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Skxawng

Help strange bulb issue for MOT

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Skxawng

Hi, not directly window cleaning related but hoping for some help from the clever chaps on here. Mot is due tomorrow and I'm getting little things sorted out but I'm having a strange problem with a reverse light bulb. I replaced the bulb with a new one as it didn't light up but it still didn't work. I've tested the bulb and it works (tried several new ones as well). I've tested the socket with a meter and get 11.8v. To rule out potential bad connection on the socket I've put 2 wires directly into the correct pins on the connector and tried the bulb- still nothing.
If I monitor the voltage, it stays at 11.8v until the bulb is in the circuit- and then it drops to 0v. Then I remove the bulb and it goes back up to 11.8v. 
Any ideas what may be going on?

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spruce
9 hours ago, Skxawng said:

Hi, not directly window cleaning related but hoping for some help from the clever chaps on here. Mot is due tomorrow and I'm getting little things sorted out but I'm having a strange problem with a reverse light bulb. I replaced the bulb with a new one as it didn't light up but it still didn't work. I've tested the bulb and it works (tried several new ones as well). I've tested the socket with a meter and get 11.8v. To rule out potential bad connection on the socket I've put 2 wires directly into the correct pins on the connector and tried the bulb- still nothing.
If I monitor the voltage, it stays at 11.8v until the bulb is in the circuit- and then it drops to 0v. Then I remove the bulb and it goes back up to 11.8v. 
Any ideas what may be going on?

 

Reverse lights can be a pain. 11.8 volts is a little low so would indicate a faulty connection if you have a good earth on your multimeter. Place to look is the connector to the tail light cluster and then the reverse switch which will usually be on the gear box. Usually, if the voltage goes low on the bulb then its a faulty earth.

 

Looking deeper, it could be a faulty reverse switch although they usually either work or they don't.

 

Edited by spruce

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Skxawng

Thanks, I've tried using a good earth instead of the common on the connector but same issue. There seems to be an issue on the positive wire. The voltage should be enough to light the bulb (the bulb above it has the same voltage and works fine) but I'm assuming that the current must be too low..

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spruce
33 minutes ago, Skxawng said:

Thanks, I've tried using a good earth instead of the common on the connector but same issue. There seems to be an issue on the positive wire. The voltage should be enough to light the bulb (the bulb above it has the same voltage and works fine) but I'm assuming that the current must be too low..

 

Your multimeter is only drawing a fraction of the current a bulb does. When you load the circuit the voltage dropping to zero indicates a faulty connection somewhere in the positive circuit, a semi short in a wire or a problem at the reverse switch. In itself 11.8 volts will light that bulb up but it won't if the volts drop to zero. If you are showing a higher voltage across the battery terminals then there is a resistance in that circuit.

 

I would start at the reverse switch. Having you got full voltage into the switch and what happens to the current exiting the switch?

 

You might also try the fuse in the fuse box as it might not be making contact. Usually a relay will either work or not.

 

I'm not 100% sure on the new MOT rules since May but having a working reversing light doesn't apply to all vehicles with reversing lights. I had my Nissan Micra 54 plate MOT about 2 months ago. The reverse light also refused to go on although the after market parking sensors still worked. It was the coupling to the tail light cluster. I ended up putting a little twist in that pin so it made firm contact. I mentioned my drama to the MOT inspector and he told me it was not a point of test on that year of vehicle. The same applied to the rear wash wipe. He told me what year it was applicable from but I can't remember what it was tbh.

 

Nissan had a bad batch of reverse switches that fit on the gearbox.

Edited by spruce

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Kleenwell

Had your van got a screen to tell you, you have a dead bulb?    It’s called can bus.  It feeds a low current to the bulb to see its its ok.   It will read as zero when a load is applied.    I suspect your reverse switch on the gearbox personally.  Had same issue on my car.    There is no solid wire between the switch and the light.    A digital signal is fed between a fuse box under the bonnet and a fuse box in the rear.    I would pull the plug out of the switch and short it with a bit of wire.  

The reason I believe this is the problem is because can bus uses around 11v to test the bulb.

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Skxawng

Thanks for all your help, still haven't figured out the problem but it went straight through the MOT 🙂

 

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