Welcome to our resource for MG Car Information.



TR parts and Triumph parts, TR bits, Triumph Car Spares and accessories are available for TR2, TR3, TR3A, TR4, TR4A, TR5, TR6, TR7, TR8, Spitfire and Stag and other TR models are available from British car spares and parts company LBCarCo.

Triumph TR6 - A 'puzzler' for you

On my car, everything electrical works, charging checks out at the battery - however!!, ever since I've had her, when driving at night, the headlights occasionally dim slightly, then brighten up again. Heres the rub - just noticed that when the lights DIM, the voltmeter PEGS, when the voltmeter drops back to normal, the lights BRIGHTEN. What in hell is going on.Does'nt affect driving or fun but has me mystified. Anyone ?? Dan M ??
TIA Peter G
Peter Gooch

I've seen something similar - thought it was the alternator voltage regulator momentarily switching off due to sensing a fully charged battery. During that time the lights would operate off battery voltage (12- V) rather than alternator output voltage (13+ V) so would appear dimmer.

Brent B

But would the voltmeter PEG in that scenario? After all, now we're on battery alone so it should drop a tad.

Methinks a voltage regulator MISsensing full battery. Brent's gotta be on the right track though.

Why not just throw a new alternator at it. 78-80 Ford Fiesta is a dead swap. $60 at a real auto parts store here on the mainland (prolly more there in paradise). Since I went to the Fiesta alternator (it's a Bosch by the way) I've had NO troubles whatsoever.

Well, there was that time my back hurt....

Jim (all charged up and in storage)
Jim Deatsch

Jim Deatsch

I keep bringing this back to the top on accounta I want to hear what's happening with it Peter.

How you making out?

Anyone have other thoughts about this besides myveryownself and Brent?

Jim Deatsch

I agree with Jim in keeping this up top, just got a new bosch alternator and plan to redo that side of the electrics this winter. So interested to see what happens I will replace volt reg., fuse box, alt, and refresh the steering column grounds at the same time.
Keith Dixon

Iv'e been thinking about this problem ever since you posted it. The voltmeter you think is a volt meter is in fact an ammeter or ampmeter depending upon where you live. It reads current flow and relates to what the alternator is putting out. The alternator has a limited output and when this is reached the meter "pegs". I take it the meter is moving to the left showing full flow. I think you have a temporary short that is consuming all that the alternator can supply. When the short takes place the alternator cannot supply all that is required to keep the lights to full potential and therefore they dim. My second guess would be a poor fitting at a battery terminal as this would shunt the alternator and battery amperge sources back and forth. Have some one disconnect and reconnect the battery while the engine is running with the lights on and see if this duplicates the problem.
Good luck.
Joe Justice
Joe Justice

Keith, you're going to love that alternator.

Joe is absolutely right. It's an ammeter unless you've added a voltmeter.

That having been said, I wonder if any short to ground that would cause this situation wouldn't cause an overheat condition in said short to ground and take a fuse or the wire out.

Jim Deatsch

Up through '72, TRs used an ammeter; from 73 on, they came with a voltmeter.

This is a puzzler. Right now, I'm engaged in the "perfect storm" of commitments (every commitment I have is now due at the same time), but as soon as I can get some free time, I'd like to look into this a bit. I don't know if it can be solved without some testing and some voltage measurements.

What year is the car in question?
Dan Masters


Dan strikes again. Come to think of it, my 73 is a voltmeter. Thanks Dan, for bringing us to reality.

Jim Deatsch

Dan and Peter,
I stand corrected. If your TR-6 has a voltmeter and it pegs there can be but one problem. An unregulated alternator actually puts out 120 volts alternating current. Hence the term alternator verses generator. The votage is reduced by the regulator and changed to direct current (sort of) by the diodes. If you are experiencing regultor failure you stand a fair chance of igniting something you don't want to. I changed from the original alternator to the Ford Fiesta in aneffort to get more amperage and brighter lights. The alternator is a drop in fit although I had to change the wiring connector end. It was an easy swap but didn't give me any better lighting. I have the old alternator and will be more than happy to ship it to you if you want it. No charge, I try to do something for someone else every week.
Joe Justice
Joe Justice


If you will please, allow me to be a bit pedantic.

The term "alternator" is a marketing term, rather than an engineering term. In an industrial environment, you'll rarely hear the term Alternator used, even though what we refer to as a "generator" has gone the way of the carrier pidgeon. However, in the automotive field, the two terms have taken hold and there is a clear distinction between the two.

There is no fundamental differance between an alternator and a generator: they both operate on exactly the same principles, they both produce AC voltage (DC voltage cannot be generated by a rotating machine), and both convert the AC to DC internally. There are ony three significant differences between the two:

1) In a generator, the field coil windings are stationary, while the output windings rotate; in an alternator, the field coil windings rotate, while the output coil windings are stationary.

2) The AC voltage in a generator is converted to DC by segmented slip rings; in an alternator, the conversion is done by diodes.

3) A generator has soft iron pole pieces to provide the "bootstrap" magnetic field to initiate current output; an alternator requires current from the battery to provide this field.

These seemingly minor differences make all the difference in the world to the relative performance of the two. I have penned a rather lengthy description of alternator operation which you may find informative on the VTR website at:

I'm not sure what you mean by "unregulated alternator output is 120 VAC" If you remove the regulator, there will be no output. If you bypass the regulator and connect the field windings directly to the battery, and hence to the output, the only limit to the output voltage will be the speed of the engine and the capability of the windings to handle the current. In fact, bypassing the regulator is one way of testing an alternator or a generator, but care must be taken when doing this to avoid damage.

You are correct that the only way to get a high reading on the voltmeter is for the alternator output to go high, but this doesn't explain why the lights dim - higher output voltage from the alternator would also apply to the lights, making them brighter, not dimmer. I'm still out of town today, so I don't have my reference materials handy to evaluate the TR6 wiring to see how the described situation could happen. My "off the cuff" explaination would be that there is a bad connection somewhere, dropping the voltage to the lights. This lower voltage is being fed back to the alternator regulator, causing the alternator output to rise. This explaination is not really satisfactory, but until I can do a study, it's the best I can come up with.
Dam Masters

Hi Peter and all

Not an engineer just fixing electronic stuff for the last 40 years or so as a living. For pedantics (I had to look that up, neat word always used smartass myself but pedantic is much more refined) go to this page and check out top right. Leading techs. for Asia/Americas.

Irrelevant but very pedantic..:)

Peter in my experience going to hands on tech type stuff.
As Dan points out.
A momentary short in wiring exites the field and increases out put. If its prior to the lights they will dim in my estimation. Dimmer flash to pass switch is not one of the better devices known to man? And a good start.

Slipping fan belt threw me on one of my toys for a long time. Fan belt slipped. Usually when damp at speed lights dimmed alternator decided it was at idle high load, and stepped up the voltage. Belt kicked back in real fast. Up goes the voltmeter.

As far as exiting the field goes you may want to stay away from some of your beaches on the drive. Never been there but from what I see on TV, my wife would definately dim both my lights..:)

Bill Brayford

Hi Guys, the only person who has not been following this is me (the perpetrator). We had an incredible storm, not quite expected, which took out our power for a couple of days, and left us with days of cleanup (chopping up and hauling downed trees, searching the neighborhood for stuff which simply blew away, etc etc).Anyway it's done and I'm back and into the fray.
Slipping belt it is not, first place I looked, and it was loose, so tightened it twice over the year. Another factor is that it has done this from day one w/ zero change in symptons, IE,not getting worse. My first thought was bad reg untill I noticed the corrolation of bright lites,low gauge (not low - normal) and dimmer lites w/ pegged gauge.And it is completely irregular. Hoped at this time there were enough clues for the TR brains out there. Nothing else malfunctions.
Peter Gooch

That's it. No more questions from Peter.

Father Gooses
Jim Deatsch

Trouble w/ this is that I cannot duplicate in the garage w/ test equipment hooked up. Runs a steady 14.4v (at the battery) at all engine speeds, and yes guys, mines a 74 and does have a voltmeter.So the mystery continues.
Peter G
Peter Gooch


It's the ghost of Mr. Lucas, just letting you know he's still around!

Bob (Practical Pedantic Applicator)
R.C. Blair

I think that, somehow, John is involved. He's the only other one in a warm climate.

Peter, why not just run extra long leads to your multimeter and into the engine compartment and monitor (or have your assistant) whilst driving?

Jim Deatsch

Only happens sometimes problems are fun aren't they? Hope it happens more than once a month. If by chance its a wire problem?

For myself unless the lights were on I'd likely not notice the meter peg momentarily? So it may happen otherwise?

If you can find a 24v military auto bulb and holder or any other cheap. (Check the army surplus stores) to hook into the voltmeter circuit that may help to narrow down. 12/14 normal should give a dim light and pegged should brighten enough to make you pay attention and think OK what was the last thing that happened Turn/bump? I turned on something. You get the idea. If it only happens with the light circuit powered then you have less wire to chase.


Bill Brayford

Took most of a year to notice the relationshipbetween the lights and meter. Now that I have, I watch! Jim, you may have a plan w/ the extended multimeter.Somehow I feel the current is being shunted between the headlights and the v meter. How, is the question.And is anything a bit off in daylight and I can;t tell ??????
Thanks for twisting your brains Peter G
Peter Gooch

Any chance the voltmeter is defective and is the source of the drain on the headlights? Purely guessing, but if it pegs just when the lights dim, maybe the gauge itself has the short. The guess is based on the coincidence of the symptoms.

I have an older car with an ammeter, so this is an uneducated guess.

Mark Hauser

This thread was discussed between 10/01/2004 and 23/01/2004

Triumph TR6 index