97 Chev Venture 3.1 83K Miles
You probably recognize all these devices, you will find them in most "Underhood Fuse Relay Centers" or "Power Distribution Centers" or other names that escape me at the moment. In one article here I mentioned my quick "No Start" Diagnosis Procedure that rarely fails me, check tank gauge for fuel, listen for pump prime, crank engine, check tailpipe for raw fuel smell, check ignition. This is all done without any hookup of equipment (except spark tester) and no tools put on the vehicle. I recall one such vehicle towed in (89 Ford Bronco) I had running in less than five minutes (found Ignition Module connector loose) the proofs in the pudding, I did not charge the customer for this (he is now a repeat customer).
Anyway, back to the Chevy Venture. This vehicle was driven in, and customer said he had at another shop for intermittent no start. Vehicle got a new MAP sensor, customer was charged $180. This changed nothing. Customer said after vehicle sat for 30 minutes, most times it would not start, but cranked over fine. Naturally, I get in, it starts right up. I have customer drive for 10-15 minutes and bring back to open stall. Left sitting for 30 minutes, it starts right up. Great... I let sit for another 30 minutes, it starts right up. Now I'm turning red faced. I drive for 10 minutes, and back in shop to sit for another 30 minutes. Cranks great, but doesn't even pop. Fuel gauge reports half tank of fuel, I hear no pump prime signal, whack tank, still no pump noise. Now I send power to prime wire, pump runs (did not look at pattern) since this wire sends power directly to pump, it bypasses the relay. Probably a bad relay right? Wrong.
I suppose at this point its easy to throw a relay in and see if thats the case, but thats the easy way out, and not everybody has relays at their disposal (unless you keep some popular ones in your toolbox). Simple thing to do now is check for pump power at the relay socket and relay power to the relay. Some get a ground signal from the PCM to activate, others get a power signal to activate. In this case the relay gets power from the PCM. Power circuit to the pump was available at the relay, and the pump ran with the circuit bridged. Now I look to see if the relay is getting signal from the PCM, and find no signal exists. So, is it a bad ground, or no positive signal from PCM? I voltage drop the circuit, and there's nothing, no positive signal from the PCM. So, where to go from here? The PCM sends the prime signal after it see's key on, so the PCM is at fault right? Maybe. Maybe the circuit to the relay is open? Maybe.
The simple thing to do now is verify the PCM, make sure it has the three important things it needs to be alive, battery power, ignition power, and ground. You may or may not be able to see all these at the DLC, I strictly adhere to going to the PCM connector to see them. That way you are completely sure what exists at the connector. The PCM on this application resides in the air cleaner box (I suppose it's helped cooled in there, or maybe GM couldn't fit it anywhere else) After removing the coolant tank, cruise control unit, and body brace, its easy (HA!) to get the PCM out for testing. First thing I check is battery power, then ignition power, and finally the ground. I use sewing needles with blunt ends to backprobe PCM connectors since they don't have much room to go in, most nowadays are pretty tight.
Testing found that the PCM was not getting battery power, the ignition signal was good, as well as the ground. Now I know why the prime signal doesn't exist. Now its a matter of finding the open in the battery power circuit to the PCM.
In the diagram, the circuit in question comes from the Underhood Fuse Relay Center (ufrc), and originates there, indicated by the arrow on one side of the fuse. Checking the fuse indicated power through the fuse, so it has to be lost from the fuse through the ufrc to the PCM. Now I have to get the ufrc apart to check the circuit from it to the wiring harness. The ufrc on this application is very easy to get into testing position, its mounted on a plastic bracket held with one screw and mostly held in place at a body brace. Once up and sitting sideways, you remove the bottom protective cover and reveal three big harness connectors plugged into it, the circuit I want is easy to find since the connector is numbered and I want C1, E2, an Orange wire. I hook up the scope to the circuit and find power exists. What?!! In moving the ufrc, I must have interrupted the open circuit, go to see if it starts, now it does. Well at least I'm getting close, now I'm wiggle testing the harness while its running to see an effect, and theres nothing. I continue moving things around for 10 minutes or so while the scope's trending the circuit, and nothing. Time for a whack test, I whack the ufrc with my hand several times, nothing. I'm getting red faced. I get a hammer and pound on the ufrc, the engine dies. I turn off the key, and call GM dealer and order new ufrc.