Factory Trailer Brake Controller Troubleshooting and Technical Info (from HDRAMS)

[Brutal_HO]

Full credit goes to HDRAMS member Tom2531. Reprinted below.

Brake Controller Troubleshooting/Technical Info

This post is about the brake controller on the 2021 HD Ram. First, my story. Second some reverse engineering and description of the warnings on the dash. Finally, some technical details. I would assume this information would apply to other years in the same generation of truck, but outside of...

hdrams.com

This post is about the brake controller on the 2021 HD Ram. First, my story. Second some reverse engineering and description of the warnings on the dash. Finally, some technical details. I would assume this information would apply to other years in the same generation of truck, but outside of that, I don’t know.

I was greeted with Check Trailer Brake Wiring/Trailer Brake Disconnected on our first trip out with our 2021 2500 and travel trailer combo. The same trailer on our 2018 1500 had no issues earlier in the year. At first it happened after a few hours. Sometimes it would just say Check Trailer Brake Wiring, and other times it would ding and say Trailer Brake Disconnected. I read this related post: Trailer Brake Disconnected. I found I had the “old” style 7 pin connector, and I needed to extend mine anyway – so I ordered a Hopkins HM20048 and put that on. The first time I plugged it in, I had no brakes and no left turn signal on the trailer. I bent the pins a little and got them working. Unfortunately, after a short while on the next trip, the error returned.

What bothered me is the error wasn’t real helpful. I wasn’t sure if I had a short, break, or something intermittent. Nor did I know if the problem was in the truck or trailer. I went about troubleshooting and here is what I learned.

Brake Controller Operation​

In the rest of this post, I will refer to the two warning messages and accompanying indicators/audibles as CTBW and TBD.

• CTBW: Check Trailer Brake Wiring – white text, no audible ding, the trailer disconnected icon is not lit
• TBD: Trailer Brake Disconnected – white text with red background, a single audible ding, trailer disconnected icon is lit (red trailer with slash through it in the upper right portion of the dash)

TLDR: A very intermittent short (less than half a second) may never trigger a message. If CTBW comes on, a short has existed for at least a second. If you get a CTBW/TBD cycle every 5 seconds, it’s been shorted all that time. This will stop as soon as the short is resolved. A continuous TBD is an open circuit.

I started testing with load resistors (6 Ohm 50W) that simulated trailer brakes. With 2-6 ohms, the truck would detect a trailer connected. But when I got down to 1.5 ohms, the CTBW/TBD cycle of errors would begin. So would a dead short.

If a short or high current on the brakes is detected, after 1 second you will see CTBW. If the problem goes away during that time, the message will go away. If the condition exists for another 5 seconds, it goes to TBD. If the problem goes away at this point, this will clear also. But if the condition persists, it will cycle between CTBW and TBD every 5 seconds.

While a short or high current scenario is detected, THE BRAKE CONTROLLER WILL NOT OUTPUT A BRAKE SIGNAL. If the short condition is resolved while already braking, the brakes will come on (I experienced this once – when stopping, there were no trailer brakes to start so I got into the truck brake, then the trailer brakes kicked in and scared the crap out of me). If you are braking when a short is detected, it will continue to brake assuming the short isn't bad enough. But as soon as the brakes are released the short will be detected, and the output disabled.

In contrast, an open connection to the trailer brakes will delay for one second, then you will see TBD. The red no trailer icon will persist as long as the brake wiring shows an open circuit.

Trailer Troubleshooting​

From there I connected the truck to the trailer again, and (luckily) it cycled CTBW/TBD. I cut the brake wiring and only connected the brakes on the front axle of the trailer, and the error went away. The back axle brakes by themselves caused the error. When splitting the back axle in to left/right, the short disappeared entirely, so I suspect the issue is in one of the wires. I’ll be re-wiring the trailer brakes soon to remedy the issue.

Etrailer has some good articles on expected current (amps) and resistance (ohms) for different trailer configurations. That’s a good place to start to troubleshoot the trailer wiring. I won’t rehash that here.

Truck Troubleshooting​

(I didn’t go this far, but this was my plan)
If the trailer had checked out, I was planning to connect a load resistor (6 ohms 50W will do it – just be aware it will get hot when braking) across the ground/brake pins of the connector on the truck and drive around with my “virtual” trailer. If there had been a short or break in the truck wiring, my thought was that it would show up and not have all of the complications of a real trailer to troubleshoot. It would have made troubleshooting with the dealer easier.

Technical Details​

I put a oscilloscope on the trailer wiring so see what it was really doing.
The brake controller uses PWM (pulse width modulation). With no trailer connected, it sends out a 12V DC 1ms pulse 3 times a second. When the trailer is connected this voltage will drop a little but this “sensing” pulse continues to check if the trailer is connected. I can actually hear this in my brakes – there is a slight ticking noise coming from the trailer brakes 3 times a second. When you brake, a 250Hz 12V DC PWM signal comes out, with the duty cycle proportional to your braking, as well as gain and trailer settings. The PWM sensing pulse is there all the time even if there is not a trailer connected. Even with an open circuit, the brake controller will still output a full signal when braking. But if it gets shorted or the braking current (amps) is too high, it will shut off. It still senses 3 times a second when shorted and if the short is resolved, it will begin to output again. I would assume the one second delay earlier is three tries before it gets disabled.

Edit: Be sure to see Part 2 below: Factory Trailer Brake Controller Troubleshooting and Technical Info (from HDRAMS)

 

[Brutal_HO]

Part 2:

I did some more tests. After gathering data with the scope, I tried to see what was available via OBDII. Luckily, you can query the PIDS on the ITBM (Integrated Trailer Brake Module). I have a OBDLink MX+ and I needed to use the FCA extended diagnostics add-on to get to them. This helped, as there is a PWM % PID and I could abandon the scope. I also found braking is in relation to the “Brake Pedal Status – Primary Circuit Pressure” in the ABS module. That combined with some manual readings, I was able to make the following observations.

All trailer modes will output 100% PWM above 10MPH with a gain of 10.0 when the dash indicates 100% brake. At speeds below 10MPH down to 0MPH, it gets reduced linearly to somewhere between 50% and 70% depending on the trailer type selected.

The manual lever doesn’t output PWM the same way the brake pedal does. 50% as indicated on the dash with the brake pedal uses the braking curves (a later chart) to determine what to output. The manual lever is proportional to the PWM output, taking into consideration the gain and vehicle speed.

It’s worth noting that the dash brake % isn’t the same for different trailer types. It’s pretty linear as compared to brake pressure, but there are some subtle differences between the trailer types. LE=Light Electric, HE=Heavy Electric, LEOH=Light Electric Over Hydraulic, HEOH=Heavy Electric Over Hydraulic.




800 inHg is what I could call “medium” brake pressure. 1500 inHg is what I could call “high” brake pressure. The braking curves below kind of follow these lines, but there are some nuances, so I generally ignore the dash percentage from here on and use inHg as my measured data for brake pressure. I posted this chart merely for reference.




The chart above shows the braking pressure vs the PWM output. The chart doesn’t go higher than 62% because the truck was stopped. That’s the whole 0-10MPH mentioned earlier. Let’s look at each trailer type. LE comes on slow at first, then goes up to the max with medium brake pressure. HE also comes on slow at first, but gets up to the max with a lighter brake pressure. LEOH stays on flat at about 4% PWM at first. Then it takes a high braking pressure to get to full output. HEOH comes on flat at about 8% PWM at first, and comes up to full output a little faster.

Lowering the gain has two effects. You can see this by looking at LE 10.0 and LE 5.0 in the chart. First, it delays the brakes coming on by increasing the pressure needed to first get them to activate. Second, it drops the maximum output down proportionally. A 5.0 gain will never output more than 50% PWM (above 10MPH). For hydraulic brakes at lower gains, they still have that low PWM when the brake is initially depressed – that doesn’t change. But the point at which it will ramp up is effectively delayed, as is the max output.

I wanted to know what this looked like while moving. Armed with the data logger, I came up with this chart:




The solid lines are the same LE and HE data from the previous chart done at 0MPH at 100 inHg intervals. The dashed lines are data recorded while moving above 10MPH. The data isn’t perfect because it would only poll at 500ms intervals, so I had to brake and accelerate at the same time over a 10-15 second period to get enough data. Generally, it looks scaled up as I would expect while moving, although HE seems a little more aggressive at first. Based on this, I would expect the braking curve to scale down between 10 and 0 MPH. It’s too hard to hold the truck at 5MPH and brake between 0 and 100% to determine if that is the case. I just wanted to do a test while moving and compare results.

Hopefully this helps someone, or at least you find it interesting.

 

[Brutal_HO]

Does your trailer 7-pin have the right connector?

The old style is problematic and the first thing to check.

Old style:




New Style:

 

[Brutal_HO]

Placeholder

 

[Brutal_HO]

Brake Controller Troubleshooting/Technical Info[/URL]]
https://hdrams.com/forum/index.php?...roubleshooting-technical-info.6004/post-96527

Assuming you did much of the static testing stationary? That seems to dispel the rumor that the brake power is reduced at low speeds, or was changed at some point? Did you measure the actual current as well?

All testing was done while not moving. The truck was "on", but not running. I did notice that the PWM signal never got to 100% duty cycle, even when I pulled the manual control all the way. I wasn't sure if that was related to the trailer profile (light electric) or not. It's possible that the PWM is reduced as a function of speed (especially at 0 mph) to limit current draw. Is the rumor just that as you slow down that the brake signal is reduced, given the same amount of brake applied? That would be tricky to test, but not impossible.

I did not measure the current going to the brakes from the brake controller. I only did that once with using the emergency brake switch just to get a baseline of actual current draw. I could measure current to the trailer but thats a little trickier.

 

[Brutal_HO]

https://hdrams.com/forum/index.php?...roubleshooting-technical-info.6004/post-96594

I've seen others say they measured lower voltage when stationary or below 30. I think in every case those were simple V-DC measurements done with a basic MM.

I can confirm that at 10MPH and above, 97% duty cycle on PWM is available - basically all the braking possible. From 10MPH to 0MPH it does lower the trailer brake max from 97% to 57%, linearly, as measured at the PWM output. This is on light electric with 10.0 gain. The other profiles may be a little different. The dash still says 100% whether using the manual override, or braking enough to get to 100% as indicated on the dash. So there is some truth to the rumor, but 10MPH seems to be the threshold. A lower gain also basically limits the max output. So the only way someone would see a full 12V is with a gain of 10.0, the truck moving faster than 10MPH, and with 100% braking as indicated on the dash.

I gathered some other data on the different profiles, so I hope to make that into something useful tonight.

 

[HemiDude]

Thanks for bringing this thread over, @Brutal_HO. I don't have a TT yet, but I can see that a need for some of this info may arise once I do

 

[n8zcc]

I can confirm that at 10MPH and above, 97% duty cycle on PWM is available - basically all the braking possible. From 10MPH to 0MPH it does lower the trailer brake max from 97% to 57%, linearly, as measured at the PWM output. This is on light electric with 10.0 gain. The other profiles may be a little different. The dash still says 100% whether using the manual override, or braking enough to get to 100% as indicated on the dash. So there is some truth to the rumor, but 10MPH seems to be the threshold. A lower gain also basically limits the max output. So the only way someone would see a full 12V is with a gain of 10.0, the truck moving faster than 10MPH, and with 100% braking as indicated on the dash.

I gathered some other data on the different profiles, so I hope to make that into something useful tonight.

My trailer brake controller experience is limited to the factory ITBM in my 2013 1500 (no longer own), 2015 3500, 2022 1500, and the Tekonsha Prodigy P2.

First off, and this is from my book of lessons learned, if you purchase a new trailer with brakes, bed the brakes. My RV salesperson nor the RV manual mentioned this very important fact.

When presenting an artificial load to the ITBM make sure you are using an inductive load and not a resistive load. The best thing to do is pick up an electric brake actuator from the trailer parts store.

If you suspect an issue with your trailer brakes or simply want to verify the trailer brakes are functional, you can use the brake-a-way switch. Typically, each wheel should consume around 3 amps at 12 volts.

Most folks do not have a scope and have no experience using one let alone reading the waveform. May I suggest this device, I keep one in my truck and have solved trailer brake issues for dozens at campgrounds.

https://smile.amazon.com/Electric-Dynamic-Simulation-Circuit-Testing/dp/B073FJ5NVL/ref=sr_1_6?crid=163SUBITVE6G7&keywords=trailer+brake+controller+tester&qid=1657016190&sprefix=trailer+brake+controller+tester%2Caps%2C145&sr=8-6

I do not have any data on the 2013 since I never had issues with trailer braking. Come 2015 when I bought a new 16,000-pound fifth-wheel RV and a new RAM 3500 Cummins to pull it did I discover a trailer braking issue. At times, we have to make a more aggressive stop, for example, a changing traffic signal. I discovered the ITBM would stop sending braking PWM to the trailer when the travel speed fell below 30 MPH. Once, the first time, the weight pushed me slightly into the intersection.

I broke out my Fluke and Picoscope to figure out what happened, this is how I discovered the loss of trailer braking. I documented the issue in a paper and submitted it to the NTSB. I was not the only person complaining, many of the RAM forms at the time had a lot of discussions on this problem. We discovered that RAM switch to another supplier in 2015 for ITBMs. In November of 2015, RAM quietly released a software update for the ITBM and ABS. After the software update, I retested and they changed the behavior. When the vehicle's travel speed falls below 30MPH, the amount of proportional braking is pulled back from the trailer's brakes. This makes for a smoother stop but it misses the mark just a bit for heavy trailers but is fine for lighter trailers, particularly utility-type trailers that can be loaded or unloaded.

This same braking behavior is also observed on my 2022 with the factor ITBM.

Because of this braking behavior, the 9-step procedure in the owners' manual for setting up the ITBM gain is impossible to execute. The manual slider over-rides the speed and will apply full PWM with respect to the gain setting.

Since after-market brake controllers like the Tekonsha Prodigy series, lacking vehicle dynamics data, will apply a braking signal to the trailer brakes all the way down to stop.