ABS going berserk

[sisyphus]

'01 Jetta sedan, about 280K miles. Over the winter I got an ABS light on the dash, it had been intermittent and upon scanning w/ VCDS got the code that said the speed sensor on the front left was faulty. Fair enough. So I replaced it. ABS light still on, so I reset the codes etc and it still came back as a bad speed sensor. So I replaced it again a week later. Never know what you're going to get at the local parts store, you know. STILL, the computer couldn't see the speed sensor, so I said, "Eff it," got the plug and replaced the wiring as far up as I could go before going through the bodywork inside the wheel well (soldered connections). Same result, no joy. Then the ABS light went off, in that I had ABS again for a week. Cool. Then it was on again, off again, all spring and now here the ABS kicks in when braking for no reason on dry pavement, when the car says I have ABS. If I don't have it at the moment the car brakes fine, provided I don't actually need ABS in that moment in particular. I suspect the ABS is actually doing something while I'm driving the car sometimes, making that noise. But it's hard to investigate.
Curiously, twice in this period (from about January until this morning) the brake light came on in the instrument cluster, flashing and beeping. Low brake fluid. I have no idea where the brake fluid has gone. This car has the better brakes of the two TDI's I own, no history of bad calipers, anything like that.
I'm at a loss here. Do I replace the ABS module? How hard a job is that?

 

[Diesel Fumes]

Did you lose fluid? It must have gone somewhere if you lost it? Or am I misunderstanding? My abs light was coming on randomly and it went away after I removed the battery, cleaned up the posts and connectors and there was a ground under the battery that was badly corroded so I fixed that. Issue hasn't come up again but this is probably not your issue though it couldn't hurt to check

 

[STDOUBT]

Never know what you're going to get at the local parts store, you know.

Chinese Replacement Auto Part?
Something else to consider, is VW recommends butt connectors vs. soldering.
You've seen how slender the wiring is on these cars. They're barely there.

 

[Zak99b5]

The BRAKE light will flash while beeping if the ABS module is very unhappy. I had three bad wheel speed sensors and always got the flashing (along with the solid ABS light). When I replaced two of them, the BRAKE light issue went away, and I only had the solid ABS light. Of course, that went out when I replaced the last one.

The issue you have could be a problem with the tone ring--have you checked it? Have you used VCDS to see what the wheel speed sensors are reporting?

 

[oilhammer]

In your rusty area, I would pull the rotor off and inspect the tone ring for damage.

Brake fluid is normally displaced from the reservoir as the linings wear down.

 

[turbocharged798]

Sounds like cracked solder joints in the ABS module. I spent months chasing down random speed sensor codes and random engagement. Turned out after cracking open the ABS module the solder joints for the connector were cracked. Reflows the solder to the connector and resealed the module and no problems since. And yes I would not use solder since the harnesses are not tinned.

 

[sisyphus]

Thanks, all. Yes, I've checked the tone ring; I even wire brushed the back side of it. Also observed the wheel speed sensor in action with VCDS. Reads zero, when I have the ABS light on. Otherwise it reads fine.
I'm leaning toward the ABS unit itself; it's least likely to be the wheel speed sensor.
How hard is it to get at one of these? I'm not excited about it, I can see it down there.

 

[IndigoBlueWagon]

I'd keep looking for a broken wire. I replaced the ABS unit and just about everything else to finally find a broken wire in the engine bay.

 

[wonneber]

Maybe check for continuity at the ABS controller plug to each sensor?
Also for continuity to ground. Shouldn't be any.

 

[KrashDH]

One thing to check with the wheel sensors...if the correct gap is not maintained to the tone ring, it will set the ABS off. I replaced my OEM sensor with an aftermarket one and the gap from the tone ring to the magnet is larger than the other 3. If I'm almost full lock with the steering wheel and I hit the brakes, ABS triggers. It doesn't trigger in any other situations, just sharp turn and activating the brakes (into a parking spot or something).

But check that gap. Those sensors are a royal PITA to change out too, that's why I haven't done anything about it. May not be your issue, but one more thing to troubleshoot/eliminate.

 

[sisyphus]

I thought about that; what is the gap specification?

 

[KrashDH]

I thought about that; what is the gap specification?

Couldn't tell you. You'd have to look it up in the FSM if it's in there. If not, measure your other ones