Truck Repair: 1995 Chevy van electrical trouble?, fuel pump relay, inertia switch

Question
The vehicle is a 1995 Chevy 3/4 ton panel van, 5.7L TBI engine and overdrive automatic tranny, and the problem is in the fuel injection system.  The problem is the fuel injectors are not releasing any fuel and the fuel pump is not going on when the key is turned on.  It is not the fuel pump as far as I can tell, because hooking up a jumper to the fuel prime terminal on the fuel pump relay brings it right on.  The fuel pump relay has been replaced.  The second circuit for turning on the fuel pump runs off the oil pressure sender; that was also replaced. This works properly-- the fuel pump kicks on (though injectors still do not squirt) after a few seconds of cranking.  My thought was perhaps it is short and thus started to go through the wiring harnesses checking connections.  I  found a few bad splices here and there but nothing seemed to fix it. I replaced the ground strap to the engine block: it was completely shot.  Though I did not change the fuel filter, I verified that fuel was reaching the throttle body by cracking open the line and flipping on the fuel pump-- it instantly kicked out a healthy stream.  My question is this: is it likely that the ECM is bad or that something (perhaps the inertial switch?) is telling the ECM not to squirt the injectors?  Injectors read twelve-some volts with the ignition on.  I'd like to know a little about the inertia switch and how it can be checked out, because right before this problem started the van got stuck in mud and we had to jerk it out with a chain and I am wondering if this is causing the trouble.

Symptoms prior to failure were the car alternately running normally and then rough, each alternating by a minute or two of time (which is why I thought electrical problem).  The car started hard for the day or so it was like this and then refused to start. My being an idiot (new to newer cars), however, means that no codes are stored, since I disconnected the battery before going to work on the wiring  and realized far too late to save them.  The check engine light came on at hot idle for as long as I have known this vehicle (about five years) and I always thought it was due to low-ish oil pressure.

I really want to thank you in advance for your help—it is a great thing you do Van!

Thanks much, Jeremiah


Answer
Hello jeremiah,
Great job of diagnosing, and describing things.

Inertia switch on that has to do with the air bag, so if you don't want to see it laying draped over your seat, don't mess with it.
Ford has the fuel cutoff switch that opens when you hit a big rough bump. Chevy doesn't have one.
I have seen a bunch of my fleet trucks with two or all four wheels off the ground simultaneously, and they don't quit running due to the bumps. (breaks some things now and then, but they keep running).

OK...power to the injectors is good.
Fuel pump running is good.
Oil pressure switch is good.(You would be amazed at how many professional mechanics don't even know that switch exists, so good job!)
Fuel supply sounds fine.
Have you checked for spark? Just curious, cause no gas, no run anyway.

The signal that is needed for the ECM to keep the pump running, and to operate the injectors, is the crank sensor.
Yours is probably right beside the crankshaft damper pulley. There are some screwed into the block.
But check the wire connections first.

I assume you have listened to the injectors with a stethescope, or screwdriver to your ear, and against the injector. They should click, but I bet yours don't. (Noid lights connected to the injector connectors should blink, but a test light should also).

So check that crank sensor and wires.
ECM is a possibility, but they really hold up very well.


Good luck,

Van