E46 M3 brakes don’t feel right? How to tell if it’s the ABS/DSC unit

The brakes just didn’t feel right.

Not a sinking pedal, the usual sign of a failing master cylinder, but something just seemed amiss with the brakes on our E46-chassis M3. The pedal felt soft but didn’t go all the way to the floor.

We figured we had two options: ignore it (hey, the car did stop) or investigate further.

The usual, proper first step in a situation like this? Bleed the brakes.

All good until we got to the left-rear: no fluid.

Like, none came out of the left-rear caliper.

Air pocket? Something simple? So we buttoned up everything and set out to exercise the brakes and traction control a bit.

Still didn’t feel right, and bleeding confirmed the situation.

No fluid came out of the left-rear line, either.

Was there a block in the line?

So we started unhooking things along the way. Still, no fluid moving in that left-rear channel.

And that trend continued all the way to the ABS/DSC unit. No fluid flowed out of its left-rear port.

We had a problem.

A firm called Module Master repairs these ABS/DSC control modules, but they only service the electronic side of the unit, not the hydraulic side. All signs pointed towards a problem on our hydraulic side–and it’s not something you can just crack open.

Could we dunk the unit in the ultrasonic cleaner? Would it then work properly or just work partially?

Since this involved our car’s brakes, we decided to replace the unit.

Time to buy a new ABS/DSC control unit.

We found new ones listed online for about $3000, but every outlet reported the same dire news: no stock.

So we called Rennie at Redline Bimmers since he always knows all.

He checked his sources and then called us back.

NLA.

None on the shelf.

Anywhere.

So we had to shop used.

Our M3 is a 2004 model, and our research showed that we needed a unit for a later E46-chassis M3, not the earlier variant or even a standard 3 Series.

We called our friends in the BMW community. No one had a spare.

We bought a used one for a 2003-’06 BMW M3 via eBay for $766. The seller seemed to specialize in used BMW parts. We took a bit of a gamble, but it seemed like our only option.

Now, of course, we hear that we might have been able to separate the electronics from the hydraulic unit and only replace the latter, but we bought the whole unit and had it swapped in. At this point, we wanted it done, so we made plans for a local shop to handle the job.

Since we replaced the electronic half of the unit, it would require reprograming. No trouble, the shop’s owner told us, they could handle the entire job.

And then the reality: Oh, we can’t do the reprogramming.

We were sent home with properly bled brakes but three warning lights on the dash: ABS, DSC and low tires. Apparently that’s the sign of a borked ABS/DSC system.

So, with the unit replaced but not reprogrammed, we called our local independent BMW shops.

Could they reprogram our unit?

The answers ranged from no to no.

We were told that properly reprogramming the ABS/DSC unit would require specialized software available for some five figures.

But, we wondered, could our Autel MaxiPRO MP808 scan tool do the job?

Answer: No.

Once again, Rennie at Redline Bimmer to the answer: While up our way, he brought his Autologic diagnostic tool and, in about 10 minutes, reprogrammed our ABS/DSC unit and cleared our warning lights.