Lift pump wiring schematic

[98flatbed]

Anyone know where I can find a schematic of the lift pump circuit, a schematic of the entire engine would be really cool! My 98 is stalling again, pretty sure it is the pmd but I havent ruled out the oil pres switch.

[rjwest]

On 96 and newer, PMD runs lift pump, oil pressure switch is back up only......

Lift pump should come on for a few seconds when key is turned on. than off, when pcm senses engine
rpm it turns lift pump on again.

there is a connector under hood to jumper lift pump to run with key off. it is black single wire
left side by hood hinge, may be hidden in bundle...I have copy of 96 pump schematic somewhere... don'y know how to post, ? email ?

[98flatbed]

Thanks rj, I heard all the fuss about the oil switch but I didnt know the circuit had been changed for the new models. hopefully my new pmd and homemade cooler will fix my stalling problem. If it dont stall in the next few weeks, I will consider it fixed.

[moondoggie]

Good Day!

You might want to buy the factory shop manual for your truck from Helm (http://www.helminc.com) (Click in colored text.) The one for my 95's was $120. It paid for itself the 1st time I didn't have to go to the garage.

Blessings!

Brian Johnson, #5044

[HowieE]

98flatbed

I have the factor manual and could scan a copy and send it to you if you e mail me your address. Mine is on my site noted in the signature.

The factor discription is very poorly written leading one to beleave that the fuel pump relay runs the lift pump most of the time. Looking at the wiring diagram, if this were true there would be no reason for the oil pressure switch, shows that the oil pressure switch is the prime source once the engine oil pressure is above 28 kPa. If the fuel relay drove the lift pump during normal operation there would be no means of shutting down the lift pump after a lose of oil pressure ie and accident or the noturious GM oil line failure.
Both the relay and the oil pressure switch are supplied from the ecm B fuse. The wire mentioned above to bypass and run the lift pump is circuit 490 and is a red wire. This wire will feed voltage to the lift pump through the NC contacts of the fuel relay directly to the pump.

[rjwest]

Howie ????? I'm not the best electrical guy. but I interperted the wiring differently.
I thought the PCM detected the engine running
and controlled the lift pump ( on when engine is running/off when not.
The oil pressure switch ( i believe there is a note some where ) is a fail save for the relay. IE if relay fails. Oil pressure will turn on pump.

I did pull relay, Truck started , lift pump ran
after oil pressure came up from engine start.

I will check this again, May have screwed up, it's been know to happen...

[HowieE]

The lift pump draws sevral amps and is too much for the PCM to supply. The PCM engages the relay and the relay carries the current during starting. Once started the and the oil pressure is up and the key is returned to the run position current is through the oil switch. A signal is returned to the PCM from the output of the oil switch. If that signal is dropped due to engine failure the pump is shut down and a code is set in the PCM.
It is a good idea to put a relay across the oil switch to carry the current since the contacts of the oil switch are rated at 1 amp and the pump draws 3 or 4 amps. This wil prolong the life of the oil switch.
If your injection pump was primed and your truck starts quit fast on startup it would be possible to start the truck with the relay pulled out.

[rjwest]

Howie, I have the schmatic an Notes from helms. but
do not know how to post into reply. It does say PCM controls lift pump
and Oil sw is a backup ?

I was assuming I would never have a oil pressure sw/lift pump problem on my 96, ???? Assume ??

[HowieE]

I have sent copies of the GM manual to 98flatbed.

The word backup is missleading. The pump is driven by the lift pump realy during the glowplug cycle and charges the injection pump to the point it has fuel to start the engine. Once the engine start and oil pressure come up the oil switch closes and becomes the only source of power for the lift pump.