Which fuse is for the brake lights? I only have 7 volts at the brake light switch, should be 12 volt at switch?I have turn signals but no flashers or brake lights. There is only 7 volts at emergancy flasher also. Anyone have a clue where I'm losing power or what to check first?

 

1. Check your battery

2. Load the circuit and then check voltage. You can still have 12 volts on a teeny tiny strand of wire with no load, but once you load it down you'll actually see the resistance.

3. I'd say you've got a voltage drop somewhere before the split to the hazards and BOO switch. Visually inspect your conductors and then start at one end, and eliminate pieces of suspect wire until you find the cause of the voltage drop

Your fuseblock isn't labeled? I'm not by my truck right now otherwise I'd tell ya

 

Sorry Rob, if I wasn't out and about I'd open my chiltons or Haynes and tell ya.

 

I read that the brake light fuse is the same one used for the emergancy flashers,I have 12v power at both sides of fuse. I have 12v at both sides of flasher untill I turn the emergancy flasher switch on, then it drops to zero volts. I got a short I take it? Where does that wire run to after the flasher switch?

I can't save my life when it comes to electric, very green.
Ps whats a BOO switch?

 

If there is minimalvoltage at the circuit then I suspect something other than fuse. It can be in the turn signal switch, the brake and 4 ways go through it. I have seen them go bad. check the flasher unit and the harness as well. Make sure that the big plug under the dash is right as well.

 

I have no clue on how to check or trace down voltage drop. I need like a step by step process explained for 10x dummies. Like check ;green wire at switch then goto this wire ,check: something like that. Simple,simple terms in detail.

Sorry for being SO GREEEN!

I have running lights,turn signals,all instrament lights and all gauges work,just no emergancy flashers and brake lights.
Emergancy flasher tested in turn signal spot and works, connections cleaned and greased under hood and colum conections all look good and clean.
I have 12 volt coming in at break light switch and leaving switch with red/black stripe wire disconnected(break light switch wires are cut and respliced at switch). If I disconect the red/black stripe wire from switch it shows 12 volts coming out of switch with pedal pressed but if I hook up red/black stripe wire it show no volts with pedal pressed.
Where to look next downline(spacific wire) ???
I hope I'm making sense. probably not!

 

I have 12v on red/black wire at base of steering wheel conection then wire turns color into a fat lime green wire that runs up to emergancy flasher switch but it seems to stop there at big lime green wire to emergancy flasher switch. Is there a way to bypass this switch or do I need to buy a whole turn singnal switch or ?/??/
I have 12v at red/black wire at base of steering colum with turn signal plug not attached but it won't light a extra bulb I hook to it????

 

Now I lost dash lights.They where working before all this.

 

My lights did the same thing. It got worse as I was searching for the issue. I cleaned all the grounds and used dilithium grease. I know I need to replace my turn signal switch and possibly my light switch as well.

 

Sorry, BOO switch is a brake on/off switch.

Anywho, I'm guessing you had the red lead of the meter on one side and the black lead on the other? You should have 12 volts when you do that if the switch isn't on.

Here's why: Your meter is only telling you the difference in voltage between the two leads. That's why when you touch your battery, it says ~12.6 volts, the positive terminal has 12.6 volts more than the negative terminal.

If you don't have the switch on, then there is no way for voltage and current to flow through, so if you have the red lead on one side, that will have battery power and the black lead on the other side will be at ground potential. Because you aren't completing the circuit, there is a difference of B+ voltage. This means that your voltage drop is B+ or 12 volts. When you engage the switch, the circuit is complete. If the switch is good there should be NO voltage drop (voltage drop = work done, there shouldn't be any extra "work" or "effort" done at the switch. A tenth or two is normal, nothing is perfect (your wife my beg to differ).

This "completing the circuit" is why you see 0v when you press your pedal. Try putting your black lead at ground potential. A painted surface is a no-go, you need bare shiny metal. Try scratching the paint off the bolt on your door hinge with the pointy lead. Now connect it to both sides of the brake light. On the hot side, you will always see 12v or B+. On the switched side, you will see 0v until you press the pedal, and then your are completing the circuit and then you will see 12v or B+.

The reason your bulb won't light up is there is no ground potential. Without ground, the circuit is "open" and it will not do work. Think of it like a circle track. If there is no return to the start (ground), then no one is doing any laps (work).

Now, with this in mind, with red lead on the switched side of the BOO switch and the black lead at ground (shiny metal), do you have 12v when you press the pedal down?

With red lead on RED/WHITE (aka red with white stripe), black lead at ground (shiny metal) do you have 12v all the time? This feeds BOO & hazards

 

I have been checking volts with a multimeter with red on power, black on good shiny metal ground from the begining. There is no red/white wire,mine is red/black.

I have 12v going into boo at all times.

I have 12v coming out when the pedal is pressed.

I have 12v at red/black wire at base of column when pedel is pressed only. This red/black wire turns into fat lime green at steering column.

 

I have running lights.
I have turn signals on all four corners and arrows work on dash BUT blinkers stop working when emergancy flashers are turned on.

I don't have brake lights. I have 12v at both sides of fuse marked 20a emergancy
I have 12v at emergancy flasher plug with flasher plugged in UNTIL I turn on emergancy flasher switch on column, then volts drop off.
I don't have emergancy flashers.

I pulled the turn signal switch and cleaned and dielectic grease switch and connecters,all look good with no chaffing or burnt.

 

Sounds to me like you have a short, not bad enough to blow the fuse. You will have to follow that wire out of the brake light switch where ever it goes, you will probably find it broken or burned off. Be aware while checking those wires, mechanics just love to stick their testers into the insulation making a hole , then they don't bother to tape up or seal the hole. I just changed the fuel pump on my wife's 2002 screw only to end up chasing wires , a pin hole that corroded the wires til it broke was the problem. I spent 2 frigging days figuring it out.

 

75'-78 wiring code
Wire colors are for steering column & turn switch.
Yellow/w/black stripe -- out to left REAR turn
Green out to right REAR turn
Green/w/white stripe --out to left FRONT turn
White/w/blue stripe -- out to right FRONT turn
Red/w/black stripe -- input power_from stop lamp switch. (on break pedal assembly)
White/w/red stripe -- input power_from hazard flasher
Blue -- input power_from turn signal flasher

Maybe someone else could verify ..
I can't remember if you have a 79 cab on 78 chasis or the other way around

 

Thanks for the replies! still working on it ????

79 cab on 78 chassis

All of my blinkers work good.