PeachParts Mercedes-Benz Forum - View Single Post

mpolli · #5 07-27-2008, 05:53 PM

Wow, the entire schematic on ONE PAGE!! The good old days...

Quote:

Originally Posted by jfarwell

I also have continunity from post to post on the brake light switch when the pedal is pushed but it doesn't appear that the 12.x volts connects inside the switch. I think it's the switch but I'm not sure why in the heck I get a good ohm reading when the brake pedal is depressed but no voltage??

Hmmmm... I seem to remember someone saying "Also keep in mind that an ohm meter uses a tiny current compared to the bulbs so it is not necessarily a fair test of the switch."

First, if you want to verify that it is in fact the switch just jumper over the switch (connect the 2 sides of the switch) and the lights should go on. If they do then it is the switch. If they don't then it is not the switch.

Now to answer your meter question. To some extent it has to do with Ohm's Law. Ohm's law says (among other things) that as you pass a current through a resistance it "develops" a voltage across the resistance. So if your switch has a couple of ohms of resistance your meter would read that as good but the lamps would see that as a bad source of current. In practice another thing that happens is the switch has some low amount of resistance that your meter reads as good. But as soon as the load tries to draw significant current through it the resistance increases.

I better way to test a suspect switch or connector is "under load". What you do is put the volt meter (set for volts) across the switch (one probe on each terminal of the switch). With the switch off you should read 12 volts or whatever the battery voltage is. With the switch on you should read 0 volts. If the switch is bad you will continue to read 12 volts or some voltage above 0. (a very small voltage like .1 is OK).