Left brake light out, but not sure why

[mattaebersold]

This is a weird one. Let me describe the problem... the left brake light only is out. All other light work great. Bulb is good. Brake switch at the pedal appears to be good (otherwise both would be out I believe). Fuse should be good since it govern's both I think.

with both taillight bulb assembelies out of their holder...

• Used multimeter to read voltage on the metal lines going to the bulbs.

BAD Side (drivers)

• measured 12 volts when brake pedal was down on the metal line that goes to each side of the bulb housing
• for the metal line that goes to the tab UNDER the bulb, measured 0.02 volts when pedal was down

GOOD Side (passengers)

• measured 12 volts when brake pedal was down on the metal line that goes to each side of the bulb housing
• for the metal line that goes to the tab under bulb, measured 0.08 volts when pedal was down

Conclusions:

This is where I am stumped. Both sides are behaving very similarly, except for the metal line that goes to the tab under the bulb. There is a .06 difference, which tells me that the bulb on the drivers side isn't getting enough volts to illuminate. This is odd, since it's such a small voltage, and such a tiny difference.

• Grounding issue? like it's not tight enough? Where would the ground for this be, and that seems odd since a bad ground would affect more things than just one isulated brake light
• Loose wire? Could this be the reason that it's not getting enough volts?
• problem with the housing itself? like the line isn't sending a strong enough signal to the bulb? (housing looks to be in perfect shape, but I don't know about the soldered areas that I can't see)

Any help would be GREATLY appreciated. Thank you.

[Loren]

These types of problems (especially on Boxsters) are usually caused by bad grounds.

[Ahsai]

Use two separate alligator clip wires to bring the metal line/tab out to connect to the bulb (May need an assistance here to connect the alligator clips to the bulb). Then measure the voltage on each metal part on the bulb. Suppose the bulb does not light up, then if the measurements are close to12v, you have a bad ground. If they read close to 0v, you have a bad 12v supply line.

Be careful not to short the contacts inside the bulb socket.

[Ahsai]

Actually there's a faster way. Just use one alligator wire to bring out the 12v tab under the bulb, then connect it to the bulb, then use a separate known good ground to connect to the bulb. If it lights up, your ground in the bulb socket is bad.

[mattaebersold]

These types of problems (especially on Boxsters) are usually caused by bad grounds.

Would there be any suspect grounds that relate to this? like in the trunk area, etc?

[mattaebersold]

Use two separate alligator clip wires to bring the metal line/tab out to connect to the bulb (May need an assistance here to connect the alligator clips to the bulb). Then measure the voltage on each metal part on the bulb. Suppose the bulb does not light up, then if the measurements are close to12v, you have a bad ground. If they read close to 0v, you have a bad 12v supply line.

Be careful not to short the contacts inside the bulb socket.

Thanks for this, I will try that. The odd thing is that the tab that the bulb sits on has a reading of .02 volts, and the good working side reads .08v. The metal tab surrounding the bulb itself (and touches the side metal case of the bulb) reads 12v exactly on both sides.

The thing that's stumping me is that I would think there would either be 12v or 0v on both... not .08 on the good side and .02 on the bad side. Is there a low voltage line going in to the bottom tab? would a .06 discrepancy cause a bulb to not have enough juice to illuminate?

[Ahsai]

Your test was not conclusive because

1) the 0.0x volt you measured could be the voltage on a floating wire that is not connected to anything. Unless you connect the red probe of the voltmeter to battery 12v and the black probe to the metal you want to measure. If you get 0.0x volt, then you know at least that metal is connected to the ground (although still may not be a "good" ground. See 2)

2) presence of voltage is not enough to conclude the bulb will see the exact voltage. Let's say you connect a 10k ohm resistor to the battery and measure the other end of the resistor, you will see 12v. But now if you connect your bulb to that 12v source via the resistor, your bulb will not light up because it won't have enough current going through it. The voltage the bulb sees will be close to 0.

In other words, measuring the voltage without the load is not conclusive because once the load is connected, the voltage may change (e.g., due to high resistance on the positive side, or negative side, or both)

Edited April 27, 2014 by Ahsai

[Hilux2400]

I had a similar problem with the fog light. I was getting a reading with a multimeter but the bulb would not illuminate. I tracked it down to a corroded connector in the multiplug. There was enough current passing to give a reading but not enough to light the 21w bulb. I used contact cleaner on the connectors and now it's OK.

H