C1256/1391--anyone ever try using DOT4 brake fluid?

After further research, I've ordered a new brake actuator from a Toyota dealer. With the information gleaned from this forum, it was clear my pump was running 5 seconds, off 6 seconds, on 5 seconds, off 6 seconds, continuously. New actuator cost is $1,173 delivered to my door.

So another maintenance project. I'll flush the brake fluid prior to installing the new actuator, in an effort to start with fresh brake fluid in the new actuator.

Thanks for the assistance!

 

I'll probably tear the existing unit apart, just for kicks. I'll test the electrical connections for ohm resistance compared to Repair Manual specs. Then look to see if the source of my unit failure is obvious upon visual inspection. I suspect the electrical contacts are all good. And expect to find a leaky valve seat or seal, since it was bleeding off brake fluid pressure like an open faucet.

 

I think the new actuator comes with a new resistor, that's installed way up under the driver's side dash. Anyone replace that resistor before? I'll try to replace mine, if I can reach it under the dash without having to strip out all of the dash...

Anything else I'd be wise to replace @73,500 miles, while I've got the inverter moved out of the way, and access is opened up in that area?

 

Thanks! I've long wished more people would do that.

 

I tested the electrical pins for ohms resistance, following the inspection procedure for the brake actuator assembly, from the 2006 Repair Manual.


My unit failed the bottom two ohm tests. Pins 1 and 2 (BM2 and BM1) were both supposed to read approximately 10 ohms from pin 33 (MTT). Both connections tested 500 ohms on my DVOM. Got some sort of an electrical defect in my actuator.

Will next spend some time disassembling and inspecting my actuator. New replacement actuator should arrive in a couple more days.

 

Having trouble getting the valve body cover removed. I might need to remove the accumulator first. I see a warning notice on the accumulator, "Do Not Disassemble"...

This first pic shows the valve body cover (the black plastic shell) partially separated, with some valves visible in that 1/4" of separation gap. Not sure what the secret is to removing the black plastic cover completely from the aluminum valve body.


This second pic shows the warning label, on the end of what I presume is the accumulator. I'm also not sure how the black accumulator body is removed from the aluminum housing it's fastened to. I see a nut on the end, in the middle of the warning label. Will try to remove that nut, see if that allows the black accumulator housing to be remove from the aluminum valve body.


I think the black cylinder is the accumulator. Anyone knows differently, please sound in...

 

Accumulator simply unscrews from the aluminum valve body, using the 18mm nut on the end cap. The accumulator is heavy, to contain high pressure without bursting. Here's a pic showing the detached accumulator, and what's under the plastic cover of the black plastic housing.


I'll try to separate the black plastic housing from the aluminum valve body, but I don't think there's going to be much of anything to visually diagnose. I think it'll look much like a valve body off an automatic transmission

 

Here's a couple pics of the motor. It looks to me as though both the motor and the accumulator could be swapped out from a used actuator, if one of those two parts were defective.


I tried to get these two pieces apart, and had no luck. Muscling the black plastic housing off the aluminum valve body was going to destroy the enclosed electronic circuit board. There must be a secret to separating these two pieces, or perhaps they're designed to go together and not come apart. There were 3 Allen head screws holding these two pieces together.



I think my brake actuator failure was within this valve body and electrical circuit board portion of my actuator. Since I had abnormal ohm readings across two of the terminals.

My conclusion...:
When these actuators fail, they're probably a throwaway item. It appears that the motor and accumulator could be swapped out from a salvaged accumulator. So if the motor has failed, a used motor could maybe be installed to repair the faulty actuator motor. My motor seemed to be running fine. Just cycling over and over again, non-stop.

I think the accumulator pressure is controlled and maintained by the valve body. And the valve body isn't something a DIY guy is going to be able to repair in his garage. I don't think replacing the accumulator body with a used accumulator is going to salvage a faulty actuator. The poster earlier in this thread that stated he'd rebuilt two brake actuators? I'd like to know how he went about it. Because other than replacing a faulty motor, I don't see how the units could be DIY rebuilt.

 

Hi.
Good to know. Did swapping out those accumulators fix a faulty brake actuator for you? If so, I'll hang onto my accumulator. Just in case...
I should receive a new actuator and resistor from a Toyota dealer in 2 or 3 days. From a Toyota dealer in Kansas.

 

Dxta got it further apart in this thread (with pics).

I don't guarantee it was non-destructively.

 

I ohm tested the brand new brake actuator terminals, from pins 1 and 2 (BM2 and BM1) to pin 33 (MTT), and again read 0.5 Kohm, or 500 ohms. Because the motor sounded fine on my faulty actuator. I think my ohm meter is reading accurately, with 0.2 ohms across the leads. And I've tested higher ohm resistance down a longer length of copper wire in the past with increasing ohm readings. So for whatever reason, both of my actuators ohm test 0.5 Kohm across the terminals the Toyota Repair Manual indicates should be reading ~10 ohms. Whatever...

As I stated, the motor on the faulty unit sounded fine. It cycled on and off continuously. I think my motor was healthy. I guess I'll hang onto the old motor, just in case. I didn't try reinstalling the motor after removing it from the faulty actuator. But in looking it over, I think a failing motor could be replaced with a functioning motor off a used actuator.

 

Toyota sends out a new resistor, supposedly matched to the motor in the new brake actuator. My resistor is labeled 239.4 mOhm, or 0.239 Ohms. So I ohm tested this resistor with my DVOM, and got 0.3 Ohms. I read 0.1 Ohms across the probes of my DVOM, so 0.1 + 0.239 Ohms equals 0.329 Ohms. My Klein DVOM only reads out to 1/10 of an of Ohms. All this to say, my DVOM is reading accurately, and I have no idea why the 2006 Toyota repair manual indicates the Ohm reading across pins 1 and 2 (BM2 and BM1) to pin 33 (MTT) should read approximately 10 Ohms. Both of my brake actuators read 500 Ohms across those terminals. Just something to be aware of should others try to troubleshoot their brake actuator by following the Repair Manual inspection process.

Here's a picture showing the new 239.4 mOhms resistor I received with the new brake actuator.

Gonna try to install this new resistor next, without tearing the dash out of the Prius.

 

Removing the original resistor, and installing the new resistor was easy. 5 minutes.

Here's a picture of my original resistor. Note this original resistor is 144.8 mOhms, versus the 239.4 mOhms of the new resistor. I read that Toyota matches the resistors to the actuator motor, and this appears to be true. I just read the Ohms on the original 0.1448 Ohm resistor with my DVOM as 0.2 Ohms. So my DVOM is providing accurate ohm readings.

Got the new actuator installed with the new resistor. Not a terrible job. Just gotta go about it methodically. Take a few pics while removing the old actuator, to help get all the harnesses and fasteners re-installed properly.

Now I need to bleed the brakes. Will be using Techstream.

 

Brake bleeding complete. I used around about 50oz DOT 3 brake fluid. No issues bleeding the brakes using Techstream.

Actuator pump doesn't run rampant any longer. No DTCs following a Techstream Health Check... DONE.