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1991 300 SE
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Since the OVP provides system power to the CIS control unit, the ABS control unit and the idle speed control unit, a failed or faulty relay can cause hard starting and rough idle. Typically a faulty or failed OVP relay will result in the ABS warning indicator on and rough idle due to no CIS management. Also there is a 10-amp fuse incorporated in the relay to protect the sensitive electronics associated with the ABS and CIS.

Hard starting can result when the throttle valve switch and microswitch on the throttle linkage do not work properly due to a failed/faulty OVP. Although the relay can be replaced or repaired one should remember that there may be an electrical system failure that caused the overload such as a charging system problem or a buss short to ground.

There may be mechanical problems with the cold start valve or its relay and the control valve in the fuel distributor.
 
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