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MG MGB Technical - Flasher Unit problems

I reshelled my 72 mgb roadster a few years ago. Fitted a new wiring harness and uprated alternator. It runs well with no problems other than having to replace the Lucas Flasher Unit. When I fit a new one the signals work fine for a short time then just stop. I've opened the flasher and can't see any signs of burning. Can anyone tell me if this is likely to be an overload or just poor quality flasher units ? None of the fuses, bulbs etc ever blow.
If it is an overload how do I check it out? Volts, Amps etc were never a strong point for me. Would fitting a modern Flsher Unit help me get around the problem if the Lucas Units are thought to be weak ?
Thanks for reading - any ideas most welcome.
Ian McCall

Ian - The very first thing to do is to start checking ground at all of the indicator lights, followed by the wires to the light sockets. All you need is for one light to loose its ground or the power to it and that side won't flash. Cheers - Dave
David DuBois

Ian

When you say they 'just stop', do they stop working - i.e. no light - or stop flashing...permanently on?
Dave O'Neill

Thanks for the replies.
I'll check the earths now - must admit I was so focused on the flasher unit I didn't think to look elsewhere.
Dave in Solihull - There's no light at all - both sides, no clicks or movement in the flasher unit. As soon as I put in a new flasher unit things work fine for a short period. The only other thing is that I didn't know where the flasher unit was mounted when I rebuilt the car and there are no other MGs around here to check. I simply attached it to the 1/2 inch painted brace that runs behind the dash with plastic cable ties. Presumably the flasher doesn't earth through its body ? Also, it now lies horizontally and I'm starting to wonder if it needs to be positioned at the correct angle much like the voltage stabiliser ....
Ian McCall

Ian, I have just replaced mine.

It slips horizontally into a small 'holder' screwed to the inside top of the bulkhead, above and to the right of the steering column, if it's there you will see it when looking up from underneath. It is obvious as its the same shape as the flasher unit which just slips snugly into it. I scraped off some paint to earth my unit anyway, but it still worked when just held in my hand, so I don't think it needs to be.

I don't think the actual mounting location makes a difference to function.

Even with that I had to clean all of my fuse box terminals and fuse ends to get a good flash rate. The earth route is the way to go.

Liam


Liam H

Thanks Liam - must have thrown the mount out with the old shell - I should leave building cars to those who know what they're doing !
Ian McCall

I don't know if it makes a difference for your problem but you mentioned you weren't exactly sure where it is supposed to attach so you put it behind the dash panel.
On my 78 B I have 2 flasher units. One of them is for the indicators and the other is for the hazard flasher.

I only worked out a while ago when my own indicators stopped flashing (they stayed on) but the hazards were flashed okay. I found there were 2 units that looked identical and through trial and error worked out which was which.
I swapped the wires between each unit and now I have indicators that flash and hazards that just stay on. I hope I don't need to use the hazards until I get around to buying a replacement.

Check back here tomorrow and I'll post a link to a photo showing the location in my own B roadster. I'm sure the 72 would have it in the same place, unless of course it only has one unit for both indicator and flasher then I don't know what I'm talking about. One of the experts around here can confirm/deny this.
D O'Brien

The flasher unit itself isn't earthed - at least not the original type - it doesn't even have to be in the clip. If both lights stay *on* but stop flashing that implies insufficient current through the circuit, and this can be from any of the myriad conenctors in the 12v feed to the flasher unit, or from there out to the lights, the light units themselves, 'tired' bulbs, and the earths at the light units (from the physical attachments to the wings in all but rubber bumper fronts). With marginal resistance problems fitting a new flasher can appear to 'cure' this but as they 'bed in' new ones will stop flashing as well. If a hazard flasher unit is inadvertantly used in an indicator position they should always flash, even with significant resistance or drop in battery voltage, as that is what they are designed to do. However an indicator flasher used in a hazard position is liable to burn out. An indicator flasher could also burn out if there is a partial short in the wiring out towards the lamp units, which would overload it.

But if the lights don't even come on when the indicator stalk is moved that is an open-circuit, pure and simple. As such these are much easier to diagnose. With the ignition on you should have 12v on both connections (green and light-green/brown) both sides of the indicator flasher unit. This should also be present at the indicator switch itself. When first operating the indicator switch the 12v should then be connected to either the green/red (LHS) or green/white (RHS) wires at the indicator switch to light the lamps. After a second or so the flasher unit starts ticking 'off-on-off-on' etc. FWIW a hazard flasher works the other way round i.e. when first turning them on nothing happens for a second or so then they start ticking 'on-off-on-off' and this is how they can be distinguished.

If you have no 12v on the green, or it goes off as soon as you move the indicator switch, the fault lies back towards the fusebox, which could be in a connector in the green circuit, which is maybe what you are disturbing when you change the flasher unit. If you have hazards the 12v feed runs through this in the 'off' position and the switch is a well known cause of in dicator problems. If there is no 12v on the light-green/brown, or if goes off as soon as you move the indicator switch, the problem does lie inside the flasher unit. It *is* possible to get more than one of these as bad if they are from the same batch. If you have 12v on the light-green/brown continuously after moving the indicator switch, and there are no lights or flashing, the problem most likely lies withing the switch itself if both sides are equally affected.

To check for a partial short (a full one would blow the 2nd fuse up in the fusebox) and if you have an ammeter that reads up to 10 amps put it in series with the green or light-green/brown conenction at the flasher unit. With the ignition on there should be no reading, if there is the partial short is on the light-green/brown. Then switch it first one way then the other, and when the lights are on you should see about 3.5 amps registered. If one side shows significantly more than the other the partial short is between the switch and the lamps on that side. If it is difficult to get a steady reading take the flasher unit out of circuit altogether and just connect the ammeter between the green and light-green/brown.
Paul Hunt 2

Paul and D O'Brien. Thank you both for your very helpful replies. I'll get the meter out and give it a try. The car doesn't have hazards so at least that's something I can exclude. Thanks again.
Ian McCall

Just had the meter working. I get 3.5 amps both sides. If I join the G and LGB wires all indicator lamps light up good and strong with no flickering even if I move any wires I can get my hands on. Presumably must be bad luck with the flasher units (?) even more so considering I have bought
some from Lucas UK direct and also a Halfords badged model.
Starting to think about electronic flasher with buzzer. Does anyone have any thoughts about which type to use and what the numbers mean on the base for the wiring ? Thanks again to everyone who responded and Paul in Solihull for the figures - they were spot on.
Ian McCall

Did you test for voltage on both terminals of the flasher unit with the ignition on, and with the indicator switch bothh off and to one side or the other?

I suppose you could try a modern 3-pin indicator flasher unit (the third wire gos to earth), but if it is something on the car that is destroying them that no-one here has thought of yet that one could likely go the same way. Terminals *might* be green to 49, light-green/brown to 49a, and earth to 31.

I wouldn't mind a look at one of your blown one's, Ian, if you wouldn't mind posting it to me. If so email me direct (link with this post) and I'll send you my postal address.
Paul Hunt 2

Hello Paul,
Couldn't test the voltage either side of the Flasher because I'd already taken the cap of it and tried to move the points about in the hope that they'd need either opening or closing. This didn't really help me much because I didn't know what a good unit looked like! On reflection the internals are probably too delicate for me to be fiddling with.
I took your advice and just put the meter between the G and LGB and that's where I recorded the 3.5 amps. As I'm typing this I'm starting to think that I should repeat the check with the engine running in case that changes anything with the uprated alternator fitted ?
Delighted to send you the unit, it will be in the post tomorrow. Might now look a bit damaged but you might be able to see something ?
Thanks again for all your ideas.
Ian McCall

Thanks Ian, I await the postman with mounting excitement :o). With the engine running you will probably see a slightly higher current as the voltage should be higher - 14.5v as opposed to 12.8v - but it should still be the same both sides.
Paul Hunt 2

Paul

Won't the current be lower with the higher voltage?
Dave O'Neill

No, I = V/R, i.e. increase the voltage for a given resistance and the current goes up. I = W/V doesn't give the correct current when the voltage is increased as W isn't fixed but depends on V and R, i.e. increase the voltage and the resultant wattage goes up. However the increase is not pro-rata as incandescent lamps have an element of positive temperature coefficient in them, i.e. the resistance goes up as the temperature increases, which tends to reduce the effect of the increased voltage.
Paul Hunt 2

OK, I'll get my coat!
Dave O'Neill

Ian,

I also have had these problems with the flasher on both MG B and tried the poor after maket quality, not worth buying, just lasting a few weeks on both cars
(may be the Far East manufacturer got a faulty sample from Moss and did a perfect copy?).


It took only a few minuters at a brakers yard to buy two flasher units from a Renault and have had no problems anymore.

As these usual flasher units have three pins, one remains unused on the MG B and anything is o.k.

I have removed the clamp holding the Lucas unit and fixed the Renault unit with the bolt holding the original clamp. Mounting it this way, the wires were long enough to be pluged to the 'new' flasher unit.

Ralph
Ralph

May I thank everyone who responded to my questions. I believe Paul Hunt has some observations about the flasher unit following his examination. Hopefully he'll update the site.
Ralph - I'm with you - its electronic from now on with a buzzer fitted in the system somewhere. I'll update the site if it performs well over a period of time.
Thanks again - bring on the spring and summer !
Ian McCall

I'm surprised Ralph says one of the 3 pins is unused on the MGB, if we are talking about more recent electronic flashers and not thermal types from the same era of Renaults as MGBs, although I though that other makes and model had changed to 3-pin electronic units *before* the demise of the MGB. If the 3rd pin *was* used on the Renault i.e. earthed, then I'd expect it to need earthing on the MGB as well.

As received Ian's failed flasher unit *was* lighting the lights but not flashing (therefore indicating flasher unit failure pure and simple), but by then it had been opened up and poked about inside by both of us so may have changed from the original symptoms. The normally closed contact did only need very slight pressure to open, normally it should be held closed under spring pressure from the bi-metal strip, only when this is heated enough to make it 'ping' should the contact open. The reason that it wasn't pinging open was probably because one half of the heating element had broken, although again this could have happened after being opened. If the problem were the contact failing to close properly that would cause no lighting and no flashing even if the heating element had been unbroken, it is wired in series with the contact (unlike a hazard flasher unit where it is wired across the open contact).

Whatever flasher unit you fit, you should always disconnect one of the main bulbs and check that it does give you a failure warning. With thermal types this is the remaining bulb glowing but not flashing, with modern electronic types the remaining bulb should flash at double-speed. Not all after-market units do this. Without that warning, if a rear indicator bulb fails expect to get rear-ended.
Paul Hunt 2

Paul,

I did not use an electonic flaser as an exchange, just a mecanical one to keep things simple. Most mecanical flaser units have three pins, it is hard to find a two pin version as used on the B.
A seperate ground was not necessary in my installation.

Electronic flashers are fine and I also have thougt about fitting one but did not like the additional wiring.

Ralph
Ralph

OK, 2-pin thermal flashers are available from probably any motor factor here in the UK. Whether they are any good or not is another question.
Paul Hunt 2

I bought a flasher unit from the US for my US-spec Honda 750/4, as it is fitted with higher wattage bulbs than the UK model.

I suppose I could have changed the bulbs, but I like to be seen!

Once I had sorted the under-charging problem - 11.5 volts - the indicators were flashing too fast with the existing flasher.

The new flasher unit turned out to be a 'universal' 2-pin, which doesn't give any indication that a bulb has blown!

Back to the drawing board.
Dave O'Neill

What amazes me is that with the increasing numbers of cars being produced with LED flasher 'bulbs' a 3rd type of flasher unit has had to be produced as the standard electronic flashes too fast with these bulbs. From what I have been able to determine so far the ones fitted to Land Rover and BMW (at least) also do not have any failure warning mode! It's true that LED 'bulbs' *should* be more reliable than incandescent, and there is also an array of them meaning even if one fails there should be many others still working, but it doesn't take into account failure of a connection between the flasher unit and the light unit. I was cussing a Range Rover just the other day for not bothering to indicate a right turn and stopping without warning, only to see as he turned that his wing repeater was flashing at the correct rate.
Paul Hunt 2

One last question or two please ...... I fitted a Lucas electronic flasher. It has three terminals 49 being the live feed, 49A being the outlet to the indicator switch and 31 the earth. Works fine (so far).
I also bought a Durite 12v buzzer. ( Probably don't need it as the flasher has a loud enough click and, just for your info, the fitting also responds to bulb problems by flashing quicker - so well pleased ).
Problem to be answered re the buzzer - I thought I would simply fit it in series between terminal 49A and the wire to the indicator switch but it doesn't work that way which surprises me because I thought it would pick up the electrical pulses going through this wire.
It does buzz in accordance with the flasher when I put it between terminals 49 and 49A. This may or may not be the correct way to fit it, does anyone know please ? and, just for a novice like me, can anyone explain what exactly is happening inside one of these flasher units for it to work in this manner ?
Ian McCall

Further to my last - just found that the buzzer works with its positive terminal wired to terminal 49A on the flasher ( wire to switch and lights ) with the negative terminal on the buzzer wired to earth. Doesn't seem to be any loss of light at the indicators so hope that's it.
Just pleased that MGBs don't, as a rule, have a ECU for me to get my hands on!
Those experienced auto sparkies out there must be wondering how long it will be before my car goes up in a puff of smoke.
Ian McCall

Not in series as most buzzers will present too much series resistance for the bulbs to light or the flasher to operate at the correct rate (Ohm's Law again). On a 2-pin flasher the buzzer will work when connected *across* the terminals, to avoid having to locate an earth, in which case it sounds in anti-phase to the lights flashing. However it only sounds when the switch is operated and the lights off, so you don't have to put up without going all the time when the switch is off :o)

On a 3-pin flasher it should work connected the same way i.e. between 49 and 49a, i.e. in anti-phase, but should buzz in phase with the lights if connected between 49a and ground.

However with either type if the buzzer is of a low resistance connecting it in this latter mode will make the flashers flash faster (ditto on the 2-pin), whereas connected between 49 and 49a it shouldn't make any difference to flashing speed.
Paul Hunt 2

Superb! thanks again Paul.
Ian McCall

This thread was discussed between 30/12/2006 and 12/01/2007

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