intermittent ABS light

[exdeath]

ABS light is coming on once or twice per drive session. Stays on for a min or two then goes off on it's own.

No loss of function, it's just the lamp coming on for no reason.

ABS scanner shows (using K2A or K20 key):

C1805 Mismatched PCM and/or ABS-TC Module

Drive with scanner's real time diagnostic shows that all 4 wheel sensors are functioning properly and that the ABS pump is coming on when I lock up the brakes.

Factory PCM and ABS unit, same tune I've always had that has TC disabled, no changes anything. Recently had the battery out and replace the hydroboost, but the brake system itself was completely untouched.

Any ideas?

 

[ac427cobra]

Clean both front ABS sensors with brakekleen.

:thumbsup::coolman::beer:

 

[exdeath]

Clean both front ABS sensors with brakekleen.

:thumbsup::coolman::beer:

Seriously? Even though they were reading fine in real time on the scanner with no glitches, same speed readings which agreed with the speedo, etc ? Signal noise possibly that doesn't show up on a scan tool?

 

[ac427cobra]

Seriously? Even though they were reading fine in real time on the scanner with no glitches, same speed readings which agreed with the speedo, etc ? Signal noise possibly that doesn't show up on a scan tool?

When you're dealing with intermittent stuff, you have to start with the basics.

 

[03cobra#694]

I agree on the sensors.All kinds of crap collects on them.Maybe stupid,but i`m sure you checked the fluid.

 

[CPViolation]

Sure it's the ABS light and not the traction control?
Just a thought..
Jeff

 

[exdeath]

Sure it's the ABS light and not the traction control?
Just a thought..
Jeff

Traction control is disabled in tune (button doesn't do anything).

It's the orange ((ABS)) light.

Comes on at any random time (idle, cruising, etc, not during anything special like braking traction loss).

MT2500 scanner pulled "C1805 Mismatched PCM and/or ABS-TC Module" which suggests that the PCM or ABS unit have been swapped, which is not the case.

 

[KoBra2.2blk04vt]

That happened to me also, I checked the abs ring and it was loose, cleaned axle with brake cleaner, put a couple of drops of lock tight pushed back into place evenly and that fixed it.

 

[exdeath]

I'll go ahead and pull the wheels off and get up close and personal, but I was just thinking logically and figured out why it might throw a strange code like that and otherwise be working properly:

Broken tooth or loose ring or something, sensor still works and gives a valid speed reading, but at the signal level the tooth count/width is off and doesn't match the reluctor profile that the electronics are calibrated to, hence thinking the modules are mismatched.

 

[Red Turtle]

I just went through the ABS/traction control light thing. It turned out to be my ABS module, and very thankful that it was under the ESP that I had!

 

[mitrls10]

C1805 usually shows up as a bad abs or tc module. You have to do pinpoint and pinout tests usually but since it just started happening and you said you recently replaced the hydroboost kinda makes me think you might have a slight air bubble or pocket in the abs and when it gets to a certain valve and piston in the modulator causes a pressure differential and throws the light. I think you have to have a scanner to bleed the abs on these so you might have to go into 1 and bleed the abs. I'd try that 1st, it might be a coincedence that it threw the light after hydroboost replacement but usually you can trace a prob back to the last thing that was done to it.. Good luck.

 

[exdeath]

C1805 usually shows up as a bad abs or tc module. You have to do pinpoint and pinout tests usually but since it just started happening and you said you recently replaced the hydroboost kinda makes me think you might have a slight air bubble or pocket in the abs and when it gets to a certain valve and piston in the modulator causes a pressure differential and throws the light. I think you have to have a scanner to bleed the abs on these so you might have to go into 1 and bleed the abs. I'd try that 1st, it might be a coincedence that it threw the light after hydroboost replacement but usually you can trace a prob back to the last thing that was done to it.. Good luck.

Brake system was completely untouched other than unbolting the master cylinder and pushing it aside, and reservoir was topped off the whole time. Wouldn't hurt to flush and replace the fluid and bleed it though, it's the factory fluid still.

Haven't proceeded yet, just brainstorming and organizing a list of things in my head for when I pick a day to put it on the lift. As I noted, everything actually works perfect and it's no rush, it's just the light is annoying.

 

[mitrls10]

Ok so the lines were never taken loose mmk gotcha.. Even though the scan tool reads good though sometimes you still have to get a oscilliscope and check the waveforms of the wheel sensors and the operation of the model and you can see a defect in the waveform.. Just stick to the basics and visually check everything 1st to see if there is any damage to any component to and from the module, unplugged connectors, frayed wires, cracked sensor. Stuff like that and if you can find anything electrical tests are needed..

 

[exdeath]

Front rings where caked over and the pickup looked like someone dragged a magnet through the sand for a few miles. Hosed it all off with some brake cleaner until the reluctor wheels were nice and silver again and cleaned off the pickups. Rears looked new still since they aren't next to the rotor picking up everything that comes off them and the pads.

Light still came on, but not sure if it was just the stored code. The VERUS scan tool where I did this had a dead battery and was being recharged, I'll have to stop by the other shop for the MT2500 and clear it out. Hopefully it doesn't come back this time

 

[exdeath]

So just an update (since I hate it when people post problems but never the outcome).

I ended up clearing the code after cleaning the sensors in the front and it came back almost immediately (C1805).

Just for giggles, I returned to stock tune, idled and peddled it around the block; enough time and speed for the ABS to self test and calibrate. Then reflashed back to the custom tune. This morning I took a few detours and made a 20 mile trip and no ABS light (normally it comes back the same day I clear it).

Hmm...

Which has me wondering... can the ECU flash memory get corrupted? If the ECU detects a CRC failure in the flash memory on boot does it fall back to a hard coded limp mode and perhaps not have a proper version number to reference to the ABS controller, thus the mismatch C code?

Also, for future reference, these are Bosch 5.3 units?