Electrical Please help. In CA, have to legally change my 5.0 ECU from 89 to 91, What must be done to make it work?

Five O Woe

New Member
Jul 19, 2019
San Diego, CA
Have been struggling for years to get my motor swap legal in CA. I was told by the shop that did the conversion that they used a 91 motor and wiring/ECU to match the 91 car i had it installed in. Welp, they used an 89 and the smog ref bombed me on the inspection. I now have to figure out how to switch the ECU from an 89 to a 91 to be CA legal. What all will i need to change to modify my 89 harness to add the 91 ECU?

P.S. the shop that did my swap went out of business due to scamming other customers similar to what they did to me, so there is no help there. I have a 5 speed manual trans, mated to what i now know is an 89 5.0 Mustang V8. I understand they are REALLY close in almost every way. I have already acquired a 91 harness and ECU.

Any gurus who can help me track down what needs to be done to make this work?

preemptive thank you, to any and all who can guide me.
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SN Certified Technician
Mar 10, 2000
Dublin GA
The 5 Speed ECU (A9L) is the same for all 89-92 Mustangs; the computer wiring pinout is also the same for 89-93.
Some of the 93's 5 Speed cars used a different computer (A3M), but the wiring harness pinouts are the same as the 89-92 Mustangs.

Computer & interchange information for 5.0 Mustang

Revised 16-Oct-2011 to add O2 sensor harness warnings

The Mass Air computers from 89-93 are interchangeable with a few exceptions. Just be sure you use the MAF & MAF sensor from the same series computer. The auto transmission computers use a different start circuit than the manual computers. You can use an auto trans computer in a 5 speed car with no problems or changes. I have done it in my original 89 Mustang GT, and I know it works OK. I would recommend avoiding using a 5 speed computer in an auto trans car due to the possibility of damaging the computer’ internal signal ground circuitry.

The odd duck is the 93 Cobra computer, labeled X3Z which is internally calibrated for 24 lb injectors. Use the MAF & MAF sensor from the X3Z computer to avoid problems with the computer's internal calibration. Try and avoid the 93 Cobra computer if you can.

Some cautions about O2 Sensor harnesses need to be observed to avoid damage to the computer.
Only run a 5 speed trans O2 harness with an A9L. Do not run an Auto O2 sensor harness with an A9L. Doing so will damage the computer’s internal signal ground.
Only run an Auto trans O2 sensor harness with an A9P in a car that has an Auto trans. Using a 5 speed trans O2 sensor harness with an Auto trans will cause no crank problems.
See http://forums.corral.net/forums/general-mustang-tech/1094179-oxygen-sensor- harnesses-manual-auto-differences-year-differences.html for more O2 sensor wiring harness info
If you have a damaged computer with signal ground problems, See http://www.stangnet.com/mustang-forums/749974-computer-issue.html#post7490537 for Joel5.0’s fix to the computer internal signal ground.

All the following are mass air unless they say Speed Density
8LD - MANUAL 88 5.0L Mustang Mass Air - California
8LF - Auto 88 5.0L Mustang Mass Air- California
8LG - Auto 88 5.0L Mustang Mass Air- California
A9L - 89-93 5.0L Mustang 5-spd Mass Air
A3M - 93 5.0L Mustang 5-spd Mass Air
A3M1 - 93 5.0L Mustang 5-spd Mass Air
X3Z - 93 5.0L Cobra 5-spd Mass Air internally calibrated for 24 lb injector
A9P - 89-93 5.0L Mustang Auto Mass Air
A9S - 88 5.0L Mustang California Mass Air
C3W - 89-93 5.0L Mustang Auto
DA1 - 87 5.0L Mustang 5-spd Speed Density
J4J1 - 94-95 5.0L SVT Cobra 5-spd
T4M0 - 94-95 5.0L GT Vert 5-spd
U4P0 - 94-95 5.0L GT Auto
W4H0 - 94-95 5.0L GT
ZA0 - 95 Cobra R
D9S - 88 Lincoln LSC Speed Density


You have a nice rear end there Dave.
15 Year Member
Nov 29, 1999
Mililani, Hawaii
Just need to get this straight...are you saying they check your wiring and computer when doing smog test? Insane. What ecu is in your car?


i'm familiar with penetration
Mod Dude
Feb 18, 2001
ECU in a 1991 IS the same as an '89.

Can you be more specific? A9L?

The part numbers on the ECU would reference an '89 engineering number. But that ECU was used 89-92.

I'm not sure how they can tell it's an '89 ECU? The '89 wiring harness is a PITA to mate to a 91 body harness so I'm really scratching my head at your exact issue.

Post the part decal of your '89 ECU and '91 ECU?

This is an engine swap into what exactly?
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