Chevrolet Repair: chevy blazer, oil pressure switch, crank sensor

Question
im have a problem with blazer have to spray starter in to carb to start.will stay going.i know it may be relay or crank sensor .ived looked where are they lol.and could maybe i dont have enough cranking power .just got a new battery thank you

Answer
Hello Jay,
First of all, I need to know if you have a carb, or throttle body injection. I will assume injection since you are talking about crank sensor, but that is just a guess on my part, since you didn't mention year, make, engine size, etc.
If the crank sensor was the problem, it wouldn't keep running, since in addition to the fuel pump relay, it has effect over the spark.
The in-tank fuel pump on fuel injected vehicles is operated by a relay, that is correct, but there is a backup operating system which is an oil pressure switch that sends battery power to the fuel pump whenever there is oil pressure.
I say when you are starting it on starting fluid, the oil pressure builds to the required 4 PSI, closes the oil pressure switch and operates the pump, and all is well.
So the oil pressure switch is good, the fuse is good, the fuel pump is good....the only thing you need is the orriginal three seconds of power when the key is turned to run.

When the key is turned to run, you should be able to hear the pump in the tank for three seconds. If you don't, I would say you have narrowed it down to the relay.

The relay location is dependent on the year and model, but my manual doesn't say anyway, other than relays are located in several locations.
Full grown 88 models for example, the fuel pump relay and fuse are on the firewall in front of the passengers area, under the hood.
Some smaller vehicles have it in the convenience center, which is like a fuse block under the hood.
I would expect a cover for the convenience center to be marked with a diagram though.

Another remote possibility is the ECM, but I think you would have other problems with that, so I would surely look for the relay, and swap it with another, like the blower motor relay, where you can see if it works or not.

All of this now assumes you have injection, and not a carburetor, which is a totally different ball game.

Good luck,
Van