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MG MGB Technical - Side Marker Light mystery

My right front side marker won't light. New bulb didn't help. Tested the circuit at the light socket and found I wasn't getting a current. Traced back and found a disconnected bullet connector which I reconnected and still no light. Here's the mystery.

I have current at the bullet connector. I tested the line from the bullet connector to the socket and the circuit is intact. In other words I know I have the end of the wire that leads to the socket. However, I still don't have any current at the socket. I'm testing it by touching the connector at the bottom of the socket with the probe from my voltmeter, the same way I verified the ciruit with the ohmmeter. And when I do the voltage test I have the other lead of the voltmeter grounded.

How can an intact wire not deliver any current? It seems so straightforward but it has me stumped.

Any ideas?

Dan
Dan Hiltz

It may be that the + wire is damaged. It will allow a voltage reading on your meter but it can't deliver enough amperage to allow the light to function. You could try running a bypass wire to the socket that is connected to a known good + source. RAY
RAY

Dan - As Ray suggests, you have a high resistance connection (damaged) somewhere in the line. A meter is a high impedance device, so it doesn't draw much current and cause a voltage drop at the high resistance point in the circuit. However when you put a bulb in the circuit, it draws a relatively high current through the circuit, causing the voltage to drop across the high resistance area. On our daughter's MGB GT, the high resistance was in one of the crimped bullets. You may have the same thing or the ground connection for the light fixture may be bad. I would start with the side light fixture - remove it and clean up the area where the screws go through the sheet metal - see the article, Electrical Connection Preparation in the Other Tech Articles on my web site at: http://homepages.donobi.net/sufuelpumps/ for information on how to make a solid connection to ground. If that cures the problem, you are home free. If not, you will need to work back along the feed line, checking each connection. I did this by using a long piece of wire that I could use to jumper around each length of wire in the circuit, each time jumping around the suspected connection. Cheers - Dave
David DuBois

This thread was discussed on 21/06/2009

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