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MG MGB Technical - Main beams dont always come on

Good afternoon, today I finished off wiring in main and dipped beam relays to my 1965 mgb roadster. The dipped beams came on ok but when I pushed the foot switch to turn on the main beams, the dipped went off but the mains didnt come on. Tracing my wiring and 5 mins later suddenly the mains came on all on their own...

I have not been able to repeat this and have had a good jiggle of all the wiring but couldnt get it to do anything it shouldnt. All now works as it should. I was wondering if the foot switch can "stick" and turn the supply to the dipped off but not quite making contact to put the main beams on?

THe other question is fuse ratings, I have a fuse for the mains and a fuse for the dipped, i.e. one fuse covers both sides. I have put a 15amp in the dipped and 20 amp in the mains. Are these values ok or a little high?



C Briggs

Sticky relay ?
R Walker

If they are new relays, it may have been a one time thing (hung relay). Yes the foot switches can and do stick or have bad contacts.

Fuses depend on how the fuses you have are rated. Lucas fuses were originally marked with the "fast blow" current, and were rated for half that for constant loads, i.e., a "35A" would blow quickly at 35, and was to carry 17.5A indefinitely. Later Lucas were marked with both numbers. American fuses are usually marked with their constant rating, unless qualified as "fast blow", "slow blow" etc. I do not know what markings are now used in UK, but guess at the Lucas system.

Since present standard lamps are rated at 60/55W at 12V nominal, the current draw is 5A per lamp. But a good electrical system will be above 12V (13.5-14.5), so the current may be up around 6A per lamp. That means that you are on the low side on your fuse selection; you want a bit more constant rating than the total draw of the circuit, say 15A constant for two lamps. The old standard Lucas 35/17.5A fuse is about perfect for this. Fuses are there to protect wiring from catastrophic shorts; slight over rating on fuses is far better than "no light" episodes!

Assuming that your fuses are in fact rated by the Lucas method, the only reason they didn't blow already is that there is still substantial resistance loss in the wiring. Clean all bullet connectors, and check all ground points carefully. My own extensive measurements have shown that the headlamp pigtails and plugs on MGB etc are themselves responsible for a drop of 0.33V, due to undersized wire.

The 0.33V drop in the lamp pigtails is for OE Lucas pigtails, which are not quite the equivalent of (US)18gauge wire (14 .010 strands vs 16 for standard 18 ga)). Many repro ones appear to be about 20 or 22 gauge, much worse for voltage loss. This drop alone is worth 8 to 12% loss of light, depending on where you are in the voltage curve. Changing the pigtail leads to 14 gauge gives a near zero voltage drop. The pigtail is the complete wire attached to the lamp, from where it (blUe/White or blUe/Red) plugs into the harness, to the lamp plug, and back to where the Black wire joins the harness or is grounded to the body/chassis.

When replacing the pigtails, create additional ground points to the wing mounting bolts just above where the front harness plugs are; keep the ground connections to the harness black as well.

If you like, send me an email and ask for "Electrobabble" and I will forward a copy of all this and more.

FR Millmore


You have mail.

The car had a new loom when rebuiult in 1990 so hopefully isnt that bad. I measured my voltage at 12.2 / 12.3 volts today when doing the checks - the car was not running so I guess this may be why the fuses didnt blow. I am using the normal modern spade type fuses so can get some higher rated ones quite easily.

As I have just spliced into the loom to fit the relays, I will take a good look at the earths etc on the lights next weekend. They already looked much birghter than before and hopefully the switch will now last for a lot longer!

SO thats headlight relays (almost) done, rear brake light relay fitted, just the overdrive to fuse and that should be me pretty much protected!

Thanks for the help.


C Briggs

The lifetime of the connector sleeves appears to be about 25yrs, even if they are up under the dash and protected - the brass age cracks. Modern sleeves from some sources are nickle plated steel and won't crack. Anyhow, cracked ones will work but give excessive voltage drop.

On this side, the modern fuses will be rated American style - the marked number is for continuous use, so 15A or 20A is good for two standard lamps. This might be standard as ISO or somesuch.

You will find that while your lights are now much better, you can expect about the same (additional) improvement by attending to the other details. And, a frequently unthought of bonus is that the rear lamps will improve as much or more, since you are not running headlamp current through the harness and switch(s).

When you're "done", check voltage engine hot @ 2500rpm, system fully loaded = everything ON. V should be near 14V everywhere.

Electrobabble on its way.

FR Millmore

One of the benifits of wiring relays into the Headlamp circuits, is to by-pass all the connectors and switches to the lamps. i.e., the wiring, connectors, lamp switch and dip switch only have to carry enough current to activate the relays. The feed to the lamps can then be made more direct. Either via a fuse from the battery itself, or from the battery feed from the alternater, again via a fuse, thus reducing the losses associated with switch contacts, cable length and bullets.
amps x volts = Watts

This thread was discussed between 29/08/2009 and 30/08/2009

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