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Hi everyone. Im new here to start with. I am currently in the process of converting an 87 4 cyl to 5.0. I have a 5.0 ecu harness but not sure what year it was donated from. it has the usual 2 plugs that go to the dash. But what confuses me is that I have a grey, round, 6 pin connector that is right there with the big main ecu plug. I can identify every other connector except for this one. Nor can I find where it plugs in to. Any help would be greatly appreciated. Thanks in advance

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I saved this from back in the day not sure it helps but none the less something to reference.

DAMN People!
All of these Gearheads with all of this knowledge, ON a Mustang Site, and NO-ONE knows the differences between the EFI Harnesses that came in the 87-93 Foxbody Mustangs.

MODS: Please Note and make this a "Sticky", PLEASE!

87-89 (EXCLUDING California Mustangs, which in 1988, were MASS AIR), are Speed Density. To Convert these to Mass Air, you "have to have" an EFI Harness from a 1989 Mustang or one from a California 1988, ONLY! The only other option is to buy a Painless Wiring Harness. NOTE: The 1988 Calif. MAM "Plug" is OBLONG and can be converted to the 1/2 moon shape with the proper connector.
Now, from here on in, this information is for the purpose of Converting a 4 Banger Mustang (I DO NOT CARE If it is a COUPE, HATCHBACK or a CONVERTIBLE)! AND, I could CARE LESS "What You Have Been Told"! I've done HUNDREDS of these Conversions MYSELF and "I" have been doing so for the past 20 plus Years!

87-89 Use a 1989 EFI Harness. How do you tell? It will have the Round Gray and Round Black Connectors near the Brake Booster BUT, NO WHITE Connectors anywhere on this Harness.

1990 is a "one year only" Harness. How do you tell? It will have the Round Gray and Round Black Connectors (just like the 87-89 but WITH 2 White Connectors on the Harness near the MAM) near the Brake Booster. Build Date of the car will be within 1990.
NOTE: NO, you cannot use this harness on a 87-89!

91 through Mid 1992. How do you tell? It will have a Brown Connector near the Brake Booster. Due to Factory "Overlap", look also for the Fuel Pump Relay that will be a Black/Green "BOX" under the Drivers Seat which is the fuel Pump Relay. If it is not there, you have a Mid 92 through 93 Harness.

Mid 92 through all of 1993. How do you tell? Look for the Brown Connector near the Brake Booster but also Confirm that it Does NOT have the Black/Green "BOX" under the Drivers Seat. Also, Confirm that there ARE 2 Black "Boxes/Relays" by/near the MAM.

QUESTIONS? Feel free to e-mail me at [email protected] Don't bother trying to make it sound like you want to buy something. I don't care, tell me what you want to do, and or confirm.
YES, I have ALL of the Harnesses that you will need and/or Information! I just ask that you do not waste my time.

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