I’ve been having a slow crank on my 2015 F250 KR, (not due to weather as it is warm here right now) replaced both batteries to no avail. Brought the truck in to the shop and was told the starter needs to be replaced as well as the battery harness due to high resistance in the battery to starter cable. Apparently this wire does not come by itself (part of the harness) and I am quoted at $2800 for the replacement of the harness. Is there an alternative option for this?/ how long can I wait to do this? Or should I just bite the bullet

 

It might even be your alternator not charging your batteries enough. So next time before you cold start check the voltage on both batteries. Alternator hardly breaks, normally it's the brushes that need to be replaced every 50k miles.
Things you can do:
1) clean all terminals related to starter, alternator and both batteries. The electrical connectors on starter and alternator clean using contact cleaner or 91% isopropyl alcohol, same for both batteries, on the cables clean using the same thing, on the terminals use wire brush a little since the batteries are new like you mentioned.
Then you can check resistance (under 5ohm is good, closer to zero better). A stock setup on dual batteries are not isolated.
2) check for parasitic draws.
3) remove the starter and take it to auto store for free bench test
4) you can even use battery charger to charge both batteries before starting, or even jump start it from another vehicle, just to see if it cranks slowly still.

 

Both batteries are new so I ruled out that as a cause. I’m just replacing the starter now to see if corrosion on the terminals caused the resistance in the wiring. Hopefully that’s the case and it’s a simple replacement. Will post an update soon.

 

No don't rule that out. If both batteries (even 1 week old) are not being charged by your alternator adequately, they need to charge by a battery charger for the moment.
That's why you should use a voltmeter and check what voltage you get just to confirm.
You also shouldn't replace the starter (motor + solenoid). All you need is to do some cleaning on the solenoid.

 

You could have a new good battery and starter and still have a slow or no crank problem.
If the battery's power cannot get to the starter components , the problem you are seeing could be caused by the corroded cable(s) , which, if internal, will require replacement... No need to replace entire harness, just the heavy duty part of the cable. Does not need to be an expensive fix. Can be done fairly quickly and neatly.

 

Would a bad alternator/ batteries not charging cause higher resistance in the wiring to the starter? Or would that just cause less amperage going to the starter

 

I was quoted by multiple shops all in the 2-3K range for harness replacement because the part of the harness that “appears” to be faulty does not come on its own. How hard would it be for me to splice new wiring in where the old one is bad?

 

What kind of tools do you have to start a wiring change ?
Most times the corrosion is in the cable coming from the battery positive post to the solenoid on the starter.

We are assuming this is what they checked doing voltage drop testing to verify.

Does the vehicle have any other kind of electrical problems ?

 

According to what the shop told you, high resistance is the wire, not the battery, not the alternator. You just need to use ohmmeter to measure resistance section by section.
Repairing rear window defroster lines for example, you can use ohmmeter to find out where the break or weak spot is when you can't tell by visual inspection.
Have you heard for grounding kit for cars? Most people would recommend you to NOT replace the existing ground wires, you just add a new ground wire to the same path. You can do the same thing to your battery cables so that you don't have to pay $2k for just the oem harness. You can even use heavier gauge size from 4AWG to 2AWG. So basically you don't have to replace the entire harness, just find out which section, and you add a new cable just like installing the grounding kit.

 

The reason I asked if there were any any other problems is:, we don't know what that positive cable might be supplying power to the inside of the vehicle and to just cut and remove could open a can of worms. What Heiko suggested makes good sense and just providing an alternate path with another piece of cable might be a cheaper and just as effective fix.
A PHYSICAL look at the wiring is something that should be checked before attempting any "surgery".