Having some electrical issues...

[2_Liter_Turbo]

I'm having a little electrical trouble, and can't seem to pin-point the problem. Curious if anyone else has ran into this, and if so, if they found a solution...

This involves my 92 240SX with an SR20DET running on AEM EMS:

I turn the ignition switch to the full "on" position to run the fuel pump (engine not running of course). And the tachometer jumps around 500 to 1000 RPM, bouncing with the fuel pump signal. Pull the fuel pump fuse, and it sits still. So far I've checked all grounds in the engine bay and the ECU, and everything is good there. I'm assuming it's a grounding issue. Tomorrow I plan to cut the fuel pump ground and ground it separately through the chassis (been meaning to do this anyways). If it's not that however, I'm out of ideas.

Any suggestions?

[daemonyk]

Sounds more like a short than a ground issue. Sounds like your fuel pump power wire is touching the tach signal wire somewhere. Try locating and tracing those wires, see if they have any places where the insulation is worn through and they're shorting out on eachother.

[2_Liter_Turbo]

I've been working on it all day, and I just found that between the ECCS relay (pin 3) and the ECU connector (38 and 40 something) I was not getting continuity (this is the power to the ecu circuit). Which is weird, because the ECU powers on. I can actually start the car just fine. It doesn't run that great atm, but it does run. I checked the continuity from the relay and the engine/chassis harness connector E9/F8 and it was good. However, the circuit from that connector to the ECU wasn't good. So I think this "may" be my problem. Not sure how it runs without it, but I'm hoping I can get the issue fixed tomorrow :/

[Klot]

If you can't figure out the issue with the grounding idea then try daemonyk's idea. It'd be easiest to find the wiring diagram for the subharness and figure out where the fuel pump and tach wires are routed by each other.

Grounding issues can cause feedback voltage, which sends power back through a ground circuit in the reverse direction it should normally be flowing. This is really common when taillight bulb filaments melt to each other, and will cause "possessed" things to happen with the electrical system, like when you step on the brakes, the horn honks/blinkers flash/etc

[2_Liter_Turbo]

I actually got it figured out yesterday (finally!). It turns out that when I had the engine harness done back in 2005, that the people who did it had the ECU, o2 Sensor, and two other wires all powered off of one switchable 12V source. The ECCS relay wasn't actually doing anything! So I moved the ECU power back on the ECCS relay circuit, and everything works great now. Took a lot of continuity testing to find that one, ha ha.