The electrical circuit for the lift pump involves several main components (refer to Figure 8-25):
• The oil pressure switch/sending unit (threaded into a lubrication system passage at the rear of the cylinder case)
• A relay (mounted near the junction block on the vehicle cowl)
• An in-line fuse (also mounted near the junction block)
• The lift pump itself (mounted under the vehicle on the inside of the left framerail)
• Terminal G of the Assembly Line Data Link (ALDL) connector (vehicles equipped with the 4L80-E Transmission
Control Module [TCM])
When the vehicle driver moves the ignition switch to the CRANK position, the lift pump circuit is completed through the relay contacts (refer to Figure 8-26, view A). During this time, oil pressure is building to the point of closing the contacts of the pressure switch/sending unit. A minimum of 28 kPa (4 psi) is required to close the switch contacts.
When the ignition switch is returned to the RUN position, the oil pressure of the running engine maintains electrical power to the lift pump (refer to Figure 8-26, view B). If engine oil pressure drops below 28 kPa (4 psi), the engine will run poorly or stall when the lift pump circuit opens.
