Cracking a Toyota's code

Jan. 1, 2020
For more than three decades, I almost always have looked forward to working on problems with new systems on a vehicle. After my first Prius brake-by-wire job, I’m a little more cautious these days!

For more than three decades, I almost always have looked forward to working on problems with new systems on a vehicle. After my first Prius brake-by-wire job, I’m a little more cautious these days!

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Verifying this brake complaint didn’t take long. There were no brakes. Well, maybe just a little if you don’t mind an adrenaline rush!
Buzzer noises filled my ears and every warning indicator even remotely related to chassis or hybrid systems were ablaze when the vehicle was readied up in power-on mode. As a bonus, the brake pedal pulsed more radically than a car in a panic Antilock Braking System (ABS) stop.

I’m no stranger to hybrids or ABS, but because the two systems on this vehicle are married together for the benefit of fuel economy, I decided to invest a few dollars in the factory web-based info to print out most everything Toyota published on the subject. With a few mouse clicks (thanks to the National Automotive Service Task Force’s (NASTF) www.oem1stop.com) I’m inside Toyota’s Technical Information System website. After filling up a one-inch, three-ring binder to the point of bulging and running my printer out of ink, I decided to give America’s tree population a break and begin diagnostics on this very unique and challenging system.

Not wanting to perform brain surgery before taking the patient’s blood pressure and pulse, I decided to give the vehicle a routine visual inspection and Diagnostic Trouble Code (DTC) check followed by a thorough check for Technical Service Bulletins (TSBs).

Preliminary Findings There were no TSBs on the symptoms. “Looks like I’ll be heading into uncharted waters,” I thought to myself. The 12-volt battery checked great at 12.65 volts, and every fuse under the hood and
dash was good with available power. I did notice a body ground connection displaying some rust and corrosion on the left front inner fender.

On to the DTC check. My scan tool pulled three current DTCs. All three would come right back after being cleared with the tool. There also were no TSBs related to these DTCs. After this quick prelim/visual check I thought to myself, “I have a Mastertech with Toyota factory software. I’ve downloaded all the factory service information on the system. How hard can this be?” After all, I am an expert by the humorous definition. “Expert = a drip under pressure.” Yep, that’s me.

Under pressure doesn’t begin to describe the challenge on this car. Not only is this car overly complicated, but Teresa, the lady who drives this little Prius, is absolutely the most important person in the world to me — she’s my wife! We’ve all heard the saying “a mechanic’s car is usually the last to get fixed.” Not true with this car. We had owned it only a couple of months, and with 78,000 miles on the odometer, I wasn’t even expecting brake pad wear out (thanks to regenerative braking) much less a total braking system loss.

The DTCs I found were in the Skid Control Module (SCM). These active codes were C1300, (ABS ECU malfunction), C1310 (high voltage system malfunction) and C1313 (open circuit in main relay). I also found codes stored in history. The first was U0129 (no ABS communications), found in both the hybrid control ECU and cruise control module, and the second was U0121 (lost communication with the ABS module) in the Electric Motor Powering Steering (EMPS). Not wanting to dive in too deep too quickly, I swapped relays (known good relay for the ABS main relay) with no change although the pulsing pedal would go away with the ABS main relay removed.

Because the factory scan tool could communicate with the ABS and SCM,even displaying wheel speed PIDs changing with road speed and pedal stroke sensor changing with pedal presses, it at least appeared that the ABS controller was not completely brain dead. Just to be on the safe side, I decided to connect my AES Wave LineSPI Diagnostic Link Connector (DLC) break out box and scope out the high-speed Controller Area Network (CAN) data bus.

As expected, it looked as good as any CAN bus pattern I’ve scoped. The Toyota factory manual says if C1300 is cleared and comes back again, the Skid control module must be replaced. This little feat requires a total instrument panel removal and $1,280 (wholesale) for the module. Nearly $1,300 is a lot of money so if I suspect the SCM is the root cause (by sheer nature of an ECU malfunction DTC) I had better be 100 percent sure before proceeding.

Back to the Books
Looking at the factory theory of operation, I see the system works in the following manner:

“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 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 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 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.”

Now I must have had a very rare exception because my “failsafe” didn’t feel too safe with the rest of those hydraulic brake components giving me back up to the electronically controlled system. One thing was certain though: this was my first problem on a brake by wire system and it had to be fixed right.

Being an old GM guy at heart, I began to rephrase the Toyota components and theory in GM terminology the best I could. Looking at the schematic, I observed the wheel speed sensors connecting into the skid control ECU along with the typical Toyota Vehicle Stability Control (VSC) inputs such as steering angle, yaw and lateral acceleration. The main brain for the hybrid system (Hybrid ECU) also reported to the Skid Control Module via the CAN bus.

The wheel speed sensor input told me that the Skid Control Module was in charge of ABS. The hybrid ECU communication relationship told me the regenerative braking is performed by varying amounts of field activity to MG2 (Motor Generator 2 – connected to the final drive/wheels) to give progressing amounts of what I would call engine braking (electric motor braking actually), which takes care of 85 percent of the vehicle’s stopping force (front brakes usually do about that much on a FWD car). This is while the remaining 15 percent is accomplished with hydraulically applied friction brakes (the ones we all know and love) courtesy a commanded apply of the Brake Actuator Module.

Looking at that module, the GM terminology side of my brain said, “Oh, an ABS hydraulic modulator assembly.” Well we all know that today’s ABS-equipped vehicles can apply braking force either in an ABS stop or even in a less than locked up situation (dynamic rear proportioning) by simply running the modulator pump motor and activating the correct solenoids in the modulator assembly to apply hydraulic br
aking force to the necessary drive wheel(s) in the event of a traction control event. Thinking back to the original high-voltage vehicle, the GM EV1, I recalled that this was similar to how it accomplished ots hydraulic braking. Only the EV1 used a ball/screw motor-based (instead of solenoid and pump based) Traction Control System (TCS) like brake-by-wire system.

On this Toyota, aside from a failsafe mode of driver’s foot application actually getting the attention of the brake hydraulics, the electronics are completely in charge. Years ago I tested an early Prius system’s pad apply during a road test by rigging a test light to glow when a front brake caliper applied a pad to the rotor. Its front pads never touched the rotor until the car got below 5 mph. Now that’s regen braking!

Now fast-forwarding to this vehicle, I can fully see how if Murphy’s Law applied to too many components in this brake-by-wire system, you might not have much, if any, braking ability. Looking under the hood at the Prius’ braking system, I saw a pump on the Brake Actuator (ABS modulator assembly in my mind) located just behind and under the hybrid inverter/converter assembly. The brake actuato
r assembly contains the usual solenoids for ABS/TCS duties plus the typical electric pump motor, along with a pressure accumulator and pressure sensor. There is also something called a linear solenoid valve that does as the name implies – applies varying amounts of braking force instead of full on or off like a system completely controlled with solenoids so that the apply of the brakes via the driver’s foot or even TCS/VSC can be accomplished with a bit of progressive finesse. (No bouncing pedal.)

Hey, I have a bouncing pedal on this car! Another critical part I discovered on this car was the back-up power supply (22 12-volt capacitors in parallel in a black box) next to the 12-volt battery in the right rear fender/interior section of the car. That makes sense. If you have a brake-by-wire system running on 12-volts what happens if the power goes down? You will need to have some backup power much like airbags do with their energy storage modules.

To give the driver seamless transition between brake-by-wire, regenerative braking and back-up hydraulic braking, there is a master cylinder stroke simulator. No matter what the case is for braking, the driver gets the same feel at the pedal. The last and most critical part of this challenging system I noticed was the master cylinder stroke cut valve. It’s important role in this system is to cut (or should I say redirect) the flow of master cylinder applied hydraulic pressure from the stroke simulator into the actual brake lines going to the front calipers and rear wheel cylinders. The Skid Control Module controls this master cylinder stroke cut valve. Now it all starts to make sense, but what about the other DTCs for things like relay problems and so forth?

Let’s Get Serious About Diagnostics Enough theory; my head is hurting! I called a Toyota factory hotline acquainta
nce and posed the problem to him. I also posted the problem on iATN (International Automotive Technicians Network) and sent emails out to a couple of other trainers who specialize in hybrid vehicles. The response from the group made sense: multiple DTCs in the same system are often the result of wiring issues such as power and ground problems.

Let’s look at that suspicious ground right off the bat. It didn’t look all that bad when it came off, but I gave it a good cleaning and tightening. No change. Next was checking the power feeds to the relays, the Skid Control Module (main brain) and brake actuator assembly. There was good continuity to the grounds and 12-plus volts to each component. “But what about a load induced voltage drop,” I thought to myself.

I grabbed a load simulator box and wired 1 ohm in each high current circuit powered up. Every circuit seemed good with a tenth or less of a volt of voltage drop except for the circuit supplying the brake actuator module. Getting to that module, by the way, involved powering down the high voltage system by removing the hybrid battery pack service plug (sans 1000 volt class 0 gloves) and removing the inverter cover (after the wipers and cowling are removed) so that I could measure and verify the absence of high voltage (those big capacitors should discharge).

My arm is going to be lying across that inverter (cover removed) just to remove and probe the brake actuator module’s connector and I don’t want any nasty surprises.

With the inverter cover safely removed for access, I load tested the brake-by-wire circuit and discovered it had nearly 1.0 volt of voltage drop. This might be the problem. Wouldn’t that be nice? Find the bad connector somewhere and avoid replacing that $1,300 Skid Control Module? Wouldn’t that be sweet? After following the schematic, I saw where the circuit feeding that brake actuator module traveled from an under hood relay then passed through the bulkhead. Next, the circuit went through a fuse called out in the manual as “F16,” then back through the bulkhead and to the under hood brake actuator assembly.

Now how much sense did this make? Run a wire from a fuse/relay center that’s under the hood through the bulkhead to a fuse and then run it back through the bulkhead to the device needing the power under the hood. That just didn’t make sense to me, but who am I to argue with a service manual that so far has been 100 percent accurate?

Now comes the hard part: finding fuse F16. I checked the under dash fuse panel, removed all sorts of lower dash pads and trim pieces, and finally even removed the entire IP dash pad. There was no sign of a fuse by that name (or continuity in this circuit) in any fuse panel, in-line fuse holder or even the sign of a fusible link or similar protection device anywhere I could find. But I still had a 1 volt drop on that circuit (when powered up with a substitute load) so where to now?

The Human Side of Diagnostics The driver of the little Prius (as you recall, my wife Teresa) is getting impatient at my embarrassingly long diagnostic process. As an afterma
rket trainer, my time at home is sometimes weekends only and on top of that, I’m just plain slow! After a few “discussions” about why she’s not back in her car after several weeks she blurts out, “Honey, maybe the dealer knows what they are doing a little more than you. Couldn’t you just tow it over there?”

My pride hit the floor and then I got mad. Next it was, “Honey, can I do anything to help you with the Prius? I can hold a flashlight or hand you tools — anything.”

“Finally, she understands,” I thought to myself. But now I feel bad. She’s right. I’m getting whipped by this thing. It’s time to just try something, but $1,300 is a lot of money to just try something. So I hand her the Skid Control Module and tell her to find me one of these parts used and cheap.

Thirty minutes later she tells me she found one on an auto recycler’s eBay store for $75! For that money, I’ll try it. “You’re a genius honey!” I screamed to her, forgetting the low blow comment she made about towing it to the dealer.

The part came two days later, and I plugged it in. Next I took my Toyota/Mastertech scan tool and perform the required initialization of the linear solenoid and calibration of the steering angle sensor and I was ready for the moment of truth! A 1 volt drop on that circuit with the phantom F16 fuse or not, here we go. Brake apply, power button pressed, car fixed! No telltales, no DTCs and no pulsing pedal. I hate to say this but once in a while the sensible thing to do is to stop “splitting atoms” and just try a part that makes sense.

The Last Word
Finally it was time for a road test, which went without a hitch, and then back to the shop to drop that Toyota smart key fob for that overly smart brake-by-wire Prius back in my common sense smart wife’s hand. After a big hug she admitted, “Honey, I’m sorry about the ‘tow the car to the dealer’ comment. I never lost faith in your mechanic skills – I was just trying to light a fire under you to get my Prius back!”

For the technical last word, now that I understand the system, it makes sense the Skid Control Module could cause all the symptoms experienced. But what about the 1 volt drop on the power feed to the Brake Actuator Module? Was it the cause of the Skid Control Module’s demise or a normal condition in my measurements that could possibly be explained by some information missing from the service manual? Will this good used Skid Control Module fail someday, too? Good questions to ponder but for now I’m focused on celebrating two accomplishments: the return of marital bliss and cracking the code of Toyota’s hybrid brake-by-wire system.

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