PeachParts Mercedes-Benz Forum - View Single Post - Strange electrical problem--need advice

mpolli · #2 11-16-2008, 06:00 PM

In two years of trying to teach electronics over the internet I have learned one important lesson: I cannot teach electronics over the internet. However, like the moth drawn to the flame, I keep trying.

Now I cannot solve all the problems you describe but you do ask one specific question I can try to answer. The best way to explain is to use the water analogy. Electricity is electrons flowing through a wire, similar to water flowing through a pipe. Voltage is the pressure. Amperage is the rate of flow (I realize you didn't ask about amperage). You have a situation where you seem to have pressure one minute and then it vanishes mysteriously when you try to draw any current through the bulb. What you have is a "high impedance" connection somewhere "upstream" from the bulb (or a high impedance ground connection "downstream"). High impedance means too much resistance. As a way of explaining, picture a garden hose connected to a faucet, and on the end of the hose you have a sprayer nozzle. You turn on the faucet and let the hose fill up. Now you turn the faucet ALMOST all the way off. You go the the nozzle and just press it a tiny bit and water sprays at high pressure. But when you press the nozzle a lot the pressure dies. You have the same situation in your circuit. Somewhere between the battery and the bulb there is a high resistance (bad) connection. AND IT COULD EASILY BE A BAD GROUND CONNECTION. The electricity doesn't favor positive vs negative. It wants a complete circuit. My advice is leave the bulb in, then start measuring the plus and ground lines all relative to a known good ground. What that means is have the black lead on the frame. Trace the plus side until you find good voltage. If it always reads good the trace the ground side until you find 0 voltage.