GM Electric fuel pump circuit testing.

[wayne petty]

i have found that an important test for GM electric fuel pumps has been skipped somehow..

engine idling...  pull the fuel pump relay or remove the wiring connector from it....  the engine SHOULD continue to idle.. and run as normal.

if the engine dies..  change the oil pressure switch..

look at this first diagram.. just the upper half..



see the orange wire.. that sends power to the fuel pump relay.. but also to the fuel pump control terminals on the oil pressure switch..  see those below..

the OIL pressure switch should be handing most of the current for the fuel pump circuit.. not the tiny pins of the relay..

this redundant circuit is to make sure that you get where you are going without stalling out..

can you see the bent lead in this image.. that is for the oil pressure switch. for the gauge circuit.  its the BAR and the outer 2 contacts in the cap that we are worried about..





when the oil pressure comes up slightly. probably 6 or 7 psi.. the BAR is pushed against the terminals inside the socket..  creating a complete circuit to power the fuel pump.. the problem is..  its NOT a wiping contact..  the spots where it touches are the same each time..  so it can build up corrosion and oxides to prevent proper contact.. when this happens...   all the current for the fuel pump supply circuit will be sent through the fuel pump relay...

this is what a lot of gm cars and trucks use for a fuel pump relay..  some have larger terminals.. some a micro sized case.. theory is the same..



the contacts get dirty..  

when the oil pressure switch fails.. all the fuel pump current is sent through this side of the harness..  look at the size of the pins.. they are tiny.  the connectors in the socket get loose from thermal cycling.. then you burn up relays..  leaving you stranded sometimes.  or odd stalling issues.. where the engine will just quit..  and then restart..

in the diagram above..  you will see the dark green and white striped wire  to the coil windings of the fuel pump relay..  the computer on most GMs.. sends positive voltage to the fuel pump relay coils to activate the relay.. which is reversed from almost everything else controlled by the computer..  please be VERY careful in diagnosing the green and white striped wire.. if your test light pulls more than one amp...  you can burn the power transistor out inside the computer.. creating an unseen problem that will drive you crazy..

you will also see that one of the wires runs to the ALDL connector.. thats the fuel pump prime circuit.. i don't know if all the GM cars with ALDL connectors and electric fuel pumps are equipped with that.. most of the tuned port injected cars are with OBD1 connectors..

there is another source of power for the fuel pump.. thats really not well known.. its shown on the diagram close to the bottom of the upper section..  thats the HOT fuel module..

that is mounted under the hood someplace..  it keeps the fuel pump running when the engine is shut off and fairly hot.. to cool the injectors from heat soak from the engine..  also stops the fuel from boiling in the fuel lines from the heat when shut off..  its hard to tell which cars are equipped with it..  this diagram is for the 7747 ECM used on a lot of gm port fuel injected cars.. and a lot of PFI and TPI swaps..

i hope that this tidbit of info and 5 minutes of reading time save you some headaches by finding an invisible issue before it happens...

one thing..  if you look at the threads of the oil pressure switch above..  they are WRONG... who ever rolled those threads rolled them to 1/2-13 UNC... not 1/4-18 NPTm  it's close.. it will thread in.. but it will also send off big turnings of steel when installing it into the STEEL adaptors that GM has on their engines..  so feel the threads on any oil pressure switch you buy..  if they are NOT smaller in diameter slightly at the first and larger at the top of the threads.. don't buy it..  i know there might be an application for the 1/2-13 straight thread.. but somebody screwed up with this one..