1987 Mustang GT Mass Air Conversion Help

[mustangmike6996]

I am currently working on a friends 1987 Mustang GT that he converted to mass air and need some help with a fuel pump issue...

Backstory:

He swapped out a mass air engine harness and PCM after changing heads/cam/intake. I beleive the harness was from a 1989 Mustang GT but I am not 100% sure. The car was running before, everything functioned properly.

After he swapped the harness the car's fuel pump would no longer prime when moving the key to the on/run position and the fuel pump would not run at all during cranking.

He needed the car moved so I pulled the relay and jumped the high current terminals to get the pump to run. The car got parked and sat for a few years and I now have it in my garage to sort this issue out.


Current situation:
The fuel pump still does not run as described above. I was able to run a serparate keyed 12v to the relay control side and ground the relay so the pump ran when you key the car on.

The problems:
#1-The control power wire only has 4vdc when it is keyed on. I checked back at the inertia switch and that also has 4vdc supplied to it.
#2- The control ground wire does not have continuity to PCM pin #22. It also gets 6vdc when the car is keyed on. There is no continuity to ground on that wire. Jumping the VIP DLC connector will not turn the pump on either. It has no continiuty to that connector either. The PCM pin 22 has continuity from pin 22 to the VIP DLC. This wire does have continuity to pin 19 (fuel pump monitoring circuit, which is odd).

He does not have the original speed density harness so I am stuck with the car as-is. He has a spare PCM and spare engine wiring harness as well. The spare PCM give the same results.

I have looked over a few fuel pump posts in here and they have helped me get to this point but I am stuck trying to sort this out.

My next steps are to trace the relay ground wire back to its origin and see what it connects to. I will also be tracing the keyed power wire back to see where that is getting its low voltage as well.

Any input would be greatly appreciated.

 

[Noobz347]

My suspicion is that you have a harness mismatch.

I cannot think of a way to diagnose on-line, what was put into what.

Best I can think to do at this point is to direct you to the wiring diagrams here:

Mustang FAQ - Wiring, Engine, Air Bag, ABS Tech Home

so that you might be able to confirm which diagram you have.



My other thought is that when the harness was installed, perhaps the ground points were never installed or not installed correctly. This would cause stray voltage in all kinds of different ways and it sounds like you're probably energizing even the body of the car at this point.

I would not plug any more ECUs into that car until you get the EEC Pinout to power and ground(s), sorted out.

 

[AeroCoupe]

Should have kept the 87 harness and just done a mass air swap and added the wiring needed.

https://lmr.com/item/PMA-EMAWH/1986-88-Mustang-50L-Mass-Air-Conversion-Wiring-Harness?utm_source=google&utm_medium=cpc&adpos=&scid=scplp&sc_intid=&gad_source=1&gclid=Cj0KCQiAj9m7BhD1ARIsANsIIvDtl8woabhnQnUvOYN082RVxRSJB2FUgpD6wih3wenywjjEG0qF2VsaAvdgEALw_wcB

Even has instructions...but you no longer have the 87 harness.

I want to say that the 89 harness was specific to the 89 only but can be used on an 87 or 88 to do a mass air conversion. I believe it will plug into the dash harness connections by the brake booster and the wiring is correct but I am not sure about that.

What you really need is the wiring diagrams for the 87 and for the 89 so you can trace wires to confirm. Once you know all the wiring is good then you can try and troubleshoot.

Can you pull codes?

 

[AeroCoupe]

Found this:

Electrical Thread 'Maf Conversion On An 87 W/ 89 Harness Question'

Sep 13, 2016

Hey guys,

Im sure I will take some heat, but I have spent days reading through old posts and am just still having some questions about what I need to do to make my MAF conversion correct so please be gentle lol!

I have an 87 coupe that I converted from speed density and AOD to MAF and 5 speed back in 2004. Back then I purchased an 89 engine harness/computer and just swapped the complete thing versus installing the MAF plug kit. Im pretty sure I just plugged my stock (87)oxygen O2 sensor harness back in and I know I didn't do anything about the VSS, which my car already had, being a...

• 87notcher
• Replies: 15
• Forum: Fox 5.0 Mustang Tech

 

[Mustang5L5]

Did the engine harness plug into to the 1987 body harness? A later 92-93 engine harness will have the fuel pump wiring routed differently, but that harness won't plug and play with the 87-89 body harness. I assume it did directly plug inbecause you don't mention splicing any new connectors.


Here's is the 1988 EVTM. SHould be similar to 1988 and has some of the wiring for '89 with regards to cali MAF

Resource icon

1988 Mustang EVTM

Jan 18, 2024

Electrical and vacuum troubleshooting manual

• Mustang5L5
• Mustang Reference Material

So lets try this

#2- The control ground wire does not have continuity to PCM pin #22. It also gets 6vdc when the car is keyed on. There is no continuity to ground on that wire. Jumping the VIP DLC connector will not turn the pump on either. It has no continiuty to that connector either. The PCM pin 22 has continuity from pin 22 to the VIP DLC. This wire does have continuity to pin 19 (fuel pump monitoring circuit, which is odd).

Pin 19 and pin 22 shouldn't have continuiity. Look at the above and you'll see why.

Pin 22 is a switched/isolated ground. It won't connect to main ground unless the ECU closes that pin. You should howver have continiituy along wire 97 the entire way. You seem to indicate you lose it between the Self-test connector and the relay. So You'll want tot check the tan/light green wire at connector C135 and C228

C228 is in the driver's kick panel. Black connector 12-pins according to the EVTM

C135 is in the pass kick panel. Black connector 8 pins

You are looking at the tan/light green wire in both of these harness connections and testing for continuity in both directions back to pin22 as well as to the fuel pump relay tan wire.

 

[AeroCoupe]

@Mustang5L5 - From what I have been able to find on the web today is that the 89 harness is a drop in on the 87 & 88 cars to get mass air. I found several mentions of the dash harness connectors at the booster plugging directly into the ECU harness and no mention of repinning. Once the mass air wiring kits came out this died out on the 87 & 88 cars as it was way less work than swapping a one year only harness into these cars.

Again, what I have read via the internet and I have no personal experience with this swap on an 87 or 88 car.

 

[Mustang5L5]

That was my understanding as well. The 1989 harness would be the only drop in harness that would work.

Now, since he is missing the connection, but my hope is that this is something simple and the pins in one of those two connectors pulled out or maybe one of those plugs isn't even connected? But typically i start at the easy stuff. no connection between pin 22 and the fuel pump relay, then gotta check the wire all the way back.

 

[AeroCoupe]

Would he have an issue with the O2 wiring harness being it was an automatic car and now its a standard? I cannot remember which way that goes but I want to say if you get it wrong it damages the ECU and not sure if it will do anything else.

 

[mustangmike6996]

Should have kept the 87 harness and just done a mass air swap and added the wiring needed.

https://lmr.com/item/PMA-EMAWH/1986-88-Mustang-50L-Mass-Air-Conversion-Wiring-Harness?utm_source=google&utm_medium=cpc&adpos=&scid=scplp&sc_intid=&gad_source=1&gclid=Cj0KCQiAj9m7BhD1ARIsANsIIvDtl8woabhnQnUvOYN082RVxRSJB2FUgpD6wih3wenywjjEG0qF2VsaAvdgEALw_wcB

Even has instructions...but you no longer have the 87 harness.

I want to say that the 89 harness was specific to the 89 only but can be used on an 87 or 88 to do a mass air conversion. I believe it will plug into the dash harness connections by the brake booster and the wiring is correct but I am not sure about that.

What you really need is the wiring diagrams for the 87 and for the 89 so you can trace wires to confirm. Once you know all the wiring is good then you can try and troubleshoot.

Can you pull codes?

I wish he had kept the harness. I have an old OBD1 EEC Rotunda scan tool. I can pull codes but have not yet.

 

[mustangmike6996]

My suspicion is that you have a harness mismatch.

I cannot think of a way to diagnose on-line, what was put into what.

Best I can think to do at this point is to direct you to the wiring diagrams here:

Mustang FAQ - Wiring, Engine, Air Bag, ABS Tech Home

so that you might be able to confirm which diagram you have.



My other thought is that when the harness was installed, perhaps the ground points were never installed or not installed correctly. This would cause stray voltage in all kinds of different ways and it sounds like you're probably energizing even the body of the car at this point.

I would not plug any more ECUs into that car until you get the EEC Pinout to power and ground(s), sorted out.

That is what I was thinking as well. I dont see any missing grounds. I did find an 8 volt drop between the EEC power relay and the positive relay control wire. I think the failure is located at the firewall or under the dash. I have teh dash pulled away to trace the wire back to see exactly where the voltage drop is. (I am assuming it is a pinched power wire). I still have the mismatched pinout on the ground control wire for the relay that I will also be tracing back. I need to get a full pinout of both EECs and verify they are similar.

 

[mustangmike6996]

Would he have an issue with the O2 wiring harness being it was an automatic car and now its a standard? I cannot remember which way that goes but I want to say if you get it wrong it damages the ECU and not sure if it will do anything else.

I believe you need to rewire to keep a good idle. I ran across this topic when looking up this issue. He did not repin the wire harness, so it was just a factory harness swap at this point.

 

[mustangmike6996]

That was my understanding as well. The 1989 harness would be the only drop in harness that would work.

Now, since he is missing the connection, but my hope is that this is something simple and the pins in one of those two connectors pulled out or maybe one of those plugs isn't even connected? But typically i start at the easy stuff. no connection between pin 22 and the fuel pump relay, then gotta check the wire all the way back.

He was under the impression that it was a direct drop-in harness. He bought it and did the work so long ago it is hard to remember the details. I'll have to look for the Ford engineering number/part number and see what year it came from if possible. So far, there are no pulled/unseated pins.

 

[mustangmike6996]

Did the engine harness plug into to the 1987 body harness? A later 92-93 engine harness will have the fuel pump wiring routed differently, but that harness won't plug and play with the 87-89 body harness. I assume it did directly plug inbecause you don't mention splicing any new connectors.


Here's is the 1988 EVTM. SHould be similar to 1988 and has some of the wiring for '89 with regards to cali MAF

Resource icon

1988 Mustang EVTM

Jan 18, 2024

Electrical and vacuum troubleshooting manual

• Mustang5L5
• Mustang Reference Material

So lets try this

Pin 19 and pin 22 shouldn't have continuiity. Look at the above and you'll see why.

Pin 22 is a switched/isolated ground. It won't connect to main ground unless the ECU closes that pin. You should howver have continiituy along wire 97 the entire way. You seem to indicate you lose it between the Self-test connector and the relay. So You'll want tot check the tan/light green wire at connector C135 and C228

C228 is in the driver's kick panel. Black connector 12-pins according to the EVTM

C135 is in the pass kick panel. Black connector 8 pins

You are looking at the tan/light green wire in both of these harness connections and testing for continuity in both directions back to pin22 as well as to the fuel pump relay tan wire.

Thanks for the schematics and diagrams. I have access to the factory site but it only goes back to 1991.

I do not know where pin 22 wire runs at this moment, I need to pull the wire harness and physically trace it back.

I am also unsure as to why the FP ground control wire runs back to pin 19. I don't see any fox schamatic that would have those pins swapped.

Most of the wire coloring does not match the entire way back, but the wire starts as Tan/light green (or possibly blue, it's faded). The VIP connector is tan/green. I'll have to look at the car when I get home but I beleive from the floor relay connector to the kick panel connector it changes at the plug from tan/green to dark grey/green. From there, I beleive it is that color to the PCM. I will confirm later.

 

[mustangmike6996]

Found this:

Electrical Thread 'Maf Conversion On An 87 W/ 89 Harness Question'

Sep 13, 2016

Hey guys,

Im sure I will take some heat, but I have spent days reading through old posts and am just still having some questions about what I need to do to make my MAF conversion correct so please be gentle lol!

I have an 87 coupe that I converted from speed density and AOD to MAF and 5 speed back in 2004. Back then I purchased an 89 engine harness/computer and just swapped the complete thing versus installing the MAF plug kit. Im pretty sure I just plugged my stock (87)oxygen O2 sensor harness back in and I know I didn't do anything about the VSS, which my car already had, being a...

• 87notcher
• Replies: 15
• Forum: Fox 5.0 Mustang Tech

Thank you, I ran across that as well. It helped point me in a better direction.

 

[mustangmike6996]

Should have kept the 87 harness and just done a mass air swap and added the wiring needed.

https://lmr.com/item/PMA-EMAWH/1986-88-Mustang-50L-Mass-Air-Conversion-Wiring-Harness?utm_source=google&utm_medium=cpc&adpos=&scid=scplp&sc_intid=&gad_source=1&gclid=Cj0KCQiAj9m7BhD1ARIsANsIIvDtl8woabhnQnUvOYN082RVxRSJB2FUgpD6wih3wenywjjEG0qF2VsaAvdgEALw_wcB

Even has instructions...but you no longer have the 87 harness.

I want to say that the 89 harness was specific to the 89 only but can be used on an 87 or 88 to do a mass air conversion. I believe it will plug into the dash harness connections by the brake booster and the wiring is correct but I am not sure about that.

What you really need is the wiring diagrams for the 87 and for the 89 so you can trace wires to confirm. Once you know all the wiring is good then you can try and troubleshoot.

Can you pull codes?

Unfortunately, I cannot find a 1987 Mustang GT engine harness to buy and test as a sanity check.

 

[Mustang5L5]

He was under the impression that it was a direct drop-in harness.

Is this a factory harness, or a Ford Racing MAF conversion harness?

Also, is this a 5-spd auto, and what ECU was put into the car? A9L? A9P?

 

[mustangmike6996]

Is this a factory harness, or a Ford Racing MAF conversion harness?

Also, is this a 5-spd auto, and what ECU was put into the car? A9L? A9P?

The car was an auto GT, it is now converted to a T5. It has an A9P. He used a factory engine harness.

 

[mustangmike6996]

Did the engine harness plug into to the 1987 body harness? A later 92-93 engine harness will have the fuel pump wiring routed differently, but that harness won't plug and play with the 87-89 body harness. I assume it did directly plug inbecause you don't mention splicing any new connectors.


Here's is the 1988 EVTM. SHould be similar to 1988 and has some of the wiring for '89 with regards to cali MAF

Resource icon

1988 Mustang EVTM

Jan 18, 2024

Electrical and vacuum troubleshooting manual

• Mustang5L5
• Mustang Reference Material

So lets try this

Pin 19 and pin 22 shouldn't have continuiity. Look at the above and you'll see why.

Pin 22 is a switched/isolated ground. It won't connect to main ground unless the ECU closes that pin. You should howver have continiituy along wire 97 the entire way. You seem to indicate you lose it between the Self-test connector and the relay. So You'll want tot check the tan/light green wire at connector C135 and C228

C228 is in the driver's kick panel. Black connector 12-pins according to the EVTM

C135 is in the pass kick panel. Black connector 8 pins

You are looking at the tan/light green wire in both of these harness connections and testing for continuity in both directions back to pin22 as well as to the fuel pump relay tan wire.

 

[mustangmike6996]

So, just a recap of the two current issues:

1- Low voltage to the fuel pump relay control: Dropping 8vdc between the firewall and driver kick panel. Possibly a pinched wire from previous work or on it's last leg since the dash was moved out when the EEC/Engine harness was replaced. I have not isolated the exact failure point yet, but I might cut the wire and run a dedicated power wire from the splice in the engine harness to the relay depending on the root cause of the next issue.

2- No relay ground control/fuel pump- No continuity to EEC pin 22, has continuity to pin 19.
I traced this back today, it has continuity from the relay connector to the 12 pin grey connector at the driver kick panel, the color changes from tan green (or tan blue) to dark grey (maybe dark brownish) with green tracer. It runs under the dash across to the passenger side kick panel green connector. From there, the PCM/Engine harness changes color and runs to pin 19 on the EEC-IV connector.

Pin 22 on the EEC is the proper color and has continuity to the VIP/DLC connector under the hood.


At this point I am 99% sure the engine harness is not compatible with this car's dash harness. I need to source a replacement 1987 GT engine speed density harness and get the MAF wiring upgrade kit in the link above.

Thoughts?

 

[mustangmike6996]

Here is the second portion of why I think the harness is incorrect... From the VIP/DLC connector the tan/green wire runs to the black connector by the booster. Then swaps to yellow/red. That wire then runs to the cruise control module under the driver side of the dash.