New to me '95 C4 convertible. The interior lights sometimes come on when a door is opened, and somtimes not. This morning, nothing when I tried them before leaving for work.. Later today, they worked. The car had not been moved or started or anything. It is a crap shoot if they work or not. If they do not work, neither door makes them work. If they are working, either door will make them work. All or nothing. They do work always when using the switch on the dimmer control.

I woule immediately think a door switch, but since it happens on or not with either door that does not make sense. What am I missing? Are there any other sensors or switches involved that could be affecting this? When they are working, the driver door makes them work and then there is the delay before they go off. When working, the passenger door makes them come on and off with no delay off. I assume that is the way it is supposed to be.

Any thoughts from the brain trust here?

 

How 'bright' are the ambient light conditions when they don't work? There is a photo-cell on the panel above the fuel and trip buttons. Among other things, this photo cell is used for Courtesy Light control.

>> When they don't come on, try this to confirm that it is the photo cell causing them to stay off: Close both doors. Wait about 15 seconds or so. Cover the photo cell with your finger and open a door. They should come on.
>> When they do come on: Close both doors. Wait the 15 seconds. Shine a flashlight in the photo cell and open a door. They should stay off.

The photo cell influences the behavior at the time that the door is opened. The photo cell will not have influence once the door is open, or change the state of the courtesy lights. Ie if they are off and you cover the photo cell they won't come on. You have to close the doors and wait the 15 seconds or so for the photo cell to again have influence.

If the above tests pass, you now know what it is.

Another parameter you can confirm is the "Battery Saver" feature for the Courtesy Lights. Open a door. The Courtesy Lights must be on (photo cell). Set a 15 minute timer in your phone. Leave the door open. Come back after the 15 minutes, and they should be OFF.

The delay on driver side, and no delay on passenger side is correct and normal.

 

Cannot thank you enough. This was the piece I was missing. Did not know that photo-cell even existed. Each time I tried this before it was in the garage, sometimes with the garage door open and sometimes closed, changing the ambient light. Played with it this morning and it all works as you said. Again, many thanks for explaining this, and saving me from buying any new door sensors. Case closed!