Electrical 93 LX Fuel Pump Issues w Relay & Fuse Link

diesel farmer

Member
Nov 11, 2021
31
3
18
51
Metamora, Illinois
I recently purchased a 93 Mustang LX with 5 speed manual which as the A9P computer. I don’t know if the computer was swapped in for another computer since this is not an auto car – it doesn’t look like there was an auto in the car. As a side note, I have replaced the three capacitors in the computer and I still don’t get any codes to dump.

The problem I am troubleshooting – the fuel pump was constantly running with the key turned on so I started trouble shooting wiring. Surprisingly, the fuel pump relay is under the seat (I’ve read lots of posts saying it is by the MAF in a 93 model). The fuel pump relay was removed (there wasn’t one installed) and there was a hot wire from the fuse block to the green wire on the fuel pump relay which must have been making the fuel pump constantly run.

I bought a new fuel pump relay and installed it and unhooked the jumper wire to the green wire – and the fuel pump does not turn on with the key.

Looking at wiring diagrams, it looks like there should be a blue-orange wire with fuse link (coming off the yellow wire) from the starter solenoid - but there are only three wires coming off the yellow wire (see pic) and none of them are the blue-orange wire. I found the blue-orange wire at the fuel pump relay (see pic) under seat, but I can’t find a blue-orange wire coming from the solenoid. I traced the blue-orange wire to the plug under the dash (which was obviously tinkered with in the past) and pulled it out (see pic). The blue-orange wire dead ends at the plug and there is no wire in the other side of the plug which should be sending current to the blue-orange wire (so someone must have removed it).

Solutions? Do I buy a new fusible link and run a blue-orange wire from the solenoid to the fuel pump relay? Any other thoughts? Thanks for your help!
 

Attachments

  • 5.0 wiring 1.jpg
    5.0 wiring 1.jpg
    345.6 KB · Views: 21
  • 5.0 wiring 2.jpg
    5.0 wiring 2.jpg
    395.8 KB · Views: 18
  • 5.0 wiring 3.jpg
    5.0 wiring 3.jpg
    683.1 KB · Views: 19
  • Sponsors(?)


Mustang5L5

Put lubricant all over the balls
Mod Dude
Feb 18, 2001
36,820
12,748
224
Massachusetts
The relay is supposed to be by the MAF in 1992 and 1993. The fact that it's under the seat suggests the car has some, or all, pre-1992 wiring in it. A9P was also a pre-93 ECU, so that's not original to the car as well.

Is this an original 5.0 car? Is the 8th digit in the VIN an E? E= 5.0, M = 2.3.

On the drivers door, there should be a door tag. Listed under "TR" will be a code. This is transmission and spring codes. Look at the first digit. A T= automatic, 2= manual transmission

Mixing and matching wiring harnesses can cause all sorts of issues, and even fry the ECU. You really need to ID what you have first, and what should be there.
 

diesel farmer

Member
Nov 11, 2021
31
3
18
51
Metamora, Illinois
Mike - thanks for the reply.

VIN on title, windshield and door: 1FACP41E2PF...

The door tag TR code: 2BOBB

I agree it is odd. It is a 93 model, but yet has wiring from an older car and if the door tag is correct, it is a 5.0 car w/ manual trans from the factory.
 

Mustang5L5

Put lubricant all over the balls
Mod Dude
Feb 18, 2001
36,820
12,748
224
Massachusetts
Yup. that's a factory 5.0 5-spd.

However it appears to have non-original wiring for some reason. There's no sign of a relay under the MAF next to the WOT relay?

This is an interesting one. Can you get a photo of the O2 sensor harness jumper loop? The O2 harness connects under the MAF as well and will have a small loop of wire like this. Need to see what pins it's connected to. This will be a clue as to what is going on

See below thread

 

General karthief

wonder how much it would cost to ship you a pair
Mod Dude
Aug 25, 2016
20,642
7,071
203
polk county florida
I would send the computer out to get checked, ecuexchange on flebay just to make sure it's good and they will up date it with better stuff.
Also make sure the O2 wiring is pinned correctly, there is info in the tech tips forum.
Damn, that's the second time today you beat me to the post
Makes me wanna toss my kindle out the window. But then I would have two with cracked screens
 

diesel farmer

Member
Nov 11, 2021
31
3
18
51
Metamora, Illinois
The wiring harness throughout the car is in pretty good condition. I have an 84 SVO that I have spent a LOT of time tinkering with and the harness is in better condition that the SVO (of course the 84 cars were known to have wiring gremlins).

The only noticeable wiring issues that I have seen are in the below photos - where multiple wires have been cut in two separate places by the driver side lower panel (same place where I pulled the connector off to find the missing blue-orange wire for the fuel pump relay). I have not yet tried to track down what these cut wires are or where they go to.

The wiring harness in the engine bay appears to be in good condition and in tact. When I pulled the computer from the passenger side it had been out of the car before (it wasn't mounted into place) which makes me wonder if someone burned up the original computer and replaced it with a different computer.

It also appears like the motor/trans has been removed from the car in the past based on some of my own amateur observations.

I guess the car could have been in a fire or had water damage which would require a new or different wiring harness? Nevertheless, I am trying to nail down the best approach to getting the fuel pump running properly.
 

Attachments

  • 5.0 wiring 4.jpg
    5.0 wiring 4.jpg
    396 KB · Views: 9
  • 5.0 wiring 5.jpg
    5.0 wiring 5.jpg
    718.1 KB · Views: 9
  • 5.0 wiring 6.jpg
    5.0 wiring 6.jpg
    803.5 KB · Views: 12

Mustang5L5

Put lubricant all over the balls
Mod Dude
Feb 18, 2001
36,820
12,748
224
Massachusetts
Test continuity on the ECU. Pin 46 to pin 40 and pin 60. You should have it on all combinations.

Mixing and matching the wiring harnesses can sometimes fry these ECU's by sending voltages to pins that shouldn't see them.

As for the wiring, i'm trying to figure out what wiring digram to provide you. I do think the fuel pump wiring colors did change, so that might help narrow it down. I'll have to provide some diagrams later on. I'm assuming it's 90 or 91 wiring, as 89 and older is incompatible and would require a total rewire of the car. Assuming you have air bag sensors and plugs to them, it's likely 90+ wiring.
 

diesel farmer

Member
Nov 11, 2021
31
3
18
51
Metamora, Illinois
See below for pics of the wiring near the MAF. There is only one relay at that location which you can see in the pics (the fuel pump relay is not at that location). I have unplugged what I think is the O2 sensor you are referring to and took pics of it - there is no jumper loop wire in it. I have not yet checked the pins in the 02 sensor plug, but can do that tonight.

Thoughts?
 

Attachments

  • 5.0 wiring 9.jpg
    5.0 wiring 9.jpg
    328.9 KB · Views: 12
  • 5.0 wiring 8.jpg
    5.0 wiring 8.jpg
    391.1 KB · Views: 10
  • 5.0 wiring 7.jpg
    5.0 wiring 7.jpg
    190.2 KB · Views: 11

Mustang5L5

Put lubricant all over the balls
Mod Dude
Feb 18, 2001
36,820
12,748
224
Massachusetts
That's the AC WOT relay, an airbag plug and one end of the o2 sensor harness. The other side that plugs into it that plug would have the jumper. In fact i think i see it down by the sway bar in one of your pics
 

Mustang5L5

Put lubricant all over the balls
Mod Dude
Feb 18, 2001
36,820
12,748
224
Massachusetts
So only 5 wires in that harness? That's not good. Suggests that's a 1987 O2 sensor harness as the jumper only started in 1988.

Can you confirm that plug is in fact the O2 harness?
 

diesel farmer

Member
Nov 11, 2021
31
3
18
51
Metamora, Illinois
Seven wires total in that plug/harness and it looks like the 8th wire was maybe cut off - see pics. It is going to the O2 sensor on the exhaust pipe.
 

Attachments

  • 5.0 wiring 11.jpg
    5.0 wiring 11.jpg
    354.3 KB · Views: 12
  • 5.0 wiring 13.jpg
    5.0 wiring 13.jpg
    405.4 KB · Views: 12

Boostedpimp

15 Year Member
May 8, 2003
1,287
483
124
NJ
The relay is supposed to be by the MAF in 1992 and 1993. The fact that it's under the seat suggests the car has some, or all, pre-1992 wiring in it. A9P was also a pre-93 ECU, so that's not original to the car as well.
Not that it matters but the fuel pump relay location changed mid production year in 92. So early 92 to mid 92 had the fuel pump relay under the seat as the 91 harness did. You can use a 91 with that 92 harness and have no problems but mid 92 to 93 had the fuel pump relay on the pass side tower by the maf :)
 

Boostedpimp

15 Year Member
May 8, 2003
1,287
483
124
NJ
I would send the computer out to get checked, ecuexchange on flebay just to make sure it's good and they will up date it with better stuff.
Also make sure the O2 wiring is pinned correctly, there is info in the tech tips forum.
Damn, that's the second time today you beat me to the post
Makes me wanna toss my kindle out the window. But then I would have two with cracked screens
I would also suggest this as they don't just replace the caps, they run a full diagnostics and replace what is needed.
 
  • Like
Reactions: 1 user

diesel farmer

Member
Nov 11, 2021
31
3
18
51
Metamora, Illinois
If needed, I can/will send the computer for diagnostic repair, but isn't it prudent to nail down the wiring issues before installing a refurbished/repaired computer? Otherwise I might end up smoking something in the refurbished computer.
 

Mustang5L5

Put lubricant all over the balls
Mod Dude
Feb 18, 2001
36,820
12,748
224
Massachusetts
Yes, however based on your continuity test results you may find the ECU will need to be repaired as well.

But you are right they you should id the problem before you start with the fix
 

diesel farmer

Member
Nov 11, 2021
31
3
18
51
Metamora, Illinois
I pulled this ECU testing language in regular text from another post and put my results on bold text.

How to test the wiring: With the power off and with the battery positive cable disconnected, measure the resistance between the computer test ground (black/white wire) on the self test connector and battery ground. You should see less than 1.5 ohms. -- I have 2.2 ohms.

On the computer, measure the resistance between pin 46 and pins 40 & 60: it should be less than 1.5 ohms. -- I have 0 ohms between pin 46 and 40. I have 0 ohms between 46 and 60. I can get ohm readings between other pins and 40, e.g. with pin 44 or 45 and 40 I have an ohms reading, but I get nothing when touched between pin 46 and 40, and between in 46 and 60.

Based on this, does my ECU need to be repaired or am I not testing correctly?