I was replacing my car's monovalve last night (to restore heat to my '91 420SEL - 126 chassis), and noticed a contact switch mounted more or less above the monovalve. This switch is visually the same as the door-jam switches that turn on the dome lights when a door is open. This particular switch operates when the hood is open.

I assumed that it was for an underhood light of some kind, but there is no light visible on my car, and I couldn't find a bulb or socket anywhere. Plus, when I look in various M-B parts catalogs and web sites, I see no reference to either an underhood light, nor to this switch.

Does anyone know what it is for? If it is for a light, where is the bulb supposed to be?

In my 126, this switch is located between the two firewalls, toward the center of the car from the windshield wiper fluid reservoir and battery area. It is also basically just behind the firewall, behind the transmission dip stick, and is mounted on a bracket that also holds a diagnostic port connector (as I said above, just above where the heating system monovalve is).

Thanks for your help! BTW, repairing the mono volve with the monovalve repair kit solved my lack-of-heat problem. Sure enough, the diaphragm on the monovalve piston insert was ripped, leading to highly intermittent operation. Since the monovalve is actually pulsing fairly continuously during most normal heating conditions, a sticking piston resulted in too little heat.

Bob Schleicher
bullswin5@aol.com
1991 420SEL, 174,000 miles