Throwing this out there in case anyone has a suggestion, my car (3800SC) will drain the battery with the fuel pump fuse left in overnight. pull the fuse out and no battery drain. New fuel pump put in because the old one stopped working (didn't change the battry drain problem). The typical oil pressure switch (3 wire) was replaced when a new gauge (autometer) was put in so there is a oil pressure switch installed with only one wire going to the gauge. so it seems as though that should rule out the typical oil pressure switch malfunction causing the drain by keeping the pump running.
Problem appears to be focused in the pump circuit because of the fuse situation already mentioned. Other than maybe some loose pins on the C203 where else does one look?

below is a picture of the oil pressure switch that is located next to to the oil filter.

ummmm that is a oil pressure sender for your gauge not the switch.

If that is connected to you fuel pump you would have a small drain all the time and the fuel pump would not work.

Also the tag on the wire says C203 E is for the gauge.

When the new oil pressure sender was installed were the two unused wires (Orange/Black and Tan/White) NOT spliced together? They should not be spliced together.

Does the fuel pump run with the fuel pump relay unplugged? It should not.

Can you hear the fuel pump run when the fuse is in key off?

Remove the fuse and insert an amp meter in place. What is the current draw key off? It should be zero. If not does the current go to zero with the fuel pump relay unplugged?n Does the amp meter go to zero with C502 (the fuel tank connector) unplugged?

Thanks for suggestions, will be a couple days before I can check them out. Will report what I find.

Out of curiosity tho, if the old 3 wire connector for a 88 oil sender/ switch is left unconnected is that an issue?

[This message has been edited by Grantman (edited 06-01-2014).]

Nope that should be just fine as long as the connector isn't touching something or wet.

The two outside wires on the connectors were the power in feed (Orange/'Black or Orange) and the feed onto the pump (Tan//White or Tan)

The center wire was the one that went to the Fiero gauge.

I'm getting about 3.38-3.42 with relay hooked up or unplugged. Pump doesn't run with relay unplugged.

3.82 Amps ? with the relay unplugged and nothing connected to the oil pressure sender?

You should be reading zero.

exactly. haven't disconnected C502 yet and tried it. Need to try that next. still wondering if something has pulled apart at C203. PO said everything worked fine for the 1st year to year and a half and then the battery draining started.
what I thought was the oil pressure wires is not, so I need to find those to see if they were somehow wired together or left exposed when the gauges were switched.
wires somewhere must be touching that shouldn't be touching.

You are measuring this current with the key off right?

So start unplugging things and see where the current drops to zero.

C502
Oil Pressure Switch
Fuel Pump Relay
C203 (don't try till trying the first three)

Key is off. Thanks much for walking me through the process.

unplugged the C502, still shows big draw at the fuse box. bigger priority right now is my other thread, car stutters and having a PO300 random misfire on cyl 3 and 6.