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MG MGB Technical - Headlight relays

The wife screwed up yesterday and turned me loose in a junk yard. I was looking for stuff for my 52 Pontiac restoration, but came upon a Cadillac with a whole bunch of 20-30 amp relays. So for five bucks what's a fella to do? Anyway I want to relay the headlights on my 73B. The way I read the wiring diagram I need two realys, One for the low beams and one for the dip beams. I'll tap into the blue/red for the coil on the low beams and the blue/ white for the dip beams. Question is: where is the best place to get the hot (brown) wire? The top side of the fuse block with the rest of the browns? I'll put an inline fuse on it regardless of where it comes from (25 amp?). I also assume that we leave the parking lamps as is. Anyone have a wiring diagram that's done it before?

As usual, Thanks in advance,
Mark

By the way- it's a super nice day today, will be driving the car all afternoon, not working on it!
Mark Thomas

Did a quick search for 'Headlight Relays' and came up with several sites, the first shows a wiring schematic for installing the relays http://www.rowand.net/Shop/images/HeadlightRelayWiringDiagram.jpg
and the second includes instructions on how to install them http://www.rowand.net/Shop/Tech/WiringHeadlightRelays.htm
Should cover what you need to know

Bruce
Bruce

...and:
http://www.mgbexperience.com/electrical/relays.html

Also:
http://www.mgb-stuff.org.uk/
Look under SpannersElectricsHeated Rear Window, and ...Horns

~Jerry
Jerry Causey

Excellent information guys. Will do the project this week. Thanks much.
Mark
Mark Thomas

I'd wire them almost exactly as you suggest, however putting a single fuse in the brown would mean loss of all headlights - unless you have the presence of mind when suddenly plunged into darkness to pull on the headlamp flasher and it wasn't the main beams that had failed. I put the relay just ahead of the main fusebox to keep the unfused brown as short as possible, then one fuse per filament (four) between the relay contacts and the lamps just behind the radiator panel. To avoid cutting into what was a brand-new loom I made up a sub-harness that brought the blue/white and blue/red supply wires back to the relays from the connectors by the right-hand headlight, two new wires from the relays to the fuses, and four new wires from the fuses back to the connectors by the headlights. If you were really picky you could but a standard 17/35 in the feed to the relays and use half the rating in each filament fuse, hoping that in the event of a short the smaller fuse would blow before the main fuse. Or a PTC thermistor as the main 'fuse'.
Paul Hunt

Paul, I like the idea of a new harness. Would keep things a little prettier and indeed will keep me from totally destroying the main new harness. Couple of questions though. Did you upsize any of the wiring going to the headlamps? Does it need to be? Also do you remember the fuse sizes for each filament? As you stated, leaving the fuse out of the brown wire is a good idea to avoid failure of the entire system, so the relays will be located directly beside the fuse panel to keep it short.

What would we do without you Paul?
Mark
Mark Thomas

Just some more relay sites:

http://lighting.mbz.org/tech/how_to/relays/

http://www.autooptiks.com/relay.html

http://www.mgb.bc.ca/electrical
Bob Muenchausen

If it ain't broke, why fix it? Relays add to the complexity of the system and all mechanics know that the more parts you have, the greater the chance for failure. The increase in performance is not that great for the work involved. If you do go through with it, make sure that you draw up good legible wiring diagrams of your modifications and put a copy in the owner's manual and one into your shop manual. You'd be surprised how easy it is to forget how you wired in some modification after a few years, and pity the next owner ( I know--you never intend to sell it, but things happen) who has to puzzle it all out. You should always document all deviations from stock.
R. L Carleen

I used standard-size wire as it is only a couple of feet in each case and, I think, about 15 amp blade-type fuses for each filament. Measuring the volt-drop in some sample lengths of wire - ignore the connectors, of course, should tell you how much benefit a thicker gauge would be.

It might not be broke but relays - certainly with uprated headlamps which was why I was asked to do it - make a huge difference to brightness and how quickly they reach it.
Paul Hunt

Actually, R.L., it is broke. The entire Lucas wiring system is broke to some extent (hence all the jokes about Lucas). There is another post on the BBS this week about dimming lamps, and I would suspect a faulty switch before a bum alternator. I also added that my problem involves a hot switch (opened it up and found burned contacts).
Adding relays to any old car is normally good practice, as switch technology was not reliable enough for the current they were required to pass. Fusing was also almost non existent. Ask any old timer about the 30's, 40's and 50's cars and he'll probably have a story or two of a fire under the dash at sometime (remember too that most were 6 volt = high current).
Relays have proven time and time again as a welcome addition to the MG headlamp system. Maybe that's why there is so much documentation on the subject. I am also for retrofitting halogen lamps to the car, as we all know how hard the little LBC's are to see anyway. There is no way the existing headlamp switch could survive the increased current load of the halogens. If these relays are installed according to Paul Hunt's suggestions, they will actually enhance the reliability of the entire system, as it is more unlikely to burn up the entire circuit at once.
I do agree however, that it wouldn't hurt to add a sheet to the manual to reflect the changes made to the harness. I printed one right off this thread that is true to life (and in color).
I'm not much on making modifications to my LBC, but I believe this one is well justified.

Mark
Mark Thomas

It's totally, totally worth it. My last MG (before I started tossing care to the wind and fiddling around with V8s and such) was bone stock but for the Carello E-code H4 headlamps that had been installed. They only glowed a dimmish yellow with the stock wiring. A friend of mine noted that they were very good headlights and urged me to try putting in fat wiring and relays. I could not BELIEVE the difference -- it was like, suddenly there was ten times more light. On my current MG I run the same setup but with the Cibie E-Code lights and they are amazingly bright even with the stock bulbs ... they throw a beam further down the road than my wife's modern BMW. If you set it up correctly, it's entirely possible to do the rewiring in such a fashion that it's reversible ... but there's no way you'll want to un-reverse it. The Lucar connectors are perfectly OK for the circuits that you'll use to turn on the relays. (Though you want fat wires to carry the new current to the headlights themselves, to avoid redrilling the headlamp bowls you may just want to use a few inches of the wiring that's there already -- the strain relief that the factory made in that key location is probably better than you can make up.)
Ted

I ran into a couple of fellows with V-8 MGB's at the GOF in Palm Coast a few weeks ago. On of them had installed a Painless Wiring harness in his car and he told me that all of his lights were now extremely bright. The other gent agreed and added that the difference when following him at night was dramatic and his car was much more visible than any MG he'd ever seen. It sure was a clean looking setup. I know it's not original, but it's reversible and I like it.
David

From the "Relay King" ;-)

The use of relays has its advantages for all the reasons already mentioned.

As for the poor next owner, well, he will only be as bad off as the efforts of the current owner to properly install and wire these relays. If the installation is handled with an eye on removal being a possibility, it is not hard at all to choose means and methods which allow an easy recovery and avoidance of the butchery of many people's hamfisted and unthinking attempts.

When I installed these relays in my car, I chose an area unoccupied with other "stuff" and near enough to the headlights to take advantage of the existing wiring harnesses. (see photos here: http://hometown.aol.com/bobmunch/index2.htm)Others have done me one better and mounted theirs up under the lock panel ahead of the radiator.

In any case, the relays can then be triggered by the current coming from the column mounted lighting switches, and using the same type of Lucas connectors as OE. All connections from the switches to the relays (trigger/low current side) and battery power connections directly to the headlamps (higher current side) are made where the main harness connects to the headlight sub harnesses under the lock panel. The power from the battery (starter solenoid mounted battery Positive + cable connection is probably the easiest and most efficient pickup point) can be carried to the relay's main power switch input inconspicuously with some # 12 or # 10 ga wire (Brown would be a good color, but red is a good second choice).

You would then have the advantages of modern relay controlled power to your lighting, while having an easily uninstalled modification that would do no damage to your wiring harness, nor, for most observers, to the look of your car or its resale value. FWIW.

Check out these sites for more info:

http://www.mgb-stuff.org.uk/
http://www.mgb.bc.ca/electrical
http://lighting.mbz.org/tech/how_to/relays/
http://www.autooptiks.com/relay.html
Bob Muenchausen

I like to keep things simple so, I simply ran a hot wire (heavy gauge) from the starter to the relays. I fused this line. The relays I located under the radiator support (on the right hand side of course). The original harness has headlight connections at this point. This is where I broke into the harness to add the relays. The wire coming from my dash switch now triggers the relay. I connected the original wire which goes to the headlights to the "switched" side of the relay. I like this setup because it's somewhat hidden and clean. It's also very simple to change it back to the original - although I have no idea why anyone would want to! I also added halogen sealed beams. I now have excellent headlights - well worth the effort!

Good luck!
Jim
Jim Budrow

BTW - an error in the website link I gave above. Use this one: http://hometown.aol.com/bobmunch/index2.html
Bob Muenchausen

If one uses fuses in the power wires from the starter solenoid what amperage fuse should be used?
Frank

High i.e. 35 amp or greater, with lower ratings e.g. 15 amp in each filament, otherwise if you get a short you will loose all your headlights, dip and main beam, unless you have the presence of mind to pull on the headlamp flasher when suddenly plunged into darkness on that winding mountain road.
Paul Hunt

Paul,
I have read that it may be best to leave out the fuses in order to avoid a disaster if the fuse were to blow on a dark winding road and leave you blind. Would you or others out there discuss the relative merits and disadvantages of the fuses when adding the relays...as I am definitely convinced I want to add them (relays).
Thanks!
Frank

Well,
without a fuse, if a short circuit occurs, you will lose your lights AND have a melted harness and possibly a burning MG... I'd say install a high amperage fuse to the main supply as Paul Hunt suggests, and smaller fuses for low and high beams.



Kari S

Kari is right. I had a dip switch short in a Triumph TR 250 several years ago, lost the lights, car interior filled with smoke and fried the under dash wiring harness. I have the lights in both my MG's fused, it isn't much fun replacing a bundle of burned up wires in a wiring harness. FWIW, Clifton
Clifton Gordon

A circuit breaker is the best way. That being said, I just use 30A fuses.
Ted

I always thought that the purpose of adding relays to headlights on their own little mini circuit was such that the large current demands were removed from the switch to the relay, leaving only switching current (minimal) at the switch itself.

Therfore the possibility of shorting the circuit and causing a fry-up is greatly reduced, unless for a prob with a relay which are very very rare.

I didn't fit a fuse into my headlight relay circuit - i couldn't see the point. If you put a fuse on the power side of the relay and it blows, you lose both the lights that work off that fuse - potentially V dangerous.

If you wanted to add fuses, then i would suggest puting one per filament (i.e. two for dip, two for main, AFTER the feed) but then you are adding so many connectors into the circuit that the potential for voltage drop through bad connections is surely much higher....

KIS this! ;-> i.e. keep it simple...

I have had my relays for about 8 months or maybe more now, and my new headlamps (cibie reflectors and lenses with 100/80w xenon bulbs)for about 3 or 4 months - they are just amazing!! The dip beam are very good, even in fog and the main beam are far better than anything new i have driven on the road in this time, which considering a lot of cars are using those *fish-eye* lights is quite impressive. Drove home yesterday in the dark on a fast road, and the distance you can see ahead is staggering, you will not go back to the original set up if you try this - if you are worried about originality, just be sure to follow the instructions to be able to put it back again and keep the relay boxes hidden.

~PHIL
Phil


I agree with Phil. I am also considering relays not just for a smaller voltage loss, but also to help my Lucas switches (including the expensive and currently unavailable ign switch) last longer.

I may not remember all this correctly off-hand, but it seems to me that originally for example headlight current goes through a fuse, then through the on-off switch, then through dipswitch, then through numerous connectors etc. Brake light current goes through ign switch to the brake light switch and then to the lights, horn main current is earthed at the steering wheel with the horn + wire always hot, heater fan current goes through the ign switch and the on-off switch and so on.

And there is way too much unfused brown wires sneaking around the car, ready to cause a China syndrome in the wiring.

SO, my "current" plan is to install relays for low and high beam, heater fan, horn, and possibly for the brake lights, parking lights and wipers (wiper motor pulls 3 to 5 amps, about as much as a single headlight bulb). This way I would be using the original harness only for controlling the relays, while the "real" current would never have to enter the under-dash area (except for the wiper motor) or the dash switches. The relay system would be fed by a fused line (or maybe two lines), equipped with a 50 to 100A fuse. Brake light relay would of course be mounted in the rear, with its own power line from the battery (again via a fuse).

I still need to figure out how to improve the situation with coil current going through the ign switch and tach(?).
Kari S

Phil, any wire has the potential (ho ho) to short out. If it is unfused then the harness will burn. This applies equally to the original factory wiring without relays, and modified wiring with relays. The purpose of the relays is to reduce the length of wiring carrying high currents and hence reduce volt-drops in them, and to reduce the currents, especially from uprated lighting, through the main and dip switches. I did not use a main fuse on a conversion I did, only individual filament fuses, and this was a deliberate decision as I had positioned the relays and individual fuses close to where I picked up the brown at the fusebox. Hence the conversion sub-harness was very short, and being wrapped in loom tape I considered that the risk of shorting was very much less than in the original wiring. Bear in mind that the original wiring, which now only operates the relays, is *still* unfused and will *still* burn the harness if it shorts out. Fusing this would put you back in the 'total loss of lights' situation again. The only comprehensive way to fully protect things is do what modern systems do and connect 12v permanantly to each component from its own fuse as close to the battery as possible, using grounds from the switches to control the components.
Paul Hunt

FWIW, When I wired up my relays, I did so primarily for the reason that Phil (UK) has mentioned. However, even though the large current wiring was actually totally separate from the OE wiring harness, I still felt it good practice to do as both Paul Hunt and others have said, and placed 35 amp fuses inline on each filament's feed, ahead of the connection to the relay input terminal. The chance of OE wiring other than the actual headlamp sub harness wiring being affected is non-existant, but it still makes good sense to prevent melted wiring regardless if it is in the OE harness or in separate modification wiring to the relay circuits.

BTW, In my setup, each pair of filaments (Hi beam, Low beam) get their own relay and relay supply wire from the battery, so if one goes down, the other, in theory at least, should still be alive. Each relay is controlled by just the switch for each beam, rather than having a single relay controlled by the main lighting switch on the dash. A bit more wiring, but a better safety factor on that fabled ditch along a cold dark lane in the rain we all fear we will meet someday. ;-)
Bob Muenchausen

for clarification, do you fuse the wiring from the starter that will power the lights, or do you fuse the wire that will power the relay?
william fox

William, Normally the wiring that provides power for the lights is fused. Clifton
Clifton Gordon

... but only when the individual filaments are fused at a *lower* rating ...
Paul Hunt

Is there a good reason not to take the power from the brown input to the fuse box, rather than from a more direct source? It would certainly be easier than from under the car.


Tony
Tony Bridgewater

Paul, what current rating then for the individual beam filament circuits? I have a feeling that mine are much too high.
Bob Muenchausen

Tony - the cable up from the solenoid to the fusebox is of sufficient gauge, but may require a piggy-back connector or similar to extend this onto the relays if there is not a spare spade. Whichever is done it could be argued that this is an extra connection and one that could go high-resistance in the future, which is true, but the option I used nevertheless. Picking up direct off the solenoid with a large lug on the same post as the main battery cable, if that is feasible, would be another option, but you must bear in mind that the solenoid moves in relation to the chassis so the wire must be covered/routed so it cannot rub through the insulation and short out.

Bob - I think I used 15 amp blade type fuses for the filaments, working on about double the ohms-law current at 12v and the rating of one main beam.
Paul Hunt

I added fuseable link wire right at the starter switch where all of the main power comes off just as done in modern cars. Although not intended for overloads, fusable linke wire will protect against short circuits. If it goes, though, everything goes.
Blake

This thread was discussed between 27/04/2003 and 10/05/2003

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