c1253/c1256 after 12v battery replacement

I have been tracking this since I replaced the 12V pack at 71k miles (it was still good, but hey, it's been 4+ years of service!) The guys at e-learnaid deserve major props!

The battery replacement went fine, I used a small 12V backup device to keep all of my settings. Instead of being smart, I used a a current limited bench supply set for 12.00V @ 5.5A.

The battery replacement was very uneventful, in fact my concern about current limiting was completely uncalled for as toyota was smart and used powerpole style connectors on the positive terminal, eliminating the risk of the positive wire hitting the body after removal. Good idea guys!

A few hundred miles later, I noticed that the brake pump was running a LOT more than usual. Being the geek that I am, there just happened to be an Autoenginuity scan tool with the toyota enhancement that just so happened to be within arms reach.

I noticed that there was the c1253 and C1256 error codes, not sure if they were there before. The brake pump was running normally before, but now it's running a long time. So, the symptoms match the new codes.

I have seen anecdotal evidence of the same error codes or similar problems after battery replacement, since this is an expensive part, some sleuthing was in order before shelling out almost $3k for something like this, right? Even then, I'm not cool with the unplugging the !@$!@% (!!!) hydraulics and re-bleeding the 8 bazillion lines running into this monster. So, it would most likely be a dealer thing.

So... in the scantool goes..

OK... before fellow priuschatters go off on a tangent.. DO NOT EVER TRY THIS IF YOU DON'T FULLY UNDERSTAND THE RISKS... READ THE ENTIRE SERVICE MANUAL, AND MAKE SURE THAT YOU HAVE ALL OF THE SOFTWARE, TOOLS AND SKILL BEFORE EVEN CONSIDERING THIS... EVEN THEN, FOR SAFETY'S SAKE TAKE IT TO A DEALER!!!

What I observed was that The pressure accumulator was 'leaking' somehow. every 1-2 minutes or so, one could see Accumulator Pressure Sensor 1 go from about 3.51V to about 3.39V, this was regardless of brake usage, the voltages would drift even when sitting on the driveway while in ready without brake pedal applied.. At about 3.3V, the booster pump would run (for longer than usual, to bring it back up to 3.51V.) This would indicate that there was a leak in the accumulator, or one of the high pressure lines. No brake fluid leaks could be found anywhere, and the level in the tank did not budge any.

Note that no warning lights came on at any time during this episode, but the error codes were there... odd. Still it's not good to have the brake pump run like that...

Time for more snooping.

My assumption is that one of the solenoids was 'stuck/leaky'. Each caliper is fed by a 'half bridge' of proportioning solenoids. (i.e. one valve provides something from the high pressure (SLAxx), and the other is 'no pressure' (SLRxx.) Turning these on in sequence allows the software to modulate hydraulic pressure going to each cylinder. Of course, like any half bridge, there is also potential for a leakage path.

Monitoring cylinder pressure revealed nothing unusual, just that pushing on the brakes increases the pressure in all cylinders normally. No change in mileage over this time indicates that the calipers aren't stuck, or otherwise messed up. Putting the car up on jackstands and giving the wheels a spin would have been too obvious, no?

Although the stuff below is not mentioned in the service manual for those error codes, I wanted to run them to satisfy my curiosity. Be sure to follow the procedures outlined in the service manual for active test on the braking actuator! The instructions/hints are in the 'Brake Control' section at around page 48 or so, the ones to look into are ACC pattern and pump pattern, but there might have been some other ones. The repair manual is not very clear on this, but read up on it..

After much clicking and what sounded like a garden hose running.

...........silence.........

If you've ever bled the rear brakes on the prius, you'll know the sound...

I left the data list running for the next 100 miles or so, and it appears that whatever was causing the leakage went away. The pressure was absolutely constant when you didn't push the brakes. So everything appears normal.

So far, I've put about 6k mi on it, and no strange brake pump noises, and no error codes. Actually, the brakes feel better now than they have in a very long time.

So, there you have it.. Hope this is helpful to someone out there.

Major props to the autoenginuity too, it's paid for itself almost 10 times over already!

 

I drove for awhile during the whole pump cycling fiasco, partly gathering data, partly worried that I would be spending an unplanned $3k. Forcing the cylinders to do hard braking was one of the first things I tried after thinking about how odd it was sounding. Normal, even very hard braking was no use in clearing this. I did not try actuating ABS though. That would be insane, it might work, but it might not.

So, I think that running active tests might have reseated one of the not so obvious proportioning solenoids. There are a lot more than the 8 driving the cylinders, there are some gating ones for failsafe. Actuator test cycles all of them IIRC.

The garden hose noises during self test made that pretty clear.

Thanks to fellow priuschatters, I think the prius is probably one of the most well understood high tech pieces of equipment out there. This is just another piece to add to that body of knowledge...

 

One of the reasons I got the Autoenginuity was the ability to bleed the brakes. And yes, it does work. You should however follow the directions in the service manual, if I remember correctly, There is a small glitch in the software though. At least when I did the bleeding. It asks you to remove the relays for both front and back brakes. The service manual says otherwise though. It says to put the relays in for the back. This makes sense, since the pump has to be running to get fluid out to the back. Also, the arrow keys aren't the keys on your keyboard, there are some buttons on the screen that you click on. I went with the service manual instructions and all went well. The fluid was surprisingly clear after 71k miles. For those of you that are curious, it's been >32k mi since I started this thread, and the accumulator is still fine. No error codes, and no strange noises. So, I think the problem was actually fixed with that test.