sounds like you wired something wrong with the fan install. I would start by disconnecting the power wire from the battery to the fan. Then check for voltage drop from the battery to the solenoid, down to the starter... also check the grounds for continuity. Check for any + wires under the hood that are melted, grounded, YOu may have to unwrap them from the loom. electrical tape. FInally make sure you rewired the wires going to the solenoid correctly.