Simulating a load dump circuit
Here OP is talking about ISO 7637 PULSE 5a, which is simulated version of load dump situation in automotive.

Specification of pulse 5a 12V(two wheeler) and 24V(4 wheeler) are shown below:

As it shows maximum of 174V can be produced for 400 ms in case a load dump situation occurs in the vehicle. And if load dump occurs all the electronics circuits will be fried if not protected.

Now,


  
    What is Load Dump?  
  


The worst instances of surge voltage are generated when the battery is disconnected when the engine is in operation, and the alternator is supplying current to the power line of the vehicle.  

To test the circuits whether they comply with ISO 7637 PULSE 5a, it needs to be tested with Pulse 5a generator, like the one shown here.


  
    Pulse 5a Protection 
  


There are couple of good documents on internet about protection from Pulse 5a, and a suggestion 5KP series diodes are suggested to use, and some times two diodes in parallel are also used for protection.