Questionhi dick,
to answer your question about the fuse block.i have 12 volts at the fuse block on both sides of the fuse until you press on the brake or you turn on the head light switch then it drops to zero.i have no voltage going to the back of the truck.so as you can see i cant check the voltage going to the back because everytime i turn on the light switch i lose all the voltage on both sides of the fuse.also i have already replaced the light switch and the results are the same.
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Followup To
Question -
hi dick,hope you can help.
i have a 64 chevy truck that all of a sudden by tail lights quit working.i have turn signals but no brake lights or tail lights.
i checked power from the fuse box and i have plenty until you depress the brake pedal or turn on the headlights then there is nothing.
any advice would be greatly appreciated.
Answer -
I'm a little confused as to what you mean about the power going away when you step on the brakes.
Let's back up here and let me come at it from another direction:
If you put your meter or test light on the fuse for the brake lights, you should find 12 volts on both ends of the fuse all the time, regardless of whether you are stepping on the brakes or anything else. If the 12 volts goes away at any end of the fuse, there is a problem.
The tail lights are on a separate fuse on most vehicles, but I'm not an expert on trucks so I'm not sure about your vehicle. Usually, if the turn signals work and the brake lights do not work, the problem is either in the turn signal switch (the brake lights are wired through the turn signal switch) or in the brake light switch itself.
The tail lights are on a totally different circuit, and use separate filaments in the bulbs, so there is no way one can affect the other unless something is wired wrong, or the grounding to the tail lights has failed.
To avoid confusion and extra work, let's fix the tail lights first, they are a much simpler circuit. Turn on the parking or headlights (either one should bring on the tail lights, as you know) and take your voltmeter or test light and probe the two terminals inside the bulb sockets in the tail lights. You should see 12 volts on one terminal, and nothing on the other terminal. If you do, and you plug in a known good bulb, and it does NOT light, you've lost the ground to the bulb. Probe the outer copper shell of the bulb itself, you will see 12 volts on it if it is not grounded. Ground it (to the chassis or bumper - any clean bright bare metal that is bolted to the chassis) with a temporary wire and your tail light bulb should light up now.
Go through this on both sides, and fix the ground problem. It can be either because the socket is dirty - the socket is grounded but the outer copper shell of the bulb is not making good contact with it - clean it up and try again, or if the socket is not making good contact with the light's mounting bracket, clean that up and make sure there is good metal to metal contact all the way from the bulb outer shell to the chassis.
Back up now: IF, when you probed the contacts inside the sockets, you did NOT see 12 volts on any terminal, there is a problem at the headlight switch or the wiring to the back end of the truck. Make sure that the fuse for the parking lights is OK, and making good contact with the fuse clips in the fuse box (these are famous for looking good but having a thin coating of oxide on the clips preventing contact). If you see 12 volts on both ends of the fuse, but not on one of the fuse mounting clips, that's your problem. Disconnect the battery and then clean ALL the fuse clips with fine sandpaper, then blow out the dust and try all the above again.
If tail lights are still not working, go to the next step.
Check to see that you have 12 volts to both of the power inputs to the headlight switch (B1 and B2). If you do, but not at the tail light terminal with the switch on, the headlight switch has failed. I doubt very much this is your problem, but it is possible.
All of the above should lead you to the cure for the tail lights failing.
Once that is solved, check your brake lights again. Make sure there is 12 volts to one terminal on the brake light switch when your foot is off the brake, and to both terminals your foot is on the brake. If you have that, and the brake lights are still not working, jiggle the turn signal lever while someone watches from the back of the truck to see if there is a position where one or the other or both of the brake lights come on. If you have to jiggle the lever to make the brake lights work, your turn signal switch has failed.
I'll stop typing now, and wait to hear from you before we take this any further - hopefully you have found the problem by now. Let me know, please - I'm not quitting on you, I just don't want to waste a lot of time talking about problems you don't have!
Dick
AnswerOK, I thought that might be the case. The fuse clips are not making good contact with the fuses. Disconnect the battery, remove all the fuses and clean the clip contact surfaces with fine sandpaper, and also clean up the fuse ends the same way. Blow off all the sandpaper residue, and I'm betting your problems are over.
Let me know, please. If that doesn't fix it, I'll have to come up with some other idea, but I don't know what it would be at this point.
From your report, we know that the source of 12 volts to the fuse is very high resistance, and the most likely place for that resistance to creep in is at the fuse clips.
Trust me, that is the case in most old cars, and it often shows up in wet weather.
Dick
