What is the purpose of C203 'L'? C203 'B' is +12V to the relay pin 'D'. When the relay closes it feeds +12V threw to the fuel pump, ALDL pin 'G' and C203 'L'. For what purpose? What is being told that the FP is, or should be, running and has +12V supplied to it? I can understand why the ECM would want to know, but what else? Why is C203 L in the loop?

In my swap the 4.9 Caddie ECM F1 and E13 control the closing of the fuel pump relay.

Joe Crawford

Fuel pump relay pin B has the tan/white power wire that runs to the fuel pump. C203 pin L is just a convenient break in that wire for production purposes, that's all. Somewhere in between the relay and C203 pin L, there is also a splice called S509 where the wire branches off in two further directions. One runs to the oil pressure bypass switch which allows power to be fed to the pump if your relay isn't working, and the other runs up to ALDL pin G. ALDL pin G is just a handy place to be able to measure whether voltage is being applied to the pump. You won't be able to tell whether it's being applied through the relay or through the bypass switch from there, but at least you'll know whether it's getting power or not.

Is it needed then?

In addition to Bloozeberry's post -

The fuel pump relay is energized in the "START" position only. The oil pressure sending unit features a circuit that is "CLOSED" when the oil pressure sending unit is pressurized, allowing the fuel pump to run. The fuel pump relay is only used when starting, the oil pressure sending unit keeps it going after startup ("RUN" position).

quote Originally posted by josef644: Is it needed then?

C203 is an in-line connector on the way to the pump so if you disconnect it you'll cut the power wire going to the pump (ie, you need it). If on the other hand you're asking whether ALDL pin G is needed, then the answer is no.

quote Originally posted by Bloozberry: C203 is an in-line connector on the way to the pump so if you disconnect it you'll cut the power wire going to the pump (ie, you need it .

OK, I just figured this out. You were correct, I just didn't understand it at first. The power to work the fuel pump either from the OP sw, or the relay, comes back inside the cabin threw C203 'L' and is switched over to the C500 body harness where it emerges and ends at connector C502, the Fuel Pump /Gauge connector. I just checked this out with my meter.

Thanks to you both

Joe Crawford

[This message has been edited by josef644 (edited 10-09-2009).]