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No (interior) lighty, no likey...


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I searched and found quite few threads about interior lights not working, but nothing quite like the issue I seem to have. 

I have an ex-utilities 110 TD5 hardtop and the interior light doesn't work.

It doesn't work if you switch it to "on" and it doesn't work on the door switches. The bulb is good and I'm getting voltage to the light unit. I get 10.7v at the purple/white wire and 12.4 at the purple wire.

I've just bought a Mud white and red LED interior light to replace the original unit, thinking there must be a problem with the original one. When I connected it up I get nothing from the white light, and a very, very faint glimmer from the red. I've tested the Mud unit on a spare battery and it works perfectly.

It's as if there's voltage but not enough current to power the lights. Anyone got any thoughts on what's going on and how I can fix it?

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So the Td5 light wiring works like a lot of modern cars - it uses a switched earth, its a bit of a pain to work with in my opinion but I'm betting this is part of your problem.

I need to dig out a circuit diagram - if you've got a Haynes they have a half decent representation. LED's are also sensitive to voltage if there isn't enough then it wont work properly - doe it work ok with a filament bulb? this could just be bad earth syndrome.

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59 minutes ago, Mo Murphy said:

Did you measure across the supply and the lamp earth wire ?


Yep, I took the original light fitting out completely, then I put the multimeter across the lamp earth female connectors and purple wire female connector, then across the lamp earth and purple/white wire female connector. The readings I got are the ones in my original post.

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59 minutes ago, Bowie69 said:

Thing is, if one of the wires is ground (or should be), it shouldn't have voltage on it, hence my stating there was a wiring fault.

finding a known earth will help to identify this...


my favourite tool for this sort of thing is: https://www.eurocarparts.com/p/sealey-pp1-auto-probe-6-24v-SEAPP1?type=shopping&gclid=EAIaIQobChMIprS524q09AIVnYBQBh3hnwsgEAQYASABEgKSXvD_BwE

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14 hours ago, Nonimouse said:

That's going on my Christmas list 😊.

Multimeters are very versatile I like the simplicity of the Sealey probe 👍

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Do you have an alarm? I don't thing that you do as it's a hardtop. Alarm means that the circuit is more complicated.

If you don't have an alarm then with the switch in "ON", the lamp is earthed directly through a black cable in the loom. If it is in "DOOR" then the lamp is earthed through one or both of the door switches on the A posts, through the purple/white wire.

Both of these switch positions need the "other end" of the lamp to be at +12V, alway there and fed with the purple cable.

The quickest check is to provide your own earth at the lamp at the end of it that is connected to the black or the purple/white (depending on that switch position). You can do this with the switch in either position using a length of cable from a known bit of metal, touching the bulb or the connector. Just be sure that you know which end of the bulb to touch. If you get the wrong end then the fuse will blow.

This will eliminate the other earth paths and will check just the power (+12V) supplied on that purple wire. If it works then it's a problem on the earth side and if it doesn't then something on the supply side is a problem.

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Right chaps, I've been doing a bit of testing on the interior light issues today.

Firstly, regarding the "has it got an alarm" conundrum...

My truck is an ex Utilities hard top TD5 but it does seem to have an alarm system of sorts. It has a blipper thing on the key ring, and an ultrasonic sensor to the right of the driver's head. However, none of it works. I don't know if it's been disabled (it that's possible) or if it's buggered. It doesn't work anyway...

Is the alarm unit a green box about the size of a pack of cigarettes screwed to the bulkhead behind the clocks?

Anyway, on to the testing...

I ran a lead directly from the earth on my Mud LED light to an earth on the bulkhead. No difference. 

I detached the headlining and A pillar trim and wiggled the cables and the light came on. But it was very dim. Really dim! 

It worked in both the "on" and "door" positions. On "door" the light stays on for about 5 seconds after the door is closed. This presumably means it's connected to some sort of delay timer.

The wiggling of the cables revealed that the connector on the body side is dodgy. When I disconnected the cables a sliver of metal fell out, so I need to get a new one (from where I don't know!).

So in conclusion:

I need a new connector for the body side of the interior light loom.

There is power to the interior light.

The door switches and delay work.

The light is still way too dim!


Where to go next???





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Yes it sounds as though you do need a new connector and the green box you mention is the alarm; 10AS is the name.

The door switches, which still connect to ground when the doors open, are connected into the 10AS. It can therefore sense when a door is opened and it switches the light on for you but then delays the switch off to give you time to find the ignition etc. As soon as you switch the ignition on, the 10AS switches the light off. If you don't switch it on, the 10AS holds the light on for longer. On my 300TDi that is 15 secs. It's a similar action when you leave the vehicle and close the doors.


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Power probes are very handy and I use mine often, but bear two things in mind,

1. you can easily send 12v where it shouldn't be and fry PCBs 

2. they do not flow much current so can occasionally give odd results on things like spotlights.

If you would like any T-shirts for the above problems please send a stamped and addressed envelope to...... 🙈





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If we put the connector issues to one side, do we think the fault if likely to be with the green alarm box? 

If so, is it worth having it repaired or should I just bypass the alarm box and wire the interior light directly to a live feed and the door switches?

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10 hours ago, Filbee said:

If we put the connector issues to one side, do we think the fault if likely to be with the green alarm box

No. With the Switch in DOOR position, the alarm isn't in the circuit. In this case it is fed on one side with +12V on the Purple and the circuit completes through the Black directly to earth. 

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Here's the circuit. As I say it's 97MY but I'm pretty confident that yours will be the same. If you follow the Purple wire from the fuses, it connects to the bulb in the lamp unit and with the switch in the dotted position it would complete the circuit to ground (0V) on the Black though the header joint K109 to an earth stud C556 which is actually on the engine side of the bulkhead (see the second pic).

So you can see that in the ON position the lamp is not dependant on the alarm at all.

EDIT: A TD5 person might come along and tweak this advice to suit your vehicle.





Edited by Peaklander
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