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Zipper-suited sun god and
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2,033 Posts
Discussion Starter · #1 ·
I just got done putting a meter to the relay leads - I've got hot constant 12.5v to the pnk/blk lead, both the red and lt.blu/org read 1.5v until ignition is on, then read 11.5v, but the dk.grn/ylo fuel pump power stays at 0v. I tried about four different relays (including one from my '95 Mustang) and no luck, but when I jumpered the pnk/blk to dk.grn/ylo the pump turned on, so I don't have a continuity problem anywhere. Is 11.x volts not enough to energize the relay? I just replaced the PCM power relay as well (that's directly upstream of the fuel pump relay), no change, and checked all the grounds... but it wouldn't be a ground, since I can jumper the relay to turn the pump on.


What the heck??
 

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FFCobra Craftsman
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3,806 Posts
What year is the harness? I don't have any diagrams in front of me, but it sounds like you are missing a ground. The computer applies a ground to one of the wires to closed the relay. If you have power on three wires, you are missing a ground. Are you checking for power with the relay plugged into the harness? Check with the relay disconnected. Two of the wires should be hot with the key on. The one wire goes to the pump. The fourth wire should be grounded by the computer for 2 or 3 seconds after the ignition switch is turned to run. It should also be grounded when the engine is cranking or running.
 

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Zipper-suited sun god and
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2,033 Posts
Discussion Starter · #3 ·
check page 6-32 fig 95 of the chilton's wiring diagrams, on the far right side of the figure. There is no ground in the traditional sense, but perhaps the ground is thru the PCM. The ground on the computer harness is good - goes to the 2x2 crossframe above the footboxes and shares a ground with just about everything else behind the dash, all of which still work btw.
 

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FFCobra Craftsman
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3,806 Posts
Correct, the fuel pump relay coil is grounded through the computer. You should be able to apply a ground to the relay coil by jumpering one of the wires in the test connector directly to ground. The test connector was located near the master cylinder in the donor.
 
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