Found this on another forum....thought I'd pass it on in case it's of interest to anyone else.

Okay, first things first. There are two different types of FSS. Stage 2 and Stage 3. Stage 2 came with the 104, 119, 120, 606 diesel, and all 111 engines after 6/1/97. Stage 3 comes with the 112, 113, and 137 engines. I'm going to copy this from the FSS manual.

"The FSS is an engine oil maintenance system based on actual vehicle requirements. This means that in addition to the pure time and mileage criteria, various other factors are taken into consideration in determining the actual inspection intervals required. These include quality factors coded with the HHT as well as other evaluation factors which take the type of operation and therefore actual wear and deterioration into consideration. After dropping below a warning limit the remaining mileage or remaining time appears in the instrument cluster and the tool symbol is shown in the display.
The Stage 3 FSS also has a filling level indicator for the engine oil. If the oil level is too low or too high, it is indicated in the text field and the oil can symbol appears in the display.
The complexity of the FSS makes it necessary for it to be equipped with its own processor in the instrument cluster.
FSS obtains the time information from the electronic clock integrated into the instrument cluster. The other data required is received via the CAN data bus from the traction system control module and engine control module:

-4 wheel speed signals for calculation of speed and mileage
-Coolant temperature (a cold climate will make the FSS come up sooner)
-Engine oil temperature(Cold oil will make the FSS come up sooner)
-Oil level(Running the engine with low oil makes the FSS come up sooner)
-RPM(High RPMs will make the FSS come up sooner)
-Load torque(Heavy throttle foot will make the FSS come up sooner)
-Oil correction factor (Stage 3 only)


EVALUATION FACTORS(dependent on actual operating conditions)

-Load torque
-RPM
-Coolant temperature used only as substitute value for engine oil temperature in Stage 3 systems.
-Oil level only (Stage 2)
-The oil level, engine oil temperature, and oil correction factor are provided by the engine control module via the oil sensor (oil level/temperature/quality) Stage 3 only
-Engine oil temperature, Stage 3"

What the "correction factor" is is this: Let's say your FSS says you're due for an A-service in 5,000 more miles. But you don't want to wait, so you take your car in for an oil change sooner. The FSS oil sensor will recognize this new oil, and extend the A-service until a later mileage. It's like a "bonus". But the absolute maximum mileage between services is 20,000 miles. So if you change your oil every 3,000 miles, the FSS will keep extending your next service until 20,000 miles have gone by. That's the point you will have to take it in for an A or B service. This also works for topping up the oil. If you're a quart low, the car will tell you it's a quart low. Once you add that quart, the sensor will recognize a quart of clean, fresh oil and extend the FSS due date accordingly.

Also, if you don't drive your car much, it will call for a B-service if the maximum amount of time goes by. I recently did a B-service on a 2000 S500 with just over 8,000 miles on it. The car hadn't had a service since it was new in January of 2000. The FSS timer recognized this and called for a B-service.

Read more: Flexible Service System operation - MBWorld.org Forums

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14 E250 Bluetec 4Matic "Sinclair", Palladium Silver on Black, 158k miles
06 E320 CDI "Rutherford", Black on Tan, 177k mi, Stage 1 tune, tuned TCU
91 300D "Otis", Smoke Silver on Tan, 144k mi, wastegate conversion, ALDA delete

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