Hmmmmm, been over two months. Looks like it's time to re-post my fuel pump info:


Here is some TBI fuel relay information I saved from other replies I have written. Other fuel systems have many similarities in running there fuel pumps but there are some differences.

The ECM controls the fuel pump relay.

The truck has a fuel pump relay that supplies power to the fuel pump when:
- Anytime the ECM receives a distributor reference pulse. (Engine running / engine cranking)
- The key is turned on engine not cranking – relay applied for 2 to 20 seconds (most trucks 2 seconds), depending on emissions equipment. Trucks over 8500 GVW and some other vehicles have and electronic Fuel timer module that allows the pump to run up to 20 seconds.
The key must be turned off for a certain amount of time before it will prime the system again when turned back on.

There will only be power to the fuel pump for a few seconds when you first turn the key to the on position. If you are turning the key on then walking around the vehicle testing for voltage to the fuel pump circuit there will be no voltage and should not be power to the pump.

The ECM should store a code 54 if the fuel pump relay fails.

When the engine has oil pressure a redundant circuit supplies power to the fuel pump. This circuit goes through an oil pressure sensor/sender. This circuit is independent of the fuel pump relay.

If you remove the fuel pump relay, the engine will have to crank until it gets oil pressure, and then the fuel pump will get power.

Once the engine has oil pressure the fuel pump should get power even if the relay has failed.

If you pull the fuel pump relay wire while the engine is running it should keep running. If it dies the oil sender or its wiring is bad or the engine lacks oil pressure.

If you unplug the oil sender while it is running it should keep running also. If it dies the fuel pump relay or its circuit is bad.

Keep in mind there is a redundant circuit (two circuits) supplying power to the fuel pump when the truck is driving down the road.
1 – The oil pressure sender
2 – The fuel pump relay

If I errored, feel free to correct me.
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Note: My Blazer is a '89 2-tone gray w/TBI. Other years (or colours) may vary.....

 

Good info, but isnt this all searchable on the forum?