Fuel Pump Not Energizing Intermittently

[NewfyGuy]

Hello there,
I have a 1996 Safari with 4.3L that has served me incredibly well. Just about 1.5 years ago the fuel pump suddenly failed for the first time. It happened while driving and had been after it had been getting noticeably noisier. Had shop replace pump and was fine till recently.
Just about a month ago she wouldn't start and I noticed I wasn't hearing the fuel pump energizing with turning the key. Towed to shop and a new pump was installed and worked fine for about two weeks then same problem, wouldn’t start, not energizing, after fueling up at a gas station. Towed to shop and started fine next morning when they went to bring it in shop. They put it on the computer anyway and couldn't find any problems and said had to be not working to really find the problem.
I went ahead and replaced the fuel pump relay thinking that must be the problem and drove it for about a week then same thing wouldn't start one morning, not energizing. Never has happened while driving.
I actually had kept the 1.5 year old pump that was replaced a month ago. and I decided hook it to a battery and sure enough it worked fine. When talking to the shop they said "yah but it wasn't supplying the right pressure when they checked it". Somehow that seems fishy to me because when I was originally stuck a month I could tell the pump was wasn't kicking in period. I wonder if the low pressure thin is a crock and whilst working on it what ever intermittent problem simply righted itself as it has done recently.
The only work that was done just prior to these problems was a new ignition installed that had a new key as well as the guy didn't have the proper tumbler pieces to match my old key.
When I turn the key everything else lights up like it should and the starter turns over just fine but still no pump energizing.
Is there a way I can jump some juice and see if the pump can be energized separately?
Any wiring in the ignition area that is specifically for the energizing of fuel pump he may have damaged?
Is it likely a faulty computer be causing this?
I got under it and moved all the wiring harnesses and cleaned the ground wire and no luck.
Will try tomorrow unplugging the harness connector along the chassis rail and see if power is at least getting to there. I'm a fairly experienced backyard mechanic but electrical is not something I've done a ton of.
Really would appreciate any help as we're having one of our coldest winters on record here in Vancouver, minus 12 Celsius, and I use this for my home based small business.
Thanks and a Merry Xmas!
Glen

[Blue Bowtie]

Welcome Aboard!

They put it on the computer anyway and couldn't find any problems and said had to be not working to really find the problem.

If you towed the vehicle in with a shredded right front tire, would they also first plug in the scanner? Their use of a scanner to diagnose the problem demonstrates a lack of understanding of the system. The only thing the PCM does is cycle power to the pump relay. That can be determined a lot more effectively with a voltmeter than a scanner.

Since you have replaced the relay, that would tend to indicate a different problem. You still need to verify the relay is energizing, and that it is closing the pump circuit contacts. When the ignition is turned on, the relay should energize for two seconds to build pressure in the system. It will energize again as soon as the PCM sees a crankshaft sensor input (from cranking the engine), or the PCM receives a "Crank" signal from the ignition switch, or the ignition power is cycled off/on again. You may have to wait up to 20 seconds with the ignition power off before cycling it on will operate the pump. If the relay operates reliably under all these conditions, the problem has nothing to do with either the PCM or the the ignition switch.

Once you verify relay operation, you can jumper power to the pump at the relay socket. The "30" terminal on the relay should have constant battery power. The "87" terminal provides power to the pump. Installing an appropriate jumper between those terminals in the relay socket should operate the pump. You can leave the jumper in indefinitely while you diagnose the circuit.

There are several connectors in the circuit, four to be precise, and most are near the fuel tank. The connector at the top of the in-tank pump unit is particularly suspect, but if the entire unit was replaced with the upgraded design, this should not be a problem. If only the pump was replaced, this is a point of concern, since these connectors are prone to overheating and becoming resistive.

There is another connector within two feet of the pump unit which is accessible without removing the fuel tank. This connector is generally reliable, but the ground connection for the pump emerges from this connector and grounds at a chassis crossbar nearby. This ground connection is also suspect.

Verify power at those points to help isolate the problem.

If the pump IS running and the truck doesn't start, verify a minimum 61 PSIG fuel pressure.

[NewfyGuy]

Thanks Blue Bowtie,
Was actually attempting to put some juice directly to the pump at the 4 pin harness connector just before the tank but didn't know which color wire was the hot wire for the pump. I used my meter to check the socket side coming from the front of the vehicle and was getting constant 8 volts in one wire but no constant 12 volts anywhere else? Any reason I'd be getting 8 volts and if not what it might indicate?
I was going to head to the library to get the wiring schematic but today all libraries are closed due to a snow storm...yes 20cms here in Vancouver!
Would you happen to know the break down of the color coded wires at this 4 pin connector? Is it safe to bet that the one getting the 8 volt is the hot for the pump?
Thanks in advance.

[NewfyGuy]

Want to thank you guys for the posted and private messages.
Put juice directly to the pump wire at the harness just before tank and pump kicked in right away. Removed the ground wire from the frame and cleaned better than I had originally and then went and tried ignition. Sure enough fired up perfect. Having forgot to try the ignition again before doing any of this leaves me questioning whether this really has solved the problem as this problem has mysteriously temporarily fixed itself before.
I am confident this is not a pump problem and I will be prepared in future with jump wire and quick connectors to bring juice right to the pump wire as a means of getting the vehicle back to the house for further analysis.
We got almost 30cms in the last 2 days here in Vancouver!! That's like crazy mang. We normally get lots in the local mountains but it stays just above freezing down here in the city.
Well I hope you all have a very Merry Xmas!

Glen