1987gt40ttop

87 gt project car, i put a 255lph fuel pump in it a few weeks ago and ran it quite a few times since then but the car has been up on jackstands so i havent been doing any normal driving with it. To day i finished the suspension and and hooked up the new 0xygen sensor harness and new hangers for the exhaust. ran the car ran fine everything sounded great. shut it off. stopped working on it for like an hour, came back and put the wheels on it and lowered it down to the ground to see what it looked like with the new lowered suspension. went to start it and noticed the fuel pump didnt prime turned the key and the motor just turned. added sum fuel thinking that maybe the fuel gage got shifted around while being out of the car and was reading wrong maybe i ran it pretty much dry. finished adding the fuel tried it again still no prime. checked the battery source. 12.6v pulled the relay 12.3v entering the relay. put the relay back in tried it again and noticed the relay pulling in or clicking but also noticed it sounded as if it were clicking twice. went to the rear of the car pulled the connector at the harness where it attaches to the fuel pump. and i got 8.0v at the connector. and thats where i stopped. i dont get why there would be a 4+volt drop from the relay to the pump. does anyone have anything im missing? my initial thought was that since the pump is brand new maybe the connection is bad or something is loose since it ran fine in the air but didnt run at all on the ground. but that still doesnt explain the 4v drop. it should get 13.5v when the engine is running. 8volts is not enough to prime the pump correct? sorry for the long discript.

MustangBradley

When you got the 8v, where you testing during the 2 second prime cycle? Or where you testing while someone cranked? Or just with the key on?

1987gt40ttop

just with the key on, i was by myself so i couldnt have someone key up the source for the prime cycle but shouldnt it be a constant 12v either way?

1987gt40ttop

today i measured it again 8.1v on the tan or yellowish wire and black. nothing on the red with black tracer and its black. if im right the 8 volts is on the control circuit right? the red is the power circuit i thought and that has nothing on it? or does the power circuit not get powered up until the control circuit shows the eec the pump running status and then it establishes a ground to power the circuit? either way i thought id try and pull the fuel line and see if there was any flow at all. nothing. i ran a 12v source from the battery to the fuel pump harness and tried to power both the power and control circuit and still heard nothing. i guess at this point that means i have to drop the tank and check the pump. is there anything else before i do this? could the harness have a bad ground? i belive its located up above the tank right? still i doubt its that, just weird how it goes from running fine to nothing at all

MustangBradley

I am going on memory here, so be prepared for a BS answer.

The fuel pump gets power from two sources: The computer during the 2 second prime, and the fuel pump relay during cranking and normal running. You should never have 8v. Either 12v or zero.

Since you gave power to the pump directly during your diagnosis/testing work, and it still did not run, you either have a bad ground, or a bad pump. It is easier to check/fix the ground so tackle that first.

Next, you need to confirm you are getting 12v at the plug for the pump during the 2 second prime and during cranking. You'll need a friend to work the key.

EDIT POST:

Seems I was wrong. I checked the wiring diagram, and the 1987 did not use a separate prime circuit. That was introduced in the 88 if I am reading the diagram right.

1987gt40ttop

well there is deff two circuits, a power and a control one circuit has 12.1v with the key turned but it only lasts for a second and then the voltage drops to zero i figured that was the power circuit but i dont understand why it doesnt have constant power, the control circuit im figuring (tan or yellow and black) has 8.1 volts on it constantly with the key in the run position. i took a 12v source straight to the fuel pump harness and nothing. i even supplied the harness with a ground incase that was the prob and got nothing. pulled the line to see if maybe i just wasnt hearing it and still nothing. i think im gonna just pull the tank. i was trying to avoid it but i dont think theres any choice at this point its gota b the pump

MustangBradley

This is the diagram I found. http://www.veryuseful.com/mustang/te...ng-ECC-EFI.gif

Do these colors and wires match your 87?

1987gt40ttop

yes it seems to be similar. well i pulled the tank and pump i did a couple more tests first had some one turn the key got 12.1 on the power circuit that went to 0v after a second, but on the control circuit i was getting a constant 8.1v with the key in the run position. i put 12v to the pump harness connector one more time with the fuel line off to see if there was any flow and nothing. so i decide to pull the tank. get the pump on the bench and wouldnt you u know i put 12v to it and it fires right up. so i check it all out and it all looks brand new no probs. i test it out a few more times and it works every time. i go to the harness and take a contiuity reading since i just put 12v to that and the pump didnt prime. the measure out good. i take the pump put it back in the tank and decide to power it up one more time b4 i put it back in the car. put 12v to the harness connector and get nothing. nothing at all. i take the pump connector off and take a continuity reading on the wires from the harness connector to the pump connector again they come up good. i plug the pump connector back in and i noticed it had a good amount of dielectric grease in there. try putting 12v to it again and nothing. i have heard before that dielectric grease when used in excess can actually act as an insulator and get inbetween contact points. so i take the connector apart and clean the connections real good and put it back together and put 12v to it. fires right up. even i had to laugh after this an i have a bad temper, all that work for a bad connection from some grease. i just hope now that was the only problem and not a combination of things, that 8v still just doesnt sit right with me