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MG MGB Technical - A Slightly Different Turn Signal Problem

To the old gurus on the board.

I haven't been on the board in a long time. Some of the older guys might remember me. Just been entirely too busy with work...

Have a '73 B that has developed a problem with the turn signals that is a bit different than the usual problems that I have seen discussed.


About 6 months ago the original flasher started flashing rapidly for a few days and then down she goes - no more direction indicators.

So, I go to my local MG parts supplier (I actually have one that stocks everything in the Moss catalog) and get a new flasher (newer style 2 prong unit Lucas P/N SFB114). Plug it in. Voila! signals work great.

2 mos. later out of the blue it starts clicking strangely - not really fast but lighting up for a shorter time than normal. About 30 seconds later, it dies.

Back to my supplier who gives me another one for free as we agree that it is probably bad.

Plug it in and voila! back in business!

Now 2 mos. later the second one has done exactly the same thing (just got a 3rd one today and paid for the 2nd one because it is obviously something wrong in my car).

About 8 mos. ago I went around the entire car (took about three weekends) and cleaned every electrical connection I could lay my hands on - even replaced a few connectors. Re-assembled each one with electrical grease. Man, what a difference! Everything worked better.

Voltage running down the road is a beautiful 13.8-14V (I have a voltmeter in my car).

Now I really don't want to plan a trip for a flasher every 2 mos.! Yeah, I could buy a bunch at one time but that's not a solution.

There is a clue here but I just can't figure it out.

Any ideas?
Richard Smith 1

You cleaned all the connections and put electrical grease in them? Well, that's the problem right there. The system is designed to work with dirty, corroded connections and just can't handle the extra electricity afforded by such improvements.

Just kidding. You might consult a wiring diagram and make sure you haven't swapped some connections relevant to the turn signal circuit. -G.
Glenn G

I have had a couple of flashers go as you say, When I installed the flasher into the car I did not bother at the time to install it back into its "cradle" bracket mounted to the did not last long. I wondered if it failed if the flasher needed to be case grounded. Maybe not and never took it apart to see but this one has lasted a couple of years so far. FWIW
Eric Willis

Similar problem that i got through 3 flasher units in a matter on 2 months and i only use the car on an odd sunday !!. I did as you and checked every thing that i could and have put it down to a dud batch of flasher units. The only thing that i can say was that i took extra care to make sure the right cable was on the right terminal with the last one.

Thanks for the comments so far.

Regarding the connections to the flasher it self and the mounting:

1) I did mount it back in the clip holder but have been concerned that pushing it into the clip far enough does cause the back of the flasher case to run into the screw for the clip. Still I left it in the clip because I like everything secure.

Does anybody know if the clip on the case IS the ground for it? If so I will be careful to confirm the ground when I install the third one.

2) Regarding the two wires being on the correct spades. I note that the back of the flasher has the the spades marked 'B' & 'L'. According to the schematic in my original owner's manual (lucky enough to have one of these because none of the schematics in the Bentley's manual totally agree with the one in my manual), the solid green (G) goes to the 'B' spade and the light green/brown (LGN)goes to the 'L' spade.
That's the way I hooked it up.

It is at least a little satisfying to learn that others have had a similar problem.

After reading a lot in the archives about correct bulbs, I have considered going round the car and replacing all the direction indicator bulbs with brand new bulbs.

I know I have bulbs that are 20-30 years old that still function...
Richard Smith

Just a wild guess Richard, but have you got the correct flasher for your year of car.

Are the lights on the side of your car wired to flash with the indicators. If so the current draw will be greater than on the cars without the lights on the side so the flasher unit would need to be rated to take account of that.

I may be confused here between US spec car as originally supplied and what people do to US cars that are brought back to the UK. (Any orange light on the side of the car here must flash with the indicators or it will fail the MOT test.)
David Witham

Perhaps you've just been getting dodgy flashers? Try going to your local CarQuest/Advance Auto/Pep Boys/whatever and get a plain ol' generic 552 flasher. It looks a little different but plugs up right and will even fit in the bracket. I've seen "proper" flashers from British parts suppliers that are nothing more than reboxed (& repriced!) 552s...

Let us know what you find!
Rob Edwards

According to the VB catalog, the flasher is the same for years 68-80. My local suppliers flasher is a Lucas and he has marked on the box "MGB 68-80". Cannot cross the VB part no. with the Lucas part no., however, so that still may be a valid question.

Would've gone out and confirmed whether the side marker lights flash together with the tail lights but I haven't yet put the new one in. If memory serves they do not. Wiring diagram indicates they do not as well. But will check when I put the new flasher in.

I just might get a generic 552 flasher for the time when this one goes bad (probably about 2 mos.) from now.

Who knows, the presence of the generic flasher in stock at the house may just scare the new one into working forever...
Richard Smith 1

Right - over here we can no longer get the original flasher unit. Apparently they are nolonger manufactured. The new (nearest)replacement unit says 35 +5 on it, if I recall (I guess it was Watts ?). The +5 bit, a repeater apparently, was never on the original. I am on my unit 3rd this month, first one died completely, second I took back and swapped, third does just the same, weak fast flash, but improves if you rev the engine ? I have now repaired the original unit and put it back in - now works just fine.
The casing is not connected to any of the internal electrics, so does not need earthing/grounding.

I swapped the hazard unit for the indicator unit to get a useable one for the road. That might help, as I have only ever used the hazards once.

R I Jones

Not sure this is relavant, but some of the flashers sold today are solid state rather than purely mechanical as the OE units were. The OE units worked on a bimetallic strip which current caused to heat up and make/break contact, and then the cycle would reset, and repeat ~ flashing the lights.

My guess would be that there is much more current passing through your flasher than intended for either a mechanical flasher or the newer electronic flashers, and that causes them to burn up.

If your wiring is correct, then the current load should be correct. However, if you have halogen lamps in use for signals, they do draw a bit more current and that may be a problem. AFAIK Moss doesn't seem to sell the halogen bulbs for signals and perhaps their thinking is that the unit offered is correct for standard incandescent bulbs and that should be sufficient.

I DO use the halogen bulbs on my 68 GT, and use a plastic bodied Tridon electronic flasher and it has worked just fine for years. I am thinking that there is an extra load or perhaps a polarity mismatch going on if your replacements happen to be an electronic type.

You might want to direct your question to Rick Astley,, as he is an automotive electrical engineer. He has a great site for MGB electrical at
Bob Muenchausen

A 73 *should* have a 2-pin flasher, not a 3-pin, they were only used on Mk1s.

Green to 'B' (Battery) and light-green/brown to 'L' (Load) as you have done.

Neither of the factory flashers needs a ground, the spring clip is simply to make the clicking more audible. There are post-MGB 3-pin flashers, and the third pin on these *is* supposed to be grounded. These are the electronic units that Bob refers to, and these flash at double-speed when a bulb or the connection to it fails.

Normally the problem with old connections and bulbs is that the flashing rate *slows*, sometimes to a stop when extra load is applied to the alternator like idling with the brake lights on. If any bulb is used that draws more current than normal, like the halogens Bob refers to, then they may flash a bit faster depending on how much extra current they draw. But this will be all the time, not the 'now you see it now you don't' that you seem to have.

It is not unknown for a new flasher to 'correct' a problem, only to fail in the same way shortly after. Two causes for this - new flashers do have slightly different characteristics initially then settle down to 'standard', and of course Hunt's Fifth Law which says "Many break-downs occur soon after a car has been worked on; 'new' parts can be faulty when you receive them; 'new' parts will sometimes fail soon after fitting; 'new' parts almost certainly won't last as long as the originals."

Rapid flashing on standard flasher on a 73 implies either a faulty flasher, or a partial short in the wiring. If the replacement flasher is from the same supplier and batch as the previous then it could well have the same fault. But a good replacement flasher on faulty wiring with a partial short should flash quickly fropm the word 'go', not behave normally then start playing up.

The problem with using a hazard flasher in place of an indicator flasher, and some so-called 'heavy duty' flashers, is that they will flash at the same rate regardless of how many bulbs are operating. All OEM indicator flasher units are designed to flash at a different rate if one corner is out, and using a flasher that does not do this is a safety hazard. If you replace the indicator flasher you should check that it gives a clear change in flashing speed if one corner is disconnected.
Paul Hunt

"There are post-MGB 3-pin flashers, and the third pin on these *is* supposed to be grounded. These are the electronic units that Bob refers to..."


Can you give us more info on this? Here in the US, I have never encountered a replacement 3-pin flasher that required the 3rd pin to be grounded, mechanical or electronic. In fact, of all that I know of, grounding that pin will place a direct short to ground on the input.

I assume these are OEM units, rather than off-the-shelf replacements, such as the 550/552 and equivelant electronic units that we have here?

I know you well enough to know that you know what you're talking about, so I'm not questioning you, just wondering. I fear that someone here will buy a 550 and ground the pin on your advice and then wonder why the fuse keeps blowing.

While we're on the subject, I'd like to point out a problem with some of the 550 types here in the US. Many of the newer models are made incorrecly, and unlike the older models, the "indicator" pin is ON when the flasher is OFF, and vice-versa. The older models, and the 3pin flashers used in some MGs and Triumphs, have the indicator pin ON when the flasher is ON. I've had several TR owners contact me to ask why the indicating lamps stays on after they replaced the flasher with a new 550.

I have new and old 550 flashers in my shop, same brand, and the old ones work as they should, but the new ones don't. If anyone is having this problem, the only 550 that I know of that works is a Tridon in the "short" can. A 550 Tridon in a tall can works just backwards,ie, the indicator is ON when the flashers are Off.

Whatever the reason for the difference, I find it inexcusable that a manufacturer would give the same model number to two different types. Regardless of manufacturer and can size, ALL 550 types should be the same.
Dan Masters

I agree, Dan. I made the mistake of grounding a newer electronic flasher and it burned up pretty quickly! Once I put one in and connected it correctly, it has worked flawlessly for 6 years now.

'69 Roadster
Roger Hotelling

Thanks to all again.

As usual Paul comes through with the definitive info and I am in agreement that if there was a short it wouldn't take 2 months to make the flasher fail.

And it is VERY probable that all the flashers I have had (3rd one now) come from the same batch. My supplier is literally out in the woods, has three tin shop buildings, scads of British iron everywhere and one of the tin buildings has just about every part for any British car in it. I know a lot of his stock has been around for a while because he has a lot of NOS stuff like crown wheels and pinions for diffs.
Actually a strange fellow - don't know how he survives and supports an inventory like that in the outskirts of a 186,000 population smaller city with not many British cars.

As far as Hunt's 5th law goes, it is SO true. I would go so far as to replace the word 'almost certainly won't' with 'will not' in the part about how long new parts will last. Original flasher lasted 31 years - now on the 3rd replacement. Original alternator lasted 17 years - now on 3rd or 4th replacement (been so many I can't remember)

And yes, I want to maintain the safety feature of having an indication of a burnt bulb.

For Bob M.

I don't have halogen bulbs in the turn signals and all of the new flashers have been mechanical and not solid state.

I have again confirmed polarity and the cleaning of the terminals a while back definitely made everything brighter rather than dimmer as I would think an increased load somewhere would do.

The new flashers do have the number 0303 on one side and a rather cryptic rating on the side - 21W x 2 + 5W. I assume that this means it will handle a 21W load on each side of the car plus 5W extra?

Also this one failed exactly the same way as the first one did. There is a very narrow (1/32 in) thin strip which supplies the current to the actual bi-metallic plate that has a wire connection in the middle. This little strip must bend back and forth with the bi-metallic plate and the failure always occurs with the strip breaking right at the wire connection in the middle. The engineer in me tells me that, for all the world it looks like a fatigue failure (explains why it takes a while). Appears to be a either a poor design or a defective batch of flashers.

Which Tridon flasher are you using?

For David W.

Confirmed - the side marker lights do not flash with the turn signals.

For Dan M.

Do you happen to know if I will run into the polarity problem with a 552?

Thanks again to all!
Richard Smith 1

"Do you happen to know if I will run into the polarity problem with a 552?"


With the 552s I have, polarity is not a concern - they work just fine either way. Of the Lucas 2-pin flashers I've seen, some work OK with reversed connections, and some don't - same part numbers.
Dan Masters

Further of note on the flasher saga. A closer look at the failed flasher this morning with a small microscope revealed exactly what I suspected - simple fatigue failure of the small flat conductor with the wire attached in the middle. Not burned at all and the fracture appearance is just like that of any piece of thin metal bent back and forth in the same place many times - all of you know what that looks like. I am also very confident that my analysis is spot-on.

Plain and simple - the design of this particular Lucas 2-blade flasher is poor and any chance of longevity is in the hands of the gods. The attachment of the wire in the middle of the strip is definitely a stress riser. Maybe some will have good luck but others will and have had similar problems.

I WILL be replacing it with either a Tridon or a 552 flasher. I currently have about 2 mos. from yesterday (yes, excessive fatigue stress is fairly predictable)to find a suitable replacement.

Final note - beware of the Lucas 2-blade flasher with P/N SFB114 on the box (other numbers on the box - 32/03 381 and 748895) and with 21W x 2 + 5W on the side and 0303 on another side. All of mine have come in a green Lucas box.

There may well be later iterations as Lucas may well have discovered the problem and corrected it - a simple and cheap design solution would be to place the wire connection at one of the ends of the strip rather than in the middle...

Thanks to everyone who took the time to reply. You made me put my thinking cap on. When I first posted this, I said "There is a clue here but I just can't figure it out."

The discussion was good in that it was also a good refresher for my mechanical engineering skills at a time when I am looking for an engineering job.
Richard Smith

Dan and Roger - the 2-pin flasher used on Mk2 and later MGBs had a Lucas model number of 8FL and is 'square'. The later 3-pin electronic flasher was (and may still be) used on many makes and the Lucas model number is similar to the above but has a 'later' digit (9FL is a hazard unit and should not be used as a turn flasher unit). These should have pins labelled 49, 49A and 31. Light-green/brown goes to 49A, green to 49, and 31 goes to ground. My 1989 Celica has one of these 3-pin units with the 3rd pin connected to ground, ditto an early 90s Metro (still 2-pin on an early 80s Metro). This type can be used as a retro-fit on an MGB, and although it does overcome the problem of slow flashers it isn't a 'cure' as the original problem of bad connections or incorrect bulbs is still present, and hence is only a work-around, and results in a dim flash albeit at a 'normal' rate.

The earlier MGB 3-pin flasher is an FL5, is cylindrical, and the 3rd pin on those is used to flash the tell-tales and must *not* be grounded. I can accept that if the wrong pin on a post-1980 3-pin electronic unit is grounded you *will* burn out the unit. No knowledge of those 550/552 numbers, although I see Google references to 3-pin 550 heavy duty and a 2-pin 552. The 550 seems to take from 1 to 4 27 watt bulbs and has a 1 second delay in first flash, which makes it a hazard flasher not a turn flasher. By contrast indicator flashers instantly light the lamps when the switch is operated, then turn them *off* after a delay to begin flashing.

Richard - the 21W is the standard rating of a flasher bulb in the UK at least, times 2 of course, the tell-tale is 2.2W I believe. An early 80s Metro diagram shows a 2-pin flasher with 2*21W main bulbs, a 5W wing repeater bulb, and a 1.2W to 2.2W tell-tale which puts it slightly above the +5W. But the extra load would help to keep the flashing rate up :o) The early 90s 3-pin electronic flasher has the same load. '0303' is probably a manufacture date of week 3 in 2003.
Paul Hunt

About a week or so ago I started a thread witch asked about a turn signal problem. They worked one week and not the next. Well today I got the "B" out of storage and was going to work on the problem with all the great advice I got from these boards. Well force of habit when making my first turn had me using the turn signal and of course you know what happened, THEY WORKED! Lucas, heal thyself. Spent the afternoon on a nice drive, with the top down, even though it was only 45 degrees and still snow on the ground. I guess I should still clean all the contacts but it isn't such a priority now.

I could find no definitive model number on the Tridon unit I have, just some misc numbers and the name Tridon molded into the plastic case. And, of course, the packaging is in the landfill long ago. However, I found that I had another electronic flasher of similar specs in my stuff and it carries this info.:

Buss Electronic Signal Flasher
Input lead {marked by an "X" on the bottom} must be fused Max. 25 amps, 12 volt

The need to fuse the hot wire is common to both units and both use the "X" marking to show which spade on the flasher this should be connected to.

Both of these electronic flashers are simple two spade connector flashers unit with a plastic case and phenolic base insulator. There appears to be no use of a separate ground as in the 3-wire units mentioned.
Bob Muenchausen

It has become obvious to me that what I described above as "electronic" flashers (and so have Buss, also) are actually, probably more correctly electro-mechanical flashers. To avoid confusion, you might want to take a look at the info below from Trico (yes, the wiper people), the parent company of Tridon.

Electro Mechanical flashers ~

True Electronic flashers ~

The Trico/Tridon line of flashers ~

And a bit more, if anyone is interested:

Grote Industries ~

Seimen's tech info on Flashers ~ ~

Wiring Products flashers ~

Hopefully this will be helpful.

Bob Muenchausen

Wow! Enough info to swamp someone. You guys are great!


Thanks for the explanation of the rating.


Thanks for more info than I ever wanted to know. But, at least you made it obvious that this is not a black art and one need not be constricted to exact OEM replacements.

Since I am not really interested in tricking out my MG, I suspect I will go to my local AutoZone and get the electromechanical one (appears to be a basic Tridon) that they have for about $8.

I might, however, be interested in the halogen lamps for the tail and turn signals. One of the things I have noticed, particularly in the last 10 years, is the dramatic increase in lighting level on modern cars. As you drive down the road at night, every vehicle seems very bright. Then you see an old MGB like ours with its single red tail light bulbs on each side on a dark road and it reminds one of the old days of travelling briskly on a two lane road and running up too fast on an old farmer in an old pickup truck with a single 5 W tiny read taillight that you can hardly see.

Has anyone had any experience with the new headlights VB is selling? The ones with the replacable halogen bulbs and, reportedly, modern lens design? I have the latest halogen sealed beams from Sylvania that will fit my car. Although, they definitely improved the lighting, they are still no where close in performance to the headlights in my wife's Trailblazer.

Perhaps, from a safety standpoint, all of us should consider doing something to our lighting systems that will significantly increase our visibility to others.

By the way, a little info on alternators. I have retained the Lucas alternator but I using the one suitable for an '80 model (the rebuilt one from AutoZone is rated at 45 amps) for my '73. And yes, Paul, I did check the brown leads for the alternator and my car is wired with a double brown lead.

The best thing I did was to get the alternator pulley for an '80 model. It is about 1/2 - 3/4 inch smaller in diam. and definitely improves alternator performance at idle and high load. Doesn't solve it completely but is a big improvement.
Richard Smith

Thought I'd throw in a couple of comments here, since I just replaced the flasher in my '63 Volvo P1800 over the weekend. I brought the original Lucas FL5 unit to the local auto parts chain, where the closest thing they could find was a Buss 550 three-spade flasher. At $4, I felt I couldn't go wrong, and I installed it straight into the three-slot socket under the dash of the P1800. It didn't work any better than the Lucas, the main symptom being that the right blinker would stop when I pressed the brake pedal.

After a little head-scratching, I decided to check the grounding of the taillight housings. The grounding was poor, and I improved it by removing paint and rust from the inside body surface where the fixing screws go through. I installed star washers coated in conducting grease under the fixing nuts. When I finished, the blinkers worked perfectly regardless of whether the brakes were on or not.

I was tempted to put the Lucas flasher back in, but decided that 22 years was enough. According to my log, I had installed it in 1983.

The Buss 550 flasher has two peculiarities. It is not as loud as the Lucas unit, and when operating, it makes the indicator and the blinkers flash alternately rather than simultaneously. -G.
Glenn G

"when operating, it makes the indicator and the blinkers flash alternately rather than simultaneously."


Replace it with a Tridon 550 in the short can, and you'll solve that problem. NOT the Tridon in a tall can, as it works just like the Buss, which is to say incorrectly! As far as I know, the short Tridon is the only 550 that works as it should.

I believe Auto Zone and Advance Auto Parts carry them.
Dan Masters

This thread was discussed between 17/03/2005 and 22/03/2005

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