I thought i already posted this here, but my comp screwed it up i think..

Ok, my top mount brake lights are not working, so i thought maybe it was the bulbs themselves, so i went to autozone and brought two new ones. well i pulled the cover off and the bulbs looked fine, but i recplaced them anyways. NOPE still didn't work. So i get my digital multimeter and test the voltage. 11.5 volts when brake pedal pressed. 13 volts when car is on BUT,, this is with the bulbs OUT. i put the bulbs back in, tested the voltage, probbing the SAME place, ZERO volts.

it makes no since, so i take out just one bulb, and leave one in. still ZERO volts, i don't get it. when the lights are in, the voltage goes down to zero, take them out, it goes up to 12.

wut is causing this? its so confusing

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- David
88' GTA 5.7L TPI MODS---> air foil, K&N, Shift Kit, 180* therm, TB bypass, Gutted CAT, Flowmaster 80 Series

http://www.geocities.com/david_angel_16

 

Bad connection (high resistance) somewhere in the wiring leading up to that point... enough electrons can squeeze through to make a 10 megohm DMM read "normal", but nowhere near enough to light a 120 ohm light bulb

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"So many Mustangs, so little time..."
ICON Motorsports

 

That high resistance connection could also be in the ground path, so don't overlook the ground wires for the rear hatch. Some years were equipped with ground connections on the rear hatch struts, and some were on the contact block near the latch. Those contact blocks can be problems, so make sure the contacts and plates are clean:



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Later,
Vader
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If you want to beat the World, it might reach up and pull you down...
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Ok, i cleaned the contact terminals, like the ones shown by vader, im still getting 11.81 volts on the base connector. however, i measured the resistance of the two outer terminals on the upper connector, and it was 800,000 ohms!! however the resistance of one bulb was .1 ohms. so could my problem be in the upper part of the thingy?? could someone measure the resistance for me of the outer upper terminals? this is gonna suck replacing that upper part if thats the problem.

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- David
88' GTA 5.7L TPI MODS---> air foil, K&N, Shift Kit, 180* therm, TB bypass, Gutted CAT, Flowmaster 80 Series

http://www.geocities.com/david_angel_16

 

Breath,

In theory, the resistance between the two contacts in the lamp sockets should be infinite, but 800,000 is close enough for 14VDC systems. One of those contacts should be the ground terminal, and one should be the power supplied by the switch on the brake pedal.

The ground contact can be metered to a known good chassis ground to verify continuity. The power contact could be metered back to the brake pedal arm switch if you have meter leads that are about three meters long.

Instead of that, install a bulb in one of the sockets and check the voltage at the switch on the pedal arm. The switch has two pairs of contacts. One set is used for the service brake lamps and is routed through the turn signal switch. The other set of contacts is routed directly to the CHiT (Center High-mounted Tail) light. There may be other connectors between the pedal arm switch and the socket, so there are more potential places for poor contact. Measure voltage on both sides of the switch while depressing the brake pedal with the bulb installed. If you lose voltage at the switch when the switch is closed, you may have a problem at the fuse panel as well.

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Later,
Vader
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If you want to beat the World, it might reach up and pull you down...
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