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Hummer Knowledge Base

The Hummer Knowledge Base

Lift Pump Testing Procedure:

Lift pump test procedure:

  1. Unplug the two wire connector for the fuel pump.

  2. With a volt meter connected to the two pins on the supply (truck) side, turn the key to crank. If you read 12V then the supply side fuse and fuel pump relay are OK.

  3. Next start the engine cold. After the engine reaches the typical cold maximum oil pressure (usually 55 PSI) immediately switch off the engine (you should run the engine for about a minute, or less). You should continue to read 12V for a short period of time as the oil pressure falls. An engine that holds oil pressure good will take about five seconds to fall to zero. If the 12V is missing in this step, and not in step 2 above, then the oil pressure switch is bad (common failure).

  4. Take the fuel pump side of the connector, with the engine off, key off. There should be one gray (or brown) wire and one black wire. Ground the black wire with a short jumper. Next connect the other wire to a 12V source. An easy to use source is the CTIS fuse which is at 12V unswitched. If the pump fails to run (you'll hear it) then replace the pump.

Expensive? parts are about $20 or so, labor can be high if it is a turbo engine where the sender is located under the intake (requires removal of the dog house to get at in that case.) A non-turbo engine has the oil pressure switch located on the side fo the block, above the oil filter (next to the oil cooler lines). Can be changed from below, but it's a bit of a reach, I've done it several times now.

Peter H.


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