Ford Pick-ups and Broncos 1987-1996 Repair Guide

Dual Battery Diesel Engines



See Figure 1

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Fig. Fig. 1: Diesel dual-battery jump starting diagram

Ford pickups equipped with the 6.9L or 7.3L V8 diesel utilize two 12 volt batteries, one on either side of the engine compartment. The batteries are connected in a parallel circuit (positive terminal to positive terminal, negative terminal to negative terminal). Hooking the batteries up in parallel circuit increases battery cranking power without increasing total battery voltage output. Output remains at 12 volts. On the other hand, hooking two 12 volt batteries up in a series circuit (positive terminal to negative terminal, positive terminal to negative terminal) increases total battery output to 24 volts (12 volts plus 12 volts).

NEVER hook the batteries up in a series circuit or the entire electrical system will go up in smoke, especially the starter.

In the event that a diesel pick-up needs to be jump started, use the following procedure.

  1. Turn all lights off.
  3. Turn on the heater blower motor to remove transient voltage.
  5. Connect one jumper cable to the passenger side battery positive (+) terminal and the other cable clamp to the positive (+) terminal to the booster (good) battery.
  7. Connect one end of the other jumper cable to the negative (-) terminal of the booster (good) battery and the other cable clamp to an engine bolt head, alternator bracket or other solid, metallic point on the diesel engine. DO NOT connect this clamp to the negative (-) terminal of the bad battery.

Be very careful to keep the jumper cables away from moving parts (cooling fan, belts, etc.) on both engines.

  1. Start the engine of the donor truck and run it at moderate speed.
  3. Start the engine of the diesel.
  5. When the diesel starts, remove the cable from the engine block before disconnecting the positive terminal.