My TBI truck's fuel pump keeps cycling ??? Any ideas why

[rfmaster]

Originally Posted by Rumble

i kinda suck with electical stuff, so basically if i get a voltmeter to check it then what should i expect when i check the grey , basically nothing right? no current?
sorry for the dumb questions, just gotta make sure i do it right, the truck has already killed one battery and I gotta keep the ground disconnected on the new one ( i know, reconnect it before i check current) LOL

Well, we all have to start somewhere. If you disconnect oil pressure switch connector from the oil switch this will disable oil pressure switch function. ECM controls fuel pump (FP) relay and when ignition is OFF ECM removes power to FP relay - opening FP relay contacts. If engine is cold and you have thick (SAE 30 or 15W-40) oil than oil pressure switch may keep FP operating for a few seconds until oil pressure drops below 4 PSI. If you still get fuel pump operation than it is likely that fuel module is malfunctioning by providing current around the oil pressure switch. You can measure DC voltage (not current) with your DVM by probing oil pressure switch gray wire with one test lead and the other lead tied to ground. Alternatively, a 12V test light can be used to verify if the switched end of fuel pump circuit is receiving power from FUEL MODULE. The 1 ton is defiantly HD in my book so its more than likely that it was equipped with a fuel module.

If cycling stops after you disconnect oil pressure switch connector - it is likely than not that oil pressure switch is sticky....

I have attached a schematic of C/K fuel pump control circuit (bottom of the page). Main control over fuel pump is provided via FP relay with oil pressure switch providing a backup in case of FP relay failure. On HD models, if equipped with fuel module, a third control element provides power to the fuel pump only when ignition key is ON position (unless PO hacked the harness)! From what I have seen fuel module keeps fuel pump operating for about 30 seconds after ignition has been shutoff.

//RF