Bogus lightbulb failure warning - '89 Volvo 240DL
I've got this really annoying warning light indicating a lightbulb is out which comes on every time I hit the brake pedal, yet, the lights are functioning properly.

I know there is a sensor unit designed to detect a lighting failure, I know what it looks like, but where is it located? I can't find it. The wiring in the underside of the 240 looks like a web made by a spider on acid.

Where is this lighting sensor located and can a replacement be obtained - or, can I just yank the bugger and be done with the warning light altogether.

 

itgs a round can a little smaller than a beer can, its under the dash on a 240, I think. you can't remove it, because the bulb circuits go THROUGH it. you can unplug the bulb in the dash, however.

the common reason it goes off is resistance at a bulb socket or connector, so the current is different between the left and right bulbs. always use chrome not brass base tail light bulbs, and always replace them in pairs. clean any connector contacts with a soft pencil eraser.

 

On my '92 240 the bulb out sensor is under the dash....kinda behind the steering wheel.

The location on your '89 may be different.

 

I have found that between the flex circuit in the tail lights and the bulb holder you get a slight resistance problem. Also between the bulb holder and the bulb. So clean that area and check to make sure the contact is in good shape. I also change the flex circuit if I see any wear issues. Never had a bulb sensor unit fail since I bought the car in 1989 and even with 372 K miles on it.

 

Just to build on what others have said...

Make sure all your contacts are clean and the bulb wattage is similar, left to right.

If that doesn't fix it and you're sure the bulb failure relay is the reason, you can very easily fix the relay itself if you're handy with a soldering iron. Just pull it apart, and re-flow all the solder that you see on the relay.

I had this issue and I had to clean the contacts first. The problem creeped up again and I ended up fixing my relay.

As a side note, re-flowing the solder on old relays is a good idea for preventative maintenance. The oddly-named radio suppression relay is one that comes to mind.

 

re relay locations on 240's, here...
Dave's Volvo Page - Volvo Relays

 

Take out the bulb, an inelegant but effective cure for slight voltage glitches.