Trailer brake voltage

[Barnbuilder]

The new trailer has the self adjusting brakes. But I still had to tighten them up even after the builder said they were adjusted when I picked it up. So not sure how it works.

 

[ModMech]

I have a 99 and a 15 F350. I have an enclosed gooseneck that I just put new brakes on. And I have a new 10 ton gooseneck. Neither of these trailers seem to stop well with either truck. I was checking voltage and at the junction box on the trailers I was getting 9 volts and at the axles 8 volts. Is there that much voltage drop? I can't get the brakes on either to lock up. The 99 truck has a P3 and the 15 has the factory controller.

Sir, THAT is a problem! The MAXIMUM allowed voltage drop on any power circuit is 0.5V on the power side and 0.25V on the ground side - all measured from BATTERY GROUND on the power vehicle.

So, if you measure the voltage at the receptacle on the truck with the cord off (open circuit) and get 12.5V, the LOWEST acceptable voltage at the wheel end of the brake power wire is 12.0v, anything less indicates a serious wiring issue - high circuit resistance.

You also need to measure the voltage on the GROUND wire/frame with all lights on and some trailer bakes applied to assure you don't get more than 0.25V showing, if you do get more then there is a grounding problem.

All the talk about having to measure amps, resistance etc is hogwash, although knowing the amps is handy few people have a meter that can handle direct readings of more than 10 amps and therefore it is not possible to do. And no, inductive meters are NOT good enough, but they can give you a relative idea of the amps.

 

[oliver1555]

Sir, THAT is a problem! The MAXIMUM allowed voltage drop on any power circuit is 0.5V on the power side and 0.25V on the ground side - all measured from BATTERY GROUND on the power vehicle.

So, if you measure the voltage at the receptacle on the truck with the cord off (open circuit) and get 12.5V, the LOWEST acceptable voltage at the wheel end of the brake power wire is 12.0v, anything less indicates a serious wiring issue - high circuit resistance.

You also need to measure the voltage on the GROUND wire/frame with all lights on and some trailer bakes applied to assure you don't get more than 0.25V showing, if you do get more then there is a grounding problem.

All the talk about having to measure amps, resistance etc is hogwash, although knowing the amps is handy few people have a meter that can handle direct readings of more than 10 amps and therefore it is not possible to do. And no, inductive meters are NOT good enough, but they can give you a relative idea of the amps.

I agree with, and have no problem with your statement concerning voltage, but respectfully disagree with your statement about amperage. Amperage is what does the work in the magnet. Most low-end multimeters will measure up to ten amps. By hooking it up in-line on each magnet, you can see the amperage draw. I lifted a couple of charts from Dexter axle that show the proper amperage per magnet under full application and none of them are over 3 amps. A low or high amperage reading will point to several things that need to be checked. Just having the voltage does not insure proper operation. Too small of wire, bad connection, bad internals in a magnet, etc., may allow the voltage but restrict the amperage. My MaxBrake controller displays total amperage as the brakes are applied.

 

[Barnbuilder]

Sir, THAT is a problem! The MAXIMUM allowed voltage drop on any power circuit is 0.5V on the power side and 0.25V on the ground side - all measured from BATTERY GROUND on the power vehicle.

So, if you measure the voltage at the receptacle on the truck with the cord off (open circuit) and get 12.5V, the LOWEST acceptable voltage at the wheel end of the brake power wire is 12.0v, anything less indicates a serious wiring issue - high circuit resistance.

You also need to measure the voltage on the GROUND wire/frame with all lights on and some trailer bakes applied to assure you don't get more than 0.25V showing, if you do get more then there is a grounding problem.

All the talk about having to measure amps, resistance etc is hogwash, although knowing the amps is handy few people have a meter that can handle direct readings of more than 10 amps and therefore it is not possible to do. And no, inductive meters are NOT good enough, but they can give you a relative idea of the amps.

So how do I measure the ground voltage?

 

[ModMech]

Agreed that Amps does the work in ANY power circuit. Ohms is generally a useless measurement, except for sensor circuits.

If you have 2.5A/brake, and you attempt to directly measure that on a 4 brake system, you will very likely damage a 10A meter. Doing each individually would work ok.

With regard to measuring ground voltage, take the (-) terminal of the meter and connect it to the B- on the BATTERY of the towing vehicle, pull on the lights and brakes (~50%) then measure the voltage on the trailer FRAME, ground wire on the trailer and ground pin on the truck's receptacle. More than likely you will discover voltages much higher than 0.5V; indicating a serious issue with the ground circuits between the trailer and tow vehicle battery.

Note: It is perfectly OK, in fact a good idea to wire the ground pin on the trailer cord directly to the B- terminal of the battery. Just NEVER do that on the power (B+) side!