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 MANUALSFORD1991CAB & CHASSIS 5.8 H, STANDARDREPAIR AND DIAGNOSISENGINE PERFORMANCESYSTEMENGINE CONTROLS - SYSTEM/COMPONENT TESTSTFI-IV SYSTEM IGNITIONCOIL TEST
1991 Ford Cab & Chassis 5.8 H, Standard
Coil Test
1991 Ford Cab & Chassis 5.8 H, StandardSECTION Coil Test
- Attach negative (-) VOM lead to distributor base and measure battery voltage. Turn ignition switch to RUN position. Measure voltage at coil positive (+) terminal of ignition coil and turn ignition off. Voltage should be 10.5-12 volts. If voltage is as specified, go to next step. If voltage is not as specified, repair ignition switch or wiring circuit to coil. See appropriate wiring diagram in WIRING DIAGRAMS article in ENGINE PERFORMANCE Section.
- Ensure ignition is off. Disconnect coil primary connector and inspect both contacts for corrosion or damage. Service or replace as necessary. Using a DVOM, measure primary circuit resistance. If resistance is not .3-1.0 ohm at 70°F, replace coil and retest system.
- Remove secondary coil wire from coil tower and inspect contact for signs of arcing or corrosion. Using a DVOM, measure resistance between coil tower connector and coil negative (-) terminal. If resistance is not 8000-11,500 ohms, replace coil.
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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.