You're right, just with work and the time I have after, I don't have much time to try this. It is a solid piece of advice though and I'll try when I'm able

 

Pffff okay a lot to note here

I forgot to note, A while ago I had used a I used an inertia switch from my '91 F250 and still the wires got hot. Today i used a paperclip to bypass the switch and the wires still got hot, but the thermal camera showed one wire heating up before the other, at the point of the butt connector. So I shortened the wire eliminating two splices, used solder butt connectors on each wire under the inertia switch, and STILL it gets hot.
I thought to look at the HP pump and sure enough the PINK wire gets hot soon after startup . Didn't see thermal trace it back much further than the pigtail, so would a new pigtail fix it? It is it the HP pump failing? It's not too old.

Then a co-worker suggested maybe that wire is warm because it is carrying voltage(current). So I checked the battery under the hood which was cold comparitively. At that time I noticed the blower motor relay I put in was very warm. Loose butt splice so I replaced it with a solder butt connector and it solved that one wire being warm,.BUT there is one wire going to the starter solenoid that gets hot after the fusible link and before the solenoid

I thought to look at the relays for FP and ECM and the ECM was hot, see pictures




There's a lot there. Any ideas?















I tried to re organize photos using my phone but it's a nightmare to do so..I hope it makes sense

 

So besides the in tank pump you also have a frame mounted pump and if so does it use the same relay as the tank pump?
We can see the wire to the HP pump is hot but what about the in tank pump wires and they getting hot?
If not getting hot and use the same relay then the HP pump would be the problem.
I did not know there was a HP pump sorry.
Dave ----

 

Using charm.Li, I cannot find the wiring of the relay itself. The relay simply says "To fuel pump". I'll check my Haynes manual later. Also this evening I will take the access panel off and check the in tank/LP pump wires.

The wire near the blower motor that gets hot is a YELLOW ignition wire. That is the whole reason I installed a relay for the blower motor , negative switched as opposed to hot side switched with the help of Chat Gpt as much as I despise it. It used to be that whenever I turned on my blower motor, that one particular fat YELLOW ignition wire would get really hot. The YELLOW wire hadn't gotten hot until I noticed it yesterday, but looking at the wiring diagram I wonder if it's the same wire

Thanks for your help so far. Lifesaver

 

I believe that yellow wire is the main feed into the cab of the truck.
It may go to the fuse box but not sure.
Dave ----

 

After a minute or so of idling, this is the result. I do not believe the LP wires are getting hot which may mean like you said, that the HP pump is failing and drawing too much amperage

That YELLOW wire eludes me though and it's been getting warm for some months or years now

 

Installed a new high pressure fuel pump because it just gave out on me. The wires under the dash still get hot. Any ideas now? Relay faulty?

 

What if you try and bypass the relay and see if the wires get hot.
Other than that I dont have anything else it could be?
Wonder if the wires got weak from getting hot from a bad pump?

Have you asked this problem in the 87> truck area as there has to be more people in there that may had this problem or have a answer for you.
Dave ----

 

Will see if bypassing it will help, working in another project at the moment
I haven't really found out how to navigate this site yet but if bypass doesn't solve it, them I will.

Or or or, I could swap ECM and FP relays right under the hood and that will tell me

 

Here's a link to the 87> truck area https://www.ford-trucks.com/forums/forum28/
Dave ----