C1241, C1256 help please

I would have said more like 0.6 to 0.8 or so; it's always a good solid fraction of a volt in my experience. Larger in ON than in ACC (and in READY, the sign changes, as the MFD is then upstream of the battery).

I'm just home from a trip and kind of skimmed this thread ... where did it say that?

 

I missed this on my first skim through, but: were you talking about the two-page "cic12565.pdf" that dolj posted, and you were looking at readings of the wheel pressure sensors and the accumulator pressure sensor?

How did you read those pressures exactly? I notice that two-page document only gives steps for reading the accumulator pressure sensor using the "hand-held tester" (precursor of Techstream; you can use Techstream the same way). I didn't see that it offered any other way to get the readings ... did you find a pinout somewhere else and measure with a meter? What readings did you see, and under what conditions?

Freeze-frames were available for two of the codes in the screenshot you posted in #2. Raytheeagle asked in #3 about the inf codes in those freeze-frames. Have you had a chance to retrieve those?

The essence of the C1256 is the skid ECU is telling you the accumulator/pump system can't get the pressure up and keep it up. The main differential-diagnosis question is: is the skid ECU telling you the truth? (then replace the actuator assembly) or is the skid ECU imagining things (then replace it).

You should be able to get a pretty good sense one way or the other by doing things that affect the accumulator pressure (like using some up by applying the brakes, and watching the pump bring it back up) and watching the pressure sensor values.

 

Sounds like we have a winner

 

Something still puzzles me though, because if the "inspection procedure provided earlier in this post" is the two-pager that dolj posted, it only describes how to read one pressure sensor, using the handheld tester (or, nowadays, Techstream). Here you've mentioned checking more than one pressure reading, and you haven't said how. It's harder for us to follow along at home if we can't be sure what you're doing.

Skid ECU has two separate "systems" inside, this pertains to "system 1". The linear solenoid could not get enough current during braking, or, voltage measured at System 1's BS input dropped below 8.9 V when not braking, for 3 seconds or more, or, the 12 V backup source is measured below 8.5 V when the main relay is opened (again, as measured in the skid ECU), or the output of system 1's internal +5V regulator has gone outside the range 4.7 to 5.3 volts for 50 ms or longer.

It's worth noting the conditions on the BS input make it clear we're not talking about any sort of imperceptibly brief pulldown of the aux battery voltage by transient currents; if anything pulled down the whole 12 V bus below 8.9 V for 3 seconds or more, you'd certainly notice. That's why a code like this is more likely to be reflecting some issue in how the power gets to the ECU, and the repair manual lists trouble areas like the ABS no. 1 relay, the wire harness and connectors, the backup capacitor source, etc., rather than pointing any fingers at the battery.

Accumulator pressure is less than 14.62 MPa for at least half a second "when motor relay is malfunctioning" (I'm not sure I trust the English translation here, but this may indicate the ECU has tried to start the pump and hasn't succeeded yet), or the pump is running, nobody's using the brakes, so the pressure ought to be rising, but it's below 14.62 MPa and not rising much over a second or more, or any wheel cylinder pressure was below the target value for at least half a second (so, not just an instant) while braking and the vehicle in motion, and the accumulator pressure is low.

What are the various voltages shown in your C1241 freeze frame as of the moment the code was recorded? What are the various target and actual pressures in your C1256 freeze frame as of the moment that code was recorded?

The manual does advise that certain causes of a C1241 might also lead to a C1256 being recorded, so it could be that you have one primary problem and another one secondary to it.

 

The snowflake doesn't take you to live readings, it takes you to "freeze frame" readings. That is, it is showing you exactly what all the live readings were at the exact instant the computer decided to log that trouble code. ("Freeze frame" ... snowflake ... get it?)

To view actual live data happening now, you can use the "live data" button over at the left of the screen.

 

You changed the battery and accumulator and still have a problem?

 

I’m no Toyota certified mechanic but the fact the pump runs every 10 seconds for 5 seconds seems like the accumulator is bad to me. If anyone here knew they would say.. so change one or the other and see what happens. Sometimes shotgunning a repair is the best you can do. Plus I can assure you it’s still cheaper than taking it to the dealer.

 

If you had a car with a normal pedal operated brake system and the petal went slowly to the floor at a light, what would you do? The accumulator pump is basically the same. It just keeps the pressure in reserve, like a brake booster holds vacuum in reserve until you need it. I think you already know what the problem is.