Caterpillar 621 - Electrical | Gauges | Starters | Alternators | Generators. If you are not finding what you need for your Caterpillar 621 Contact us here.. You may also want to check out the Lights and Related or Battery Categories in the previous menu. Amp gauges (Ammeter), Water Temperature Gauges, Oil Gauges, Fuel Gauges, and Tachometers are critical to monitoring performance of your equipment. Sending units and tachometer cables are also available for many Caterpillar.