can you splice the harness that connects under the dash that goes to the ecu? swaping s5 t2 in to 87 n/a i have the motor harness for the s5 but cant connect the plug that comes from dash do i have to swap out the rest of the harness or can i get it to work by splicing a s5 plug to the s4 harness?

 

Yes. Compare the pinouts between the years and you can make it work.

However my favourite is a slightly different approach. Pull apart the EFI (emissions) harness and simplify it to the point where it plugs into the engine, plugs into the ECU, and then has just a power and ground that connects to the car. In this way, you can literally swap that engine into ANY car regardless of how the factory wiring is set up. This does mean that you will need to add the ignition coil, CAS and some other misc wiring to the harness that you construct, though.

 

https://www.rx7club.com/2nd-generation-specific-1986-1992-17/how-splice-s5-tii-engine-into-s4-chassis-703692/

 

Like FC3SDRIFT said, it can be done.

But your question leaves me questioning what is going on. The series five EM harness won't connect to the series four front harness nor the series four ECU, but you just mention the problem with the Front harness plug(s). So, what ECU?are you using? Which Front harness plug?

There are two plugs from the front harness that connect to the EM harness, that are located about six-ten inches from the engine firewall. Then there's the plug from the Front harness that connects to the ECU.


And you know that in the series five and series four FSM's FUEL SECTION, there is a sub section called CONTROL UNIT that shows the function of each pin of each of the three plugs that connect to the ECU??? Say yes. That right there should solve almost half of the *problem* of splicing wires b/t a series four and five ECU.

The other half of the *problem* is how to splice the OTHER TWO plugs on the series four Front harness to the series five EM harness. That can be solved by looking for the plugs called FEM-01 and FEM-02 in the wiring schematics and comparing b/t the two series cars.

Why, you'll be through by supper time and up and running.

Then again I've never seen a JDM EM harness and maybe it has but one plug near the firewall that connects to the Front harness instead of the two on a series four car.

 

heres a jdm S5 harness...one FEM plug