97 F150 Blows Engine fuse at 75 MPH

jimmy57 · May 25, 2014

97 F150 4x4 4.6L 5 spd manual 251 K miles.

I have a weird one. The 30A Maxi-Fuse, pos 24 in engine bay CJB will blow consistently at 75 MPH. It got a new fuel pump for this problem but that had no effect on the problem. I think the shop just used SWAG to decide fuel pump.

I inspected wires and harness to injectors, coils, purge, EGR controller, all 4 HO2S's, MAF, IAC, and intake control.

I used a mirror to inspect harness behind engine that goes to ox sensors and it is not near anything and is still secured at original harness clips and ties.

I drove it with a jumper loop and amp probe and when you get to speedo indicated 74-76 mph the current jumps to 60 to 100 amps and will come down when it coasts down to 72-73 mph. Sometimes the engine cuts out when it shorts but comes back in an instant when the current draw drops. At other times the current on the fuse is 9-14 amps.

I know of nothing that is switched on at that speed. No vibration starts at that speed. The truck is not free of vibration as it needs some tires but no vibration amplitude or frequency change occurs in that speed range where short or excess current draw comes on.

Just throwing this out as you never know if someone may have encountered something unless you ask.

Thanks guys.

8WA Sman · May 26, 2014

On a 97 there is two other places I would look both kick panels inside cab and the wire harness runs below drivers side frame. With that high of an amp draw the only thing that comes to mind is a short to ground. Also make sure the VSS pid reads the same as the speedo @ 74-76 mph.

jimmy57 · May 26, 2014

I found it.

I decided to run it on the lift (Risky but tough problems take tough investigation) and I could get the high amp draw at bit over 75. A storm was brewing and a dark cloud blew over and I didn't have shop lights on. I ran it up to speed one more time and I saw a reflected flash under hood to the left. I looked more closely and I found where the harness was laying on a stud on the top of LH valve cover at rear most position. I dug in to see if there was damage. It looks like the oxygen sensor heaters have a 10 gauge wire before the splice and that was the wire. One spot so all I did was corrugated sleeve and tape and replaced the unused stud head bolt with a regular bolt. This is a beater the neighbor's 16 year old son bought with his earned money. I offered to repair it and he gets to shovel horse crap out of horse stalls.

Brad Clayton · May 26, 2014

Looks like the stars and moons aligned just right for that repair! :thumbup:

jimmy57 · May 26, 2014

The part that threw me off was the fuse blowing precisely at 75 mph. There is a stretch of road near me with 75 mph speed limit and the truck would blow the fuse when he hit 75 mph. When I drove it with amp probe I saw the same thing.

There is some vibration that hits its peak or at least jumps up in amplitude at that speed. I had moved the harness in the area before but it is in a place where you can't see it for the EGR pipe and some other plumbing. That it got moved and went right back to the position ready to buzz on the stud is a bit amazing. Of course, in 251K miles and 17 years the wire bundle has hardened and likes to be in its long time position.

I am happy it got dark and the arc occurred to get my attention. I was unlikely to be watching while it is on lift going 75 mph. That is a bit too scary.