I have a '90 300E that keeps blowing fuse #3 which is for the dash lights, controls lights, right side parking lights, and headlight wiper motors. The first time it blew was at night about 1/2 way between work and home. I replaced the 8amp fuse and it was ok for a couple of days before it blew again. Once again at night on the way home. Since then, when I replace the fuse, it blows as soon as the parking lights are turned on. When it blows I lose dash and console lights, right side parking lights, and the door buzzer.

The wiring diagram would seem to indicate this is most likely one of three things: 1) Light switch; 2) Headlight wiper relay; 3) lighting control relay (the $180 baby). The problem is that I can't find a listing anywhere for a "headlight wiper relay" so I don't know if this is actually the interior light relay (which apparently works fine) or the

I'm thinking it is most likely the light switch. Opinions ? Other ideas ?

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Andy_M
'98 W140 S320 Black/Black
'06 W164 ML500 AMG White/Grey
'03 Porsche 986 Midnight Blue/Grey

Light selector switch seems quirky. Also sounds like you could be shorting/grounding out, try to find the switched 12V contact on the light switch that causes the blown fuse and with the battery disconnected (and a new #3 fuse) check for continuity between ground and the 12V contact. Then you have isolated that it is shorting out. Post back and I'll see if I can find some good isolating troubleshooting points to test from there. Seems that because it was an intermittent problem at first then continuous, a loose wire shorting out is a rational possibility.

There are simple diagnostic techniques used to find full time shorts. The first is to identify the circuit and divide it untill the suspect is found. It helps to have some tooling. An interesting concoction can be made with some simple pieces. Wire a high wattage bulb through a two prong VW flasher (cheap doesn't need to be VW, domestic cars also use them). Hook the device between the fuse leads such that it flashes the light.

A compase following the harness with be distrated every time the field is made or broken. Without an inductive ammeter thats the best I can do.

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Steve Brotherton
Continental Imports
Gainesville FL
Bosch Master, ASE Master, L1
33 years MB technician

Looking through the wiring diagrams, I felt like the light switch was probably the most logical source of a short. Three of the four circiuts that feed through that #3 fuse come out of the light switch. So I ordered a new switch from Autohauz. I received it on Friday and put it in yesterday. That is where the problem was located. All is fine now.

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Andy_M
'98 W140 S320 Black/Black
'06 W164 ML500 AMG White/Grey
'03 Porsche 986 Midnight Blue/Grey