Strange headlight issue

[Andrew S]

Hi All
Struggling to understand an issue with one of the dipped headlights not working
Bulb is OK and tried a spare too
I have done a continuity test for the earth black wire on the bulb block connection to the bumper and fine
Tested 12v feed at the headlight block connection earthing to the bumper and OK
Put the meter probes in the earth and blue / red wire on the bulb connection block and I’m getting just under 12v

So I just can’t understand why the bulb won’t light

Will be driving home on the M40 M25 tomorrow night, good job the mains work and the other headlight is bright

So frustrating

Any suggestions appreciated

Cheers
Andrew

[mole42]

It’s probably corrosion in the bullet connector - they aren’t terribly water proof and newer ones aren’t made well. Try taking the wires out of the 3-way block and cleaning with wire wool.

[Andrew S]

Thanks Richard
So what you are saying is that although I get a reading from the multimeter probe for an earth, it could be that the bulb terminals themselves are not making a clean connection?
The fact I can just pull the bulb off and either get a 12v reading or a confirm by continuity test that I have an earth to the bumper really doing my head in

[KOY 23]

Multimeter needs a very low current to operate, a high resistance joint will allow enough current to operate the meter but not allow enough current to light the bulb. Try the volt drop test method. Fix the positive lead of the meter to battery positive and put the negative lead on the blue red headlamp wire. If you get around battery voltage, work your way back until voltage drop is low, the transition from high to low is where your problem is. If you get low voltage at the blue red connection, the problem is in the earth side, swap the meter earth lead to battery earth and trace the earth wires for voltage drop. As Richard says the problem is likely to be in the bullet connectors.

A test lamp is often more use than a meter asit loads the circuit

[Andrew S]

Thanks John
Much appreciated
I love weekend jobs
Hope alls well
Andrew

[Phil S]

You are getting volts but not amps, good clean up of the connections

[Andrew S]

Hi Phil
Just been out to car and put an earth wire straight from battery to headlight whilst still connected to the block and nothing so can rule out an earth now
However did the same in that putting a wire from battery power side to headlight and it works so my issue is on the power feed side.
Thinking about it, I have switchable relays fitted so all 4 headlights come on full beam so could well be my starting point. Saying that, I can hear the relay clicking
Run out of time now and will be winking with one headlight tonight when driving

Thanks all for feedback and advice
Cheers
Andrew

[sujitroy]

With electrical faults I rig up a 12 volt bulb on wires. I find 12 volts on a meter doesn't mean much. Not on a Stag anyway.
Sujit

[mole42]

Hi Phil
Just been out to car and put an earth wire straight from battery to headlight whilst still connected to the block and nothing so can rule out an earth now
However did the same in that putting a wire from battery power side to headlight and it works so my issue is on the power feed side.
Thinking about it, I have switchable relays fitted so all 4 headlights come on full beam so could well be my starting point. Saying that, I can hear the relay clicking
Run out of time now and will be winking with one headlight tonight when driving

Thanks all for feedback and advice
Cheers
Andrew

You've ruled out the negative side, and because one of your headlights works, I'd suggest that the wiring and switching is okay. That leaves the bullet connectors, a known trouble spot and as people have previously suggested, that's the place to check. Take the connectors apart, clean the bullets, clean the connector housing and away you'll go. 99 times out of a hundred, the connector housing has rusted because some scrote used steel rather than brass. Rust doesn't conduct electricity very well.

[Philip Wardle]

One of my other "hobbies" is "domestic electrical installer" amd I have a Megger MFT multi-function tester, this is very handy for measuring the resistance across the various runs in a wiring loom on a Stag, and across connectors, and application of Ohm's law will often reveal the problem of volts but no current. In practice the advice to use a light bulb rather than a voltmeter works just as well.

[Andrew S]

I’m certainly going to knock up a test light

[KOY 23]

It takes all sorts

[Andrew S]

That’s what Fred Basset said

[Andrew S]

Sorted. It was the power to dip headlight failed on the switchable relay I have fitted. Fitted so that all four main beams come on. Cheap fix and spare relay in toolbox
Thanks for all the advice and suggestions