Electrical 93 LX Fuel Pump Issues w Relay & Fuse Link

Mustang5L5

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Mod Dude
Feb 18, 2001
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Sorry, I worded that a bit confusing. The engine harness is not transmission specific. The jumper position on the o2 sensor harness determines that.

I suspect it’s a 91+ harness but not confirmed yet. Let me see if I can figure out something else to check.

That particular circuit changed wire colors a few times from 1990 thru 1993, that’s what is confusing me a bit.
 
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Mustang5L5

Put lubricant all over the balls
Mod Dude
Feb 18, 2001
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Massachusetts
That’s the video. He’s spot on for the most part. I could add a few minor things I’ve noticed over the years, but fpr the most part, the major harnesses on the car should all match.

The minor harnesses (doors, transmission, hatch, etc have less variation.
 
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Mustang5L5

Put lubricant all over the balls
Mod Dude
Feb 18, 2001
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What can I say. I might know a couple things here and there

This means that o2 harness is incorrect and the pinout it has now probably contributed to damaging the ECU.
 
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Mustang5L5

Put lubricant all over the balls
Mod Dude
Feb 18, 2001
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You can probably repin it, but I’d have to sit down and really think about what to say goes where.

Here’s how the jumper should go. I’d have to think about the wires on the engine side of the harness and if any need to move

87BB9317-5EA7-42E7-B06F-9CC27E280E5F.gif
 

Mustang5L5

Put lubricant all over the balls
Mod Dude
Feb 18, 2001
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Yes.

I believe the o2 diagram I posted earlier was for an 88-90.

You may have to draw out the wire colors you have and play match up. I can look up the wires, but just not today due to the holiday
 

diesel farmer

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Nov 11, 2021
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The 02 harness is now pinned as in the attached #22 photo -- I followed the video guidance to pin it for a year 91 harness. With everything hooked back up and an A9L plugged in (note that there was no voltage to ECU harness pin 30 prior to this trial), I turned key to on position and there is no fuel pump activation and I still cannot dump codes.

Looking closely at the A9L (which is from another local car), there are no visible signs of traces being burned, but two of the three capacitors are in poor condition with green gunk around their terminals -- would faulty capacitors keep it from dumping codes and keep the fuel pump from running?
 

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  • 5.0 wiring 22.jpg
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Mustang5L5

Put lubricant all over the balls
Mod Dude
Feb 18, 2001
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Can you test power at the relay to see if the relay is being activated and if the relay is sending power to the pump?

This is 91 fuel pump wiring
8BFE4FE8-28E9-40CE-8464-8B976A086FDB.jpeg



You can test for power on the relay at the dark Gray/yellow wire at the relay or at pin 19 on the ECU. (Pin 19 is a fuel pump monitoring pin to see if it’s running)

If you don’t have it, check the relay. Red wire (361) should have constant 12v. Light blue/orange (wire 926) connects to pin 22 on the ECU and is switched ground.


If you go to the diagnostic connector and jumper the wire as shown to pin46, the fuel pump should run. If it doesn’t, strong chance pin 46 is fried.
2A31D794-575F-42E3-8F68-D4200A54AB3C.gif



Usually a fuel pump that never runs, or constant runs is a sign pin 46 is damaged
 
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diesel farmer

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Nov 11, 2021
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With the A9L installed and everything hooked up with the key on, I have no power to red, green or pink/blue. Remember that my fuel pump relay has the following wires shown in pic #3 at the beginning of my post:
-solid red
-pink w/ blue stripe
-green
-blue w/ orange stripe

The below pic #23 shows the jumper wire on the diagnostic terminal to test if there is power to the fuel pump (I think this is the correct position to jump it) -- with the jumper wire in and key on, there is NO fuel pump activation -- so does that mean I have a bad fuse link and/or a computer issue? Remember that I can run a hot wire from fused power to the green wire on the fuel pump relay to get the fuel pump to run, so I know that part of the fuel pump circuit is good!


5.0 wiring 23.jpg



Now for the GOOD! -- I was able to get the codes to dump using the jumper method (yesterday I tried it with the little handheld tool and didn't get anything) but with the jumper wire method using the A9L computer I get the following codes:
--51
--81
--82
--85
--87

The only code that appears to deal with the fuel pump is 87 (i.e., fuel pump primary circuit fault). I will dig more into this later this afternoon.
 

Mustang5L5

Put lubricant all over the balls
Mod Dude
Feb 18, 2001
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Massachusetts
I think that’s a clue. Since your fuel pump wiring doesn’t match, I think you have a 1990 interior harness.

If you go back to post 25 and compare, you can see that not only are the wire colors different, but so is the routing the wiring takes. The connectors are different. I think you have a wiring issue still with supplying the necessary power and ground from the ECU to the fuel pump relay.

Another big clue this is the case is the jumper trick did not work. It should have sent power to the relay.

Let me dig through the 90 and 91 wiring diagrams in a bit and give you more info
 

Bullitt347

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Mar 23, 2007
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I have ran into several 93 Mustangs with the fuel pump relay under the drivers seat over the years. Everything else in the engine compartment lookd correct, Air bag crash sensors, O2 harness in the correct spot, ect. but there was no FP relay out by the MAF and no indication that the harness had been tampered with. Never thought much about it until reading this thread.
 

Mustang5L5

Put lubricant all over the balls
Mod Dude
Feb 18, 2001
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Massachusetts
Here’s 1990 first. Connection C110 is where the ECU connects to the harness going towards the rear of the car. Wire 97 and 361 come out of the ECU, go into the engine bay and go across to the brake booster area where c110 lives. Then it goes into the drivers kick panel to connect to the harness going to the relay and intertia switch.

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