1994 F150 5.0L - won't start, no dash lights, cranks OK

Only driven every couple of months. Fresh gas about a month ago.

First symptom:
-after a month of no driving, cold start was perfect
-after 35 minutes of warm up on the highway, intermittent complete loss of power, flickering CEL at highway speeds and just after stoplights; frequency of intermittent-ness increased until I got back home

A few days later, as part of PM (not related to intermittent loss of power):
-replaced air charge temp sensor, coolant temp sender, coolant temp sensor, upper radiator hose
-then the "no start" happened

Next symptoms:
-won't start
-seemingly no fuel pressure on rail (not much experience with this diagnostic technique and it's a brand new gage to me)
-seemingly no spark because I used ether and not even a pop
-most dash indicator lamps don't light at "key on"
-I hear no fuel pump priming at "key on"
-my OBD1 code reader cannot pull any codes and I don't think it's even linking

Other:
-at power distribution box, I swapped relays 1 and 4, and 2 and 3 - no effect (horn does work)
-battery continues to crank engine but I also now have it on a maintainer to keep it charged while I diagnose this issue
-switching from front to rear fuel tank, and back, seems to make no difference

What component could produce all these symptoms?

 

Bad fuse at position "P" or "N" in the engine compartment fuse box or a bad Ignition switch or its plug.

 

Thank you Bill. The ignition switch doesn't seem to expensive or hard to change - I'll try to find one locally.

I don't have a fuse location map so I don't know where N or P is. Can you look at my pic and tell me which number? I did pull each fuse and all looked OK visually.

 

Here is a photo, check each side of the fuses with the key in the run position.

Attachment 280139

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I put in a new Echlin ignition switch. No joy.

Then I did your extra test. Started at fuse L. I don't remember that I got voltage. Then N - no voltage. Then O - heard some clicking and the fuel pump "hum" and definitely had voltage on all fuses L through U.

Jumped in the cab, engine fired right up.

What could explain testing for voltage in the distribution box bringing the something to life to be able to start the truck?

 

One of the fuses was not making good connection.
But "N" should have had voltage on it.

 

Bill, thanks for the pointers so far.

Is there a way to make this more reliable? I hate to be at the whim of jiggling the fuses.
Could the relays be an additional point of failure?
Do you think it explains the highway driving intermittent sudden loss of power?

 

Could be wiring under the Fuse Relay Box or bad fuse socket.
No relays in the circuit of fuse feeding #N.
Yes I do.



Attachment 280134
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I tried to read the codes with my OBD1 reader - it would not link.
I did not hear the fuel pump priming.
I disconnected the reader and tried starting the truck - no joy.
Fiddled with "taking voltage measurements" again (the trick discovered last time) - still no joy.
I did measurement ~12.51v on both sides of every large fuse in that row next to the relays.

What downstream component is perhaps not getting the voltage from the distribution box to keep the truck from starting (but allows cranking).

Where to go now?

 

If the wire #16 is dead from the Ignition switch that goes to the Ignition coil and the coil of the PCM Power relay then that would do it.




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Hope spring eternal so I tried starting the truck first thing this morning - no start.

Measured 12v at ignition coil rightmost wire.
"Key off" - measured voltages in distribution box - 12v both sides of fuses - L through T. "Key on" got 12v on both sides of U.
While measuring voltages in distribution box, I heard fuel pump priming (indicating start-able).
While start-able, I hooked up my OBD1 reader - received a 512 KAM failure code.
Unhooked reader and I was able to start engine.
Installed new Duralast ICM - no start. Put old Motorcraft ICM back in. Still no start.
With old ICM, pierced ICM lead for PIP test with LED - blinks OK; engine did not start.

What part to replace next? Distribution box? PCM? Something else? Or something else to do/check?

 

Sounds like a bad wiring connection somewhere near the Distribution box.

 

It seems that both spark and fuel are simultaneously affected.

To try to track down the bad connection, where could i physically measure voltages to discover the break in connectivity? In other words, had 12v here at "A" and was expecting it at "B" but did not get 12v.

Continued thanks!

 

C269 would be a good place to start.
Then maybe C101.
This would be wire number 16 a red wire with a light green stripe in both cases.




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It's been a few weeks since I spent time with truck... I went out to troubleshoot it tonight.

Dang it... It started right up. It did die after about 10 seconds but it started up again.

It then ran ok for about 15 minutes without issue until I turned it off. I restarted it a few times without issue. While it was running I jiggled the wires to all the safe places I could reach. No engine reaction to any of the jiggles. I also jiggled each fuse and relay, no reaction from the engine.

So, do I trust this truck more than two feet from my driveway? What more can I do to troubleshoot?