I’ve been there with our C class and think I may be able to help, if you haven’t already figured it out. It sounds like you know how to bring the various light groups on sequentially so you know generally where to look for the problem. Assuming that all the lights really do light up but the light warning light is still on, the problem I had was barely perceptible corrosion between the bulb and the socket. The light with corrosion lights up but trips the bulb out warning light. I used rubbing alcohol to clean the contacts and it was fixed. I’m sure it applies to all the bulbs, but for me personally, it’s always been the brake light bulbs, especially the center one. Cheers and good luck.

__________________
Michael

1988 300 SL (5 Speed)
1994 E320 Wagon
1997 C230

thanks for that.

i will clean the contacts as we are having some really damp weather here in england ,u.k.

i notice that one of the reverse light bulbs has a faulty connection and comes on after i tap the light assembly from the outside-but this does not trigger the warning light as far as i am aware.

my car does not have a centre high level rear brake light.

Figuring out which lamp of many on a car is the culprit can be challenging. To help narrow your search, make careful note of when the indicator light comes on. There are 4 different possible situations that will trigger the indicator light - 1) Parking lamps - these are the lamps that illuminate when you turn the headlamp switch to the first detent. 2) Headlamps - lamps that illuminate when you turn the headlamp switch to the second position (I believe this would be confined to just the left & right headlight only). 3) Brake lamps - only on when you depress the brake pedal. 4) High-beam headlamps - only on when 2 above is true and you select high-beams from the turnsignal stalk.

When exactly does the indicator lamp come on? When you switch on the parking lamps, or not until you turn on the dipped headlights?

The answer to this question will point you in the direction of the solution.

If the indicator comes on when you switch on the parking lamps, then the problem lies with one of the bulbs in that circuit. Front parking lamps, rear parking lamps, license plate lamps, daytime (city) lamps - if car is so equipped. Some cars also have lamps on the front fenders or possibly on the exterior rear-view mirror housings - don't forget those.

If the indicator comes on after you switch on the dipped headlamps, then the problem is with one of those two lights.

If the indicator comes on when you step on the brakes, then the problem is in one of the bulbs that provide brake lamps in the rear.

Note that the system is sensitive - other posters have noted issues with corrosion, which is a likely cause if present. On my car, the PO wasn't too careful when replacing bulbs. I found the wrong size bulb in one of the parking lamps in the rear - the base fit fine, but it wasn't the right wattage (although I could see no discernable difference when viewing the car from the rear). That fixed the indicator that lit when I switched the parking lamps on. However, I still got a light when I stepped on the brake. All bulbs in the rear lamp assemblies were verified to be the correct wattage and no corrosion was present. Finally discovered that the PO had replaced the high-mount stoplamp bulb with the wrong size.

Be persistent and logical in your approach to the problem and you'll find your bad bulb/connection.

thanks for your detailed stepwise and logical approach.

the warning light comes on when the parking lights are switched on by turning the headlamp switch to the first detent-so i gues it narrows it down to the front parking lights ,rear number plate lights,rear parking lights but since all these lamps are working and it is a new fault,then the most likely culprit is corrosion.

Hey you’re almost there… I’d start by checking the easy ones, the rear light assemblies. Do you see how to get at the bulbs you want to check? Often, to break the corrosion, all it takes is the friction of unscrewing and then re-screwing the bulb. I try to use a clean, soft cloth to keep my greasy prints off the bulbs.

__________________
Michael

1988 300 SL (5 Speed)
1994 E320 Wagon
1997 C230

Hi there,

I am having a similar problem. The exterior light warning is lit all the time, even when no exterior light is used. Just turn on the ignition key and taa daa it is on and remains on.
I checked all bulbs and found no burnt one. Perhaps I need to check all of them now for corrosion. It is much work when you cannot narrow down which ones.

Have a nice day!

Tex, I’m totally unqualified to say this, but if it were me, I’d start with the brake lights. Does the warning light come on before or after you put you foot on the brake? For me it’s always been the center, high-mounted one. Your owner’s manual has a diagram that illustrates which bulb is which and how to access them.

__________________
Michael

1988 300 SL (5 Speed)
1994 E320 Wagon
1997 C230

i finally tracked down the fault-one of the rear tail light bulbs had blown and was causing the dash warning light to trigger on.

i replaced both tail light bulbs-(5 w bullbs) as the other non-faulty one looked corroded and everything is back to normal.

i like happy endings!!