Fuses
02-28-2005, 11:13 PM
#1
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Fuses
My trailer fuse under the hood keeps on popping. I know that it only kills the trailer brakes and doesn’t affect the lights but I have replaced the brake box and it still pops. I have checked the wires over and over again and haven't found anything wrong. I pull several different trailers and get the same result. Do any of yall have any ideas or had the same problem? I need to get the fixed ASAP b/c I'm pulling everyday and the no brake thing is not very safe. 
03-01-2005, 01:44 AM
#2
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Get an OHMS meter that will give you a readout in milliohms. Then measure the resistance of the brake terminal of your trailer to ground.
Apply ohms law with the nominal voltage of 12.6 volts or if you like the peak charging voltage of say 14 volts.
Ohms law is amps = volts / ohms
If you have 1 ohm of resistance at 12.6 volts you will have 12.6 amps when the voltage is applied.
If you have .5 ohms (1/2 ohm) then you'll have 25.2 amps.
This is the theroetical maximum current if you connected the brakes directly to the battery. In practise, the voltage is connected resistively to the trailer brakes to allow foir a varying amount of braking. The amount of current determines how much braking force you have with the maximum current being applied if the brake controller applies all of the available voltage (I.E. it's resistance is zero) to the brake terminal. The amount of current is determined by the amount of resistance of the braking controller in relation to the resistance of the trailer brakes themselves. (I don't know what the trailer brakes should read resistance wise and would depend on how many wheels of brakes you have.
Mechanically the brakes consist of an electromagnet which is on the end of a lever connected to a cam. When the electromagnet is energized, it is attracted to the inside of the drum. When the drum rotates it drags the electromagnet which actuates the cam which forces the brake shoes into the drum.
These electromagnets will wear down and eventually you'll weardown to the coil which causes a short which blows your fuse. For this reason, the test above may not be valid since without energizing the magnet the short wont show up.
The only way I know of to find out is to pull off the drums and inspect the electromagnets.
Edwin
Apply ohms law with the nominal voltage of 12.6 volts or if you like the peak charging voltage of say 14 volts.
Ohms law is amps = volts / ohms
If you have 1 ohm of resistance at 12.6 volts you will have 12.6 amps when the voltage is applied.
If you have .5 ohms (1/2 ohm) then you'll have 25.2 amps.
This is the theroetical maximum current if you connected the brakes directly to the battery. In practise, the voltage is connected resistively to the trailer brakes to allow foir a varying amount of braking. The amount of current determines how much braking force you have with the maximum current being applied if the brake controller applies all of the available voltage (I.E. it's resistance is zero) to the brake terminal. The amount of current is determined by the amount of resistance of the braking controller in relation to the resistance of the trailer brakes themselves. (I don't know what the trailer brakes should read resistance wise and would depend on how many wheels of brakes you have.
Mechanically the brakes consist of an electromagnet which is on the end of a lever connected to a cam. When the electromagnet is energized, it is attracted to the inside of the drum. When the drum rotates it drags the electromagnet which actuates the cam which forces the brake shoes into the drum.
These electromagnets will wear down and eventually you'll weardown to the coil which causes a short which blows your fuse. For this reason, the test above may not be valid since without energizing the magnet the short wont show up.
The only way I know of to find out is to pull off the drums and inspect the electromagnets.
Edwin
03-01-2005, 08:03 AM
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dodge4x4, mine has done the same thing thing to my boat trailer lights once or twice. Everything/wiring checks out solid - who knows. Thats another good reason to do a thorough walkthru before pulling. I also carry extra fuses onboard just in case.
You gotta love chasing down trailer wiring gremlins.
You gotta love chasing down trailer wiring gremlins.
03-01-2005, 09:32 AM
#4
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Sure sounds like your problem is in the truck itself. Have you checked the plug itself for a possibility of a crack or frayed wire on the brake line? If it happens with multiple trailers, it should not be the trailer. What controller do you have? If you have a Jordan, monitor how much current is being drawn when the brakes are applied. What is the voltage at the plug when you apply brakes with no trailer attached? My bet is a short in the truck lines somewhere.
03-01-2005, 06:34 PM
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While I hate to admit it, I agree with BrakeSmartLess Bob (FiverBob). However I have a slightly different theory. Over time, moisture can build up in the truck's trailer plug and cause all sorts of corrosion problems. Take it apart, clean it and reassemble everthing with dielectric grease.
CAsey
CAsey
03-02-2005, 12:50 AM
#7
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Originally posted by DODGE4X4
I replaced the plug and about 90% of the wires and that seems to have fixed it. Thanks for the help on this guys.
I replaced the plug and about 90% of the wires and that seems to have fixed it. Thanks for the help on this guys.
Edwin
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