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 http://www.mangles.net/fun/projects/2014-Phaeton/
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.”