ABS diagnostics with Appcar for U0121

[GR5677]

I think that the ABS control unit has failed (getting code U0121) on my '08. This might be a bit of a dumb question but shouldn't I be able to see ABS under the list of modules that comes up when I use AppCar? I'm assuming it's not showing up because it's dead (I've checked fuses) but before I rip it out thought I'd ask!

Secondly can I just pull the electronic unit and replace that or do I need to pull the entire pump/ valve assembly too?

 

[GR5677]

So I pulled out the module and sent it off to ECU testing. They told me there was a "hardware" fault in the unit but no obvious reason for the U0121 error. Got the repaired unit back, reinstalled it, bled brakes etc and everything worked as it should. ABS module now visible on AppCar

10 miles later, bing! All the warning lights come back on, speedo drops to zero, gears won't shift etc. Limping it home it then fixed itself after about 5 miles. First ABS light went off and speedo came back, then ESP and 4wd, just leaving MIL.

Took it out again this morning, MIL light on to start with, but after second restart that cleared too.... A few miles later, bing! And around we go again. It's showing U0121 again and AppCar can't see the ABS module.

 

[KJowner]

if the module is ok its got to be wiring or a connection at fault.

 

[GR5677]

if the module is ok its got to be wiring or a connection at fault.

Problem I'm having now is finding wiring diagrams. The ones I normally use at cardiagn,com see to have the "wrong" ABS unit connector in their schematics. Mine has fewer pins so that's got me a bit baffled. I have managed to confirm that there is power going to the ABS module.

 

[KJowner]

That's a nightmare, i only have manuals for the older Jeeps.

 

[GR5677]

So the ABS module connector is 38 pin, not 47 as shown in the wiring schematic. It actually makes a lot more sense as the large pins are the main power and ground: pins 1 & 25 +ve at one end of the connector, pins 13 & 38 -ve at the other end. The pin number assignments in the wiring schematic seem to be right e.g pin 32 tests as +ve power with ignition from fuse M37.

Not that this helps me much as I've confirmed that the connector at the module has power and earth as expected. But the ABS module is not showing up on the CAN-C bus as though it's just not there. Next thing to check is CAN connections at pins 14 & 26.

 

[GR5677]

So.. no problems with the wiring of the CAN bus back to the connector. Put it back together and had the same U0121 error.

Went for a pootle round the block, same issues as before, no speedo, no gear shifting etc. Went a couple of miles, turned round and bing! ABS light goes off, speedo comes back, I can shift out of 4wd etc.

When I got home I plugged in the diagnostics without turning the Jeep off. ABS module now showing up and reporting the following errors, most of which look like the other side of U0121. So I've got an intermittent communication fault that doesn't seem to be a result of the wiring... I'm a bit stumped.

 

[Airwreckc]

So.. no problems with the wiring of the CAN bus back to the connector. Put it back together and had the same U0121 error.

Went for a pootle round the block, same issues as before, no speedo, no gear shifting etc. Went a couple of miles, turned round and bing! ABS light goes off, speedo comes back, I can shift out of 4wd etc.

When I got home I plugged in the diagnostics without turning the Jeep off. ABS module now showing up and reporting the following errors, most of which look like the other side of U0121. So I've got an intermittent communication fault that doesn't seem to be a result of the wiring... I'm a bit stumped.

 

[Airwreckc]

Did you ever figure out what the problem is? My 2012 isn’t showing the ABS system either (same U0121 code). It runs and drives fine, and I believe the fuses are fine. It has only 43,000 miles on it so I’d be surprised if the wiring or electronics are bad, but haven’t spent any time further diagnosing the problem.

 

[GR5677]

Not yet... It's weird as so far it has failed four times, since refitting it, and every time has been when going slowly (sub 5mph). Each time it has sorted itself out again after a while, first ABS comes back with speedo and gearshift, then traction control/ESP a bit later. When it fails it's not drivable as it doesn't seem to know whether it's moving or not. I'm pretty sure it's not a wiring fault as I've checked all the wires going between the ABS unit and the TIPM so far as possible. It seems to be some kind of CAN bus communication problem, which makes me wonder if the "main" ECU is the culprit but as to how to test that? Mine has 30k miles on it and I have had to replace the transfer case control unit and fit a fuel pump relay bypass, so low mileage is clearly no barrier to electrical gremlins!

It's currently sat on my drive whilst I decide what to do with it - I'm rather bored of trying to fix it for now and the byways are dry so I don't need it at the moment.

 

[Airwreckc]

Not yet... It's weird as so far it has failed four times, since refitting it, and every time has been when going slowly (sub 5mph). Each time it has sorted itself out again after a while, first ABS comes back with speedo and gearshift, then traction control/ESP a bit later. When it fails it's not drivable as it doesn't seem to know whether it's moving or not. I'm pretty sure it's not a wiring fault as I've checked all the wires going between the ABS unit and the TIPM so far as possible. It seems to be some kind of CAN bus communication problem, which makes me wonder if the "main" ECU is the culprit but as to how to test that? Mine has 30k miles on it and I have had to replace the transfer case control unit and fit a fuel pump relay bypass, so low mileage is clearly no barrier to electrical gremlins!

It's currently sat on my drive whilst I decide what to do with it - I'm rather bored of trying to fix it for now and the byways are dry so I don't need it at the moment.

If you are willing to be without the PCM for a bit, there are places that will diagnose if it has a repairable fault.

 

[GR5677]

I've spoken to ECU Testing this morning and they want me to sent the ABS unit back in (under warranty) so they can have a poke around, so that's the next step for now. Cost me a couple of knuckles and a lost 10mm socket last time...

 

[GR5677]

Got a call from ECU Testing yesterday afternoon - they had put the ABS unit on the bench to test it and bingo! "intermittent communication fault". The chap I spoke to didn't know the ins and outs as he wasn't from their testing Lab, but apparently comms to the unit were dropping out and then reconnecting, which is exactly how the problems were presenting in the car. I would guess they didn't leave it connected up long enough last time for the fault to occur (and they had found a different problem). Anyway it has been "fixed" and was delivered back to me this morning, so fingers crossed!

 

[GR5677]

Right, so clearly I am cursed. Put the ABS unit back in, brake pipes al connected, TIPM refitted, and then I dropped the +ve power lead as I was reconnecting it to the TIPM and it hit the body of the ECM. Shouldn't have been a problem as I had the battery disconnect open, but "flash" sparky, bugger... Finished putting it back together, and sure enough ECM is not showing up on diagnostics and car won't start. Oh, how I laughed. ABS module shows up now though.