So i just bought a 96 blazer about a month ago. Ive owned blazers in the past and am sorta familiar with the common fuel issues. The first week i owned her i changed the spider inj/ FPR, mani gaskets, and cleaned the tb. She ran fine for a couple weeks. Then it died as i was driving, checked the pump no noises. Changed the pump and replaced the 4 pin pump connector. when she was all put back together she didn't want to start. The next morning she started right up, ran it for half an hour no problems. When i shut it off, the key stuck in the ignition. I have to press the key release everytime to get the key loose. She will only start after sitting for at least 12 hours, if im lucky, the engine will crank,and spark fine but the fuel pump wont power.
Ive already checked the ecm fuse, read for power and ground at the pump connector, "Horn checked" the relays in the glove box, and hard wired the pump. Pump is good, relay is good, but no power to the pump. Also the battery keeps draining. I cant find any thing quite like this when searching this forum, nor can i find any readable wiring diagrams anywhere. Any help would be greatly appreciated.

Im thinking its either the computer or some ignition switch issue.

 

Is the key still stuck in the ignition? Im not sure if your Blazer has it but some cars have a security system that shuts off power to the fuel pump if someone tries hot wiring the car. Maybe with the key stuck in there, the computer thinks the ignition is being messed with. I could be wrong, its just a thought.

 

Mine would run fine and then for no reason it would stall. I would remove the fuel pump relay in the glovebox and put in a new one. I could start the motor and then shut it off. I then replace the original relay and the truck would start again and crank every time for no specific time. The problem would occur fairly often so I had to keep an extra good relay on hand. I can offer no explanation of why this occured but installing a new relay for one start kept the issue at bay.

 

ignition switch?

 

Unplug the fuel pump relay: Check voltage on the dark green with white stripe wire in the relay socket. It should show battery voltage for ~ 2 seconds when the ignition is initially turned to the RUN position, then drop to zero volts. Post your results.

 

-They key gets stuck almost every time. But it comes right out if i press the release under the ign switch.

- I Will check power to the relay contacts in the AM.

I noticed it started both times i had it on jumper cables, so my game plan for now is to change the battery and throw about 4 more gallons of fuel in to make sure it hasn't ran dry. Could be as simple as low batt voltage to the relay. Will post results tomorrow night.

I also noticed that when it starts i clearly hear the relay "click" from the glove box, and i do not when it doesn't, so that narrows it down to something on the ign switch, wiring, or battery, or possibly the VCM not energizing the relay.

 

Update-

Today i put in a new battery and 5 gallons of gas( which i might soon regret) and of course she started fine and died after 5 mins. then she started again and ran for ten mins. Then she woulnt start again.

Heard the relay and the pump everytime it started. And didnt hear either of them when it didnt start.

I get no voltage(prime signal) at the relay contacts when the key is switched on. but have good grounds

The sticking key issue seems to have been resolved by the battery.

What controls power to the f/p relay? Is it direct power from the ign sw or is it computer controlled?

 

The PCM supplies the relay with a trigger. Make sure it is consistent, (each time you turn the ignition to the run position). See post #5 and post your results.

 

OK im not getting ANY voltage to the relay. Pump power or relay control.

What logic does the vcm need to prime the fuel pump? Ive seen mentions of the oil preesure switch cuasing fuel shutoof through the vcm. Any truth to this?

 

This applies to 1996 & 1997 S&T body vehicles.
Unplug the fuel pump relay. Probe each circuit in the fuel pump relay socket:
Orange wire must show battery voltage at all times.
Green with white stripe wire must show battery voltage for ~2 seconds when the ignition is initially turned to the RUN position, then drop to zero volts. It must show battery voltage constantly while in the START position.
Black wire must show less than 5 ohms resistance to ground at all times.
Grey wire goes directly to the fuel pump. When you apply battery voltage to this wire, the pump must run.
Red wire, (short dead end pigtail) is connected to the grey wire inside the relay when the ignition is OFF. When you apply battery voltage to this wire, (relay must be plugged in) the pump must run.

Check each wire and post your results.

On certain 4.3L engines, (your 96 is included) the oil pressure sending unit supplies an additional circuit for current to flow to the fuel pump when engine oil pressure is above ~7psi. It can not and does not control the fuel pump. GM eliminated the circuit on 1998 and newer 4.3's, possibly realizing it wasn't necessary. GM has not used the oil pressure sending unit to control fuel pump operation since the 80's.