Interior light delay unit fault?

[ChrisHC]

Investigating why my car had a flat battery, I found that the post holding the end of the festoon bulb in one of my interior lights had a bodged repair, allowing the bulb to slip sideways and touch one of the metal arms that make contact with the end cap, depending on the switch position. This meant that the bulb was permanently on and had flattened the battery.

I have removed the bulb until I can replace that light unit, the other interior light is sound and the three position switch works as it should, putting one or other of the arms into contact with the end cap, or neither when the light is permanently off. However if either of the arms connects, the light is permanently lit. The delay unit was working prior to this, but could it be that the lengthy period the faulty light was on has damaged the delay unit, so that it is now permanently on? If not is there any other likely cause? I can hear a relay operating when either door switch is pressed in, so I presume that these switches are functioning properly, although I seem to remember that this relay serves some other purpose than the interior light.

[mark_i]

yes, the relay also activates the timer when the drivers door handle is lifted.
sounds like the control box is permanently grounding the timed output to earth...

[Brianmoooore]

Interior lights have a permanent live at one end of the bulb, and a switch arrangement at the other end, which either earths the bulb, making it light, or connects it to a wire from the interior delay unit, assuming one is fitted. The interior light unit contains a relay which earths this wire when it is activated.
If the non live end of the bulb had slipped and earthed itself, this would have no effect on the delay unit whatsoever, even if the pole in the switch connected to the delay unit was shorted to the earth as well - the wire just goes to the open switch contact of a relay.
The relay that clicks when you open a door is to switch the power supply for the doors and roof from an ignition switched source to a permanent live one. Again, nothing to do with the light delay unit - that even uses separate contacts on the door switches from the ones that switch the lights.
I've no idea why your remaining light is lighting when the switch is in the auto position without further testing, but it's not as a result of your theory above. Check that the brown and brown/purple aren't shorting together at the light you've removed and the brown/purple isn't shorting to earth anywhere else, including at the passenger door switch and rear door switches, if you have them.
Unplug the delay unit (next to the central locking ECU, inside the sill, under the driver's side speaker to confirm that the delay unit is innocent, or otherwise.

[ChrisHC]

Thanks Brian, I will investigate further along the lines you indicate. I had to replace the central locking ECU which had been flooded in the past owing to blocked drain holes, perhaps the delay unit was also submerged and the damage has only now had an effect.

[ChrisHC]

I have resurrected this thread to report further developments. I replaced the faulty interior light and the door switches in case they were the problem to no effect, so went back to the delay unit. I bought a used unit and left it on the kitchen table for a month while I did other things and it got acclimatised. I tried it today and the interior lights now work, coming on when a door is opened and going out when it is closed, but they go out immediately with no delay. This is a much more acceptable fault than not going out at all and I can live with it, but it would be nice to have the delay working.

I presume this must be a fault with the second hand unit and it is a sealed unit that resists any repair, so the only thing to be done is to get another delay unit and try that, I assume they are not available new.

EDIT
It turns out it is not that sealed and I sprung the cover off my old unit. The relay looks in very good condition but the electronic components at the other end look as though they might have suffered the same soaking the central locking ECU went through.

[Brianmoooore]

Unit is definitely not sealed, and comes in two versions, which are identical from the outside. Both are repairable at a component level, although circuit board corrosion problems may require a bit of ingenuity.
A point that most people aren't aware of is that the delay should only work on the driver's door, and only when you enter the car. The lights should go out instantly when you close the door on exiting the car.
This feature is controlled by a crude switch operated by the exterior door handle, and if this isn't working, or the signal from it isn't reaching the delay unit (door A pillar plug and socket problems), then the delay won't work, in or out.
The door switch connects to the module on a brown/green wire, and a quick check is to short this wire to the brown/yellow from the door pillar switch, which should make the delay work when both leaving and entering the car.

[ChrisHC]

Mant thanks Brian, I will investigate further on those lines. I did have to take out and clean up the window motor which was mounting a go-slow, and I might have disturbed something in the process.

[ChrisHC]

Shorting the brown/green and brown/yellow wires did indeed produce a delay. I decided to make that a permanent connection as taking out the door lock seemed like a lot of work when the crude switch might not turn out be repairable and anyway I have no objection to the light delay operating when I exit the car. Thanks once again for your help Brian.

[Brianmoooore]

Most people prefer the delay working on both entry and exit, so it's arguably an upgrade. Also an improvement to link in the passenger side switch as well, so that the delay works from the other door(s) as well.

[ChrisHC]

Of course that prompts the question, how would you arrange to link the passenger side, presumably run a wire from the two cables already linked to the brown/violet cable of the N/S door switch?

[Brianmoooore]

Brown/violet has to be disconnected from the passenger door switch, and the connector pulled back out of the sill, so it can't short to earth and turn the lights on. Run a new wire from the switch across the dash, and splice into the brown/yellow to the delay unit.