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Here's a procedure for doing the swap:
This section is for people who have a 1989 or 1990 240SX and want to install a later model KA24DE. Before we get to each of the engines, here’s the car prep that needs to be done.
-The accelerator cable needs to be changed. The SOHC cable is shorter and doesn't have enough travel. I have heard that people have stretched it across the engine. Maybe it can be done, but it will probably look like crap.
-The power steering lines need to be changed over. The S13 lines work better, but with a little bit of finesse (adjusting), the S14 P/S lines will work in the S13 coupe. The S13 DOHC P/S reservoir will bolt right on the strut tower. The S14 reservoir doesn't fit quite as well, but can be made to work. If using an S14 engine, the P/S pumps have different connections for the P/S lines. I.E. if you get an S14 engine and want to use that P/S pump, you’ll need the S14 lines. If you’re using an S13 engine and are using that pump you’ll have to get S13 P/S lines. The lines won't interchange with the pumps, but the pumps will interchange on the engines.
-The tachometer will need to be changed out to a DOHC S13 unit. The signal is different between the two. The tach can be removed from the cluster, or the whole cluster can be changed. (The S14 tach won't work, because the physical size of the tach is different.)
-The dash gauge water temperature sensor from the SOHC engine needs to be used; the resistance is different. If you use the stock sensor from the DOHC engine, it will register on the gauge as being higher than with the SOHC unit.
-Go ahead and remove the coil assembly from the car. The SOHC one won't work on the DOHC engine. I'll explain more on this down further
S13 DOHC engine
To swap an S13 DOHC, all the above needs to be done.
-Use the coil setup from The DOHC donor car.
-The engine wiring harness and ECU will need to be replaced with one for the S13 DE engine. The dash connector will need to be cut off from the SOHC engine harness (where it meets The dash harness), and soldered to the DOHC engine harness in the same location. There will be wires that are left floating so don't worry. All The wires that need to be connected will be color matched. For example: just put red to red, and white to white, etc.
-If using a stock exhaust system, the down pipe from the SOHC will bolt to the S13 exhaust manifold. The EGR will work as well.
-The MAF from the DOHC car will need to be used. The SOHC MAF won't work with the DOHC ECU. If you want to change the plug you can even use a S14 MAF; it supposedly has a bigger opening. NOTE*** I understand from other people that the SAFC II shows both MAFs to be the same type. (type 5 in the SAFC library)
-The heater core hose locations are the same and will work between each.
-The S13 P/S pump has to have S13 P/S lines. S14 P/S lines won't bolt up without modification.
95 S14 DOHC engine
-The 95 engines have the same electrical connectors on the intake as the S13 DOHC engines. You can use an S13 DOHC wiring harness and ECU with no problems.
-If you use the S13 engine harness you'll need to change the distributor to an S13 DOHC unit and add the coil assembly. (All S14 distributors use an internal coil).
-If you use the 95 S14 harness there are a lot more wires to weed through. You’ll also need to consider an update to an S14 tranny if you want to keep the ECU happy, because the diagnostic crank angle sensor is located on the tranny bell housing. The S13 tranny doesn't have a provision for a diagnostic crank angle sensor. If you don't care about the check engine light, then use an S13 tranny and don't worry about the crank angle sensor.
-If you want to use a 96+ harness and ECU, then all the sensors have to be changed including the TPS. Best thing is to not use a 96+ harness and ECU.
-Regardless of which harness/ECU combo, the MAF will need to be updated to a DOHC unit. Easiest thing is to use the MAF that fits the plug (minus The SOHC unit).
-The EGR tube will need to be changed because the angle and placement to where it connects with the exhaust manifold is in a different location between S13 and S14. I don't know if an S14 aftermarket header will have clearance issues on a S13. Best thing to do is to change the exhaust manifold/header over to an S13 unit and change the EGR tube or eliminate it altogether. (I don't condone this, I'm merely pointing out the possibilities.)
-The heater core hoses are different sizes between the S14 engine and S13 chassis. You’ll have to rig something up to work. The location of the outlets on the engine is different as well. On the S13 engine/intake plenum they come out the back of the engine. On the S14 engine/intake plenum they come out the side underneath the plenum.
-The S14 P/S pump has to have S14 P/S lines. S13 P/S lines won't bolt up without modification.
96+ S14 DOHC engine
-The easiest thing to do here is replace the intake manifold with an S13 manifold and electronics and just follow directions above.
-And again, the 96+ engine harness and ECU can be used but the wiring is more involved. With the exception of the engine harness and electrical connectors, everything else swaps just like above.
-The S14 P/S pump has to have S14 P/S lines. S13 P/S lines won't bolt up without modification.
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A question on this thread
Thank You Rogoman
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