The oil pressure sending unit, (OPSU) supplies a redundant circuit to the fuel pump. It is spliced into the fuel pump circuit after the relay,(grey wire). It has nothing to do with picking or holding the relay. If the relay is chattering, there's either a problem with the relay itself, the coil in the relay, coil ground, (rear of left cylinder head), voltage to the relay, (orange wire) or the trigger circuit from the ECM, (dark green with white stripe wire). There is a very simple way to determine if the OPSU redundant circuit is working properly: With the engine at idle, unplug the fuel pump relay, if the engine stalls, the circuit from the OPSU is not working. To determine if the fuel pump relay is supplying voltage to the fuel pump: With the engine at idle, unplug the the OPSU, if the engine stalls, the relay is not supplying voltage to the pump.

The fuel level buffer is part of the fuel level sender in the fuel tank. It uses battery voltage, not Vref.

 

The oil pressure sending unit/switch latches the pump on once the truck is running. I do not know if the fuel pump relay stays energized all the time. If the relay was clicking, you may have a problem with the ignition switch interrupting power to the ECM which makes it think it should energize the fuel pump relay as it does during the starting cycle. Or just a weak relay like stated above.

 

Following the wiring diagram for a 1997 Blazer, The fuel level sensor utilizes a 5vdc reference.

 

I stand corrected, thanks!

 

No problem. I was hoping we werent going to have a shooting match!

 

The fuel level sending unit doesn't use any voltage in and of itself. It is just a resistance to ground circuit.

 

Here’s a little info about the fuel pump relay and OPSU, hope it helps.

The fuel pump relay is activated by the ECM when the ignition is initially turned to the RUN position. If the ECM does not receive data from the crankshaft position sensor within 2 seconds, it will deactivate the fuel pump relay. The relay is activated continuously in the START position. While the engine is running, the relay remains activated provided the ECM is receiving data from the crankshaft position sensor. Whenever the data is terminated, the ECM will deactivate the relay after 2 seconds, (That’s why the pump stays on briefly after you turn the ignition off).

The same orange wire feeds both the fuel pump relay and the OPSU. Voltage is supplied through a 20 amp fuse labeled ECM BATT in the instrument panel fuse block, it is hot at all times. When oil pressure is above ~7psi, the orange wire is connected, (inside the OPSU) to the grey fuel pump feed wire. A common misunderstanding about the OPSU is that it will deactivate the fuel pump if/when oil pressure drops below ~7psi. This is only true if/when a fuel pump relay circuit fails and cannot supply the fuel pump with voltage. If oil pressure drops below ~7 psi, there will be no voltage supplied to the fuel pump and the engine will stall. The redundant circuit merely supplies the pump with an additional path for voltage and amperage while the engine is running. If there is a failure in a fuel pump relay circuit, it will cause extended cranking time before the OPSU can supply voltage to the pump on a re-start. (Engine oil pressure must satisfy the OPSU before it can supply voltage to the pump.)

 

I just bought the OPSU, and I was about to cange it out until I read all this. I pulled out the relay whle the engine is at running and it cuts out. I still dont understand what I have to XACTLY DO to troubleshoot this issue. All this seems like its the ECM now?

 

It means the OPSU is not supplying voltage to the fuel pump. With the ignition in the RUN position, the orange wire at the OPSU must show battery voltage. With the engine at idle, and the OPSU plugged in, the grey wire must also show battery voltage but, the grey wire from the fuel pump relay needs to be isolated from it when checking.

 

The grey wire can back feed voltage from the relay if it is installed.

I would try to test the normally open switch in the OPSU with the engine running using a multimeter set to continuity. If the switch doesn't close with the engine running, then it is bad. You want to test across pins C & D which correspond to the orange & grey wires in the connector.