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BMW's ELECTRONIC DIPSTICK

If you have never worked on a late-model BMW, you will be in for a surprise when you try to check the oil on certain models. You can't because there is no dipstick on NG6, N62TU, S65 and S85 engines. The "old-fashioned" dipstick has been replaced with an oil level sensor in the bottom of the oil pan. If the oil level is low, the CBS system illuminates a service ­reminder light and tells the driver to add oil.

In theory, it is a good idea because most oil warning lights that work off oil pressure don't come on until the crankcase is almost out of oil — which may be too late to add oil because the engine has suffered bearing damage. BMW's approach is to alert the driver if the oil level is down more than a quart. Unfortunately, the oil level sensor has been troublesome, sometimes creating false low oil warnings as well as various other driveability issues.

If the sensor is not reading accurately, it may indicate the oil level is low when it is not low. Since there is no way to actually check the oil level in the engine (no dipstick, remember?), most people simply add a quart of oil and wait for the warning light to go out. When the oil level still reads low, they think it needs a second quart, so in goes another quart. You can see where this is going. If the oil sensor is faulty, it may mislead the vehicle owner or a technician into adding too much oil and overfilling the crankcase.

BMW's service procedure for checking the oil level sensor does not require a factory scan tool or any other fancy electronic diagnostics. You simply drain the oil out of the engine, measure it, and then pour the correct amount of oil back into the engine. If the oil level sensor still reads low, replace the sensor and the problem is fixed.

Electronic problems in the CBS oil sensor circuit can also cause other problems. BMW’s technical service bulletin SI B 12 33 06 (issued October 2006) ­describes some of the seemingly unrelated problems that can be traced back to faults in the oil sensor ­circuit. These include:

• Engine won't crank (E46, E53, E83, E85, M54 & N62 models);

• Charging system is not working (E53, M54 and M62 models);

• "Engine temperature fault" message displayed, but the engine is not running hot or overheating (E60, E63, E64, E66 & N62 models); and

• False oxygen sensor fault codes (E60, E63, E64, E66 and N62 models).

All of these seemingly unrelated electrical shorts in the oil sensor circuit or the sensor itself can cause problems. If the oil sensor circuit fuse blows, it also kills power to other systems that share the same power supply (such as the starting circuit and charging circuit). So the fix is to check for a blown fuse in the oil sensor circuit, then track down and repair the fault that caused the fuse to blow (which usually turns out to be a short in the oil sensor wiring or sensor itself).

On E46, E83 and M54 models, check Fuse 30 and also Fuse 9. On E53 and M62 models, check Fuse 5. On E60, E63, E64, E65, E66 and N62 models, check Fuse 008 located in the IVM.