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MG MGB Technical - Relays & flashers

Hi All

I am building my own electrical panel for my 74.5 MGB and I am wondering if there is a difference between the hazard flasher and turn signal flasher units. They are both after market and look the same. Is the flashing rate different or something like that?

I am using micro relays on my new panel which have 3 3/16 terminals and 2 ¼” terminals. The flasher (I have) terminal configuration does not work and I have not been able to locate flashers for a micro relay. Does anyone know where I can locate flasher units for micro relays?
Looking at the relay, if I could solder on a piece of metal and make the terminal wider and then cut it down to fit into the micro relay socket, does anyone see any problem with this. I would have to do an extra one at the same time to have as a spare.

Comments appreciated.



Bruce - The hazard flashers are what used to be called 'heavy duty' flashers and are meant to handle the current through multiple light bulbs without the flashing rate changing as it does when trying to handle more than two bulbs with the standard turn signal flasher. Cheers - Dave
David DuBois

Indicator flashers are designed with the contacts normally closed so the bulbs light as soon as you start it up and current flows. In the mechanical ones a heating element in series with the bulbs gets hot from the current and this causes the contacts to open. Current stops flowing, the heater cools and the contacts close again. That's why when an indicator bulb blows the flashing gets slower or the remaining bulb always stays on. There isn't enough current to make the contacts open fast enough or at all. That's by design so you can tell a bulb has gone.

The hazard flasher is different since it has to power all the indicators (or less if some are broken in an accident) but at the same rate regardless. These have the heater across the contacts which are normally open. When you start a hazard flasher the bulbs are off.

The heater takes a very small current, not enough to make the indicator bulbs glow but enough to heat it up. When it's heated enough the contacts close and all the indicator bulbs light. Since the heater is across the contacts which are now closed it is shorted out and no current flows though it. The heater cools and eventually the contacts open again, the bulbs go off and the heater start heating again.

It's quite good fun taking one of those little cans apart to see all this!
Simon Jansen

Presumably the flasher is on the secondary circuit of the relay?

Those described above (bi-metallic strip type) can be replaced by more modern electonic ones that do away with the vagaries of heated metal! Tho these do need an earth contact.
Michael Beswick

"Presumably the flasher is on the secondary circuit of the relay?"


Unfortunately hazard flashers and so-called heavy-duty flashers will flash with anything from 1 to 4 bulbs (as any one or more may be disabled after an impact) so some people chose to fit them to the indicators when the get problems with the original flasher units not flashing due to bad connections, to which they are very sensitive, and is a deliberate safety feature to detect a blown bulb. Modern electronic 3-wire flashers are much more immune to these bad connections, and indicate a blown bulb by flashing at double-speed. Unfortunately some people (none here I hope) are too stupid to recognise that. Hazard or heavy duty flashers should not be used as indicator flashers as they won't respond to bulb failure either. As already said the two are easy to identify in use as indicator flashers come on as soon as you operate the switch, then after a short pause start flashing off-on-off-on. Hazards operate the other way i.e. there is a short pause with no lights when the switch is first operated, then they start flashing on-off-on-off.

Are you trying to find a flasher unit that plugs into a 'micro relay' (whatever that is) socket? As far as I know they plug into standard Bosch relay sockets with four or five identical spade sockets. Are also taking about soldering strips of metal onto the spades of a standard flasher unit, then bend and cut them to plug into a small socket? Bad idea I suggest, as the flasher unit is likely to flap about under its own weight and snap these or just pull them out. If you must use a small socket instead of the correct one, then mount the flasher unit securley somewhere and use a mini wire harness to connect it.
P Hunt

Thanks for all the info.

The micro relays are 5/8 x 7/8 x 1" ( comparison to the mini relay which is 1x1x1.

Yes I am trying to find a relay which would fit into the micro relay socket to save space on the new electrical panel. But I have been unable to locate any.

I think the best thing to do is to go with the Bosch relay socket. It will take up a bit more room but allow me to be able to buy a replacement flasher from almost any auto parts store. Which in itself is a good enough reason to try not to fabricate my own pin configuration on a standard flasher unit.

The standard SPDT Mini relay is
Power = 30
To Switched Unit = 87
From Switch = 86
Ground = 85

The Micro relays have a different numberings system. 1-5
Am I right with my interpretation?
Power = 3
To Switched Unit = 4
From Switch = 1
Ground = 2


Paul I couldn't quite see why a relay was necessary. But I think this is a custom rewire. I guess you could trigger the primary circuit with the indicator switch and fit the flasher unit in the secondary circuit. I don't think the current drawn by the primary circuit would be sufficient to "work" a bi-metalic strip type unit?? Or have I got it wrong? Yes regular Bosch sockets would seem the easiest-its what I fitted-but not for indicators! M
Michael Beswick


"Yes I am trying to find a relay" should read
Yes I am trying to find a flasher

Definitely a custom re-wire. Several years ago I ran all the wiring to behind the glove box. This year I am cleaning up the maze of wires.

The panel will use 12 relays.
Hi Beam
Low Beam
Turn Signal flasher
all feed from the alternator

Drivers Door (shaved door, trunk handles)
Pass Door
Cooling Fan
fuel pump
Emergency Flasher
all feed from the starter

In addition the panel will have 12 fuses. 4 as per stock MGB wiring configuration. 4 for the headlights, 1 for the door solenoids and 3 spares.
All wiring entering and exiting the panel to be wired to multi pin connectors so the panel could be removed fairly quickly

What did you use for the signal flashers Michael



Bruce I have the original arrangement for the turn flashers. It works adequately though I noticed last year that the rate differs slightly from side to side If it gets worse I shall fit an elecronic flasher ( and check the earths!)

I have relays for hazard, air horns, electric fan, dip beam main beam. The relay primary circuit is fed by the car's switch (and hence the original fuses), and the power to the secondary circuit is taken from a +'ve stud I have fitted in the engine bay (connected to the starter main feed)via modern blade fuses. Vehicle Wiring here in the UK supply fuse/relay sockets that clip together that I attached to a panel mounted beside the original fuses.

The car is a 69 roadster (supercharged) so some items (rear fog, air horn, electric fan, hazard) were not standard, so I added "new" wire and switches. The headlight relays were used because I didn't like 10+amps through old wires ( I replaced the switch)and a steel dash!

Michael Beswick

Still confused as to the need, Michael. Full bulb current is passed through the flasher unit, either original 2-wire or modern 3-wire. That heats up the bi-metal strip enough to start the former flashing, or generates the approriate voltages and currents internally for the latter.

Looks like the voltage to operate the relay is applied to 1 and 2 (85 and 86 in Bosch parlance). Looks like it can be in either direction as the winding is resistor quenched and not diode quenched. Some Bosch relays are diode quenched, connect those the wrong way round and at best they don't work, at worst you blow the supply fuse and the internal diode. I can't see the terminal configuration well enough to see how the other connections are arranged. It depends on what you want the relay to do as to what goes where. For a simple 'normally open' operation to say light a bulb when the relay is energised again the two wires can be connected either way round, but the convention would be for the 12v supply to be connected to the common terminal (30 in Bosch parlance), the load connected to the normally open contact (87) and nothing to the normally closed (87a).
P Hunt

I just rewired my whole car with an Advance Autowire kit:

It would do what you want and has a nice relay/fuse panel. Despite being relayed the wires are all MUCH thicker/better quality than a standard loom. This can make it harder as you end up with a massive bundle in the engine bay and a lot of wire under the dash.

You also need to cut route, cut and terminate all the wire ends which takes a long time. Took me several weeks working on and off since I carefully ran, spiral wrapped then crimped, soldered and double heat shrunk each termination.

I did the final wiring of the alarm and door actuators yesterday. Everything worked first time except the ignition bulb and that turned out to be a blown bulb! I used LEDs for the other indicators but the ignition light is bi-directional and I believe also needs to pass some current to the alternator windings to kick it off so I used an LED bezel, cut the legs off the led and slide a normal bulb behind it. That shines through the LED in front easily.

The Advance kit provides extra wiring for the hazard lights using a normal double pole switch, fuel pump cut off, ignition cut off and a cooling fan override. I made an extra switch panel that lives under the fuel gauge for these switches.

In the engine bay I made an aluminium cover for the power block to cover all the relays but leave the fuses accessible. If I can find something suitable I will put a clear cover over these. They also provide you with those LED fuses where the thing glows if it burns out.

One small trick I found though was regarding the large battery isolation switch I fitted. That's on the heel board behind the drivers seat. I set mine up to isolate the negative lead. I think at the time my logic was then with that off if I stupidly tried to undo the positive battery lead first and slipped and shorted to the body it wouldn't actually short out.

However when I fitted the alarm I wanted it to work with the isolator switch turned off. That way when I park the car I can remove the battery switch (kind of the ultimate immobiliser) but still enable the alarm and door actuators (I shaved the external locks). If I ran the negative of the alarm to the negative on the battery directly to get around the switch everything would work except for the flashing indicators! Those are earthed through the body.

The solution was to swap the battery leads and isolation switch around so it switches the positive battery lead. Then you run a separate wire to the alarm positive. That way everything still works correctly.

The alarm and door electronics are all up on the passenger side above the foot well. I am going to make a trim panel to bolt up there and hide all that and let me tuck the wires neatly out of the way.


Simon Jansen

This is what the dash and extra switch panel to the right look like. The alarm wiring is all dangling and just needs to be tucked up in the passenger footwell.

You can also see the loop of wire going to the right hand door. I found the hinges actually have cut outs in them so you can run a wire through the hinge and loop it down the A pillar in a long loop that comes back up to the grommeted hole in the front edge of the door. That should minimise the flexing of the wires.

The wire duct taped to the transmission tunnel is the alarm positive feed direct to the battery. Also the little box in front of the gear lever is a microwave sensor for the alarm. I think it should actually work through the carpet. It won't work through metal though.

I also need to make a custom steering wheel cowling.

Today though, carpet!

Simon Jansen

This thread was discussed between 17/01/2010 and 18/01/2010

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