{"product_id":"mercruiser-voltage-regulator-for-470-485-488-3-7l-inline-engines-replaces-99502a13-86255a3-sierra-18-99421-1976-1989","title":"Sierra 18-99421 Voltage Regulator for Mercury\/Mariner Outboard Engines, Replaces 77305A2, 703579","description":"\u003cp\u003eThe Sierra 18-99421 Voltage Regulator is a direct-fit electrical component engineered to regulate charging output on compatible Mercury\/Mariner outboard engines. Voltage regulators control the amount of current sent from the alternator\/stator to the battery, protecting the electrical system from overcharging while keeping the battery properly topped off during operation. This regulator operates at 12 volts, matching the standard charging system voltage used across the supported engine range.\u003c\/p\u003e\u003cp\u003eThis part is built for engines including the 1976-1984 470 with BBL carburetor, 1980-1984 485 and 488 with BBL carburetor, 1985-1986 170 and 190, 1987 165 and 180, and 1988-1989 3.7L Mercury\/Mariner outboards. Because voltage regulators are wear items subject to heat cycling and vibration, replacing a failing unit with a properly rated component like the 18-99421 helps prevent downstream damage to the battery and other electrical accessories caused by improper charging voltage.\u003c\/p\u003e\u003cp\u003eThe Sierra 18-99421 cross-references to multiple OEM and aftermarket part numbers, making it a straightforward replacement for boat owners and marine technicians servicing older Mercury\/Mariner outboard charging systems.\u003c\/p\u003e\u003ch3\u003eSpecifications\u003c\/h3\u003e\u003cul\u003e\n\u003cli\u003e\n\u003cstrong\u003eBrand:\u003c\/strong\u003e Sierra\u003c\/li\u003e\n\u003cli\u003e\n\u003cstrong\u003eManufacturer Part Number:\u003c\/strong\u003e 18-99421\u003c\/li\u003e\n\u003cli\u003e\n\u003cstrong\u003eWeight:\u003c\/strong\u003e 2 pounds\u003c\/li\u003e\n\u003cli\u003e\n\u003cstrong\u003eVoltage:\u003c\/strong\u003e 12 Volts\u003c\/li\u003e\n\u003cli\u003e\n\u003cstrong\u003eApplication:\u003c\/strong\u003e Mercury\/Mariner outboard engines\u003c\/li\u003e\n\u003cli\u003e\n\u003cstrong\u003eOEM Cross Reference:\u003c\/strong\u003e 703579, 77305A2, 86255A2, 86255A3, 99502A4, 99502A7, 99502A8, 99502A9, 99502A12, 99502A13\u003c\/li\u003e\n\u003c\/ul\u003e\u003ch3\u003eFitment\u003c\/h3\u003e\u003cul\u003e\n\u003cli\u003eMercury\/Mariner 470 with BBL Carb, 1976-1984\u003c\/li\u003e\n\u003cli\u003eMercury\/Mariner 485 and 488 with BBL Carb, 1980-1984\u003c\/li\u003e\n\u003cli\u003eMercury\/Mariner 170 and 190, 1985-1986\u003c\/li\u003e\n\u003cli\u003eMercury\/Mariner 165 and 180, 1987\u003c\/li\u003e\n\u003cli\u003eMercury\/Mariner 3.7L, 1988-1989\u003c\/li\u003e\n\u003c\/ul\u003e\u003ch3\u003eReplaces (OEM)\u003c\/h3\u003e\u003cul\u003e\n\u003cli\u003e703579\u003c\/li\u003e\n\u003cli\u003e77305A2\u003c\/li\u003e\n\u003cli\u003e86255A2\u003c\/li\u003e\n\u003cli\u003e86255A3\u003c\/li\u003e\n\u003cli\u003e99502A4\u003c\/li\u003e\n\u003cli\u003e99502A7\u003c\/li\u003e\n\u003cli\u003e99502A8\u003c\/li\u003e\n\u003cli\u003e99502A9\u003c\/li\u003e\n\u003cli\u003e99502A12\u003c\/li\u003e\n\u003cli\u003e99502A13\u003c\/li\u003e\n\u003c\/ul\u003e\u003ch3\u003eFrequently Asked Questions\u003c\/h3\u003e\u003cp\u003e\u003cstrong\u003eWhat does the Sierra 18-99421 voltage regulator do?\u003c\/strong\u003e It regulates the charging current sent from the outboard's alternator or stator to the battery, keeping the 12 volt charging system operating at the correct output and preventing overcharging.\u003c\/p\u003e\u003cp\u003e\u003cstrong\u003eWhich Mercury\/Mariner engines does the 18-99421 fit?\u003c\/strong\u003e The Sierra 18-99421 fits the 1976-1984 470 with BBL carburetor, the 1980-1984 485 and 488 with BBL carburetor, the 1985-1986 170 and 190, the 1987 165 and 180, and the 1988-1989 3.7L Mercury\/Mariner outboards.\u003c\/p\u003e\u003cp\u003e\u003cstrong\u003eIs the Sierra 18-99421 a 12 volt regulator?\u003c\/strong\u003e Yes, the Sierra 18-99421 voltage regulator is rated for 12 volt marine charging systems.\u003c\/p\u003e\u003cp\u003e\u003cstrong\u003eWhat OEM part numbers does the Sierra 18-99421 replace?\u003c\/strong\u003e It replaces OEM and cross-reference numbers 703579, 77305A2, 86255A2, 86255A3, 99502A4, 99502A7, 99502A8, 99502A9, 99502A12, and 99502A13.\u003c\/p\u003e\u003cp\u003e\u003cstrong\u003eHow much does the Sierra 18-99421 weigh?\u003c\/strong\u003e The Sierra 18-99421 voltage regulator weighs 2 pounds.\u003c\/p\u003e\u003cp\u003e\u003cstrong\u003eWill this regulator work if my old one shows OEM number 77305A2?\u003c\/strong\u003e Yes, part number 77305A2 is listed as a direct cross-reference to the Sierra 18-99421, so it is a suitable replacement.\u003c\/p\u003e\u003cp\u003e\u003cstrong\u003eCan a failing voltage regulator damage my battery?\u003c\/strong\u003e Yes, a failing regulator can allow overcharging or undercharging, which can shorten battery life or damage other electrical components on the engine; replacing it with the 18-99421 restores proper charging control.\u003c\/p\u003e\u003cp\u003e\u003cstrong\u003eDoes the 18-99421 fit engines with a BBL carburetor?\u003c\/strong\u003e Yes, it fits the 470, 485, and 488 Mercury\/Mariner engine models equipped with a BBL carburetor within the listed year ranges.\u003c\/p\u003e\u003cp\u003e\u003cstrong\u003eIs the Sierra 18-99421 an aftermarket or genuine part?\u003c\/strong\u003e The 18-99421 is a Sierra-branded replacement part designed to match the specifications of the original equipment voltage regulators it cross-references, including OEM number 703579.\u003c\/p\u003e\u003cp\u003e\u003cstrong\u003eWhat symptoms indicate I need to replace my outboard's voltage regulator?\u003c\/strong\u003e Common signs include a battery that will not hold a charge, overcharging or undercharging readings, or dimming\/flickering electrical accessories; installing the Sierra 18-99421 addresses these issues when the regulator is the cause.\u003c\/p\u003e\u003cp\u003e\u003cstrong\u003eDoes the 18-99421 fit the 3.7L Mercury\/Mariner engine?\u003c\/strong\u003e Yes, it fits the 1988-1989 3.7L Mercury\/Mariner outboard engine.\u003c\/p\u003e","brand":"Sierra","offers":[{"title":"Default Title","offer_id":48988460351707,"sku":"SIE-18-99421","price":547.25,"currency_code":"USD","in_stock":true}],"thumbnail_url":"\/\/cdn.shopify.com\/s\/files\/1\/0839\/7586\/4539\/files\/CDI-194-9502_lg__04256.1748445884.500.750.jpg?v=1785188235","url":"https:\/\/www.goodvibesmarina.com\/products\/mercruiser-voltage-regulator-for-470-485-488-3-7l-inline-engines-replaces-99502a13-86255a3-sierra-18-99421-1976-1989","provider":"Good Vibes Marina","version":"1.0","type":"link"}