May 292012

When an issue pops up on an older BMW, there is no reason to be in the dark as to the cause. There are many resources to help you diagnose the problem on your car, and as the expression goes knowledge is power. Even if you don’t intend to try and fix the problem yourself, you’re in a better position if you’ve researched probable causes and costs of repair.

My E36 M3 and E39 M5 have been fairly trouble-free lately by BMW standards.  Recently though a couple of issues have popped up. On my M3 the cruise control stopped working, and on the M5 the Dynamic Stability Control (DSC) and brake lights lit up on the dashboard.

It was time to start researching. Online forums hold a wealth of knowledge, and its advisable to search all DIY sections and threads before starting a new thread on a question that may have been discussed many times before. A Bentley repair manual is the gold standard of information on your car’s systems and repairs, and is available in electronic formats. is also a great resource — just type in your car VIN number and see full diagrams of every part of your car, complete with part numbers and estimated prices.

I don’t use the cruise control very much, so I don’t know exactly when it went out. Compared to some other issues, there was not a lot of online discussion threads in the forums on this topic. Initially I was thinking maybe the computer module went bad. Then I read a suggestion to visually check the engine bay to make sure the cable hadn’t become dislodged. It’s smart to work through possible causes, going from easiest to hardest.

Cruise Cable

No luck on the cable as far as I could see. I believe the most probable cause is the cruise control switch behind the foot panel, near the back of the brake pedal. In 2010 I replaced the brake light switch, which was a real PITA. It’s right next to the cruise control switch, and I may have dislodged or damaged it unintentionally. Or it may have just died — my car is own it’s third brake light switch.

Full diagram from RealOEM — cruise switch circled in red

Shopping around can save a lot of money on parts. The BMW MSRP for this switch is $80, but I’ve found it online for $10. It’s on the way, and hopefully it will fix the cruise control. Even if it doesn’t, at that cost it’s smart to replace the switch.

For the M5, the lights are off now. But they probably will be back. When I first saw them, it reminded me of the ABS module repair I did on my 2001 530i. In this case however the ABS light wasn’t on, just DSC and brake. The brake light alone could be pads, but then DSC wouldn’t show. The most benign cause possible was low brake fluid, so I checked that first.

BMW makes that a little tricky on the E39, since the brake fluid reservoir is located under the driver side microfilter assembly. The owner’s manual tells you the location, but not how to access. It’s not hard, but you have to remove the plastic assembly and be careful not to damage the hood latch sensor which is attached to it.

Microfilter assembly

Brake fluid reservoir

As I suspected, the fluid level was fine. Another potential cause was the wheel sensor. That one was easier to rule out. Since I had previously unlocked my instrument cluster, I could test the sensor by choosing the option that shows speed in kilometers per hour, since this is measured by that sensor. It worked, so the sensor should be good.

The research continues on that one, and I’ll see how long it is before the malfunction returns. Fortunately for me neither of these issues affects the safe operation of the car, so I’ve got plenty of time for diagnosis and redress. This kind of sleuthing is smart for any BMW owner not covered by an original warranty, and should be easy for anyone accustomed to looking up information online.

If I can fix these issues myself, I’ll find that DIY success very satisfying. But like I say above, even if you don’t do the work yourself you should educate yourself on the problem. That alone can save some dollars and headaches.

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