ECM options for KA24DE stock 240sx ('93)

[GRcoffee]

Hi guys,
long time owner of a bone stock '93 240sx fastback. I've kept it around since 2008. It was somewhat running on it's own until 3 years ago but now I'm ready to fix it up. OEM+ and occasional city cruiser is my goal (and visual kouki conversion).

It's an USDM 1993, manual, fastback w/ the original ka24de at 200k+ miles.

Issue: Car cranks but no fuel. fuel pump relay, fuel pump and wire continuity from 12V, fuse, relay to ECM (right kick panel) was confirmed okay. The problem is at key "ON", the ECM doesn't ground the pin for the Fuel Pump relay for ~5 seconds (prior to starter/crank). The ECM is getting power and ground, it even does the diagnostics code for "5 5" which is no issues (but the check engine lamp is on). If I hand crank the engine w/ key in ON position (engine not running), I can hear audible pings suggesting the crank position is being picked up.

1. What else can I check to confirm what could prevent the ECM(ECCS?) from grounding that Fuel Pump Relay?
2. If ECM replacement is the route, can anyone share pros/cons for looking for a direct replacement or going standalone?(what is available in todays market?) (Again, I'm not doing a crazy drift build or huge power; just enjoying the car stock+ so I would like something that allows me to keep using the Nissan FSM.

Thanks in advance!
photo for attention from my actual college days.

[VStar650CL]

See if you have 12V (through the relay coil) at the ECM drive pin for the relay. If not, either the relay coil is open or there's an open in the wiring between the ECM and the relay coil. If the voltage is there, try grounding the circuit with a jumper wire and see if the relay and pump turn on (the ECM output is open collector so you can't harm it by pulling ground, only by pushing power). If that gets a response then the ECM is definitely causing the problem. There's a "start signal" fuse on most of the old stuff which can cause the ECM to stay dead even though the starter is turning, check that next.

[GRcoffee]

See if you have 12V (through the relay coil) at the ECM drive pin for the relay. If not, either the relay coil is open or there's an open in the wiring between the ECM and the relay coil. If the voltage is there, try grounding the circuit with a jumper wire and see if the relay and pump turn on (the ECM output is open collector so you can't harm it by pulling ground, only by pushing power). If that gets a response then the ECM is definitely causing the problem. There's a "start signal" fuse on most of the old stuff which can cause the ECM to stay dead even though the starter is turning, check that next.

Thanks for the reply!
Following the FSM, I did this step which confirms the continuity from ECM pin to the relay coil is ok.

To your point, I put the key in ON position and then effectively shorted the ground of pin 104 (on the ECM) and heard the relay clicking in the engine bay. So, it does narrow it down to either 1) ECM circuit is bad for the fuel pump relay, or 2) ECM is expecting something before it does the pump prime.

I will look into the start signal fuse next. From the FSM, my understanding is ECM will listen to crank position feedback (5V~0V) and START signal to keep the fuel pump on. If either is missing, I guess it kills the pump. I believe this logic is independent from the initial pump priming (~5 seconds) so I'm not hopeful since that should just ground the pin at key ON regardless of crank position (because it's not moving yet).

I appreciate the input!

[VStar650CL]

That's correct, priming is generally an independent function. It's part of the ECM's boot sequence and not the start/run behavior, and I'm pretty sure it's unconditional on everything Nissan. So if the relay clicks when you ground the ECM end of the wire, then either your ECM pin-fit is bad or the ECM itself is bad. There aren't any other obvious choices. Don't disregard pin-fit. That wire carries relay coil current and sees high energy whenever the relay cycles off, so a bad pin on an old ride is distinctly possible, especially one that's been sitting for 3 years. You might have problems in the tank as well, but turning the pump on is first-things-first.

[GRcoffee]

That's correct, priming is generally an independent function. It's part of the ECM's boot sequence and not the start/run behavior, and I'm pretty sure it's unconditional on everything Nissan. So if the relay clicks when you ground the ECM end of the wire, then either your ECM pin-fit is bad or the ECM itself is bad. There aren't any other obvious choices. Don't disregard pin-fit. That wire carries relay coil current and sees high energy whenever the relay cycles off, so a bad pin on an old ride is distinctly possible, especially one that's been sitting for 3 years. You might have problems in the tank as well, but turning the pump on is first-things-first.

Thanks for the affirmation; it helps. pin fit and more or less the ECU and wire harness looks pretty clean despite the sitting so I've got a replacement ECM on order. It should be interesting if that will immediately prime the pump once swapped. Thanks for the advice.

[GRcoffee]

Hey guys,

Just circling back on this for closure. purchased a direct replacement ECM (refurbished) from Module Mechanics. Plugged right in and fuel pump relay is operating at key on. The only thing I had to do was transfer the ECM brackets over to the new ECM.