The terrible hell of an intermittent fault

Hello,

Welcome to my own personal electrical hell.
I have a Defender 90 of 91 vintage with the original 2.5na petrol engine. It's only done 47k in the last 24yrs and has been well looked after.
I've been going around it over the last 12mths righting wrongs and replacing or refurb ing bits that have inevitably succumbed to time, wear and rust.

My latest gremlin concerns the headlights. (Well it's always done it it's just a bit worse now)

I've read about a hundred posts on multiple forums over the last few months but I'm no nearer a solution. Basically with the headlamp position switch (small stalk lhs steering column) flicked forward I get 30% dim dipped headlights and sidelights as per design. However when I flick it forwards to what should be 100% dipped headlights..... Nothing. At least not until the thing warms up for five minutes, then magically it comes on, and switches off and on with the switch without issue until you park up and it goes cold again. Then you are back to square one.

I've stripped and cleaned the headlamp connectors and earth points on the inner wings, new bulbs, good 7.5amp fuses and connections on all the dipped headlamp circuits. I swapped the headlamp relay around with known good ones, no different.
Lots of posts suggested the two light stalk switches were at fault so I stripped, checked and referbished them where necessary. No change other than my indicators now self cancel again (yay, small victory!)

Lots of folk suggested the dim dip resistor. It's the big one mounted on the bulkhead behind the dash so disconnected that. Same fault, except I just got sidelight bulbs and no 30% dipped headlamp, which is even worse. At least I can drive on side streets on 30% dip, the lights normally come on properly by the time I reach proper roads.

Sometimes just warming up the Landy isn't the only thing which completes the circuit. Occasionally if it's nearly warm, stamping your feet, flicking the brake pedal so it springs back and makes a jolt, or just hitting the steering wheel is enough and the relay in the fuse box will click and the lights come on. Sometimes if it's not quite warm enough the relay will just whirr as it try's but fails to energise the circuit with resulting flickery headlamp effect.

It's doing my nut as I have to sit on my drive with the engine running for more of my life than I'm happy with. Presumably at some stage whatever broken bit that's hanging in there will give up and I'll have no full power dipped headlights at all.

If this strikes a chord with a maverick auto electrician or anyone who can help there would be much kudos indeed.
My only alternative would be to fit a relay controlled headlamp loom run straight off the battery or alternator energised by the first sidelight switch position. But then I would have no sidelights which would prob incur an MOT fail I'm sure.

HELP!!

Ps I have absolutely no idea where that B&Q hyperlink came from. Not me!

I've only got a partial circuit diagram, so with that limitation...

Does the oil pressure light come on with the ignition and go off as it should?

Basic test. Take a wire from the battery +ve and dab it on the blue/red wire connector at the dim/dip unit. Dip beam should be on full.

Let me know how that test goes.

If you have glass fuses then the fuse box can go high resistance- mine did very similar

I ripped out old fuse boxes and re did the lot

Hi, thanks for the reply.
I'll try the bypass wire later in the week. Didn't think of that one. They are blade fuses in the box. Pretty much the same wiring as the 200 tdi apart from the petrol/diesel unique bits.
Ive poured over tdi and even series three wiring diagrams for clues. I suppose I should check (when I find my multimeter) whether its the high or low side of the relay too. Im pretty sure from the way it behaves its the lower draw ignition switched side as opposed to the higher current lighting side of the circuit. But I could be wrong.
I'll let you know how it goes. I'm determined to sort it rather than pay an auto electric fraudster. Been stung that way before and got a motor back no better than when it went on and got charged a couple of hundred for the trouble!

Oh and yes, all the other electrics are 100% working as they should including the oil light. A company did fit a towbar and associated electric set up before I bought it. Could that have affected something? Its invoice is with the docs and looks like a pretty good tidy job by a reputable firm.

have you stuck a multimeter on the headlamp when it doesn't work to see if you get any juice? then again when it does?

I eventually got to the bottom of this with good old trial and error.
The headlight relay in the transfer tunnel fuse box had an intermittent fault. New relays sorted it right out.

It took two years!!!? Awesome stamina, but very glad you found it, and always good to file away these discovered fixes for when mine does it, as it probably will some time.....! Nice one, A

Yes, all my electrical and electronic faults are intermittent. Or at least most of them are. It's actually quite a pleasure to go looking for the trouble and find something disconnected or looking burnt. Most of the time, it seems, faults are undetectable to the naked observer.

I was asked this week to have a look at a friends Citroen 2CV. Botched connections everywhere and almost every wire was green - and I mean the factory wiring. Didn't they do coloured wiring in France?

Probably depends what they had on the bench at the time !! Had a real nightmare trying to fit landy tail and indicator lights on my French trailer (makes it easier and cheaper to replace when I bust em working in the forest), had all sorts of colours but most of the lives were black FFS. Only way I sorted what was what was with a battery, long lead and clips and a light bulb. That is on a 2 year old trailer, hardly any wiring on it really, took me an hour to work out.

Brown said:

Yes, all my electrical and electronic faults are intermittent. Or at least most of them are. It's actually quite a pleasure to go looking for the trouble and find something disconnected or looking burnt. Most of the time, it seems, faults are undetectable to the naked observer.

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Is this where you're going wrong ?