C1253/138 After Brand New Actuator

I recently entered into the Prius world by buying a 2005 with about 198000 miles on it. It came to me with a known bad brake actuator and my first order of business was to get it swapped out. After trying two different used units from junkyards, I gave up and bought a new Toyota unit. I installed it, bled the system, I'm pretty sure I did all the relearns and resets, and all was good. For about 3,000 miles or so.

This is the car I bought for my daughter to drive to college, so she's put most of the miles on it. She told me that after a hard braking event the three brake/ABS/VSC lights came on. I brought it back into the garage and found that it had a C1253 with INF code 138: MTT input is 3.5V or less for at least 1 sec (0.2 sec during initial check) when main relay 1 monitor is 9.5V or more and motor relay 2 is on.

Knowing that things get wonky with old batteries, I put a new battery in it and everything seemed okay again. I couldn't get it to fail on me so I sent it off. It happened again while she was away at school so, since she's a less-experienced driver, I hooked up the trailer and went and fetched it (and her) to get it back into my garage to work on it.

I followed the flowchart:

1. Active test of motor relay 1 and 2. Worked fine - relays clicked, pump ran.
2. Inspect ABS-1 and ABS-2 fuse. Checked resistance all along the fusible link assembly and had 0 Ohms.
3. Inspect brake actuator assy. 6 different resistance tests, and they were all (probably) good. Probably? Well, BM2 to MTT and MB1 to MTT were both 500 Ohms exactly, as opposed to the "about 33 Ohms" that the flowchart calls for. It's a brand new unit from Toyota and I saw another thread or two where others indicated that the 500 Ohm reading was expected. So I moved on to the next test.
4. Check harness and connector between the skid ECU and the brake actuator. Three resistance tests, and all three passed.
5. Use the scanner to measure the voltage from the accumulator pressure sender. This was also within spec

The only thing left after that is "replace skid control ecu". So I went out to the local junkyard this morning and pulled a unit with the same part number off of another '05. After figuring out how to get to all three bolts without removing the steering column, I bought it home, got it installed in my car and... same code showed up.

I fiddled around a bunch with trying to clear the memory, reset the yaw sensor, re-run the linear valve offset learn, to no avail. If I cleared out the codes, they would come back immediately if I ran the manual activation of the ABS MTR relays from my scanner. Eventually, I performed the memory clear, left the battery unplugged for a while, and installed the resistor that came with the new brake actuator. I reconnected the battery, then did the linear valve offset relearn and the yaw sensor relearn. No codes came back, so I then went through and manually activated the MTR relays about a dozen times. Still no codes. I gave it a quite drive for a couple miles, including some hard braking. Still no codes.

I don't know if I did anything or if it's just going to have the same problem again. The only thing that really changed is that I have the resistor replaced now, but I haven't read anything so far that indicates that the resistor would make any difference other than what it sounds like. I suppose it's certainly possible that I grabbed a used skid control ECU that is broken in the same way as my original one, but I haven't seen any way to test one - it seems like if you don't find any other problems, replace that.

Is there anything else that might not be on that chart which could cause this sort of problem? I have no other codes, but could a system that isn't totally bled get that sort of result? I hate to call it "fixed", because it's done this to me before, and I'm very open to suggestions!

 

I've got an Autel MaxiCOM MK808S and I used its bleed function where it ran me around the all four corners and actuated the various valves.

I will say that I've noticed that when the system is working, there's a lot of groaning noises from the rear brakes. Not like a metal on metal noise, more like the noises that the linear offset valve relearn process makes. I have no idea if that has anything to do with anything.

After letting the car sit overnight, I just started it up and the lights are back on, same code/INF. I'm kind of at my wit's end here...

 

I pulled the relays off and bench tested them and they worked perfectly. I wasn't able to pull the full amperage, so it's entirely possible that the contacts aren't.. uh.. contacting. For good measure, I swapped out the relays with ones I harvested from another car, but had the same problem. I can absolutely hear the relays clicking on and the pump running, though, when I do the commanded activation of the MTR1 and MTR2 relays with my scanner.

 

I spent some time glaring at the car and calculating how far into the parking lot I'd need to move it so that I didn't catch my garage on fire when I doused the car in gasoline and tossed a match on it. And then got back to work.

The freeze frame data has indicated every single time that the MTT voltage is exactly 0. So I pulled off the cowling, backprobed the MTT pin on the accumulator connection, powered up the car, and started pumping the brake pedal. As soon as the brake pump activated, I read 7 volts. I'm not very good with numbers, but I think that is greater than 0. That tells me that the accumulator is probably doing what it needs to be doing.

I went in and backprobed the top connector on the skid ECU next, again selecting the red wire which is the MTT one. Powered up the car, started pumping the brakes, and as soon as the brake pump turned out, I got the same 7 volts. That tells me that the wiring is solid from the accumulator to the skid ECU. I had also cleared the codes to make sure that I wasn't just wiggling loose wires and fixing the problem as I observed it. Sure enough, the code came back with the same 0 MTT volts.

This leads me to believe that the accumulator is good and the wiring is good but that I have managed to score myself a skid ECU that is bad in the same way as my original one.

 

I did swap the relays around and grabbed a set from another car and tried them as well with no change in the behavior. I also only checked from the top of the bus bar to the bottom of the fusible links, and didn't try to load test the circuit from the positive terminal all the way to the actuator.

That said, though, I'm measuring 7.something volts on the MTT wire at the skid ECU connector while the ABS system is reporting that the MTT value read by the skid control unit is 0 volts. I don't fully understand how the system works, but is it a bad assumption that if the actuator is coming on, giving good brake pressure, and outputting >3.5V to the MTT circuit that the relays, fuses, and wires from the PDC to the actuator are testing good? And further, if I'm reading >3.5 volts on the MTT pin at the skid control ECU connector then either the connector is faulty or the skid control ECU is faulty?

 

Well, I wasn't driving the car at the time, so it's kind of a 'customer states' situation, but my daughter did say that she thought it happened after she had to slam on the brakes. I don't know if it was hard enough to engage ABS, for example, or even if it happened at some point previously and that's just when she happened to notice. The ABS buzzer was removed before I bought the car so I just left it rattling around in the glove box, so it's not like there would have been any auditory notification.

I did inspect all the connectors and there's no corrosion or wear that I can see. I used a bit of very thin wire to "front probe" the connectors as well as back probed them. Not quite a pin drag test, so it's possible I assume that the socket in the connector could be wallowed out. But visually inspecting it, there doesn't appear to be any damage, but it is an awfully small socket, so it's hard to see.

The wiring diagram I have shows this:

And the Toyota test procedure is to check resistance from pin 33 at the actuator -- the source of the voltage -- to pin 29 on the S7 connector on the skid control ecu. That measured at 0 Ohms of resistance:



I haven't tried a voltage drop test across that wire, so it's possible there could be a drop, but I do get the exact same voltage reading when I backprobe the A2 pin 33 and the S7 pin 29.

Interesting, though, that maybe it's trying to read the voltage at a time when the pump shouldn't be running... Like it's getting (or not getting) some other signal that makes it think that the brake pump should be running, it's seeing that it's not, and then it's throwing the code. I'll have to do some more reading to see what condition causes the skid control ECU to see what the actuator motor is doing.

 

Unfortunately, I can't find a value for the MTT voltage in the live data anywhere; it only appears in the freeze frame data. Techstream might be able to pull that, but I haven't remembered to have my laptop, the car, and my cable all in the same place at the same time.

I do, however, have a scope, so that's a great idea. That's also a really good point about the error referring specifically to the MTR2 relay. If it only uses the second relay for ABS function (unless the first isn't working), then sitting still shouldn't be commanding ABS activation. There is something about the voltage reading being under 3.5V for 0.2 seconds "during initial check", so maybe when the module powers up, it's trying to fire the relay as part of a boot process or something? Either way, you're absolutely right that I need to verify that.

The car is going to be away from me for the next week or so, but as soon as it returns, I'll break out the scope and see what I can see in there.

Also, even though I hate to fire the parts cannon, I did order up another new skid control ECU with what I think is the latest revision (89540-47130). That way it'll be there by the time the car comes back to me and I won't have to wait around with the car clogging up the garage while I wait for it to ship in.

 

The car came back home the other day so I grabbed the new-to-me Skid Control ECU and plugged it in. I didn't bother physically removing the old one yet; I just pulled the lower dash panel and popped the four connectors (S7-S10) off the installed unit and plugged them into the new one. After the reset and relearn, I had the exact same problem. So the issue is not likely to be the skid control unit.

Next up, I hooked up my scope and connected channel 1 to the MTT pin and to ground, channel 2 to the MR1 and MR1+ pins, and channel 3 to the MR2 and MR2+ pins. So, in theory, I should be seeing the input to the skid control ECU from the motor as well as the outputs to the relay control for each of the ABS motor relays. Baseline:


When I power the car on, both relay control channels spike, but the MTR1 voltage remains high.


After pumping the brake pedal a few times, I could hear the motor run and got this result:



So far, so good, in theory, right? So on your recommendation, I pulled out the MTR1 relay and then pumped the brakes a bunch of times. The motor did not run at all:



So I can confirm for certain that the motor does not run unless the MTR1 relay is in place. I checked continuity between my backprobe and the pin on the connector so I can confirm that the backprobe is inserted fully.

 

This might be a rather embarrassing resolution...

I was continuing to look for problems... Ground was good and I was probing the right pins. I noticed, however, that the s10 connector was missing its locking tab and was slightly higher than the others. I gave it a little more of a push and ran my tests again. Voltage on MTT and no codes.

Rather than pull the whole dash apart again to get to the upper retaining nut, I decided to try just connecting the old new skid control ECU and see what happened.

No codes, normal operation, and I even took it out in the snow and did a lot of ABS activation. Still fine. We'll see if that is the real fix, but if S10 was a little loose, that would prevent it from activating the MTR2 relay. It assumes that there is some other way to activate that relay because the manual activation from the scan tool does turn the motor on. But I suppose time will tell.