[Sully!]

Hello -
This is my first time on this forum and I am blown away by the technology and knowledge available! That said, I do have a question related to Foxbody 5.0 Mustang ECUs.

I have a '91 5.0 (with a t-5) and it seems the ECU is in need of replacement. I happen to have a spare '88 ECU from an '88 (AOD). Would the '88 ECU (S/N E7SF-12A650-C1A) be a direct swap for the '91?
Thank you -

Sully

[cgrey8]

I don't think the 88 is a candidate. The 89-93 era Mustangs are the ones that got MAF. Prior to this, the EECs were speed-density and I believe were also less capable. I don't recall all the details, but I recall that there were enough differences in the hardware between the two era EECs that you couldn't just "chip" an 89-93 tune onto an 88-earlier EEC.

The swap between manual and automatic can also be problematic, even within the 89-93 era EECs. Early model A9Ls and A9Ps were cross-compatible. However I believe there was a difference in the wiring harness that had to be made otherwise a direct swap would cause electrical damage. I don't recall exactly what the issue was, but it seems like it had something to do with the pinout for neutral switch. It also seems like the issue was only in one direction (going auto EEC in a manual or vice-versa). However once you understand what the issue is (you may have to do some asking around or google-searching), you can just fix the issue in the wiring.

That said, I have not had this experience. I've got a T5 neutral and clutch-neutral switch in my truck that wire together to send the neutral/drive signal to the EEC. There's no power on that line, so been able to swap between A9P and A9L with no issues...and both have detected the neutral state without a problem. So I don't fully understand what the issue was and what sub-era of vehicles in the 89-93 lineup introduced this.

Later on, cost-cutting measures were taken (somewhere around the 91-92 time period) where vehicles that didn't need certain components lost them. The one that comes to mind is earlier versions of the EECs from this era had the circuitry to run an electric fan. And while the tune had this disabled, it could be enabled. But since the Mustang never made use of this, they eventually pulled those components off the circuit board to save like $.02/EEC or something (no clue what the actual savings was). So obviously those EECs lost the ability to control a fan, even if you attempted to activate it in the tune. The only thing that would get activated are lines on the EEC circuit board that dead-ended into pads that didn't have the necessary components to get the signal out onto a wire. It seems like there were other similar issues to this, but I've forgotten the details. I'm sure someone will remember. There's a chance that Google's web-scrapers have picked up that detail and if you ask Google's AI, it'll find that info or similar.

[Sully!]

Cgrey8;
I appreciate your explanation. Last week, I had heard through a very knowledgeable source that the '88 ECU is not compatible yet hadn't received this information. I now plan to send the '91 ECU out for refurbishing. After 34 years or so, it is plausible some components in the ECU have degraded. I've replaced the ECT and ACT sensors since the problems I am experiencing (idle and tach dropping to zero and coming back, stalling, hard start) only happen when the motor is cold. Once the motor warms to 150 degrees or so, these issues disappear. I do believe that the ECU is at fault yet may need to search for additional sensor possibilities.

Thank you,
Sully