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MG MGB Technical - Trailer indicator lights

I have a 1973 MGB GT, onto which I have recently fitted a trailer hitch. I also installed a new electrics socket and all lights work. So far, so good. However, the problem I have is that the trailer indicators are very dim and all indicators on the car flash very quickly. I realize this is probably down to the increased resistance in the circuit etc, which is upsetting the indicator unit, but would appreciate some advice on a simple and (hopefully) foolproof method of overcoming this. I could (I suppose) fit a switch to transfer the juice from the MGB’s rear indicators to the trailer when in use (assuming that's not illegal!), but a possible downside could be me remembering to switch it back again. Also, what about fitting a heavy duty flasher unit and if so, which one would you recommend. Finally, I had thought about replacing the trailer indicator bulbs with LEDs. Any ideas would be most appreciated.

Many thanks

Paul
PJ Eades

Paul

Have you changed the flasher unit? If not you need to change that first. The original flasher unit (2 pin) works on a simple bi-metallic strip and the current flowing through it to operate. Get a 3 pin electronic unit such as item No 270779941898 from ebay, lots of others available. You'll need an earth wire for the flasher unit and I'd suggest you get one that has its own mounting bracket, this'll save you making one.

If you decide to go down this route let me know and I'll find a wiring diagram that shows the pin connection. It's really simple you use the 2 existing wires plus the earth wire.

As an added benefit your flashers will be much more regular.

Bob
R.A Davis

What you need is something like this...

http://www.just-tow.co.uk/audible-towbar-buzzer-relay.html

It connects into the rear indicators and allows the trailer indicators to flash without affecting the indicators on the car. It also buzzes to let you know that the trailer indicators are working properly, which is a legal requirement.
Dave O'Neill2

Dave

If this unit is anything like the ones I've fitted in the past all it does is sound a buzzer if the trailer lights are working. Although it says relay in the description none that I've fitted have relays in them and rely on the flasher unit being up to the job, I'm willing to be corrected if this unit actually has a relay. I come back to my point that the original 2 pin flasher unit leaves a lot to be desired, they work well if the if the system is charging but will almost grind to a halt while sitting at a junction or traffic lights.

The original flasher units are designed to operate with a load of approximately 47 watts ie 2 x 21 watt indicators and dash repeater 1 x 5 watts (or is it 2.5 watts). If you either add or remove something the flash rate will change. If one flasher bulb fails the other working one will just stay on or flash very slowly. The electronic unit doesn't suffer this problem, although you could argue it doesn't give you any indication a bulb has failed. I still prefer the nice steady flash rate while sitting at junctions and bear in mind that the law requires the flashers to operate between 60 and 120 flashes per minute ie 1 to 2 flashes per second.

Bob
R.A Davis

"The electronic unit doesn't suffer this problem, although you could argue it doesn't give you any indication a bulb has failed."

The standard electronic unit i.e. no towing facilities *does* have bulb-failure warning - at least the one for incandescent lamps does - it flashes at double(ish)-speed, but whether the driver notices or realises what it means is debatable. I suspect the flashers used on cars with LED indicators do *not* always have this feature, having nearly rear-ended an Range Rover that turned with no indicator at the rear, but I could see the side flasher was going.

If your standard 2-pin flasher is stopping at idle, then there is something wrong i.e. too much resistance/not enough current in the circuit. They *should* really flash with the ignition on and the engine stopped. This is not a legal requirement, but flashing at idle is. Remember changing to a 3-pin flasher may well keep the flash rate up, but the problem causing the slow flashing with the 2-pin is still there, and causing the lamps to be dimmer than they would otherwise be.

The term 'relay' is often used when there is strictly speaking no relay, many refer to a 'flasher relay' when it is't not, it is a flasher unit. Whatever technology is used, the trailer flashers must be supplied from alternative source, with a switch operating in time with the vehicle flashers, as otherwise the 50% increase in current (not resistance which has been lowered) will cause the standard 2-pin flasher to flash very quickly, and probably burn it out in short-order. If the trailer flashers are dim then there is some other problem that *is* causing increased resistance, or the are the wrong (too low) wattage. Maybe just as well as otherwise the flasher unit would probably have burnt out instantly.

LED lamps on the trailer *could* simply be tapped onto the indicator wires in the boot as they take a very low current and will not upset the flasher unit. However like the Range Rover indicators above, you will have no indication that they are working, and I doubt the 'audible buzzer relay' above would work with them. Given the description I also doubt it is anything to do with actually powering the trailer flasher, but is simply something an add-on to give an audible warning.
PaulH Solihull

What is the lamp configuration on the trailer(per side)? Over here they commonly have only tail and a common bright for stop/turn (2 filaments), which does not work with cars that have tail/stop/turn (3 filaments)lamps. There are converters to deal with this, but I much prefer having 3 or more filaments on the trailer.

How many terminals has the plug/socket?

Is there a dedicated earth lead in the socket? Common that there is not, relying on the hitch, and even when there is, I fit a separate earth from trailer to tow vehicle - fixes all sorts of problems.

You need a heavy duty flasher for more than 2 indicator filaments.

FRM
FR Millmore

Towing 'modules' used to require a separate 12V feed, although I'm not sure if that was to power the indicators or the warning light on the dash, which used to be what was used before the more recent buzzing variety.

Having said that, I don't think the current versions have the separate supply, so may well just add the extra load to the indicator circuit.

It may be better to fit a 7-way bypass 'relay' as recommended for modern cars with CANBUS wiring.

http://www.ebay.co.uk/itm/7-WAY-SMART-BYPASS-RELAY-TOWBAR-TOWING-ELECTRICS-MULTIPLEX-RELAY-/110850239922

The last two towbars that I have fitted - to modern cars - I used vehicle specific wiring harnesses, which cost more than the towbars, but don't void the manufacturers warranty.
Dave O'Neill2

FRM

Trailer electrics in the UK have 3 filaments per side - stop, tail and turn.
They generally use a 7-way plug and socket with a separate ground wire.
Dave O'Neill2

Dave-
OK, We have 4, 5, 6, or 7 terminal plugs, and different configurations of each, plus whatever the idiot doing the wiring dreamed up. Trailers and many especially older vehicles commonly have only the two filaments/side.

His problem of the indicators working but dim tells me that either something is in parallel, or a bad earth(s), or bad power feeds from the car. This last could well be the usual N/W/G and fusebox/bullets problems.

FRM
FR Millmore

Paul

The electronic flasher units I use are rated such that they supply a varying number of bulbs, the unit supplied on ebay I provided an item number for (270779941898) is capable of supply lamps up to a total of 98watts, a maximum of 4 x 21watt bulbs and 2 x 5watt repeaters, more than adequate for Paul's needs without requiring any additional switches or relays. The flash rate of these is unaffected by the number of lamps and is controlled at a legal rate by the electronics.

When I referred to the unit probably not containing a relay I was referring to the audible warning device as suggested by Dave. Most electronic flasher units do contain relays, although completely solid state units are available. This is however irrelevant as I was trying to point out that unit suggested by Dave if used in conjunction with a 2 pin flasher won't help.

When I've replaced 2 pin units because of slow flash rate at idle in the past the problem has always been the flasher unit.

In my opinion if your still using one of the old 2 pin units and it's working properly you're lucky but the electronic units are much better. They were ok when these cars were still in production but it's like using radial tyres and brake pads with asbestos in, you could still use them but why would you want to.

You're correct the electronic unit does flash faster if a bulb fails, I tried it this evening, the reason I didn't know this is I've never had a bulb fail with one of these units.

Bob
R.A Davis

I did of course mean cross ply tyres not radials

Bob
R.A Davis

Bob - I have checked out the unit you refer too and it appears to be a three pin unit. I assume one pin is the earth. Can you advise what the wiring configeration should be please to sync up with the existing 2 pin MGB wiring?
Thanks
Paul
PJ Eades

Paul

The 2 existing wires are green which is the positive supply and goes to pin 49 and green/brown which goes to the indicator switch and connects to pin 49a. The only additional wire required is an earth that connects to pin 31. The correct colour for the earth wire should be black if you wish to be pedantic. You can connect the earth wire to any convenient point, I use the same screw used to mount the flasher unit.

If you still have issues after this check it works without a trailer first and if you have problems once the trailer is fitted check the trailer wiring.

It's a legal requirement to have something to indicate, the trailer indicators are working, I'd suggest the audible warning device or similar as suggested by Dave. I like the comforting sound of the buzzer as it indicates the trailer is still attached. A friend of mine had a trailer uncouple and it was several miles before he realized and I've had one come loose fortunately at low speed.

Bob


R.A Davis

Many thanks Bob and thanks also to all of you who contributed to solving my little problem!
Paul
PJ Eades

"The flash rate of these is unaffected by the number of lamps and is controlled at a legal rate by the electronics."

So no lamp failure warning then.
PaulH Solihull

Messy here.
"Three pin" flashers used to be those that had a third terminal to operate a single warning lamp on the dash; these have bat, load, and ind terminals, no earth.
If some new ones have three terminals but require an earth, then you can get scrambled.

FRM
FR Millmore

<<"Three pin" flashers used to be those that had a third terminal to operate a single warning lamp on the dash>>

Or even two separate warning lights on the dash via separate contacts in the indicator switch.
Dave O'Neill2

>>Or even two separate warning lights on the dash via separate contacts in the indicator switch.<<

Never saw one like that, or a switch like that. Two warning lamps are normally just tapped off the main indicator circuits.

FRM
FR Millmore

The Mk1 B has a LGP wire from the third pin on the flasher unit which then splits to both warning lights. There is a LGY wire from one and a LGU from the other which go via a separate pair of switch contacts in the indicator switch to ground.
Dave O'Neill2

Here's a pic of the switch

Dave O'Neill2

Eight you are.
Didn't remember that one - either it is so good I never had a problem with one, or I forgot it because it is such a silly way to wire it!

FRM
FR Millmore

hers have said, yr going to have to change the flasher unit, or units, if the car has hazard warning.


Get a dedicated towing flaser unit with 4 pins and wire as follows


49 live supply green

49a to switch light/grren/brown usually


31 earth new wire


C or C1 to new trailer warning lamp or a buzzer.


Easiest way I've found,


SR Smith 1

This thread was discussed between 26/03/2012 and 01/04/2012

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