Alright, as you all may have seen, I've had a bit of a nightmare of a time with replacing the heater core in my car. First we broke the new part trying to install it, then it wouldn't start, then people sent us incorrect parts, and finally it came down to having simply missed a couple wires behind the relay panel upon reassembly the first time and we finally got her to roar back to life.

Well, now another electrical issue has cropped up, as the title implies, the ABS and Brake lights are now making a Christmas tree out of my instrument cluster. Only problem is we didn't touch any wheel/brake components, just the wiring under the dash, making the the obvious conclusion that its merely an electrical mixup somewhere under the dash (yay...)

Now, you may be thinking to yourself "well why did you put the knee panel and everything else all back together if you know this is a wiring problem?" Well, the problem didn't make itself visible to me until after everything was back together. Just starting the motor didn't activate the lights, so when we finally got it to start again, we didn't know the lights were on, so we assumed everything was good and put it all back together. They didn't light up until I started driving home from where we had been working on this job.

First impressions of the problem: As mentioned, they don't light up until after the wheels are moving. Paying close attention to this, I noticed two things: First, they light up after exactly one full rotation of the wheels. Second, they light up at exactly the same time. Also, the brake light illuminates when starting the motor only because the e brake is engaged, when its disengaged, it goes off as it should, but as soon as the wheels make that one revolution, it lights right back up in sync with the ABS light. Lastly, the driveability of the car is not impacted at all. I haven't noticed any odd sounds or effects or anything else that wasn't already present before the job. Though I'm pretty sure the ABS is definitely not functioning, but this conclusion could be premature. We had a pretty reasonable snow storm here last night and roads today weren't yet fully cleared and upon trying to stop for a light at only about 30mph, I started fishtailing like mad with no ABS kicking in. I say my conclusion is premature because I wasn't trying to stop in snow, I was trying to stop on a sheet of ice, and I had barely tapped the brakes so its possible that because I didn't fully apply the brakes to stop, the abs sensors didn't activate.

I should also note that before the heater core job, the ABS light only has ever come on when a) the ABS has engaged when braking in snowy conditions (as it should) and b) when all of the DIS lights (CEL, ABS, Brake, Battery, and Seatbelt etc) all come on at once for a couple seconds after starting the motor until the computer verifies that the systems are functioning and they shut off.

Conclusions: Its definitely wiring related, as the only components of the braking system that were remotely fiddled with during this job was the wiring. Either something is STILL not plugged in somewhere, or something is plugged in where it shouldn't be. ImTheDevil's first guess when I told him about the problem was that it sounds like the ABS controller isn't getting power, which is very feasible and if in fact the ABS *should* have kicked in when trying to stop earlier despite my gentle application of the brakes, it would make sense that the ABS controller isn't getting power and that's why the system didn't engage, causing me to damn near do a 180 degree spin across 4 lanes of traffic through an intersection (fortunately I was only going about 30mph as said and I was able to engine brake with clutch feathering through second gear and Tokyo drift style steering)

So, any thoughts on where to start poking around to remedy this? If someone could post up a wiring diagram for the ABS system that'd be one of the most useful things since it would allow me to trace the wires under the dash to make sure they're plugged in at the right places.

Much much thanks in advance

 

In all honesty, ive engaged ABS lots of times, and never had the light show up. Ill gig through the service manual and see what diagrams I can find

 

Do you have access to VAG-COM? Seems to me like you could potentially dispel your theory in a matter of seconds after hooking one up.

I went through a PLETHORA of ABS issues with my car. ABS module, both front ABS sensors were bad/damaged, broken front ABS ring, and ABS sensors not pushed in far enough. From my own experience chasing ABS crap, my guess is it's a sensor that's not reading. But you won't know for sure until you hook up a VAG-COM.

 

Thanks for the replies, any and all comments/suggestions are always welcome. Devil, if you read this, you've got a Hayne's manual right? Someone, possibly you (sorry, can't keep track of who tells me what anymore, school is baking my brains haha) told me that the Hayne's manual is an excellent source for wiring diagrams, can you confirm this and or shoot me a PDF scan of the diagram for the ABS systems?

Yea, I've had it not come on at times too, but when it has come on when engaging, its when its applied for a long time. If it just engages for a quick second, it usually doesn't come on. Thanks for offering to look for the diagrams, I greatly appreciate that, if you can't find anything though its no big, I'll figure out a way to get them eventually I'm sure.

I don't personally have access to VAG-COM, but my usual mechanic that I take my car to to have work done (punk wouldn't touch the heater though cuz of how much of a PITA it is, he's more of an engine/mechanical/performance worker so I don't blame him for not wanting to dicker with that job) has it but he's been closed for the holiday and now weekend so first thing Monday after class I was planning to head over there and see what we can find out. I'm really hoping for the computer to pop up a friendly message saying something along the lines of "no power getting to ABS controller" (though I'm pretty confident it WON'T say it that easily/blatantly, but I can hope haha) so I know for sure that it is in fact just another missed wire like the starting issue came down to.

 

Justin, I'll see what I can find in there and scan anything I can to send you. I was thinking again about this - an S4 I worked on last weekend had an ABS controller issue and not only did it have the DIS lights on, but the car also had a CEL code (I think it was P1606). He's having the module rebuilt. Point is, that code was the result of a fault in the controller. I have a feeling that if the module wasn't getting power, you'd have that CEL code.

What I'm curious about is whether, when the dash was dismounted to do the heater core, a wire or wires were pulled/pinched (yeah, back to that again). They'd have to come into the cabin en route through the fuse/relay panels and up the bundle to the ECU.

 

Awesome, if you can find something that'd be most excellent. If you can't get it into a PDF, its no big, I just prefer PDF cuz I have a PDF reader on my phone and I'd be able to far easier reference the diagrams on my phone while upside down under my dash hunting for wires than I could reference a computer.

As for the code, I'm going to swing by my buddy's house later tonight after I get off work and hook up his basic OBDII reader to see if there is a new code; having a preexisting code from the stupid SAP issue I have yet to figure out (been snowing and cold as hell this whole week so I've severely lacked the motivation to try and recheck the hoses for cracks) makes it impossible to know if the DIS lights are on in conjunction with the CEL for an ABS specific code so I'll check it out tonight and report back on if something else is there besides the P0411; If not, hopefully VAG-COM will be a little more helpful tomorrow.

As for the wiring case mentioned, that's the exact same thought that crossed my mind too. Those other two wires that we found that were causing the starting issues were like, 22 gauge wires, they were incredibly easy to miss so I have no doubt that its more likely than not that either something got plugged into the wrong spot, or another wire has been missed. Hence the quest for a wiring diagram for the ABS system, I'm hoping that'll enable me to look at the relay panel and see if there is a wire plugged in where the diagram says there should be a wire plugged in etc.

 

The easiest way to diagnose this is to get a VAGcom and check the ABS module for codes. It will save a LOT of tail-chasing. I know you don't have one, but get a hold of one to prevent further headaches.

 

^ Yea, I'd like to invest in the software and just have it on my laptop someday, but for now its gonna have to wait. For the case at hand, I'm still planning to swing by my mechanic's place tomorrow after class and see what we can dig up.

On that note, ran the DTCs after work today and the only code present was the P0411 as always, no new codes so while that knowledge doesn't fully rule out that the ABS controller isn't getting power as Devil proposed, it has me leaning away from it not getting power and more to something being plugged in where it shouldn't.

Also, the ABS is definitely not working. As mentioned a couple posts up, its been snowing like the north pole here today so I went and engaged in some hooliganry in a nearby, unplowed parking lot, mostly just to have fun with Quattro like we're all guilty of when winter hits, but partly to test the ABS functionality without being on a main road and risking losing control in the presence of other people. Sure enough, I was cutting loose left and right and there was absolutely no ABS kicking in when I would get going in a straight line and slam the brakes as if trying to stop under conditions possible when driving on main roads. After confirming that, I resumed some drifting, donuts, and general practice of controlling the car in snowy conditions (this is my excuse for if any blue guys ever harass me for being a hooligan in a parking lot, that I'm doing society a favor by practicing in a safe area how to regain control under snowy conditions without making matters worse and thus potentially preventing unsavory collisions with other vehicles/pedestrians).

That said, I'll report back soon as I have some readings from VAG-COM hopefully tomorrow, if not Tuesday at the latest and see where to go from there. Thanks again for the help/suggestions everyone

 

Alright, got the VAG-COM reading, have absolutely no clue what sense to make of it. My mechanic that I usually talk to wasn't in when I got to his shop so I just had one of his assistants hook up the computer and have it print out what the fault was so I could post it up here. This is what it says:

Control module part number: 4DO 907 279 L
Component and/or version: ABS/EDS BOSCH 5 4171
Software coding: 0265109043
Work Shop Code: BB 34171

1 Fault found:
00301-ABS Return Flow Pump (V39)
35-10 - - - Intermittent
35-10 - - - Intermittent


Anyone able to offer some insight as to what this is trying to tell me? My shot-in-the-dark guess is that this return flow pump isn't getting power but the ABS controller is?

 

Well folks, I'm back in business full steam Got beneath the dash again today to try and figure out what wire we missed or had in the wrong spot and got it figured out. Using the information provided by the VAG-COM scan noted above, I opened up the PDF Bentley manual to see if it had any useful information on the ABS, which it didn't really, mainly just showed steps on replacing the pump in the engine bay, but it did show me where the return flow pump relay was and from that, I was able to trace the wire back into the cabin and find more or less where it linked to the system. Using my associate's access to alldata, we got a diagram for fuses and relays under the dash and found which ones corresponded to the ABS and focusing on those, ended up finding that the wire coming off of the main ABS fuse (the fat, 50amp one in slot 18 sitting pretty much on top of the clutch pedal) was accidentally put on the fuel pump relay power post, instead of the ABS relay (slot 4) so we swapped that out, the lights stayed off and I went and found a clear, icy patch in a nearby parking lot and the system activated as normal

And so concludes the nightmare that is replacing a heater core. I state again for anyone who attempts this job in the future themselves to always take pictures before unplugging the components necessary to get the dash out to get to the heater core. It will save you a lot of headache when putting everything back together

And to those of you that have helped in the process of figuring out the aftermath issues of the post-heater core replacement: I thank you all for the suggestions