Lighting Mess
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Elder User
Joined: Mar 2008
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Lighting Mess
im trying to rewire my taillights and its not working too great(just to let you know, im not positively sure how great they were b4 since this is not driven alot) i got a diagram off of autozone and wired everything up like its shows....heres what i have
-all lights up front
-passenger side brake light until i turn on headlights, then nothing
-no driver side brake lights, but taillights, brightens just a little when taillights are on and brake is pushed
-all blinkers(pass. side slow)
what is up?
-all lights up front
-passenger side brake light until i turn on headlights, then nothing
-no driver side brake lights, but taillights, brightens just a little when taillights are on and brake is pushed
-all blinkers(pass. side slow)
what is up?
Post Fiend
Joined: May 2006
Posts: 10,007
Likes: 4
From: Central Florida
I'd advise braking out the DMM and metering the wires. Write down each wire color, then meter them all when the tail lights are on. Turn the tails off, and have someone press the brakes. So on and so forth, until you have voltages for all the wires. This is important, if you don't get near 12v on them, you need to look forward to the switches and harness for trouble. Do this without the bulbs in place and the ground on DMM to a good ground.
Pass tail sounds like the brake lights are grounding on the headlight circuit.
Driver's side, the tails and brake lights are supposed to be independent filaments. Maybe a bad ground, or a bad bulb.
Pass tail sounds like the brake lights are grounding on the headlight circuit.
Driver's side, the tails and brake lights are supposed to be independent filaments. Maybe a bad ground, or a bad bulb.
Fleet Owner
Joined: Aug 2005
Posts: 23,667
Likes: 301
From: Easton,Ks
Sounds like you have a bad grounding problem.
Make sure that both tail lamp sockets are grounded to the body, do not use the frame as it may not be grounded. If the tail lamps do not have a ground wire then run one from where they are mounted to the body.
Make sure there is a ground wire (10/12 GA) from the Neg (-) battery post to the body.
Use a test light that uses a bulb for testing these lighting circuits.
A DMM will not load the circuit and will give false readings and you will be at it all day long and not get it fixed.
Make sure that both tail lamp sockets are grounded to the body, do not use the frame as it may not be grounded. If the tail lamps do not have a ground wire then run one from where they are mounted to the body.
Make sure there is a ground wire (10/12 GA) from the Neg (-) battery post to the body.
Use a test light that uses a bulb for testing these lighting circuits.
A DMM will not load the circuit and will give false readings and you will be at it all day long and not get it fixed.
Post Fiend
Joined: May 2006
Posts: 10,007
Likes: 4
From: Central Florida
I thought that only the turn signal flasher needed load. You are mostly looking for a change in polarity on the wires to indicate their function. If you have insufficient current to trip the bulbs, but still enough to show voltage, then you know you lack current. It maybe because we were discouraged from using test lights in school, but they don't tell you much other than your getting a little voltage and a little current.
If your DMM can handle 2amps or more you should be fine. The bulbs should only draw 500mA. Plus this will tell you exactly how much current you have traveling the circuit.
I would also recommend the DMM because if you get the resistance of the bulb, make the denominator, and 12 (for 12 volts) the numerator, the result is correct current the bulb requires.
If your DMM can handle 2amps or more you should be fine. The bulbs should only draw 500mA. Plus this will tell you exactly how much current you have traveling the circuit.
I would also recommend the DMM because if you get the resistance of the bulb, make the denominator, and 12 (for 12 volts) the numerator, the result is correct current the bulb requires.
Fleet Owner
Joined: Aug 2005
Posts: 23,667
Likes: 301
From: Easton,Ks
I thought that only the turn signal flasher needed load. You are mostly looking for a change in polarity on the wires to indicate their function. If you have insufficient current to trip the bulbs, but still enough to show voltage, then you know you lack current. It maybe because we were discouraged from using test lights in school, but they don't tell you much other than your getting a little voltage and a little current.
If your DMM can handle 2amps or more you should be fine. The bulbs should only draw 500mA. Plus this will tell you exactly how much current you have traveling the circuit.
I would also recommend the DMM because if you get the resistance of the bulb, make the denominator, and 12 (for 12 volts) the numerator, the result is correct current the bulb requires.
If your DMM can handle 2amps or more you should be fine. The bulbs should only draw 500mA. Plus this will tell you exactly how much current you have traveling the circuit.
I would also recommend the DMM because if you get the resistance of the bulb, make the denominator, and 12 (for 12 volts) the numerator, the result is correct current the bulb requires.
If you are very good with a DMM and you get 11.5 Volts when you know there should be 12.0 Volts there, then you know you have zero volts.
But most people that are new to working on trucks do not know this and will take it that there is power there when there is not.
We lost a full day of work on a system because the ones in the know used a VOM and it read voltage on a circuit that was dead.
When I came to work on the next shift I used a wiggens and found the problem in a few minutes. I was working as an ET at the time.
If you use a test light then it will not light in this case but the DMM will show voltage from capacitance (a wire laying beside another wire) or a bad connection.
I am not saying a DMM is no good as I have about ten of them but they have their place and the lights and the fuel system on trucks is not the place for them. They need to be used in the charging system and around the computer and so on but not where the system needs to be loaded.
Fleet Owner
Joined: Aug 2005
Posts: 23,667
Likes: 301
From: Easton,Ks
Is this the 1978 Ford truck you are working on?
If do you need any circuit diagrams?
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Fleet Owner
Joined: Aug 2005
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From: Easton,Ks
I do not know if one of these is close to the 78 or not.
Fleet Owner
Joined: Aug 2005
Posts: 23,667
Likes: 301
From: Easton,Ks
Note that the frame is not a good ground unless you make it a good one by running an cable to it from the battery.
Moderator
Joined: Jan 2001
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From: Virginia
The most simple circuit to get working first is the running lights, along with the license plate lights. All of these are fed with a brown wire, and this comes from the headlight switch. I would get these working first.
The brake/turn lights are a little bit more complicated, and if you cannot seem to get them working, then you may have steering column/turnsignal switch problems.
P.S. You can use a DMM or digital voltmeter on lighting circuits IF, you leave all the lights plugged into their sockets. Again, this is to keep a load on the circuit.
The brake/turn lights are a little bit more complicated, and if you cannot seem to get them working, then you may have steering column/turnsignal switch problems.
P.S. You can use a DMM or digital voltmeter on lighting circuits IF, you leave all the lights plugged into their sockets. Again, this is to keep a load on the circuit.
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Elder User
Joined: Mar 2008
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i tinkered around somemore and this is what i have
-taillights work
-pass. brake light did work, but blinkers didnt so i messed around with the turn sig. cam /switch and the brakes nor blinkers work
can you tell me which of these should be hot and when?
-taillights work
-pass. brake light did work, but blinkers didnt so i messed around with the turn sig. cam /switch and the brakes nor blinkers work
can you tell me which of these should be hot and when?