How to Change the Alternator on a 1990 Toyotaby Don Bowman
When replacing the alternator on a 1990 Toyota, check the amperage rating on the placard on the alternator. The alternator is available in different amperages, depending on the electrical accessories on your particular car model. When an alternator begins to fail, the diode trio is usually the first to go. There are three sets of diodes in an alternator, and when one set fails, the alternator will lower the charge rate to around 13.5 volts or a little less, but will not be able to sustain this rate when the lights and air conditioning are on. The diode trios work in a similar way with one-way valves: The alternator creates alternating current and the diodes allow voltage to pass in one direction, converting the alternating current to direct current.
Disconnect the negative terminal from the battery, using a wrench. Loosen the tension on the serpentine belt, using a wrench to move the spring-loaded tensioner away from the belt. Hold the tensioner away from the belt and remove the belt from the alternator pulley.
Remove the plastic electrical plug from the side of the alternator by pulling it out. Remove the large positive wire connected to the "B plus" terminal on the back of the alternator, using a wrench.
Remove the upper and lower bolts that secure the alternator to the bracket, using the appropriate size socket and ratchet.
Lift the alternator out of the engine compartment. Install the new alternator in reverse order of removal. Reconnect the negative terminal on the battery when finished.