Voltage Drop to Brakes

[5erWonk]

Been a long time since I've been on, obviously working too much.

This is issue goes all the way back to last August when we were going to one of my wife's art functions in Montana. On the way we found our suspension was trashed. Long story short we had to replace both axels and springs which also includes all new wheels and brakes.

When end leaving the repair facility I tested by brakes and found weak braking. They went around checked adjustment and all was in spec. Drove around some more still weak. Check again, all in spec. Finally they said thing wow old improve after a few hundred miles......well after about 1000 mile no change.

Finally started looking king at the voltage at the breaks and we only get 9.5V. So working backwards I have 13.5V at my receptacle on the truck. Checking inside the junction box where the plug wire connects to the wires that go to the trailer I only have 10.5V which leads me to believe the plug is bad. I have already used a file to clean the plug connections and made no difference.

I also wonder why there would be a one volt drop from the junction box to the breaks, but I don't think that is going to be a problem.

Any suggestions

 

[Jim.Allison]

Does the brake controller have to be engaged 100% to allow the full voltage of the batt through?

Been a long time since I've been on, obviously working too much.

This is issue goes all the way back to last August when we were going to one of my wife's art functions in Montana. On the way we found our suspension was trashed. Long story short we had to replace both axels and springs which also includes all new wheels and brakes.

When end leaving the repair facility I tested by brakes and found weak braking. They went around checked adjustment and all was in spec. Drove around some more still weak. Check again, all in spec. Finally they said thing wow old improve after a few hundred miles......well after about 1000 mile no change.

Finally started looking king at the voltage at the breaks and we only get 9.5V. So working backwards I have 13.5V at my receptacle on the truck. Checking inside the junction box where the plug wire connects to the wires that go to the trailer I only have 10.5V which leads me to believe the plug is bad. I have already used a file to clean the plug connections and made no difference.

I also wonder why there would be a one volt drop from the junction box to the breaks, but I don't think that is going to be a problem.

Any suggestions

 

[5erWonk]

Yes it does, and all my tests have been with the controller full on.

I think I'm going to run some jumpers from the truck receptical to the j-box and see what I get at the brakes before I replace the plug.

 

[Jim.Allison]

I think I would start cleaning connections with a professional contract cleaning and lubricating aerosol spray. You probably need to do that anyway. And I would see if I could get the spec on what the full output of the controller should be at 100%. It may be less than your batt voltage.

Yes it does, and all my tests have been with the controller full on.

I think I'm going to run some jumpers from the truck receptical to the j-box and see what I get at the brakes before I replace the plug.

 

[Jim.Allison]

Does this help you?

Voltage
Brake system voltage is best measured at the magnets in order to check the entire system. All voltage starts at the tow vehicle battery, passes through the controller, through the electrical connection between trailer and tow vehicle, and ultimately is applied at the brake magnets. Electrical integrity is crucial for safe stopping. The engine of the tow vehicle should be running when checking the voltage, so that a low battery will not adversely affect the measurements.

Attach the voltmeter in parallel with any of the brake magnets. The voltage should be zero volts at the beginning of the test. As the controller is activated, or the brake pedal depressed, the voltage should gradually increase to about 12-volts DC. This slow climb in voltage is termed modulation. No modulation means that when the controller begins to apply voltage to the brakes, the controller immediately applies a high voltage, which causes the brakes to apply instantaneous maximum power.

The threshold voltage of a controller, remember, is the voltage applied to the brakes when the controller is first becoming activated. The lower the threshold voltage, the smoother the brakes will operate. Too high a threshold voltage causes grabby or harsh brakes.

Amperage
System amperage is the current being drawn by all brakes on the trailer. The engine of the tow vehicle should again be running. One location to measure the current is at the brake output wire right at the controller. Sometimes, however, this is easier to accomplish at the electrical connector plug at the rear of the tow vehicle. The brake wire must be disconnected and the ammeter put in-line or “in series” with the brake wire. Make sure the ammeter has sufficient capacity and note the polarity to prevent damaging the ammeter (a range of 0 to 15 amps is sufficient for single or tandem axle trailers; 0 to 25 amps is needed for triple axle trailers with six brakes).

Individual magnet current draw can be measured by inserting the ammeter in the line at the specific magnet you want to check. Disconnect one of the magnet wires (or simply slip an inductive-type ammeter over one of the wires) and attach the ammeter. By far, the most common electrical problem is low or no voltage at the brake magnets. Common causes for this condition can usually be attributed to one or more of the following:

• Poor connections
• Open circuit
• Insufficient wire size
• Broken wires hanging below the axle
• Improperly set up controller

Another common electrical problem is shorted or partially shorted circuits usually indicated by extremely high amperage readings. Possible causes are:

• Internally shorted magnets
• Defective controller
• Brake wire shorted to ground somewhere in the system
• Corrosion between the ground wire and the brake wire in the electrical connector plug

All electrical troubleshooting procedures should start at the controller. Try to eliminate the brake controller as the problem first. Make sure the controller is set up according to the manufacturer’s recommendations. If the voltage and amperage are not satisfactory, proceed to the connector and then to the individual magnets to isolate the problem source. Twelve volts output at the controller on the brake wire should equate to a minimum 10.5 volts at each magnet. The voltage will be higher if the tests are made with the engine running as suggested. Nominal system amperage at a basic 12.0 volts with cold magnets and the controller gain adjustment at maximum, the current at each magnet should be as follows (approximately):

Brake Size Amps/Magnet
7 x 1-1/4 inches 2.5
10 x 1-1/2 inches 2.5
10 x 2-1/4 inches 3.0
12 x 2 inches 3.0

Some modern electronic brake controllers are equipped with a built-in continuity tester, voltmeter or ammeter. When the trailer is properly connected to the tow vehicle, an indicator lamp is lit on the controller. This informs the operator that there is a good connection and current and voltage can flow from the controller, through the electrical connector to the brake magnets and to ground.

Properly adjusted electric brakes, a sound electrical path and a correctly installed and synchronized brake controller all contribute to not only your safety, but to an overall positive towing experience as well. And remember, RVing is more than a hobby, it's a lifestyle!

 

[donr827]

Great article Jim. Might be good to place in HUG.Don

 

[superduty08]

I found my problem with my truck socket. After years of use the pins had compressed a had no resistance when trailer plug was pushed in. I spread the pins until I could replace the socket and my problem was fixed.


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[danemayer]

I found my problem with my truck socket. After years of use the pins had compressed a had no resistance when trailer plug was pushed in. I spread the pins until I could replace the socket and my problem was fixed.


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Thanks for coming back to share what you found.

 

[superduty08]

It was tricky because I had 12 volts to each brake but not the amperage that should be when the brakes were applied. When powered from the trailer tail the amperage was about 2.5 each as it should. That narrowed it down to the truck socket. Hope this might help someone else having low trailer braking.


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