It is unlikely that you will blow the overdrive relay in your 1991 Volvo 940 turbo wagon by putting a 30-watt fuse in it when it asks for a 15-watt fuse. The purpose of a fuse is to protect the electrical circuit from damage caused by excessive current flow. In this case, the 30-watt fuse is rated to handle a higher current than the 15-watt fuse, so it is unlikely to blow unless there is a problem with the electrical circuit itself.
However, it is important to note that using a fuse that is rated for a higher current than what the circuit is designed for can be dangerous. If there is a short circuit or other electrical fault, the higher-rated fuse may not blow in time to prevent damage to the circuit or components. Therefore, it is always best to use the fuse that is specified by the manufacturer for the particular electrical circuit.
