Fog light not working: MoT due

[jjad]

I've got a couple of problems ahead of our GTL's MoT on Thursday.

Firstly, the rear fog light isn't working. There's 12V at both ends of the fuse, but only 18 mV at the positive wire above the fog light itself. So presumably there's a problem with the wire between the fuse box and the rear of the car. Any thoughts?

Secondly (and I don't think this is a problem for the MoT) it seems like the heating is stuck on hot. The knob turns OK (though was stiff to start with), but it makes no difference. Again, any tips (ideally not involving taking out the heater matrix).

Many thanks!

 

[André4Renaults]

No the heater's not part of the M.O.T. (unless the test is carried out on a VERY hot day and the tester's in a bad mood!)

As to the fog lamp, both the fog lamp and the warning lamp on the dashboard must illuminate when then switch is operated and the car's lights are on. In other words it can't be operated independently of the car's normal lighting. When you measured the voltage at the rear bulb did you have the car's lights switched on? Another thought is that the earth connection may be faulty. Try an alternative earth & then see if there's 12 volts. Scratch away any rust or "scabby" areas around the bulb holder and its fitting.

 

[jjad]

Thanks. I forgot to mention that the earth is good (as very little resistance between outside of bulb holder and shell of car).

The fog light does only operate when the headlights are on (although the dashboard lamp for the fog lights doesn't work unless the ignition is on).

 

[jjad]

Looking at the wiring diagram, it could be the switch (which comes after the fuse).

 

[jjad]

The switch seems OK...

 

[jcarruthers2]

Hi

With regards to the heating situation. Sometimes the rubberized knob attached to the heater tap comes loose and when you turn it, it gives you the impression that you are turning the heating on and off but it is in fact just the loose knob moving but not the heater tap spindle (tighten the screw).

You can check to see if the heater tap is seized up by taking the very same knob off and trying to turn the heater spindle with a pair of mole grips. If the spindle moves freely all should be ok. If you find that it is seized up, try spraying some WD40 using the straw to get the solution into the central area. Keep the mole grips on and wiggle up and down whilst spraying. If the heater tap is completely seized up, you have no choice but to take it out completely.

 

[jcarruthers2]

*Alternatively the thermostat could be stuck

 

[malcolm]

There's an electrical connector in the fog lamp wire somewhere underneath the rear of the car that is prone to corroding. Worth checking that first.

 

[jjad]

Thanks @malcolm -- I've found two connectors in the rear for the fog lamp, and neither get 12V. I can feel the wire going into the rear loom. I'm now going to try the connector block marked 81 in Haynes. Unfortunately I don't think the colours match up though.

@jcarruthers2 -- thanks for the tip. I'll try that.

 

[jjad]

Well, that is embarrassing. Turns out I was testing the wrong switch! I had got the symbol for the rear demister muddled up with the symbol for the rear fog lamp In my defence, the wiring colours are different to Haynes. And at least I've now found the connection box where the wire goes.

I hope that can at least make you feel that your day is going a little better than mine

I've taken the heater knob off and the spindle turns freely (although it's a bit 'crunchy'. I'll see what temperature the air comes out next time I venture out.

Thanks for all your help

 

[jjad]

Having passed the MoT last week, we took a lovely 66 mile round trip today (actually to our nearest available NHS dentist), and it's not blasting out hot air So all good.

 

[reidalpine]

Quick fix for defect/leaking heat-regulator in dash is to cut rubber-hose on top of valvecover & insert a ball-valve(?) -Works better than finding a new brass-valve which only works for a short time anyhoo.. -Reid