I am not sure what initiates the fuel pump prime and how it is wired, all the wiring diagrams I have only show a red wire with the caption 'Fuel pump primer' but does not show where it leads to. I would appreciate if someone would give me a short essay on what happens when you turn the key on and after fire up, as far as the fuel pump is concerned, I am not getting any signal to the pump but am now getting a buzz from the fuel pump relay. Pump comes on when it is hot wired at the relay plug so there is not break from the plug back.

Thanks in advance

 

Sure, when the ignition key is first turned to the run or crank position the ECM will power up the fuel pump relay for 2 seconds. As the engine is cranking or running the distributor is sending reference pulses (DRP's) to the ECM.

The ECM will power up the fuel pump relay as long as it is receiving the DRP's.

Wired in parallel with the fuel pump relay contacts (the ones that switch to energize the pump) is an oil pressure operated switch. Once the oil pressure is greater then ca. 4 PSI this switch closes and acts as another circuit to power up the fuel pump.

The purpose of this switch is to activate the fuel pump if the relay fails. If the relay is bad, or even removed, the engine can be cranked until some oil pressure builds which will then enable the fuel pump allowing the engine to start.

Here are a couple of links to '89 TPI wiring diagrams:

ftp://ftp.diy-efi.org/pub/gmecm/manu...9-165v8MAF.jpg

ftp://ftp.diy-efi.org/pub/gmecm/manu...165v8tpi-4.jpg

RBob.

 

Thanks

Had a general idea of what was happening. I take it the primer lead goes directly to the computer? Also do you think a fried ign module could cause the problem? I took a spark plug and hooked it coil wire but it is not firing like I would expect, it fires like it is hooked to a regular plug wire, much longer between sparks than I would expect. I am going to do some testing but figured I would get a little more info first plus it has been cold as hell.

 

I just noticed that you put Accel computer in the subject line. I outlined what a stock GM '89 MAF computer would do. If this is truely for an Accel unit then I apologize, it may not be the same.

If the stock GM computer, the fuel pump relay drive is directly from the ECM. The DRP line goes directly to the ECM. The ECM will use this for ignition timing, RPM calculation and knowing to enable the fuel pump relay.

RBob.