Mk7 Golf (MQB) boot light failure diagnosis

[blueST]

Can anyone help me diagnose and fix this.

Late last year I bought an absolutely mint Mk7 GTI, literally the only fault is the boot light doesn't illuminate. Most irritating att his time of year. I have removed the lamp from the car and powered it from a separate battery so I know the bulb and holder are good. I even checked the resistance of the diode in the holder and it's correct and properly orientated.

My understanding is that on these MQB cars the boot light is controlled by the body control module (BCM) rather than a simple switched and fused 12v feed. There is no separate fuse for the lamp. When I open the boot, the warning illuminates on the dash screen, so I believe this tells me the that the switch in the boot latch and associated wiring is ok.

If I measure the voltage at the plug for the light fitting, with the boot closed it shows 0.5v (maybe that's some sort of continuity check by the BCM?). With the boot open it is 11.8v which seems low to me, but I don't know what the BCM would normally put out.

Any thoughts on where to look next?

[Dr G]

Wire a tester directly to the pins - any 12v lamp.

[catso]

blueST said:

With the boot open it is 11.8v which seems low to me, but I don't know what the BCM would normally put out.

A 12v bulb would illuminate at 11.8v so, something else is not right. Is the bulb the correct wattage?

[blueST]

Dr G said:

Wire a tester directly to the pins - any 12v lamp.

That essentially what i do when I connect the original light up. It doesn't illuminate.

[blueST]

catso said:

blueST said:

With the boot open it is 11.8v which seems low to me, but I don't know what the BCM would normally put out.

A 12v bulb would illuminate at 11.8v so, something else is not right. Is the bulb the correct wattage?

Yes, it the original factory little festoon bulb. I suspect for some reason the voltage reads sort of reasonable when there is no load on the circuit, but when a bulb is connected it can't sustain the required current to illuminate it. But not sure why that would be.

Wiring is in good knick, terminals are clean etc.

[Belle427]

Are you checking the voltage at the lamp positive and ground terminals?
Confusion can occur if your using a different ground point for testing.
Maybe have a look at the wiring in the loom where it enters the hatch as it can partially break here.

[blueST]

I measure it across the terminal in the loom connector.

The lamp is insidethe boot, not the boot lid.

[Belle427]

Apologies wasn`t thinking.
I cant think what else it could be apart from the bcm shutting it down for some reason with load installed, used to happen on some bulb warning systems years ago from memory.

[blueST]

I think i might have to just live with it. I don't want to pull the interior apart searching for damage to the loom.