights = +0.06v, added wipers = +0.07v the voltage drops are consistant with what i would expect. i measured at 1500 rpm and the voltage was -0.10 volts ... the polarity change is what i would expect from the system if it was charging
i measured the resistance of the circuit, it was 0.1 ohms ... why is the ignition light glowing???
do not know why I=E/R does not seem to work ... help
rocky
rocco grillo

Rocky,
I once had an alternator that was shorted internally (internal regulator) that caused this same problem. a dim glow that got brighter as the alternator output increased. Still charged but iluminated the lamp. I probably got a spare if you want to try it.
Roy
Roy Dougherty

Rocky - The regulator in your alternator is bad. You should be able to find a local auto electric shop in your area that can replace the regulator for you and check out the alternator. One of the symptoms is that your alternator is only putting out 13.3 volts when it should be putting out 14 - 14.5 volts when the engine is reved up. The reason that I=E/R for you is that the regulator is full of active devices like transistors and they don't behave like a pure resistance in the above formula.
Good luck - Dave
David DuBois

May also be caused by a bad ground causing a backfeed to ground elsewhere. Extra high resistance (from bad grounds) will give low voltage readings!

Mike!
mike!

Bad diode.

If you are not a stickler for originality, one word, DELCO...since I changed it to a 65 amp Delco..no problems.

For a while I tried a number of Bosch Ford Fiesta rebuilts. They were cr*p.


Delco...so far, no problems.

rn
RN Lipow

I finally got a 0-15 voltmeter at radio shack and a cigarette lighter plug ... epoxied them
together so they would fit into the cigarette lighter socket in the b. I marked the static
point with all items off,
started her up and the voltage read 13.9 volts. I added loads, lights
with both beams on, blower motor, wipers, brake lights and the meter never got below the
marking I put when it was static
THIS IS GOOD ENOUGH FOR ME
by the way, I added a dash panel light in series with the alternator
brown/yellow wire .... seems to charge better and the dash light acts normally
rocky
rocco grillo

I also have a Delco unit (suitably modified). like that one wire stuff but I ran an exciter (from the dash light) anyway.

FYI all, and I mean all, internally regulated alternators will self excite. That's right. But it may take a "sprint" to 2500 rpm or so to initiate it.

Mike!
mike!

This thread was discussed between 31/07/2004 and 03/08/2004

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