GM Full-Size Trucks 1988-1998 Repair Information

Alternator

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TESTING



See Figure 1

If you suspect a defect in your charging system, first perform these general checks before going on to more specific tests.

  1. Check the condition of the alternator belt and tighten it if necessary.
  2.  
  3. Clean the battery cable connections at the battery. Make sure the connections between the battery wires and the battery clamps are good. Reconnect the negative terminal only and proceed to the next step.
  4.  
  5. With the key OFF , insert a test light between the positive terminal on the battery and the disconnected positive battery terminal clamp. If the test light comes on, there is a short in the electrical system of the truck. The short must be repaired before proceeding. If the light does not come on, proceed to the next step.
  6.  

If the truck is equipped with an electric clock, the clock must be disconnected.

  1. Check the charging system wiring for any obvious breaks or shorts.
  2.  
  3. Check the battery to make sure it is fully charged and in good condition.
  4.  

There are many possible ways in which the charging system can malfunction. Often the source of a problem is difficult to diagnose, requiring special equipment and a good deal of experience. This is usually not the case, however, where the charging system fails completely and causes the dash board warning light to come on or the battery to become dead. To troubleshoot a complete system failure only two pieces of equipment are needed: a test light, to determine that current is reaching a certain point; and a current indicator (ammeter), to determine the direction of the current flow and its measurement in amps.

This test works under three assumptions:



The battery is known to be good and fully charged
 
The alternator belt is in good condition and adjusted to the proper tension
 
All connections in the system are clean and tight.
 

In order for the current indicator to give a valid reading, the truck must be equipped with battery cables which are of the same gauge size and quality as original equipment battery cables.

  1. With the ignition switch ON , engine not running, the charge indicator light should be on. If not disengage the wiring harness at the alternator and use a fused jumper wire with a 5-amp fuse, ground the "L" terminal in the wiring harness.
    1. If the lamp lights, replace the alternator.
    2.  
    3. If the lamp does not light, check for an open circuit between the grounding lead and the ignition switch.
    4.  

  2.  
  3. With the ignition switch ON and the engine running, the lamp should be off. If not, stop the engine, turn the ignition switch ON , and disconnect the wiring harness.
    1. If the lamp goes out, replace the alternator.
    2.  
    3. If the lamp stays on, check for a grounded "L" terminal wire.

      Click image to see an enlarged view

      Fig. Fig. 1: Connections for the alternator output test

    4.  

  4.  
  5. If the vehicle voltmeter shows high or low voltage readings with the engine running:
    1. Disengage the wiring harness from the alternator.
    2.  
    3. With the engine off and ignition switch ON , connect a digital multimeter set on the DC scale, from ground to the "L" terminal in the wiring harness. The reading should be battery voltage, if not there is an open, grounded or high resistance circuit between the terminal and the battery. Repair this circuit before performing any more tests.
    4.  

  6.  
  7. Engage the harness connect to the alternator and run the engine at 2500 RPM with the accessories off.
  8.  
  9. Measure the voltage at the battery. If it is above 16 volts, replace the alternator.
  10.  
  11. With the engine off, connect an ammeter at the alternator output terminal. The ammeter must have the capability to measure 115 amps of current.
  12.  
  13. Connect a digital multimeter set on the DC scale across the alternator and a carbon pile across the battery.
    1. Run the engine at 2500 RPM, turn on all the accessories and load the battery with a carbon pile to obtain maximum amperage. Maintain voltage at 13 volts or more.
    2.  
    3. If the output is within 15 amps of the rated output, the alternator is OK. Refer to the alternator specifications in this section.
    4.  
    5. If the output is not within 15 amps, the alternator must be replaced.
    6.  

  14.  

REMOVAL & INSTALLATION



See Figures 2, 3, 4 and 5



Click image to see an enlarged view

Fig. Fig. 2: Loosen and remove the nut securing the wire



Click image to see an enlarged view

Fig. Fig. 3: Don't break the locktab securing the harness connector in the alternator

  1. Disconnect the negative battery cable.
  2.  
  3. Disconnect and tag all wiring to the alternator.
  4.  
  5. If equipped, remove the alternator brace bolt.
  6.  
  7. Remove the drive belt.
  8.  



Click image to see an enlarged view

Fig. Fig. 4: Loosen and remove the alternator retaining bolts...



Click image to see an enlarged view

Fig. Fig. 5: ... then remove the alternator from the vehicle

  1. Support the alternator and remove the mounting bolts. Remove the alternator.
  2.  

To install:
  1. Install the unit and tighten the bolts to the following torques:
  2.  

On 4.3L, 4.8L, 5.0L and 5.7L engines:



Top (right) mount bolt: 18 ft. lbs. (25 Nm)
 
Lower (left) mount bolt: 37 ft. lbs. (50 Nm)
 
Bracket bolt: 18 ft. lbs. (25 Nm)
 

On 7.4L engines:



Lower mount bolt: 18 ft. lbs. (25 Nm)
 
Upper mount bolt: 37 ft. lbs. (50 Nm)
 
Upper bracket bolt: 18 ft. lbs. (25 Nm)
 
Lower bracket bolt: 24 ft. lbs. (33 Nm)
 

On 6.2L and 6.5L diesel engines:



Top mount bolt: 18 ft. lbs. (25 Nm)
 
Lower mount nut: 17 ft. lbs. (23 Nm)
 
Bracket bolt: 18 ft. lbs. (25 Nm)
 

  1. Reconnect the wire at the alternator.
  2.  
  3. Reconnect the negative battery cable.
  4.  
  5. Adjust the belt to have 1 / 2 in. (13mm) depression under thumb pressure on its longest run.
  6.  

 
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