Safety Warning
DIY auto repair can cause serious injury, fire, or vehicle damage. These guides are for informational purposes only. Always follow OEM torque specs, wear PPE, and consult a certified mechanic if you are unsure. You are solely responsible for your safety.
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HOMESERVICE MANUALSGMC1977FORWARD CONTROL 4.8L ENG VIN T, AUTOMATIC TRANS, TRANS MFR CD 400/M40REPAIR AND DIAGNOSISELECTRICALCHARGING SYSTEMSALTERNATORSON VEHICLE TESTSCURRENT OUTPUT
1977 GMC Forward Control 4.8L Eng VIN T, Automatic Trans, Trans Mfr CD 400/M40
Current Output
1977 GMC Forward Control 4.8L Eng VIN T, Automatic Trans, Trans Mfr CD 400/M40SECTION Current Output
- With test connections made as for "Charging Circuit Resistance", move negative lead of voltmeter to a good ground, then move the positive lead of voltmeter to "BAT" terminal of alternator. Start engine and operate at idle.CAUTION: To avoid damage, reduce engine speed to idle immediately after starting.
- Adjust engine speed and carbon pile until a speed of 900 RPM (100 amp.) or 1250 RPM (all others) and a voltmeter reading of 13 volts (100 amp.) or 15 volts (all others) is obtained.CAUTION: Increases in engine speed should not be large enough to allow voltage to exceed 16 volts.
Observe ammeter, current output should be within specifications. If output is less than specified, remove the alternator from the vehicle and bench test it.
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When to See a Mechanic
Stop DIY work and contact a certified mechanic immediately if any of the following apply:
- • You smell fuel, burning insulation, or see smoke.
- • Brakes feel soft, pull hard to one side, or make grinding noises.
- • The engine overheats, stalls repeatedly, or misfires under load.
- • You are missing required tools, torque specs, or safe lifting equipment.
- • You are not confident in the next step or safety outcome.