Why does an appended "abs"-block change earlier results?

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I've got a Simulink model, simply a filtered step given as reference onto a control loop with PI control. There are Delays both in the plant as in the measurement feedback. As the simulation results oscillate heavily (wrong) I take ode15s as Solver. The result is good.
Now I add an "abs"-block to the system output, i.e. not inside the loop. The result BEFORE this block still looks good, but different from before. How can this happen, which one is trustworthy?
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Swarooph
Swarooph on 26 Oct 2016
This seems like a solver related issue. Adding an Abs block maybe triggering extra zero crossing events which may affect how the model is solved as a whole compared to just that one block. Without looking at the actual model, this is just a guess of course. If you are new to the concept of zero crossing, take a look at this example and draw parallels with your application.

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