PeachParts Mercedes-Benz Forum - View Single Post

JimF · #8 10-04-2007, 01:47 PM

In my business, MTBF is a ‘sacred’ cow . . . all sub-systems are designed such that the overall system meets or exceeds the MTBF goal. Every system spec calls for a minimum MTBF be met and proven by analysis and then tested to show that it does.

Obviously, ‘cost’ plays a big part of this since it will cost more to design/build the system to meet this goal. If the goal is ‘tough’ to reach, then more ‘reliable’ parts must be used in order to meet the meet the overall system MTBF. So ‘cost’ is a very key element in meeting MTBF but it takes a back seat when the MTBF must be demonstrated.

This system described is a military system. For a commercial system, MTBF is probably treated as secondary since ‘cost’ is the key factor.

I have a favorite: the crankshaft sensor! On early MBs such as my car, it’s a ‘hunk’ of magnetic material wrapped in ‘wire’. A more straightforward device doesn’t exist! How un-reliable can a piece of magnetic material wrapped in wire be??? The MTBF should be through the roof!

In 10+ years at my tech’s shop, I’ve never seen one go bad! How could they unless it gets mechanically damaged; ie the wire breaks or the shield is punctured. Even that is not very probable since it's routed on top of the transmission so it’s not readily accessible to be damaged.

Then MB re-designed this part and made it ‘electronic’ for their newer cars. It’s a small ‘circuit’ then sends a pulse to the control module just as the previous version. Now I’ve seen a lot of them go bad . . . for whatever reasons.

This part must have been designed to lower costs involved and maybe b/c the design of the receiving end of the signal required a higher level? Not sure but to go to an electronic ckt instead of a passive ckt means that the MTBF has to be lower than the former part . . . however the cost must be less otherwise why would it be used.

As a consequence, that small example shows why some of the newer MBs aren’t as reliable as their older counterparts. When you lower MTBF, the product can’t be as reliable!

A failure analysis may show ‘cheap’ parts being used so the only fix is to use better grade of parts b/c you can’t eliminate that part. It must be improved. The consequence is that better parts will increase the overall MTBF as it also increases the cost of the part.

Kinda like chasing your tail!