Thursday, March 10, 2022

How to Change the Alternator on a 1987 Ford F-150

Facing the vehicle, the alternator on the 1987 Ford F-150 lies just behind the radiator at the top left side of the engine. The accessible location and adequate space permit you to install the alternator without jacking up the vehicle or removing any other parts. To aid in diagnosing problems, many major auto parts stores will do free on-vehicle or off-vehicle alternator testing. These stores often also offer a choice of replacement alternators: new (most expensive), rebuilt with lifetime warranty and rebuilt with one year warranty (cheapest). When buying a replacement unit, take the old alternator to the auto parts store to be sure that the replacement unit matches or is similar enough to fit. Penetrating oil may help in the removal of the old unit. Before beginning the repair, spray some penetrating oil on the top and bottom mounting bolts and the region where the bottom of the alternator fits into the mounting bracket.

Tools Used: Penetrating oil, Socket set, Breaker bar, Anti-seize compound, Replacement alternator

Change the Alternator

Loosen the clamp bolt and disconnect the negative battery cable from the battery. Place the cable end in a secure place well away from the battery terminal so that it does not accidentally brush back against the battery terminal during the repair.

Remove the top and side electrical connector plugs from the alternator. Be sure to release the clips that keep the plugs in place. Gently prying under the clip with a small screwdriver sometimes helps. If the side plug clip is too difficult to release with the alternator in place, wait until after Step 4 and rotate the alternator body for better visibility. If there is burning or cracking of the electrical plugs, replace the plugs or the wiring harness. Replacement plugs with pigtail leads are available from Ford dealers. Some alternators may have screw terminals and nuts for electrical connections. On these, label the wires before removing to avoid errors on reinstalling.

Notice the long ribbed belt at the front of the engine that loops around the alternator pulley and the pulleys of other devices. Often referred to as the "serpentine belt", it transmits power from the pulley on the end of the engine crankshaft to the alternator and other units such as the air conditioner compressor and power steering pump. A spring-loaded idler pulley just to the right of the alternator keeps this belt tight. Before removing the belt, either draw a sketch or locate a diagram (sometimes present on a sticker somewhere under the hood) documenting the routing around the pulleys. Also, before removing the belt, check for belt stretching and deterioration. The belt wear indicator located on the front of the idler pulley assembly pivot area can detect excessive stretching. Replace the belt if the small projection on the front of the moving part does not lie within the range scale stamped into the stationary part. Clean the area below the projection if the range scale is not visible. Decide about belt deterioration as follows: Do not replace the belt because of small cracks perpendicular to the ribs, but do replace it for chunks of ribs missing or longitudinal cracks parallel to the ribs. If the belt does not need replacement, mark the front edge with chalk or marker so you can reinstall it to travel in the same direction. Remove the belt by using a breaker bar and a socket on the bolt head in the center of the idler pulley (not on the pivot bolt) to relieve the belt tension enough to slip the belt first off the idler pulley and then off the alternator pulley.

On some older modified vehicles you may encounter an engine with multiple belts rather than a single serpentine belt. On these, the alternators usually have a pivot bolt and a clamp bolt. Remove the belt by loosening both bolts and then moving the alternator body to release the belt.

Remove the upper and lower alternator mounting bolts. Do not lose any spacers that may be present on the long lower bolt. Work the old alternator carefully out of the mounting bracket.

Apply anti-seize compound to the threads of both mounting bolts and keep them clean. Place the new alternator in the mounting bracket and insert the long bottom screw along with any spacers that were removed. Tighten the bottom bolt just enough to hold and then put in the top bolt. Tighten both bolts securely. Reattach the top and side electrical plugs and reinstall the negative battery cable.

Refer to your sketch or the diagram under the hood and route the drive belt correctly over all pulleys except the idler pulley. Reversing the belt removal procedure, use a breaker bar and a socket on the bolt head in the center of the idler pulley to move the idler pulley up enough to slip the drive belt under it. Check for proper seating of the belt vee-ribs on all the grooved pulleys. Also check that the belt wear indicator described in Step 3 is in the correct range.

Start the vehicle and make sure that the battery-check light goes out. If your vehicle has a dash gauge or you have a voltmeter to connect to the battery terminals, verify that the voltage is 13.5 to 15 volts and remains in that range when you turn on the headlights. It may dip slightly and then come back up. Lacking voltage readings, a rough alternate check involves starting the engine and observing the headlights. With the engine running turn on the headlights. Next turn the heater fan to medium (not high) or turn on the windshield wipers. The headlights may dim momentarily but then should regain their original brightness. Headlights staying appreciably dimmed under these conditions could indicate a problem. If you still have concerns, the auto store where you bought the alternator will probably do a free on-vehicle test.

Tips & Warnings

If the bolts seem frozen in place, apply more penetrating oil and allow it to soak in for a few hours.

Start the bolts by hand (i.e., not with a socket or wrench) to prevent cross-threading.

Take the time to clean the battery terminals and cable clamps if they are corroded.

Automotive batteries have a lot of stored energy. Electrical shorts can produce spectacular sparks and injuries can occur. Don't forget to disconnect the negative battery cable.

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