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Old 01-15-2019, 11:14 PM   #9
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Join Date: Jul 2014
Location: North-central Virginia
Posts: 353
Re: GMT400 Wiring Woes

Note the "fuel pump prime connector" just below the fuel pump relay. It should be an empty terminal on a red wire protruding out of the harness mere inches away from the relay on the firewall. Connect a wire from the positive post of the battery(probably should fuse that wire, but I never did) to that terminal, and it will energize the pump. If you don't hear the pump run, the problem has to be downstream. Either the wiring, the switching valve, or the pump itself.

Edit to add: looking at the diagram I posted, that test terminal might not be where I said it was. On my '87 v20, it was at the fuel pump relay, but it may have moved to the ALDL by the time your truck was built.

(ALDL -- Assembly Line Data Link -- also known as the obd1 port)
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Last edited by kipps; 01-15-2019 at 11:27 PM.
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