4x4jeepmanthing

hey all im trying to get my brake lights back. its a 99 auto

I have pulled out the brake stop switch and tested the pink/blue trace in which has its constant 12v supply.
whit and tan also is supplying 12v until the switch gets pressed in then 12v is cut out. this white wire is sending 12v to the rear of the vehicle and all other lights work, just not the brake filament.

there are three wires to the brake lights, driver side example-black( ground), Brown ( fused left inboard tail lamp) i get nothing on this wire with switch pressed or not, white/tan (brake lamp switch output) again 12v is here when supplied by driving light switch.


the brake stop switch ohms good. the light bult is plastic surround with two metal pins, im not sure how to make the other brake filiment glow here- again driving filament works when 12v runs through.
all fuses are good.
Im a little stuck on what im looking for now guys?

lawsoncl

That white/tan wire runs through a few connectors along the way through the body, and arrives at the brake lights as a white/tan wire. Does that have voltage when the brake is pressed (and the switch is unpressed)? You tried a new bulb yet?

BlueRidgeMark

Check it right at the place in the socket where it contacts the bulb. Check it with a test light, not a meter. Trust me on this.


Oh, come on, Lawson! Do you think he'd overlook something as obvious as that????

Yup, he would. and we've all done it!

4x4jeepmanthing

Lol, no new bulb yet, I did try swapping them for the hell of it. New bulb will be tried, probably should have spares anyway

awg

use a test lamp

does it light at the brake switch when pressed ?

does it light at the rear socket when pressed ?

If yes then no, the harness is bad somewhere, when I pulled some wires at the rear connector on one of mine, I actually screamed...all the wires had cracked insulation

find the 12V, correct any deficiencies, re-wire if needed

Dave51

Why is that?

awg

a corroded terminal might pass 12V...at a few mico-amps for instance..so your multimeter will say 12V

try passing 2amp, like a brake light, and it wont...a test light pulls much higher current

I prefer test light for 12V, but not electronics, (due to their low resistance)

Dave51

Well, it strikes me like you're talking about 2 different things:

1. Is there any power at all?
2. Does the power drop under load?

and IMO using a multimeter with save you a trip back to the toolbox (to get the multimeter).

1. Check to see if the bulb is good.
2. Check to see if there's power and how much.
3. Check if the power drops under load and how much.

4x4jeepmanthing

I first need to know where and when 12v should be present

awg

I am confused..had a quick look at a '95 brake switch diagram..appears there is more than 3 wires..one to TCM and PCM..they may be switch to earth..dunno ?

What I do know is all mechanical brake switches I have worked on switch +12V to the brake lights when the switch is either closed or open, (depending on the design)

therefore, when the pedal is actuated, the switch should close the circuit, allowing 12V to flow out of the switch, down the wire that is identified in the relevant FSM by color, to the brake lights

If this is not occuring, the switch is defective

BlueRidgeMark

You may see it drop to zero under load. A test light and a meter are both good tools. Both should be in your bag of tricks.

awg

On this point, I believe we have consensus

I use both side by side

for 12V auto electrical, (not electronic), I just find it faster and more certain to start with a test light when searching for 12V+

good example, under dash fusebox

For all other applications, I start with a multimeter

4x4jeepmanthing

Under load = vehicle running?

I have test light, digital and cheapo analog for visual.

boxburn

Under load meaning the circuit has a power draw and is under load. When checking for voltages at a connector you can easily find +12v but if you pull a current draw through it, it can drop to 0V.

I recently traced fault with tow wiring which had me stumped. At the relay I was reading +12v so at the time I thought the power source was good. It turned out that when you tried to pull power through the relay to power the tow lighting, everything died. In the end there was a bad connector under the van which had corroded causing the issues. The point being, under no load you can get false readings with a multimeter as they can't always show a true reading of a circuit under load. Had I of used my test light, I would have found the issue much quicker.

Hopefully that makes sense, it's 1:26am and I'm off.

BlueRidgeMark

No, not at all! You are right on the money!