My 1993 LS400 has a slight issue with letting fuel pumps live beyond six weeks or so, and as it's the night before I put pump #5 in, I figured I'd ask; does anyone know what the second ground wire (the one that runs to a small box on the pump harness) does? Or what the little box it's attached to does? I've been trying to check for shorts in the wire (haven't found any yet) but if that second wire doesn't do anything important I'd like to leave it disconnected and see if there's any improvement to pump life.

So far I've replaced;
-1 Fuel Pressure Regulator (no change)
- 5 fuel pumps (the denso ones live a lot longer)
- 1 EFI Relay (nobody knows where the circuit opening relay is on the '93)
- 2 fuel filters (the second one just to prove that it wasn't the issue)

I've looked for the fuel pump resistor; nobody can find it. It's not in the same place as the '90, '91, or '92, and the '94 models and later had a fuel pump controller module in the trunk (My car does not). Took a look at the ECU today, and It's brand new - replaced in 2014, so I know that isn't the issue. I'm getting really tired of trying to find this problem.

 

I only can give you an idea reading above your statemet.
Check the supplied voltage to the fuel pump when starting, idling, accelerating and constant speed running. I worry that it always gets the full power and the excessive voltage/current heats up the windings and ends up to fail quickly. The supplied voltage should be 1/2 to 2/3 of the +B voltage except the high load conditions. The voltage is reduced by the fuel pump resister stopping the bypass by the relay which is controlled by the ECU.

 

Unfortunately I was only able to measure it while hot idling, but the pump was only getting 8.5 to 8.6 volts. That's exactly what it should be, right?

 

This thread might be helpful (or not):
https://us.lexusownersclub.com/forum...elay-location/

And some pics that might be the circuit diagram (it's the internet, so you never know for sure), which might help you figure out what the wires + "box" are: