2001 V70 2.4T Brake Failure Service Urgent / Power Fluctuations / Engine Shuddering

[prwood]

Yesterday, after a 2.1 mile city drive, on the return trip, my car displayed 'BRAKE FAILURE SERVICE URGENT' on the DIM, while at the same time, the car's power (blowers, dashboard lights, displays) dimmed and then came back. The dashboard showed ABS and BRAKE warning lights. The car was drivable and the brakes were functioning normally. I pulled over and checked the brake fluid level, and it was full. I noticed that there was some washer fluid accumulated just on top of the brake fluid reservoir, so I thought perhaps some fluid had trickled underneath after I used the washers a few minutes earlier. I cleaned it up and restarted the car. The BRAKE FAILURE message was gone and I didn't have any other issues on the 2.1 mile drive home.

Today I went out again, and there were no issues on the 4.5 mile city drive to my destination, but on the drive back, the BRAKE FAILURE message came on immediately after starting the car, and I started to experience numerous power fluctuations. In addition, this time, each time there was a power fluctuation, it was also accompanied by a thunk from the engine. I pulled over to check under the hood. This time I didn't see any indication of washer fluid near the brake fluid reservoir (I didn't use the washers), so I don't think that's related. I checked on the accessory belt side of the engine. There was some power steering fluid leakage around the cap on the power steering fluid reservoir, which is fairly normal, but none of it seemed to have gotten near the accessory belt components. I restarted the car and resumed the drive home. The BRAKE FAILURE message came back, and while the car continued to be drivable and the brakes worked, it had numerous power fluctuations and engine thunks. I was able to get the car home without having it shut down or needing to stop. Once I got home and parked with the car in idle, I let it run for a few minutes. I tried revving the engine with the transmission in PARK a few times to various RPMS and didn't have any issues. I also put the car in DRIVE and held the brake, and revved the engine a few times that way, with no issues then either. So either whatever intermittent issue it was resolved itself, or it only happens when the car is underway and rolling in normal driving conditions.

My sense is that one of the components attached to the accessory belt is seizing up, which would cause the engine thunk symptom as well as the power fluctuation symptom. Now I need to track down which component it is. Any suggestions on how to track this down? My thought was to take off the accessory belt and try spinning all of the pulleys in the system to see if any were behaving incorrectly, but I'm not sure if this would be valid if the issue is intermittent, and/or only occurs when the engine is driving the belt. I'd be interested to hear any other thoughts on how to proceed.

Some may recall an earlier thread from June 2021 where I had an ENGINE SYSTEM SERVICE REQUIRED message on the DIM, along with an ECM-8410 5-volt power supply signal too low: viewtopic.php?f=9&t=97407 It was suggested at that time that it could be the alternator's voltage regulator. Since then, that message had gone away and I didn't have any other issues, so I didn't take any action to replace the regulator. I'm now wondering if the issue has progressed and the whole alternator needs to be replaced? How might I test this?

The only other thing I've noticed over the past few weeks is that the automatic transmission has been flaring a bit when shifting from 1st to 2nd gear. I don't know if that sort of issue could propagate back through the engine and cause the crankshaft pulley to seize up, or if that's a mechanical impossibility and a red herring.

I appreciate any thoughts folks have!

[motormikeyman]

These are all classic CEM failure electronic related issues. Brake failure warning, tach and speedo failure, various warning lamps, clunk in the front like you ran over a brick, no A/C or sunroof, weird shifting, trans stuck in limp-home-mode. Basically, the high and low speed CAN stops communicating and the car throws a fit. This can be caused by water getting into the CEM, corrosion on the CEM connector or it can be heat related. Xemodex has good diagnostics for this issue. In my case, I just had to drop the CEM, pull the main connector and clean both sides with electronic cleaner. It only took me a year to figure that out!

[prwood]

How does the CEM failure cause a physical clunk?

[jonesg]

How does the CEM failure cause a physical clunk?

It's probably the tyranny slamming into 2nd. Had the same issue on a Ford.
Turning ignition off and on again usually resets the computer and it stops... for a while.

[TisMe]

When I had the same terrifying brake message appear, I plugged in the OBD reader and discovered a faulty ABS wheel sensor. I see you referencing an ECM code though so you've probably already pulled codes and found no ABS faults. And no other quirks accompanied my symptom so probably unrelated to yours, but figured I'd offer my experience with that warning.

Definitely start with mikey's suggestion of cleaning the ECM contacts before you peruse Xemodex's rebuilds, the system going haywire is a nod toward electronics in my opinion. In my other car, I put my 2nd ECU into a ziplock bag to assist with moisture protection... felt a little ghetto, but so far so good.

[prwood]

How does the CEM failure cause a physical clunk?

It's probably the tyranny slamming into 2nd. Had the same issue on a Ford.
Turning ignition off and on again usually resets the computer and it stops... for a while.

Is this the sort of thing that it would cause the transmission to do several times in succession? Yesterday I was driving at constant speed down a straight, level road, and the engine would clunk three or four times with a 1/2 second gap in between each clunk, then it would be fine for a minute, then it would happen again a few more times. Each time with the lights and blowers stopping and then starting. I wouldn't have thought the transmission was causing the clunk, since I wasn't in a situation where shifting should have occurred. But if this CEM issue causes the transmission to randomly shift even when not requested, I could understand.

[prwood]

Just to clarify, are we talking about CEM (central electronic module) or ECM (engine control module) issues? As I understand it, the CEM is located in the cabin underneath the steering wheel, while the ECM is located in the engine bay in a box with the TCM. I know I had mentioned an ECM code, but that was in reference to an incident last summer (which had no symptoms other than 'engine system service required').

I actually haven't pulled the codes from this time around (yesterday I was just concerned with getting the car back home), but I can do that today and see if anything helpful comes up.

[prwood]

These are all classic CEM failure electronic related issues. Brake failure warning, tach and speedo failure, various warning lamps, clunk in the front like you ran over a brick, no A/C or sunroof, weird shifting, trans stuck in limp-home-mode. Basically, the high and low speed CAN stops communicating and the car throws a fit. This can be caused by water getting into the CEM, corrosion on the CEM connector or it can be heat related. Xemodex has good diagnostics for this issue. In my case, I just had to drop the CEM, pull the main connector and clean both sides with electronic cleaner. It only took me a year to figure that out!

In relation to heat issues with the CEM, is this the sort of thing that could be triggered by running the car's HVAC on max heat?

[TisMe]

Just to clarify, are we talking about CEM (central electronic module) or ECM (engine control module) issues? As I understand it, the CEM is located in the cabin underneath the steering wheel, while the ECM is located in the engine bay in a box with the TCM. I know I had mentioned an ECM code, but that was in reference to an incident last summer (which had no symptoms other than 'engine system service required').

I actually haven't pulled the codes from this time around (yesterday I was just concerned with getting the car back home), but I can do that today and see if anything helpful comes up.

I was referencing the CEM as everyone else, excuse my late-night fat fingers. When I saw the ECM code my brain glitched.

If you have a faulty ABS sensor, it's likely just one of a multitude of things considering the range of symptoms you have. Again, in my case the only "issue" I had was the big flashy brake warning. My money is on mikey's diagnosis. Perhaps codes will tell you more.

[TisMe]

If you take the car out again, try running in manual geartronic mode. Someone correct me if I'm wrong, but if this solves the clunking that would probably insinuate a miscommunication in the car's brain, further insinuating previous hypothesis.