Fuel pump doesn't work unless directly hotwired to ignition...

[deathsled]

So I'm trying to get this Stealth TT I've had in my garage for a while going.

One of the things I found as I tore into this car was that the fuel pump was directly powered off the ignition fuse in the fuse box left of the driver's knee. There was literally a wire run from the fuel pump up to the fuse box and a blade connector jammed in the ignition fuse socket alongside the fuse. The wire was also laughably tiny.

So one of the things I did while sorting out other issues in this car was try to eliminate this hack. I found that the fuel pump relay in the engine bay was super corroded and rusty and thought maybe that might have been the issue that lead to the last owner to directly wire the fuel pump as he had.

I replaced the relay with one I knew worked (from my VR4 when I did the relay bypass) and proceeded to hot wire the fuel pump correctly using a standard kit with proper wiring and it's own relay.

I'm trying to start up the car today and it seems pretty clear that the pump is not coming on. I removed the relay under the hood and replaced it with a jumper wire (the fuel pump relay bypass) and this didn't help at all.

Any suggestions as to where to start checking? Any common failure points? Is the stock fuel pump wiring in any way connected to the ECU? Or is it simply wired into the ignition somewhere? This car has an AEM EMS in it as well, is there any way the EMS could be setup incorrectly and is not activating the fuel pump?

I'm horrible with electronic troubleshooting so I really could use some help here haha.

[KeithMac]

AEM could have a problem with the fuel pump trigger circuit.

It basically gounds the trigger coil of the fuel pump relay to run the pump (not the one in the engine bay, that`s low/full speed relay), the fuel pump relay is part of the MFI relay (which is multiple relays in one box), in the cabin, near the engine ecu.

[deathsled]

I guess it's also possible I could have a bad MFI relay then?

[Maximal]

I guess it's also possible I could have a bad MFI relay then?

That would be my guess. The MFI relay does require a ground from the ECU but i'm not exactly sure how that's handled with an AEM so it could be the issue there too I suppose.

[stealthee]

To this day I never figured out why I lost power to my fuel pump on the SOHC. I tested and was getting power coming off the MFI, but it wasn't making it to the fuel pump, so I just jumped a wire off the MFI and ran a new power wire to the fuel pump. It still works like stock, but I actually put a kill switch midway so that I could disable the car if I wanted to.

[deathsled]

I suppose if worst comes to worst I could just feed an ignition switched power source to the relay I installed as part of the hot wire to trigger it, right? My problem now is that the relay isn't getting triggered now that I've installed the hot wire. Probably a little safer than running the pump directly out of the fuse box with a 16 gauge wire.....

Is th MFI relay in my 96 vr4 the same as in the 91tt? can I swap them as a test? Or even better, are they the same across all 3Ss and I can rob the one out of the scrap heap that was the deathsled (92 sohc)?

##EDIT##

ok a little searching has revealed that the MFI relay is the same across models but different for years. There are 3 : 91-93, 94-95, and 96+. I'll rob the one out of my 92 sohc and see if that helps at all.

[deathsled]

Ok so both MFI relays behave the same. I hear them click back off after attempting to crank. So based on what stealthee said is there supposed to be a wire coming out of that relay that is the power wire to the fuel pump?

[stealthee]

Yeah. I think its green/black or blue/black, can't remember off the top of my head. Its the same color wire coming off the MFI that feeds power to the fuel pump.

[deathsled]

Yup, it's a black wire with a blue stripe. I can see that on the harness side for the connector to the MFI that the slot for that pin is a little crispy and I can see on the wire that someone has exposed a piece of the wire previously probably to test it for voltage lol. Looks like I'm retracing someone's troubleshooting steps haha. I guess I'll test it myself and see what's up. It's possible that pin might be burned up inside the plug.

[stealthee]

Yea. Remember, that at least in stock form, there is only power on that wire when cranking the engine.