In the previous article, auto appraiser Jason Phillips noted that once you confirm power at the jamb button switches and their functionality is correct, it’s time to move to the rear. In the subject car, a 1979 Pontiac trans Am, we opened the trunk, and look to the driver’s side wheelhouse. Just beyond it and above, is the wiring harness to the taillights. It’s in this same location on many GM 70’s cars. The dome light plugs in here. It’s a two white wire connector, black end. Unplug and test. Ground one side of your tester. You should have 12 volts on one side, doors closed. when a door is opened, you should have power on both leads. If it tests out, then check the continuity to the dome light itself. With the bulb in, it should have continuity. Obviously, you have made sure that your bulbs are all good first! Always Always address the cheap and easy first, right? That’s it. That’s the whole circuit. In our particular case, we purchased an aftermarket plastichrome plated housing and harness from NPD Parts Warehouse, and it was poorly made. The chrome coating was grounding out against the steel posts in the roof of the car that the housing slides up onto. Our headliner began to smoke when we put in our light bulb! Thank GOD for quick ground disconnect switches!

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In your older GM car, are your courtesy lights stuck on? auto appraiser Jason Phillips notes that most GM button switches located in the door jambs, used to break ground to the circuit operate the same way. The switches are threaded into the cowl jamb post, and are spring loaded. Often, the left switch is a two wire, one side to the courtesy lights, and one to the ignition buzzer. Both are supposed to be hot leads, so do not be confused, thinking that pushing in the switch will cut power to the other side of that switch. Most switches on the passenger side are only a one wire connection. You can un-screw the switch and pull the wire spade connector right through the hole on either side. You don’t normally need to remove the kick panel to remove the switch. Pull them out, un-plug them and test switch for continuity. Use an electric wire wheel to clean them up while their out, freeing them of decay and possibly old repaint build up. Where the wire spade connectors are now hanging out of the holes: if you have 12 volts at all three spades, and both switches check out for continuity by breaking the circuit when they are pushed in, then you should rule out those items as the source of stuck on or off dome light problem. The follow up diagnosis is posted later in this blog.

Technorati Tags: Classic car chat, Pontiac, Techinical