This writer and his technical staff are no strangers to hybrid vehicles and their sometimes-weird problems. We have always managed to find and fix the issues, faults, defects, and other shortcomings on hybrids, but there is always a first time for everything and in this article, we discuss a case of total brake failure on a 2017 Toyota Prius that resisted all our initial attempts to diagnose the root cause of the issue. We did find and fix the problem after about two weeks or so, but before we get to the specifics of this case, let us state –
The problem Prius belonged to an acquaintance of a long-standing regular customer, who recommended us as the only workshop in town that could possibly fix the problem. We had no idea if this was true or not, but in short, the car’s owner was an estate agent who was ferrying two prospective clients to a new housing development when her Prius experienced an unexpected, and total brake failure.
Also according to the customer, the Prius failed to slow down when she took her foot off the accelerator to stop at a red traffic light. Worse, as she put her foot on the brake pedal, the pedal started to pulsate rapidly and violently, and no matter how hard she pressed on the brake pedal, the car refused to stop. It did stop eventually but in the middle of the intersection. Fortunately, the traffic flow was light, and everybody managed to avoid hitting anybody else.
It did not take long to verify the customer’s concern. We powered up the Prius while holding the brakes, and sure enough, the pedal started pulsating rapidly, just as a brake pedal under ABS braking conditions does. There was more though- several warning lights lit up the dashboard like the proverbial Christmas tree, and at least three audible alarms went off.
Disregarding the lights and alarms for the moment, we drove off slowly to test the hydraulic brakes; nothing happened when we tried to stop so we almost ran into a wall. This was something we had not seen before: we had come across regenerative braking failures on hybrids of all makes before, but in all of these cases, the hydraulic brakes always worked at speeds under about 10 km/h or so.
If you are not familiar with how brake-by-wire systems on hybrids work, here is the (very) short version of-
On all hybrids, irrespective of their nameplate, the principal braking force is regenerative, which happens when the onboard electronics switch power from the motor/generators' rotor to the field coils. This creates an enormously strong magnetic field in the field coils to act as a braking force on the rotating rotor.
This happens automatically when pressure is removed from the accelerator pedal, and on most hybrids, this braking force accounts for about 85 percent of the vehicle's total braking ability. However, at low road speeds, the regenerative braking effect is not strong enough to bring the vehicle to a total stop, so at speeds under about 10 km/h or so, the driver can apply the hydraulic brakes to bring the vehicle to a stop, and to hold it stationary.
The hydraulic brakes can also be applied manually via the ABS in emergency situations. Under emergency conditions, the combined effect of the hydraulic brakes and regenerative braking creates enormous braking forces that often exceed the braking forces even high-end luxury vehicles can generate.
From a driver’s perspective, the regenerative braking effect is undetectable because the overall braking system is configured so that the pedal “feel” correlates directly with the vehicle’s deceleration, regardless of which braking mode is in operation.
In practice, however, it is sometimes possible to operate a hybrid vehicle in such a manner that the hydraulic brakes are never activated, and we have often seen hydraulic brake systems, and particularly, brake callipers, on hybrids that have seized up through a lack of use, which brings us to-
Our first order of business was to inspect the hydraulic brake system to verify that it was working at low road speeds. This is an important point, since, as we stated earlier, the regenerative braking effect is too small to bring the car to a complete stop at speeds below about 10 km/h or so.
The image above is a fair approximation of what we saw. There was some rust on the rotors, but there were also some shiny parts that proved the hydraulic brakes were working up to the moment the entire brake system failed. By all appearances, the hydraulic brakes on both sides seemed perfectly normal, and in good working order.
In any event, even if the hydraulic brakes were not working at all, this would not cause the pedal to pulsate as violently as it did. Therefore, we connected our Toyota-specific (OEM-level) TechStream software running on a dedicated laptop to the DLC via a J-2534 pass-through device and launched a procedure to scan the entire vehicle for whatever codes we might find. We found both active and historical codes. Here are the details-
Active codes in the Skid Control Module
Historical codes
We found-
The presence of these codes made sense on an intuitive level, but since we had not seen this particular combination of codes before, we thought that the most prudent thing to do would be to start with the easiest thing, which was to switch relays to see what would happen.
We also tasked a service advisor with looking for TSB’s relating to this set of symptoms and codes, and to log into our NASTF (National Automotive Service Task Force) account to find and download all and any relevant service/repair information, which brings us to-
We found the ABS main relay, and replaced it with an OEM-equivalent unit, erased all the fault codes, and powered up the Prius. The results were less than encouraging because while the pedal did not pulsate anymore, all the previous warning lights came on, all the previous alarms went off, and the brakes still did not work. All the previous fault codes also came back.
At this point, the diagnostician thought it might be a good idea to check the condition of the 12V LSI battery since the ABS and its associated systems derived their power from this battery. He also intended to check all accessible ground connections to eliminate poor ground paths as a possible cause. As it turned out, however, the LSI battery was fully charged and in excellent condition, and the search for poor ground connections came up empty.
By this time it was late afternoon, and just before closing time the service advisor came round to tell us that despite his best efforts, he could not only not find any TSB’s relating to this issue- he could also not find online discussions about this issue on our extensive list of aftermarket sources of repair information. He (the service advisor) thought that it seemed that no Prius had ever suffered a total brake failure anywhere in the world. We thought this was possible but extremely unlikely. We thought it was more likely that somewhere, someone had found a fix for the problem, but for reasons of their own chose not to share their findings with the rest of the world.
The upside was, however, that the service advisor had found a description of the Prius brake system on an official Toyota site, and was in the process of downloading it- all 167 pages of it. The bad news was that this information did not include anything on total brake failure and therefore, there was no information on how to diagnose and repair total brake failure on Prius vehicles.
Just before we left for the day, we called the customer and told her that finding and fixing the problem was going to be more difficult than we had anticipated, and as a result, we could not give her an estimate on when she could have her car back, or what the final cost was going to be. The customer took this rather stoically but made it clear that she did not have an unlimited budget to fix this problem, and we were to do our utmost to keep costs down, which added another layer of complexity to the problem.
Thus, to prepare ourselves for the next day’s problems, we decided to take another look at the problem with the TechStream scan tool before closing up for the night. We connected the tool again, and since it could communicate with both the ABS and the Skid Control Modules, we verified that the ABS module was not completely dead, because the scan tool registered and displayed changes in readings from both the brake pedal pressure and stroke sensors.
Somewhat emboldened by this, we connected a breakout box to the DLC and scoped the high-speed CAN data bus, which turned out to be in perfect order since the two CAN lines produced waveforms that mirrored each other perfectly, which brings us to what we did-
Since the CAN bus system seemed to be working as designed, we concluded the problem did not involve wiring issues, but rather a control module malfunction. The diagnostician thought that since we found three active codes in the Skid Control Module, this module was the root cause of the problem, but knowing we had to keep costs down, we had to be 100 percent sure of this before we replaced it.
Therefore, we took a look at the factory information on the operation of the brake system. We obviously cannot reproduce all 167 pages of it here, but we can post a paraphrased version. Here it is-
"ECB (Electronically Controlled Brake) — The skid control ECU receives signals from the pedal stroke sensor, master cylinder sensor and wheel cylinder pressure sensor. Based on these signals, the skid control ECU calculates the necessary braking force for each wheel. The necessary hydraulic pressure braking force signal is sent to the Hybrid Control ECU via CAN communications. The skid control ECU receives a braking force (regenerative braking force) signal from the Hybrid Control module via CAN communications.
The ECU calculates the necessary hydraulic pressure braking force based on the necessary braking force and [available] regenerative braking force. Necessary hydraulic pressure is supplied to each wheel by adjusting the brake accumulator (hydraulic pressure source) pressure with each solenoid valve. If there is a problem with braking function, the rest of the normal operating parts will maintain brake control as a failsafe.” (Source: Toyota USA)
In translation, this means that if the electronic control system fails, the brake system will revert to normal hydraulic braking. However, in this case, the "fail-safe" hydraulic system also failed, so the vehicle had no braking function at all.
Since there was no service or repair information included in this, we had two choices; we could take the shotgun approach and start replacing parts until the system worked, or, we could start asking around to see if anybody we knew had ever seen total brake failure on a Prius, and what they did to fix it.
The first option was a non-starter for obvious reasons, so we settled on the second option, but in the interest of brevity, we can skip over the details of most of the responses we received over the next week. In short, though, most responders stated that-
It was clear that we were going to be on our own on this one, so we bought a wiring diagram from an official Toyota source, and got down to finding and fixing the problem. Here is how-
The first thing we had to do was to relate physical brake system components on the vehicle to the wiring diagram, and then to establish the relationships between the various bits and pieces in the system that made the vehicle stop. Fortunately, for us, the description of the brake system included a complete list of all implicated components, as well as the symbols used to indicate their position on the wiring diagram.
Thus, we were able to trace and/or locate the origins of all input signals and follow their respective paths throughout the system, until the vehicle came to a full stop, in a manner of speaking. Now that we understood what we were looking at, we located all the various parts and components, and particularly the wiring that tied everything together.
However, before we began testing power and ground circuits, we had another look at the factory description of the brake system, and we found the following (paraphrased) information on (about) page 137-
The brake system incorporates a master cylinder stroke simulator and a master cylinder stroke cut-out valve. The function of the stroke simulator is to give the driver the impression that pressure on the brake pedal is slowing the vehicle down, when it is in fact, regenerative braking that is slowing the vehicle down. The function of the master cylinder stroke cut-out valve is to actually direct pressurized brake fluid into the brake lines to actuate the hydraulic brake system.
In combination, these two components provide a seamless and undetectable transition between regenerative and hydraulic braking, regardless of which mode of braking is predominant at any given moment. This process is controlled, monitored, and managed by the Skid Control Module, which also oversees and managed ABS, Traction Control, and Stability Control functions. (Italics added)
The problem now became a bit clearer, so we decided to start our investigation by testing the power feed first to the Skid Control Module relay, and then from there, to the actual module. Using the wiring diagram, we identified the correct connectors, wires, and pins, and somewhat surprisingly, the power feeds and ground circuits appeared to be OK.
To be absolutely sure about our findings, we decided to load the power feed circuits with a sensor simulator and dialled in a 1-Ohm resistance into each power circuit with the system powered up. All the power feeds appeared to be fine-except the power feed to the Skid Control Module that showed a voltage drop of just more than one volt.
Since this module was connected to the harness, the problem could be somewhere in the circuit, or the module, so we started looking for the site of the problem. We can gloss over the details of this search, except for saying that it took two days of testing wiring, and not finding the problem.
Eventually, we disconnected the Skid Control Module from its wiring, and the voltage drop disappeared. Reconnecting the wiring made the voltage drop re-appear, so we concluded that the module must be defective, but we had no idea what the actual problem with it was. It could be that an undiscovered fault in the wiring somewhere induced a fault in the module, or it could be that the defective module somehow induced a voltage drop in the wiring. We had no way of knowing for sure- one way or the other.
The upside was, however, that we had reduced the problem to two possible causes, down from an unknown number. Nonetheless, we were still not exactly sure, and the eye-watering price of a new Skid Control Module precluded any guesswork. We had to be sure before we condemned the module.
Just then, though, the diagnostician thought that it might be worth the effort of calling up an old acquaintance that used to work at a Toyota dealership to see if he could get hold of an official repair manual for us. As it turned out, this person was no longer active in the car repair trade, but he did allow us to use his credentials to buy a repair manual from an official Toyota source. The information we found was both simple and illuminating.
In a section on brake system failure modes and their symptoms, we found the information that if code C1300 keeps on coming back after clearing it (as it did on this vehicle), the Skid Control Module is defective and must be replaced. We also found the information (as a footnote in small print), that a defective Skid Control Module can cause total brake failure.
We also found parts that mentioned additional codes setting in various modules, including in the Power Steering Control and Cruise Control Modules, when the Skid Control Module fails. The actual progression of the fault was not described, but we now had enough information to recommend to the customer that we replace the Skid Control Module to fix the problem.
The customer authorized the module replacement, which required an extensive relearning and integration procedure, the cost of which we waived by way of apologizing for the nearly two-week delay in fixing the issue.
Nonetheless, the Skid Control Module replacement restored full braking function to the Prius. All the warning lights and alarms stayed off, no codes returned, and the Prius is still ferrying prospective clients between housing developments.
There is one more thing though- we mentioned “old-school” diagnostics a bit earlier. While there was a fair amount of testing, probing, measuring, and scoping during this painful process, we did not mention the most effective diagnostic method of all, known simply as “IF ALL ELSE FAILS, READ THE MANUAL”.
Sadly, this method was not available to us right from the start. Had it been, like it would have been had we been in say, the American market where manufacturers are obliged to share service information, we would have had the Prius back on the road in a day or two, as opposed to in two weeks, and this article would not have seen the light of day.
On the bright side, though, now you know what to do when next you encounter total brake failure on a Toyota Prius, which is perhaps the most important thing of all.
