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MG Car Repair/74 MG alternator update

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QUESTION: Hi Barrie! ..Driving my 74 MG today and I got an ignition warn light at highway speed, and a burning smell for brief moment. I stopped and checked wiring and grounds, and manage to get to a mechanic who tests battery charge (ok) and starter (ok) but notes a ripple in the charging.
.I limp home with the light not glowing if revs are kept at 2000 or below. However if more, the light pulses bright and dim in a rhythmic cycle - the travel home was 60 miles. The alternator is only 3 yrs old.
Have you seen failures like this? any other suspect parts?
    
regards Paul

ANSWER: Hi Paul
It is possible that your alternator has developed a fault,
but it is more likely that the warning light is correct.
I think you have been given a warning which you must not ignore.

I presume that you have an overdrive fitted to your 74 MGB?
When the light showed, were you using the overdrive?
If so, then as a matter of urgency please get the wiring to
the overdrive checked out, especially underneath the car.  
This circuit is not fused and it can cause an electrical fire
in the wires connected to the ignition switch.
In the meantime, do not use the overdrive.  

I had a similar problem with my previous V8.  The wire to the overdrive solenoid was swinging around and occasionally touched the metal bracket holding the gearbox.  This caused the engine to die momentarily because it shorted the ignition circuit.  The cure was a zip tie to hold the cable away from the bracket.  I also fitted a line fuse into the overdrive circuit to protect against it ever happening again.  It is a common problem and there is a simple fuse kit sold by Brown & Gammons specifically for the MGB.

Regards
Barrie

---------- FOLLOW-UP ----------

QUESTION: thanks Barrie - I do have overdrive fitted and checked my wiring to solenoid right away. Fuse is great idea. My wiring was ok but curious that I saw the heavy braided ground cable nearby seemed fused to the casing of the overdrive. I was unsure if being grounded at the mid point would matter as it was firmly attached at both ends? I moved it away from the casing. I removed the alternator and found evidence of brown powder and  (windings?) and also a burn mark on the internal diode pack. Time to re-build?

ANSWER: I agree with you.  It looks like the alternator may be about to fail.  

Please get your batteries checked as well.  A faulty battery can cause a perfectly good alternator to fail.

... and fit a fuse in the overdrive circuit.

Regards, Barrie  

---------- FOLLOW-UP ----------

QUESTION: Well - I have a new problem ( or same old problem misdiagnosed? ) I installed a new Lucas alternator - -- but now the ignition light is on even with key in 'off' position. I dared nor start it fearing I have wired something wrong. ( my battery was checked by 3rd party and 12.6V and holds charge ) I am now heading off to read wiring diagrams! - Any tips on where to start?
 
Thanks Barry!

Answer
Hi Paul

There is either a wiring fault, or else your ignition switch is faulty.  

When you run the engine and turn the ignition off, does the engine stop?

The reason I ask is because the ignition light only has 2 wires going to it.  One is Brown with a Yellow stripe.  That comes directly off the smallest alternator terminal.  The other wire is white.  That is connected to the battery via the ignition switch.  

If the car is wired correctly then it should not be possible for the warning light to come on with the ignition switched on.  Therefore, the white wire must be getting a permanent 12 volt supply from somewhere.  It could be a faulty ignition switch or it could be a wiring fault.  

There are 3 white wires attached to the ignition switch.  One goes to the warning light, a second one goes to the overdrive and the third one provides power to the coil and to fuse #3 on the fusebox.  If that third White wire went to fuse #4 by mistake, then you have found your problem.

Regards
Barrie

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Barrie Jones

Expertise

MG Sports Cars from 1949 to 1980 - Including MGB, MGBGT V8, TF1500, TF, TD and modern Midgets. I also specialise in SU carburettors, Lucas wiring, suspension, steering & brakes.

Experience

I have owned my 1955 TF1500 since 1966. Technical specialist for the TD and TF with the MG Car Club T Register. Also owned 20 MGBs and currently own an MGBGT V8. Written 3 books on MGs and produced a DVD on how to strip and rebuild the TD/TF gearbox.

Organizations
MG Car Club.

Publications
Barrie's Notes on the 1953-55 MG TF (author). /=====/ Barrie's Notes on the MGB (author). /=====/ Barrie's Notes on the MGB GT V8 (author). /=====/ The Essential Buyer's Guide to the MG TD, TF and TF1500. (author)

Education/Credentials
M Phil, C Eng, BSc (Eng).

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