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MG MGB Technical - Lazy headlights

About this time of year my '73 BGT's headlights become lazy - by which I mean that when I switch them on at the dash then the sidelights come on immediately, but the headlights might take as long as 60 seconds to light up. When this first happened, a few years ago, I thought it must be a bad earth so I cleaned all the bullet connectors, added a better earth point and "thought" I'd solved it. Last year it happened again and it occurred to me that it couldn't be a bad earth or feed connection - because the headlight flasher works every time. I'm now thinking that maybe I need to clean the dash switch internals, but why would it only happen around this time. The obvious candidate would be condensation in the switch, but could that have this effect. What does the "team" think ? Oh, once they have lit up, then that's the end of the problem until the next day, so it suggests that the switch has dried out even though it doesn't seem damp.
Miles Banister

The switch is a likely culprit. But did you clean the bullets where the dash loom connects to the bonnet loom? Carrying that current through an old switch is not a good idea, especially if you have up-graded the lamps. Use the switch to activate relays, that way shorter wire run, fewer bullets and the dash switch has to carry a much smaller current. Get the feed from the battery or the brown wire on the starter or similar on the alternater.
Allan Reeling

Absolutely right Allan. Fit a pair of relays, one for dip and one for main. There is far too much current passing through the light switch.
Mike Howlett

Can be one or more bad connections anywhere in the circuit, not just the earth. But if the headlamp flasher lights them right away you can forget the earth, and the blue/white connectors out by the headlights. You can narrow it down still further if you can say whether this affects both main and dipped beam, or just one beam.

If both main and dip it is going to be the main lighting switch or the dip-switch and connections between them (but could possibly be both).

If it affects only the dipped beam then it is going to be the dip-switch or the blue/red connectors by the right-hand headlight.

Relays are essential for uprated i.e. higher current headlights but shouldn't be needed for standard lights, although they will be a benefit with 40-year old wiring and components. However a problem such as this may not be solved by adding relays, if they don't come on for 60 secs now there is little or no current flowing, so if the problem is round the switches it will still be there and the relays probably won't operate either. This problem needs to be fixed first, then you can decide if you are going to add relays. And if doing that you need to fuse the individual filaments as well to avoid a short burning wiring or killing the lights both sides.

PaulH Solihull

Paul, Yes the flasher works right away, and it does affect both main and dipped beams so it's looking as though I need to clean up the switch. I'll do that at the weekend and report back. Thanks!
Miles Banister

I decided not to wait until the weekend.

The switch comes out and disassembles very easily. Inside are three metal contacts - one for power in, one for sidelights out and the other for headlights out. The power in contact is quite worn. There's a sliding metal shoe which runs over the contacts and joins them as required. That also has some wear corresponding to the contacts. I've cleaned the parts and reassembled the switch with the shoe turned through 180 degrees so that its less worn part is against the most worn contact. It works in the garage so it could be OK. However, one of the plastic nibs that keeps the two parts of the switch frame together is no longer very secure so it's possible that the switch might separate in use. If that happens then it will either need a spot of glue, or a new switch.

It never ceases to surprise me what simple but effective design went into some of the parts on our MGs. It would be nice to imagine that something I'd designed would still be functioning perfectly after forty years continual use.

Thanks for all the comments.
Miles Banister

When I restored my roadster 20 years ago I decided to replace all the switches as the painted icons were worn, and I wasn't doing very well at repainting them. A couple of years ago my main lighting switch started playing up (very similar to your symptom now I come to think of it). By this time the painted logos were beginning to get worn on the new switches, the old lighting switch wasn't that much worse, so went back in and is working fine.
PaulH Solihull

I've re-built and modified all my rocker switches, when the plastic clips break i use a hot glue gun, works perfectly.
Allan Reeling

This thread was discussed between 01/11/2010 and 03/11/2010

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