TECH: Think Your Alternator Is Crapping Out?
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TECH: Think Your Alternator Is Crapping Out?
Battery Discharges While Cruising Down The Highway
1. Start the engine and bring it to up to its normal highway-cruise rpm. Using a voltmeter, check the voltage across the battery posts.
13.6-14.6 VOLTS: Go to step 2.
UNDER 13 VOLTS: Go to step 3.
OVER 15 VOLTS: Go to step 6.
2. Turn on all accessories, including the heater and air conditioning, electric fans, stereo, windshield wipers, headlights (including high-beams), and turn signals.
IMMEDIATE UNDER 13-VOLT READING: Go to step 3.
VOLTAGE SLOWLY DECREASES FROM READING IN STEP 1: Go to step 4.
OVER 15 VOLTS: Go to step 6.
3. Connect a voltmeter between the alternator's output (BAT) terminal and Ground. Test as in step 2.
13.6-14.6 VOLTS: The alternator is OK. Power from the alternator is not getting to the battery. Go to step 4. UNDER 13.6 VOLTS: Go to step 5.
OVER 15 VOLTS: Go to step 6.
4. Check the battery cables and alternator charge wire for bad connections, improper cable and wire size, or corrosion. Repair or replace as needed. Repeat the step 2 test procedure.
13.6-14.6 VOLTS: Stop--you've fixed the problem.
INITIAL 13.6-14.6 VOLT READING, BUT SLOWLY DECREASES: Go to step 5.
5. Check the alternator drivebelt. Adjust its tension or replace as needed. Be sure you have the right pulley ratio. Never underdrive an alternator. The crank pulley should be three times larger than the alternator pulley (a 3:1 ratio). Repeat the step 2 test procedure.
CONSTANT 13.6-14.6 VOLTS: Stop--the problem is fixed.
INITIAL 13.6-14.6 VOLT READING, BUT SLOWLY DECREASES: Your alternator is operating normally, but it can't keep up with the vehicle's current demands. Upgrade to a higher-output alternator.
IMMEDIATE UNDER-13.6 VOLT READING: Go to step 6.
6. Bypass the regulator. Is the alternator externally or internally regulated?
EXTERNAL REGULATOR: Go to step 7.
INTERNAL REGULATOR: Go to step 8.
7. Unplug the harness from the regulator. With the engine running, connect a jumper wire from the connector's B+ terminal to the connector's Field terminal (on most GM cars, those are the red and blue wires, respectively). Do this for no more than 30 seconds at a time.
RESULT: Expect to see an arc (don't worry; this is not a short to ground). The engine will bog down. The alternator should have an audible whir and ramp up to its max output.
ARC: Repair or replace regulator.
NO ARC: Open Field circuit or worn-out alternator brushes. Fix or replace as needed.
8. Late-model GM CS alternators don't have serviceable regulators; the entire unit must be replaced. On GM SI alternators, with the voltmeter still hooked up to the alternator BAT terminal, use a screwdriver or similar tool to ground the small tab inside the D-shaped hole in the back of the case.
VOLTAGE IS HIGHER THAN BEFORE: Repair or replace regulator.
VOLTAGE IS LOWER THAN BEFORE: Repair or replace alternator.
1. Start the engine and bring it to up to its normal highway-cruise rpm. Using a voltmeter, check the voltage across the battery posts.
13.6-14.6 VOLTS: Go to step 2.
UNDER 13 VOLTS: Go to step 3.
OVER 15 VOLTS: Go to step 6.
2. Turn on all accessories, including the heater and air conditioning, electric fans, stereo, windshield wipers, headlights (including high-beams), and turn signals.
IMMEDIATE UNDER 13-VOLT READING: Go to step 3.
VOLTAGE SLOWLY DECREASES FROM READING IN STEP 1: Go to step 4.
OVER 15 VOLTS: Go to step 6.
3. Connect a voltmeter between the alternator's output (BAT) terminal and Ground. Test as in step 2.
13.6-14.6 VOLTS: The alternator is OK. Power from the alternator is not getting to the battery. Go to step 4. UNDER 13.6 VOLTS: Go to step 5.
OVER 15 VOLTS: Go to step 6.
4. Check the battery cables and alternator charge wire for bad connections, improper cable and wire size, or corrosion. Repair or replace as needed. Repeat the step 2 test procedure.
13.6-14.6 VOLTS: Stop--you've fixed the problem.
INITIAL 13.6-14.6 VOLT READING, BUT SLOWLY DECREASES: Go to step 5.
5. Check the alternator drivebelt. Adjust its tension or replace as needed. Be sure you have the right pulley ratio. Never underdrive an alternator. The crank pulley should be three times larger than the alternator pulley (a 3:1 ratio). Repeat the step 2 test procedure.
CONSTANT 13.6-14.6 VOLTS: Stop--the problem is fixed.
INITIAL 13.6-14.6 VOLT READING, BUT SLOWLY DECREASES: Your alternator is operating normally, but it can't keep up with the vehicle's current demands. Upgrade to a higher-output alternator.
IMMEDIATE UNDER-13.6 VOLT READING: Go to step 6.
6. Bypass the regulator. Is the alternator externally or internally regulated?
EXTERNAL REGULATOR: Go to step 7.
INTERNAL REGULATOR: Go to step 8.
7. Unplug the harness from the regulator. With the engine running, connect a jumper wire from the connector's B+ terminal to the connector's Field terminal (on most GM cars, those are the red and blue wires, respectively). Do this for no more than 30 seconds at a time.
RESULT: Expect to see an arc (don't worry; this is not a short to ground). The engine will bog down. The alternator should have an audible whir and ramp up to its max output.
ARC: Repair or replace regulator.
NO ARC: Open Field circuit or worn-out alternator brushes. Fix or replace as needed.
8. Late-model GM CS alternators don't have serviceable regulators; the entire unit must be replaced. On GM SI alternators, with the voltmeter still hooked up to the alternator BAT terminal, use a screwdriver or similar tool to ground the small tab inside the D-shaped hole in the back of the case.
VOLTAGE IS HIGHER THAN BEFORE: Repair or replace regulator.
VOLTAGE IS LOWER THAN BEFORE: Repair or replace alternator.
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