2019 Ford F350 - Trailer brake issue

You know how much current the magnets should draw, now measure the trailer brake current draw while braking under the condition that causes the short circuit warning.

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Yes I did I had the brakes inspected and had all 6 wheel bearing repacked. The repair facility made sure the brake operated correctly with no damage to wiring

I have a meter to measure DC current, I have the wire to run up to the truck cab, but I don’t have an adapter to go in between the truck’s 7-pin connector to the trailer’s 7-pin cable. With the adapter, I could put the meter in line to see the current flow to the brakes while driving down the road towing the 39 ft trailer. Do you know where I can buy the adapter ? Wouldn’t it be easier for Ford to inform me what the brake current is that trips the warning on the dash ?

Fault B148A:11 Trailer brake output shorted to ground will be set if the trailer brake current exceeds 49 amps.

Once I figure out what that DC current number is, then what I can’t change it.

Installing a new brake control would require the dealer to remove the Ford OEM controller.

As soon as you plug any trailer cable into the truck’s 7-pin connector the computer senses it and a dash light come’s on dedicating the trailer.

When I took the truck to the dealer there was no fault code listed on the computer

If the trailer has surge brakes and no wire connected to the brake controller, there should not be a warning message. You would have the same condition if you controlled the trailer brakes with a separate controller.

Before you try that, identify the problem that is causing the failure.

Was the trailer connected?

I found it:


Ford Ford DTC B148A:11

Trailer Brake Output: Circuit Short to Ground
This DTC sets in continuous memory when the ignition is set to ON or during the on-demand self test if the current draw on the PWM output circuit is greater than 27 amps.
Failure Type:

Circuit Short To Ground
This sub type is used for failures, where the control module measures ground (battery negative) potential for greater than a specified time period or when some other value is expected.

When the truck is running the voltage is higher than 13 volts
More like 14 to 15 volts that extra voltage it will increase the current draw to exceed the 27 amp setting
That is why the Warning Light does light all the time

Bull Cookies.

The controller can be placed in tandem and a second 7-pin installed. Old school, like before integral controllers were a thing.

So now the hard part getting Ford to rewrite the Brake Controller to a higher trip point.

My SWAG is to have Ford rewrite and reset the trip point to 31AMPS or 32 AMPS instead of the current 27 AMPS

15 Volts /.5 = 30 Amps
13 Volts /.5 = 26 Amps

If the voltage goes up and the resistance stays the same the current goes up


12 volts/24 amps = .5 ohms of resistance from electric brakes all 6 brakes are in parallel across the 12 volt source

But if truck voltage increase because engine is running to 14.8 volts

Brake resistance stays the same .5 ohms

The 14.8/.5 ohms = current is now 29.6 amps

For the formula to work if the voltage goes up and brake resistance stays the same because you didn’t add or subtract any trailer brakes

Current has to go up so, 14.8 volts = .5 ohms X 29.6 amps
So V = IR
And 12 Volts = .5 ohms X 24 Amps
The Fords OEM Brake Controller trips when current is above 27 Amps that is what is causing the Orange Warning Message
The control works great until the current limit is exceeded
A trailer with 4 brakes the warning never appears never exceeds 27 Amps

Ford has said there is 100 and 100s of different trailers, which may be true

But there are only a few companies that make the electric trailer brakes that go on all the different trailer manufacturers

The only big difference is the size of the electric brakes based on tire size

Hard to put a large brake inside a small tire or a large GVWR trailer to use a small brake to stop the trailer but to handle the heavy GVWR they add more axles and that increase the number of brakes to stop the increased weight.

The bottom line there isn’t a larger number of different electric brakes that operate on a 12-volt system that Ford needs to interface with.

The Ford OEM brake controllers are designed to interface with the electric brake manufacturers that can be pulled by Ford F series trucks which really limits the number of different trailer brake companies Ford has to deal with.

Where are you measuring the voltages higher than 12VDC and under what conditions?

The reason I ask is that I suspect you are measuring battery bus voltage while alternator is charging and assuming that is the voltage that will eventually be applied to the trailer brakes.

If so, it is unlikely. First, modern OEM trailer brake controllers are “smart” electronic controls. They are monitoring all kinds of sensors and vehicle conditions to use in determining how much braking to apply or even if any will be applied at all e.g. if no trailer is detected.

Any electronic design like that is sure to employ a voltage regulator scheme to ensure the maximum voltage applied to the load (trailer) is consistent. It is a fundamental feature of every piece of electronic equipment in your truck. There’s no reason to believe the trailer controller isn’t employing one.

To verify, this has to be measured under the specific conditions where you have the problem. The controller uses a gain algorithm to determine how much braking force to apply (voltage) and in some cases, the duty cycle of that voltage signal. Only under worst case braking is the maximum power applied to the trailer brakes. This is when the full voltage and duty cycle, if applicable, will be present on the trailer wiring.

I suspect you may be assuming too much and following a red herring as they say…but please confirm otherwise…

In other words, the warning only appears when you make panic stops.

You’re towing a 16,000 lb trailer, if you’re making that many panic stops, you need to change your driving style.

To simplify this:
You bought 2 pieces of equipment that are incompatible.
It’s not either manufacturers responsibility, to make them compatible.
You want to make them compatible.
If no one can make that happen, you need to buy equipment that is compatible.
Do I have this correct?
I don’t know what to say, other than good luck, and best wishes. :crossed_fingers:

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I did buy 2 pieces of equipment but according to Ford, they match Fords’ limits. Fords’ 666-page manual said the truck can tow a 3 axle trailer as long as they have an electric brake system on the trailer which this Toyhauler trailer has.

The other limits to towing a trailer is that it does not exceed the max weight of 18,000 lbs which this trailer is under that max limit and that the combined weight doesn’t exceed the total combined weight of the trailer and tow vehicle of 28,700 lbs. The max weight allowed for the truck only is 11,500 lbs and the max weight for the trailer is 18,000 so in adding the two together the total max weight is 28,500 lbs and neither the truck nor the trailer weighs in at the max rated weight.

So according to the Ford service manual, they are compatible.

Ford did not say it will handle a 3 axle trailer as long as the brake current does not exceed 27 amps. When those limits were placed in the fine print didn’t exclude any current restrictions on double or triple axle trailers other than weight limits. My current setup falls below those weight restrictions.

So according to Ford, they are comparable.

Trailer brake current requirements vary by their size (i.e. stopping power). More than one source defines the requirements as:

We’ve found the most common amperage draw on a 7-inch brake magnet to be 3.2 amps maximum. The increased amperage of a 10 or 12-inch brake magnet tends to max out at 4 amps. This means that a double axle brake system would be drawing a maximum of 16 amps for a full emergency stop.

So extrapolating that to a 3 axle trailer, it should really draw no more than 6x4= 24 amps under full emergency stop, well below the designed limit of 27 amps for your truck. So the controller is designed to handle a normal 3 axle trailer with some margin.

Rather than speculating in paper analysis, some measurement is in order to prove your trailer is actually operating within specifications under the maximum conditions. Not just a resistance check of the brake magnets…