Failed MOT

[lucas1787]

Hi my Volvo S40 98yr,has failed its MOT as the rear fog light isnt work.I know it was working,but had an intermittant fault,ie worked sometimes then the next time I tryed it wouldnt.Just so happeaned didnt work when being MOTed!.I have checked the bulb,contacts etc and Iam led to belive it might be the relay under the top part of the dash.Before I spend out £45 for a new relay,has anyone else had the same problem and would recommend a relay change,as the only other thing would be the switch.(the light switch is also in the correct position) Thanks Dave

[p fandango]

Hi my Volvo S40 98yr,has failed its MOT as the rear fog light isnt work.I know it was working,but had an intermittant fault,ie worked sometimes then the next time I tryed it wouldnt.Just so happeaned didnt work when being MOTed!.I have checked the bulb,contacts etc and Iam led to belive it might be the relay under the top part of the dash.Before I spend out £45 for a new relay,has anyone else had the same problem and would recommend a relay change,as the only other thing would be the switch.(the light switch is also in the correct position) Thanks Dave

£45 for a relay YOUCH. I would take the relay out, get the multimeter out & using temporary jumper wires bypass it & see if the light still plays up.

[t5 pete]

As pedro says bypass the relay

[lucas1787]

Sorry not very good with electrics,could you break it down into a numpty step by step for me.Give me a PC to work on and I can cope with that.!!Thanks Dave

[p fandango]

Sorry not very good with electrics,could you break it down into a numpty step by step for me.Give me a PC to work on and I can cope with that.!!Thanks Dave

as said you'll need either a multi-meter or some sort of a test light, any 12volt bulb (not LED) with enough wire will do, & a notepad & pen is a good idea so you can jot down what each terminal on the relay was doing

remove the relay, it should be a simply 4pole relay. You won't need the ignition or any lights on for this. Connect one wire off the meter/bulb to a known positive. With the other meter/bulb wire probe the terminals of the relay socket. The meter/bulb should react to 3 of them, 2 will light it up brightly while the other quite dim. With the wire connected to the terminal which brings the light on dim you should notice the fog light is also lite up dull, this is one of the terminals you need to remember

now remove the positive wire off the tester & connect it to a known earth. With the ignition & foglight switch turned on (& headlamp switch in the correct position) test the 4 terminals again, it should now light up with 2 of the them, one of these will turn the bulb out when you turn the foglight switch off, this is the other terminal you'll need to remember

You now need a short piece of wire preferably with a male shape connector on each end, now place one end in the wire that light up dim when you did the first test, & the other to the terminal which reacted with the foglight switch.

This should now be bypassing the relay configuration & the foglight should come on when you turn them on as normal without the relay being in place. As you've pointed out if the light still plays up then the relay wasn't at fault, hope that helps

[lucas1787]

Update on my problem,I checked the wiring and relay as suggested and found it was the relay.I have now changed it and I now have a fog light!!!.Thanks guys for the input.