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Generator / Ignition Light Diagnostic


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1968 GT6 MKI
Generator - LUCAS C40L (2276E)
Voltage Regulator - LUCAS 340

The little car has been running quite well, until Friday.  Car did not start on first attempt, but did start on second attempt (this has happened a few times when I am in a hurry).  Noticed that the ignition light on the dash would not go out when it was running.  Searched the various threads on Club Triumph and elsewhere and on Saturday I attempted to diagnose the problem.

Could not find anything obvious so took out the meter and got these readings:

Battery Voltage
12.55 - engine not running
12.35 - engine running at idle (800 RPM)
12.18 - engine running @ 5000 RPM

Generator readings when engine running:
Voltage output - 290 mV
Current output - 40 mA

Being that the generator has a maximum current output of 25 A, does it make sense that the generator is faulty when its running output is 40 mA?

Anything else that I should be checking?  I did not check the Voltage Regulator, besides disconnecting and reconnecting the wires.

I also have a relay connected between the battery and the rest of the wiring but don't know what this would be affecting.

And unfortunately I also buggered up the timing I fear.  Car was running great, moved the vacuum advance and now it runs like crap.  Very poor acceleration, runs smoothly at 3000 RPM and then absolute garbage above that.  Think its time to go play in the distributor and reset things.  Any thoughts on this?


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Hi JB.
Can't completely cure your generator problem from afar unfortunately, but, as I see it, there are 3 components to the charging system that can make it stop charging.
One is the generator itself of course, as you mentioned. The next is the voltage regulator. The last is the wiring between the two of them, and also to the battery (at the starter relay, if I remember).

So while you might indeed have a bad generator, in need of replacing, it could be the regulator or a bad wire/connection causing the trouble.
first thing I'd do is to check for loose or corroded connections, frayed or broken wires, loose mounting points to any of the above (causing a ground/earth circuit to go missing), or even a loose belt on the generator's pulley.

Then the testing gets fun! There are tests and adjustment procedures to go through with regard to the regulator, to make sure it's functioning properly and telling the generator what to do. Then again, like a generator, it might just need replacing at this stage in it's life anyway.

The moral of the story being, you can simply replace both the regulator and generator for a quick-n-easy fix, or not. The problem with the shotgun method (just throwing parts at it) is that doing that might not fix the initial problem because you had a bad wire somewhere.

Can't tell you exactly how to adjust or test the regulator, or even if there is a simple test you can perform on the gennie to see if it's an internal fault. However, with all the extremely finely-detailed knowledge of the members hereabouts, I'm thinking it won't be long before you have your exact answer.

Best of luck. Hope it's a simple solution (as in, inexpensive!) that gets you on your way again.


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Thanks Guys

Spent some time alone today, just me and the little car, well actually just me and the little car's generator.  Did a bunch of testing, disassembled it and put it back together again.  

I have various manuals on the GT6 and followed some of the testing on it and found this fine little booklet online:  http://www.mgcars.org.uk/imgytr/pdf/lucas.pdf.  It is a gem - download it.

Definitely appears to be generator related as the output is very minimal - 0.3 V and at 3000 RPM it climbs all the way to 0.4 V (way below the 14 - 15 V it should be doing).  

One obvious sign in the work today is that the brushes are shot.  Not sure if there is anything else wrong at this point.  If I can't source new brushes on Monday then it will go to an auto electric shop for an overhaul.  I get lost when the various windings come in to play and don't have the right tools for removing bearings, pulleys and the like.

Will keep you posted.


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Shop advised that the generator needs a complete overhaul - brushes, armature and field coils are all shot.

So, am in the market for a Lucas C40L generator.  Does anyone have one for sale?

I appreciate the comments about converting to an alternator (or a dynator), but it is not something that I am considering.  I will maintain the originality of the car.

Also, don't want the C40 as it is a tad bit shorter and I don't want to mess around with fittings and shims and spacers and the like.

So, if you have a C40L (idealy Lucas Part # 22763E) please let me know.  Yes I realize that shipping will be dear, but c'est la vis.



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Thanks Nang - much appreciated.

Am sure that there must be at least one somewhere in North America.

Found a shop in California that rebuilds the old Lucas gennies.  So far they are an option, but looking for something closer to home since shipping could be dear.


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just to add a bit about the control box,  TimH  and I spent a day acuratley following all the manual settings voltages , gaps, currents, engine speeds and having fitted it back on feeling pleased with the result, a quick cuppa and run the tests again to be sure ,,all seemd fine

next day   it decides to blow all the headlamps....fit a second hand unit and bingo.....so moral is   leave the little box alone

have to say to  keep original is very valid,but in modern  driving conditions   a pair of headlamps fed direct from the battery via a relay will turn  the old  10v drop  fed yellow lamps into  14 v whiter intensity,    dont have to be  halogen
finish of with a alternator to keep things topped up all the time and night driving becomes more illuminating


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If John B's GT6 in Canada is like US-spec ones, it has sealed-beam headlights.

Sealed-beam replacement headlights for many years have been halogens, anyway.

It is interesting to read of the major operations that folks with home market cars go through to install halogen headlights, when we've been running them for a long time.

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Yes, US market sealed beams are almost universally halogen now. However most are still wattage limited to the old limits set in the days when sealed beams were the only legal headlamps on the US market. Modern separable halogen headlamps can have quite a lot more wattage now. So, it's a case of there are halogens and then there are halogens.

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