I have an issue that sometimes the fuel pump doesn't get 12v to prime. First time this happened the truck cranked but wouldn't fire. I tapped the relay (didn't have any tools and did it more out of desperation) and the truck fired right up. So I thought it's a bad relay and changed it. Now it happened again and I checked for voltage at the brown wire at the relay. The relay works fine (also the old one does, go figure) but the fuel pump still didn't get 12v. The inertia switch was not triggered as well. To make sure the switch is the little black box mounted to the firewall just above the clutch pedal, correct?

After bypassing the wire from the relay to the fuel pump with some "extension" leads with alligator clips the truck started right up. That was yesterday. Truck still starts today but I'd rather rewire and test what's in the path. I found some picture (courtesy of subford) for a 1989 showing the inertia switch is between the relay and the fuel pump. So I'd need to rewire from relay to switch and from switch to fuel pump, correct? How do I test the switch?

 

That is the location of the inertia switch.
To test ground pin #6 of the self-test plug and turn the key to the run position.
Use a loading (non LED) test light and test the wires in to and from the inertia switch.

Attachment 277922

Attachment 281923

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Maybe you could try jumping from the relay to the inertia switch only and see if it starts. If no, the trouble may well be beyond the switch so you would only need new wire from that point.

I had a sort of similar no start trouble with my '89 several years ago. Ended up being a bad wire from the factory. I had no obvious damage or nicks etc.. but it had a slightly bulging spot in the big yellow wire (10ga.I think), coming to the pump relay. After I replaced it, I cut the bump open only to find green corrosion for about 1/3 inch of the length of the wire. I had to ask a lot of questions on here before I had an understanding of where to test for power enough to run it down.

 

I had a similar problem with the wife's 94 Explorer. Occasional hot no start. Had her change out the relay, still no start. Wait a while, it started. Never would do it for me.One time, it wouldn't start after a long wait, had it towed to the shop late in the day. Next day, it started right up, mechanic used it for errands all day, trying to duplicate the fault. We went an picked it up.

Finally, it did it for me here at home. Determined that the fuel pump wasn't running during crank mode. And, apparently, the computer sends the signal to the relay through a different circuit. Checked the computer for corrosion, bad caps, etc, examined every inch of wire, jiggled things, cleaned connectors. No fixum. This problem had been going on for months, very randomly. Whenever I checked codes, I would get a 'fault in secondary fuel pump circuit". I had replaced the fuel pump and sender not long before this started, so I figured I screwed up a wire. Looked everything over, everything OK.

So, I ended up finding a circuit at the fuse/relay box that was hot during crank, and tapped that to turn on a new, separate relay for the fuel pump. Problem has not reoccurred since, and the only difference is the fuel pump doesn't shut off during the on/not running portion of start.(That circuit had always worked properly)

I had checked everything you've mentioned, plus lots more.

Good luck with 30 year old electronics

 

Did the test subford suggested and power going to the inertia switch but nothing coming out. Did a continuity test from the switch to the fuel pump and that was good. I also jumped the wires at the switch with a paper clip and truck started right up. Finally I removed the switch since I thought it was odd that I couldn't see or feel anything to push back in in case the switch was triggered. That button is missing. So I think it's safe to say that the switch is kaputt.

I googled the part number on the switch (E1AE-9341-A2B) and found some used ones on ebay for $50 and one for 5 Euro in Lithuania. Apparently these switches were used in Europe quite a bit (Fiesta, Transit and Sierra of the 80ties and early 90ties). When I look at the usual parts store websites the ones they show are totally different. Could I use those but have to mount them differently?

 

This one should work for you:

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Thank you! I assume I will have to get a new plug?

 

Yes if yours is different.
There is a plug below in the link.

 

I overlooked that plug. Ordered it as well. Hopefully everything l will be here this week. Thanks to everybody for their help and input. May need more guidance with the wiring.

 

Got the switch and plug. The plug has three wires. 2x 10g and one a bit thinner. The old switch and plug were only 2 10g wires. Am I correct in thinking that I can just connect the two wires each and ignore the 3rd thinner one? Does it matter which line goes to what? I assume not but since I'm a noob I'd like to double check.

 

The third wire went to the cluster on some trucks so you knew the switch had tripped.
Just use the two wire wires that have a electrical path through the switch when the switch is not tripped.
If it has a gray wire with an orange stripe do not use that wire.
Power in to the switch may be a dark green wire with a yellow stripe.
Power out of the switch may be a red wire with a yellow stripe.

Your old one should have had a brown wire as power into the switch and pink wire with a black stripe wire for power out of the switch.

 

The switch in your link earlier had a white/yellow, blue and white/red wire. I checked for continuity tripped and not-tripped. Tripped the blue and white/yellow show continuity and not-tripped blue and white/red. So I used blue as power in and connected it to the brown wire and pink/white as power out connected to the pink wire going to the fuel pump. I am tempted to wire a light to the white/yellow to indicate the switch had tripped.

Thank you for all your help. Everything seems to be good for now. That switch fits right into the existing screw holes. Perfect fit.