90's merc. Bulb indicator fault permanently stuck on???

[Nick12345]

Hi. Anyone got any ideas. My bulb indicator bulb is stuck on the dash without turning the lights on. Before if a bulb blows it doesn't flag a fault until I turn the lights on. Now it is permanently stuck on day or night??

[Baldchap]

Relay fault or dodgy earth gets my vote.

Or a brake light/driving light? Have you actually checked the lights?

[bolide]

The bulb fail relay can be super-sensitive. It works on the Wheatstone Bridge principle so compares one side of the car to the other. I you have a 5W bulb on one side and a 10W bulb on the other it may flag that up

In your case it may be a bad solder joint in the relay

[PositronicRay]

Dry joint is a good call.

Before you go down this route, try re-seating contacts (instrument cluster and lighting relay) with some switch/contact cleaner.

I've solved loads of annoying glitches this way, and keep a can of Servisol in the car.

[Panamax]

Some fault detection circuits can work even when the bulb isn't illuminated.

If no bulbs have blown (and assuming nobody's fitted aftermarket LEDs which these systems hate) then it might be oxidation on the central solder contact(s) on the back of the bulb. Remove bulb, polish the solder with emery paper and replace. Check also that the contact in the socket looks clean. If not, get in there with a cotton bud dipped in methylated spirit.