Brad Huff

I recently purchased a 5.0 Miata that was built using Martin's conversion manual. The fuel pump is wired as per his suggestion. I would like to convert it to a Ford wiring design with the computer controlling the fuel pump relay. I attached a breakout box to test the voltages present at pins 37 and 22. It appears that I have a solid ground at pin 22 KOEO past the normal 3 second prime period which would point to a bad ECU. The typical leaking capacitors have been replaced. My question to the group is, is this really that much of a big deal? Is this issue worth the couple of hundred bucks that it will probably cost to correct? I have other carbureted cars that always have power to the electric fuel pump when the key is on. It seems maybe the only real reason for computer control is for emergency cutoff during an accident. Or am I missing something?

engineer

Hi Brad,

Marton's method of wiring the fiuel pump is not ideal.

Best to allow the ECU to run the relay, the way Ford intended.

I think that you should have the computer fixed. it should NOT keep constant power to the pump at KOEO. An EFI fuel pump pushes about 43psi of pressure when running a return line and even more on against a dead head (about 65-80psi). A carby car by comparison does not exceed about 6.5 psi of fuel pressure to carby. And yes in an accident the fuel pump will still be pumping fuel like crazy....dramatically increasing the risk of fire.

Please correct the ECU...and wire it like ford intended

Cheers,

Gunpilot

When I did my first V8 conversion, I had McCully Racing do my harness modifications. He has a custom fuel pump relay circuit he adds to the harness. Works great. The 4th car I’m working on now, I used an SN95 harness which has its own constant control relay module to control all that. Made it easy.