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MG MGB Technical - Relay wiring

I am putting two relays for headlamps and spots so that when the main beam comes on so do the spots my query is should the control wire from the switch go in series or parallel to the two relays ?( the new brown power wire will go in parallel to both relays as the heads are halogen and the spots tungsten so a fair bit of power)
ap lestocq

You would put 2 x 12v relays with coils in parallel to the main beam pole on the dip switch. Since the relays will prolly be close to one another and near the front of the car you would re-use the wire to the main beam of the headlights and then common up the relay coils locally. On my car from memory the headlamp wiring goes to the middle of the slam panel and splits there. If you put relays in this position some rain/splash proofing would be good ides. Paul H has a similar circuit on his site and has put some thought into not losing all forward vison through the loss of one fuse. You could fuse both main beams and spots together and the dips sepratley and achiev this.Clarifying, you would always have the main beam or spots or one dip, so 4 fuses.
ps before someone else points it out yes you could put 2 6v relays in series, but that would be silly.
Stan Best

Hi,

As Stan suggested, a good place for the relays is under the slam panel, where all the headlight wires come together. This will require only one extra wire, from the battery to the relays, to supply power for the lights. As you will hace the high beam and spots on together I would strongly recommend a separate relay for each set, since you could be looking at around 400 watts when on, which is around 34 amps. Standard relays are rated at 30 amps.
A hint here, I've found that automotive type relays are available from the local electronics store, at about half the price at the auto shops, albeit loose, rather that in a bubble pack, exactly the same otherwise. Apart from fusing each light I would also fuse the main power wire, up near where it connects to battery, so that if the wire is damaged somehow the car's electrics are protected.
Two 6V relays, in series, would work BUT if one goes open circuit then the other would not work either. Anyway, I don't think auto style relays come in 6V versions.

I have attached a circuit diagram, of my car, to assist you. NOTE, I have used red wire as the power feed to the relays, and you would need 2 relays where the high beam one is.

Herb


Herb Adler

I put relays in as Herb describes and they work well. The only point with which I differ with him is that I ran two power feeds each with its own fuse, so that both the main beams and dipped beams will not both be lost if the fuse blows. With two fuses, if one blows you still have the other sets of filaments....
Rob Edwards

If you are going for 3 relays to keep the loading down with the spots you could go up to 4 relays and 2 feeds when you could fuse n/s and o/s seperatley so loss off one of the big fuses would still leave you main and dip on one side, luxury. The problem would be that you would finsh up with a lot of wires.
Stan Best

Thank you all for your helpful replies I now am starting the job -wire stripper soldering iron and wiring diagram and notes at the ready $ relay box with cover and correct coloured wires for once and weather proofing where required Fuses every where !!
ap lestocq

I don't like the slam panel as it is wet, I've had relays there on other cars have spades rot off, I mounted mine behind the radiator mounting panel.

Wiring both relays in parallel means the spots would always be on with the main beams of course, personally I would control them separately even though most of the time you probably would have them on together. I have my front and rear fog relays powered from the parking light contact of the main switch.

4 relays Stan? Not really required if you use 2 relays and four fuses i.e. one per lamp per side for the mains and spots, two more relays and fuses if you do the dips as well. Wiring is pretty simple for the main and dip, make up a harness that brings the blue/white and blue/red switch wires from the bullet connectors by the right-hand headlight to the relays, two from the relays to the fuses, and four from the fuses back to the connectors by the right-hand headlight for the wires that go to the lights.
PaulH Solihull

Yes Paul,the circuit I sketched out had the dips swithched via relays as well. Since you will be using dip driving towards an oncoming pair of headlights atmy age I need all the help I can get. It used one relay per side for spot/main beam in parallel. Splitting them out gets you up to 6 relays! You could then fit a 3 way switch slave/off/on for spots!
Stan Best

Still don't see why you are using one relay per side, as opposed to one fuse per side. OK, if a relay fails, you would lose a beam, but Bosch for example relays are pretty reliable, more reliable than the original switches I'd say.
PaulH Solihull

Hi Paul, first of all good advice about relay corrosion, I put this pic on the Fiat forum as this relay box in front of the battery is a frequent source of problems and realising its even there is a good first step. I have had most relays out of here over the years to clean the relay and socket contacts so some petroleum jelly from day one would be good idea on the MGB.
I havent put relays on my car yet but I have thought about it, modern cars have a few big fuses close the the battery before very much happens, I guess this is insurance against the sort of really solid short that can start a fire. Not very logical to worry about this on the B which simply relies on loom integrity elsewhere. But, if you did this it would be nice to have one, 100A or so, this then becomes a possible single fail mechanism for all lights so you finish up with one for each side. Then its 4 at about 35A for each filament(from memory, the sketch I did has got covered)and then to keep the sides seperate you get 4 relays. Long time since I looked at this but I never came up with a balance of conplexity/benefit/failure survival that I wanted to go with.


Stan Best

Hi Paul, first of all good advice about relay corrosion, I put this pic on the Fiat forum as this relay box in front of the battery is a frequent source of problems and realising its even there is a good first step. I have had most relays out of here over the years to clean the realy and socket contacts so some petroleum jelly from day one would be good idea on the MGB.
I havent put relays on my car yet but I have thought about it, modern cars have a few big fuses close the the battery before very much happens, I guess this is insurance against the sort of really solid short that can start a fire. Not very logical to worry about this on the B which simply relies on loom integrity elsewhere. But, if you did this it would be nice to have one, 100A or so, this then becomes a possible single fail mechanism for all lights so you finish up with one for each side. Then its 4 at about 35A for each filament(from memory, the sketch I did has got covered)and then to keep the sides seperate you get 4 relays. Long time since I looked at this but I never came up with a balance of conplexity/benefit/failure survival that I wanted to go with.


Stan Best

35A per filament? That's 35 x 12 = 420W. In the US, headlights are typically ca. 45/65W (dipped/main beam)....
Rob Edwards

Hi Rob,

My halogens are rated at 100W hi beam, X2 is 200, plus 2X spots, possibly at 100W ea equals 400W, as originally said.

Herb
Herb Adler

So you'd need about 35A steady-state capacity (not sure about inrush current) /in total/. Stan mentioned 35A /per filament/....

Cheers!
Rob Edwards

Modern cars do indeed have fusible links close to the battery for everything except the starter motor. However that feeds *all* lights (plus a load of other stuff) and is to protect the car if a main cable to the relays shorts, which is really only likely in a major impact with crumpling of the body. There is a single relay feeding both sides, then one fuse per side per filament to avoid being plunged into darkness in the event of thinner wiring out to the corners of the car being damaged in a lesser impact. There may well be cars with one relay plus one fuse per side, but it isn't universal.
PaulH Solihull

Thinking about this again, and using a spare envelope back, you could use 2 double pole relays one for dip one for main, and 4 fuses of 35 A or 20 if thats your choice and get pretty good inpendance n/s to o/s and dip to main . Thia would skip the FO fuse upstream There is still a case for putting this in the brown wire to protect everything , 100A should do it.I may do this to my car as a good compromise.
Stan Best

This thread was discussed between 15/02/2011 and 18/02/2011

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