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Brakes in 2016 Ford C-Max Energi

This year I bought a 2016 C-Max Energi to flat tow behind my RV. It’s a great car with amazing features and economy, but there seems to be an issue with the brakes. Ford says that this vehicle is fine flat tow if you tow it with it turned off, if your run it for 1 minute and don’t exceed 70mph.

When towing behind an RV one needs to use an auxiliary braking unit. Many people have towed Ford C-Max hybrids this way, but there seems to be something different about the Energi in the way that the brakes work. Between 2 different brand Aux braking systems it seems impossible to set them to either brake enough or not lock up the brakes on the Energi. Some have speculated that the Active Hybrid braking can switch on even with the system electronics turned off to suddenly increase the brake sensitivity. The Ford dealer says, “No way!” Does anyone know why the Energi and the Hybrid would have differences in how they brake from system off configuration?

A very specific question you do not seem to find the answer for given your research. Do not know if anyone here can help you, but are the brakes in the cmax activated while being towed from the towing vehicle brakes?

For tow dollies there is a hard wire to the brake in the RV, but for flat towing there are two methods. For diesel pushers they sometime hook up a vacuum line from the RV brakes to the tow vehicle (dinghy), but for the most part, and in my case, the auxiliary braking systems uses an accelerometer to sense braking and slope and apply brakes accordingly. That said, they are working independently from the brakes in the RV.

Hybrid cars recover energy from using the electric drive motor to apply a drag (brake) to the front wheels. In order that you not have wildly differing mechanical braking effort from the foot pedal, there are various countermeasures that try to balance that.

An example; when the hybrid battery is fully charged, braking must all come from the mechanical system because there is no place for the hybrid energy to go. When the battery is somewhat depleted, the mechanical brakes only need apply maybe 1/3 of the brake effort while the hybrid system applies the rest. Your foot effort for the latter is 1/3 that of the former. THAT can really mess up a driver expecting to apply the higher force when the hybrid doesn’t need it or a lower for because the car DOES need it.

The mechanical brake system is built with some method of altering the brake assist so the pedal feel is consistent.

I explained that to refer back to your C-Max. If the car is off, none of the controls that moderate that pedal pressure are on plus any power assist is also off. These combined are likely causing your sensitivity while flat towing. All I can say is, turn the gain down for the trailer brake and use more of the RV’s brakes. If that isn’t acceptable, use a non-hybrid vehicle as your towed car.

Hi Mustangman,

Thanks for your reply and your thoughts on this. Clearly Ford gave some thought to making the feel of the brake the same regardless of how much charge was stored for it to work as well as it does in normal operation mode. Kudos to them for that.

Let me explain further that the aux brake that I used was from RVI in Colorado. They have sold many, many of these to people with hybrids, including C-Max Hybrids and claim to have zero problems. Judging from what I see on the RVForum that must be the case. On the other hand there are at least 2 of us with the C-Max Energi that are having problems, so I concluded that the Energi brakes are fundamentally different than the C-Max Hybrid.

I also believe that the Active Assist is never kicking in (however the other guy with this problem on his Energi disagrees). I tested this by coasting down a long hill with the power off and applying the brakes more than 200 times and never noticing and difference in how the dead brakes respond.

Also let me explain that the only things braking are the RV (motorcoach) service brakes and the brakes in the C-Max being towed flat (no trailer brakes involved here). I used to have a tow dolly where there were trailer brakes actived by a wire from the RV, but that was too much hassle so I bought the C-Max to get away from that. However, when I used a tow dolly I got a good sense for the feel of braking from the towed vehicle (tow dolly brakes) so I know when it is braking too much or too little.

The RVI aux brake controller allows me to set the sensitivity, as you suggested, for a range of vehicle weights with vacuum brakes or a range of vehicle weights with active assist brakes. At the lowest “compact” setting for vacuum brakes my C-Max brakes lock up and smoke as brakes are being applied. On the other hand, at the highest of settings for active assist “20psi” the aux brakes do little if anything to add any braking to the dinghy to put any pull on the combination. I can test this by sending a signal to the aux brakes to do all of the braking and I feel nothing, nor any surge when the brakes are no longer being applied. I have asked the RVI to reprogram to a setting between these two and thy agreed, but 3 months have passed and they never have gotten around to this.

That brings me back to the original question. Are there any fundamental difference between how a C-Max Hybrid or Standard applies power off brakes to the way that it is done in an Energi, where 2 of us are experiencing this unfortunate inability to find a setting that brakes enough without locking up our brakes?

And, after shopping long and hard for a car that my wife finds it easy to get in and out of, that is compatible with being flat towed, and that is light enough to not add too much burden on the RV, and selling my Lincoln MKZ Hybrid, I am not about to get a different vehicle. Perhaps I will find a different aux brake controller when all is said and done. First I need to exhaust all avenues to understand better the C-Max Energi brakes.

Thanks again for your thoughts.

Unless there is a Ford brake systems engineer on this forum, and I don’t think there is, you probably won’t get an answer. And even if you got an answer, you may not get a solution.

That said, how about using a double axle trailer to put the car on? (I never recommend tow dollies, ever) Yeah, I know they can be a pain, but you would get consistent braking.


You are probably right about the likelihood of getting this answer here. I have other ideas about finding that engineer that I am also pursuing.

You are also right that I could have solved this problem with a trailer. Actually, my old dolly worked fine, but it was too narrow for the C-Max and I sold it. Not only are trailers more trouble for loading and unloading, but they are a storage problem. My RV is 37.5’ long. I am occasionally forced to put this in a space that is only long enough for my RV and the C-Max with little or no room left over for a trailer, which constrains my choices for RV campgrounds. The place that I store my RV has a space big enough for the RV, but I had to rent a 2nd space for the tow dolly and would be the same for any trailer. That said I am persistent enough that I will find an aux braking system that works eventually or will convince RVI to reprogram the one that I have. Knowing how the C Max Energi really works will help them or someone else to do that.


Mike Angles here in San Jose, Ca. A small group of us have been working this issue since June 2017. We also have pulled in tech support folks from RViBrake and Roadmaster into our group as these RV brake system companies are seeing a CMAX issue with brake lockup when we flat tow.

You are right that Ford does not acknowledge the problem. We know otherwise and I have spoken to Ford senior Mgt, or so they claimed to be. The dealers are worthless on this issue.

We have collected various Ford documents, one of which, ODBII for hybrid and plug-ins (a PDF which this site won’t let me upload) explains a lot, about how the car will turn on the electric vacuum pump. It shouldn’t when we flat tow, but we are convinced it does. See more below my name.

OK. Now that you know I am serious about this issue, we need to figure out how to get in contact with each other, so that we can exchange info if you want. If not, watch for a short article from me in the upcoming FMCA mag asking CMAX owners to call the Ford Customer Support folks to open a case that basically says your CMax brakes lock up while flat towing. Back to how to reach me. I really don’t want to publish my contact info here, and I have only joined this forum to post this info. So, you can either google my name, I am listed as a real estate appraiser in California, or you can try the email me link on our web site. You may be interested in some of my Cmax info there, so here’s the web link to my projects page

re, mike

Here’s quotes from the Ford ODB booklet, pages 187 and 189 you may find of interest:

Brake System Overview
The brake system on HEV and PHEV vehicles plays a vital role in the regenerative braking function, first as the
primary measurement of the driver’s requested braking torque, and the arbitration of how that braking torque will
be delivered, as a combination of powertrain regenerative braking torque and conventional friction based torque at
the brake pads/calipers. The ABS module is responsible for maintaining overall vehicle stability during braking,
traction control maneuvers, and ABS events, and may limit regenerative braking torque during any event to
maintain vehicle stability.

The overall amount of regenerative braking will impact the fuel economy of an HEV or PHEV vehicle. Certain
failure modes of the regenerative braking system can limit regenerative braking torque for the duration of a drive
cycle. In a PHEV, this affects the all-electric range of the vehicle, as regenerative braking is used to recapture
kinetic energy from the vehicle while braking. This reduction in EV range of a PHEV vehicle makes the
regenerative braking function an emissions control device, and therefore any component in the braking system that
can affect regenerative braking on a PHEV will have an OBD monitor. In Ford’s PHEV’s, the brake module
contains the diagnostic software and monitors for these components, and the summary of those monitors is
reported to the HPCM, which handles the OBD reporting for the ABS module (which does not have a diagnostic
identifier in the OBD range). In this manner, the HPCM will act as the gateway for all brake system OBD

“The Electric Vacuum Pump (EVP) is controlled by the ICU, and is designed to maintain the vacuum level in the
ACU to meet all braking requirements. In HEV and PHEV vehicles, the engine is not always running, so can not
be depended on to provide sufficient vacuum for the ACU in normal driving. This is especially true in PHEV
vehicles where the engine may not be started for an entire drive cycle up to ~20 miles. Thus, the EVP is used as
the primary vacuum source for the ACU, and is controlled by the braking system. In certain EVP failure modes, the
powertrain will revert to a more conventional mode of operation to reliably deliver required vacuum levels in order
to maintain braking performance.”

Mike, if you think the EVP turns on while towing then disconnect it. Simple test. Pull the fuse on the EVP, pump the brakes a few times to release the stored vacuum and tow the car somewhere and note how it brakes. If the problem is that simple, then so is the solution.

Wow, talk about initialize… has about 12 undefined abbreviations. Of which I know about half. Google got me most, but what is ICU (intensive care unit)?