Fuel Pump Problem on 93 Cobra swap

66gt5.0H

New Member
Jul 9, 2018
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0
1
Yachats, Oregon
Ok I'm almost done with my swap. Before I go into anything, Yes I checked the inertia switch and it works as designed. :cool:

-66 Mustang Coupe
-1993 5.0 Cobra Swap
-1993 T5

Also, let me say that I've read many 5.0 Fuel pump troubleshooting guides and I still am having a brain block on this. So please bare with me.

1. Fuel pump relay not sending 12v to Inertia Switch in trunk through the DK Green/Yellow wire. It actually shows 7 volts when not connected to the fuel pump but I realize that isn't necessarily relevant except to say that its not 12v. It also shows 7 volts without the relay plugged in so I'm thinking that is artifact from pin 19 on the EEC.
2. Fuel Pump runs when given 12v outside of the relay circuit. Also Engine runs when I do this.
3. Replaced Fuel Pump Relay with known good relay same behavior.
4. Pink Black Wire (Common on Relay 30) at Fuel Pump Relay has 12V at all times.
5. Red Wire (Relay Coil +V 86) at Fuel Pump Relay has 12v with key on.
6. LT Blue/Org Wire (Relay Coil -V 85) at Fuel Pump Relay has 12V with key on.
7. DK Green Yellow Wire (Relay N/O 87) at Fuel Pump Relay has ?7v?

For item # 6, I cant for the life of me figure out why the negative side of the relay coil is getting 12v in relation to chassis ground. but it does explain why it doesn't work. Hopefully someone here can provide another look at it.
 
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Rusty67

15 Year Member
Dec 3, 2002
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Seattle area, WA
Just to make sure, you are using a factory harness from the 93 Cobra right? Let us know exactly what harness you are using so we can help with a wiring diagram.

I don't know a ton about the Fox chassis wiring but I have put a factory EFI setup into a classic before. I think it is a bit odd that you are getting 7 volts when that wire is not connected to the fuel pump. I would trace that down and figure out why.

If you cross post this in the Fox forum you might get better help.
 

66gt5.0H

New Member
Jul 9, 2018
8
0
1
Yachats, Oregon
Just to make sure, you are using a factory harness from the 93 Cobra right? Let us know exactly what harness you are using so we can help with a wiring diagram.

I don't know a ton about the Fox chassis wiring but I have put a factory EFI setup into a classic before. I think it is a bit odd that you are getting 7 volts when that wire is not connected to the fuel pump. I would trace that down and figure out why.

If you cross post this in the Fox forum you might get better help.


Yep factory 93 harness. Fuel pump really next to the WOT in the engine compartment. Computer, harness, and motor are all from the same Cobra donor.
 

Rusty67

15 Year Member
Dec 3, 2002
3,749
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109
Seattle area, WA
Ok, I'll have to take a look at this later. Might be a day or two before I have time to dig up the wiring schematic. I'd look into that 7v coming out because I think that should be 0v until the car is in start/run and then it should be 12v. I think the Fox EFI used a return style fuel system and just ran the pump didn't it? It wasn't a PWM pump if I remember correctly.
 

Rusty67

15 Year Member
Dec 3, 2002
3,749
37
109
Seattle area, WA
PWM is Pulse Width Modulation. Instead of feeding a pump 12v constantly you burst it 12v in spurts and you change the timing of how frequently you pulse the pump. The closer together the pulses are, the more the pump flows. If I remember correctly that is how they do a returnless fuel system without blowing up the pump.

From what I recall when we did my friend's EFI swap the pump just always ran and the fuel pressure regulator had a return line to the tank so the pump just always ran full speed. That is why I can't get my head around your pump having 7v going to it ever. In my head it should be 0v or 12v but again, I'd need to look at the specs and wiring schematic to confirm that.