Originally Posted by deadbird
I really wish people would quit using the oil pressure switch as a reason for the fuel pump not working because it's wrong
OK - not trying to really argue the point, but rather discover.
This is based on my experiences with 3 3rd gens, a 1990 RS TBI, a 1992 RS TBI, and a 1987 Bird CC Carb. In all 3 cases, it's been my experience that unless the OPSU isn't working properly, the pump won't run.
With you're scenerio, the OPSU is a 'redundant' secondary power source - meaning that if the primary source fails, the secondary 'redundant' source kicks in. In the interest of discovery, I ask ... what device in the vehicle 'decides' when the primary source has failed and how, and then how does it 'switch' to the secondary 'redundant' power source?
It is a fact that the OPSU can go bad in such a way that it reads oil pressure when there is none. When this happens, the fuel pump will run constantly, regardless of whether the key is in the ignition or in Timbuktu. The ONLY interruption between the source of power between the battery and the OPSU is the fuse in fenderwell beside the battery-unless that fuse is blown, the OPSU gets CONSTANT power regardless of the key position. If the OPSU was a secondary source, would this event not occur unless 'the decision' was made by the primary source?
The relay and the ECM can be taken out of the picture, and the fuel pump will run - that's how we all switch to carb, remove the ECM and relay, and still utilize the intank pump without any rewiring what-so-ever. My 1992 RS is currently setup this way - I have no ECM, and no relay, and my pump runs just fine with no added or changed wiring.
It's therefore my understanding that, the fuel pump relay is controlled by the ECM, and the process of a stock car is as follows ...
1) Turn key on. This provides power to the ECM, which sends a power signal to the fuel pump relay switching the relay ON, which runs the pump to send fuel up the line.
2) Two seconds later, the ECM cuts this power signal to the relay, cutting off the fuel pump so that it's not in a constant running state. By this time, the driver has started the car. And if driver didn't start the car yet, the pump is not running constantly wasting battery energy and/or creating a danger by having fuel pumped constantly. This 'decision' I can easily see happening, for it's not a 'decision' being made by any component, but rather a preprogrammed action on a timer - an easy task for these old computers.
The ECM will only send the 2 second 'priming' power signal to the relay once for each "key ON" sequence initiated. If the key is left in an ON state for say 8 hours, and the fuel that was 'primed' up to the engine has since drained back, or evaporated - the car may not start, unless there is another "key ON" sequence to reprime the pump.
3) If vehicle is started, then there is now oil pressure. Since the OPSU gets a constant supply of power directly from the battery, as soon as there is an oil pressure reading and as long as there continues to be an oil pressure reading, the pump will continue to run, which is where the pump gets it's power until the car is shut off, and there is no oil pressure reading.
4) As assumed by many, and may very well be the case, this design may well be a safety feature - in case of serious accident, hopefully no oil pressure will be present, thus the OPSU is no longer sending a constant supply of power to the fuel pump, keeping the pump from continuing to pump dangerous gas into the engine compartment where there may be issues to cause a fire.
It's my understanding that the ONLY reason for a relay at all in the fuel powering system is that the ECM can not directly power the fuel pump since all it's functions are low amperage with very large guage wiring (small wires), thus the ECM sends a small amp power to the relay to activate it, where the larger electical requirements are thus sent to the pump.
Anyway - that's been my experience with the 3 cars I've dealt with, and to me it makes perfect sense - it's a system that works MOST of the time. The one flaw in the system is the OPSU - when it goes bad either 1) it reads no oil pressure and thus no pump function (which really isn't a flaw, but leads many to think that their fuel pump is dead), or 2) it reads oil pressure whether any exists or not, and thus the pump runs constantly, which drains the battery after about 48-72 hours.
I might would have more insight if I ever saw a wiring diagram that shows the complete fuel pump wiring for these cars, but I have yet to see one that was complete. Haynes doesn't even have a wiring diagram for the fuel system on a TBI car in their book at all (they do have for TPI cars and carbed models), and neither does Chilton's that I ever saw, and after 2 hours of searching, I never found a complete diagram in ALLDATA either.
Again - all this is NOT to create some big argument, but rather to provide information for the basis of discovery and knowledge (and friendly debate). I'm more than happy to be wrong so that any fellow TGO member can gain the insight necessary to successfully fix, maintain, and modify their vehicle to obtain their desired effect.
PS - For anyone interested...
I have made use of the fuel pump relay in my car. Since I swapped to a carbed setup, I haven't been happy with the OPSU having total control of my fuel pump. I've been left twice because of this - once just this past June at Carlisle (how embarrassing).
So, I disconnected my OPSU, and replaced it with an older unit that didn't control the fuel pump, but only sends the signal to the guage (the earlier model cars had two seperate units, one that sent signal to the guage, and one that ran the pump - later models merged these functions into one unit). I took the smaller wire from my OPSU connector, and wired it directly to the single wire connector on the oil sending unit from the earlier models. Then I took the two remaining wires from the OPSU connector and rigged them into the fuel pump relay - the other two contacts on the relay are wired to a ground and to a run-in-hot wire.
This made it so that my fuel pump will NEVER run without the key in the ON position, since the fuel pump is now on a relay that requires power from the key on state to activate the relay. BUT ... this also means that as long as my key is ON, my pump will run - thus I have no "safety" feature left to deactivate the pump given a serious accident. I also plan on tapping into the relay with wires to run in-cabin, so that I can rig a kill switch as an anti-theft device.