Hi, a quick question. I built a truck replacing a 6.0 (see thread below) and left all the parts that it ran on with the engine.

When I tried to start it all looked good but when I tried to start it would not crank. I left the ignition key on and used the purple starter wire on the the plus battery post to crank it. It cranked slow but started. I thought wth is going on but after running it for a few minutes I shut it off and it restarted with the ignition key.

Today it did the same thing again. This time it cranked with the starter wire but too slow to start. I knew the batteries were on shaky ground but I jump started it and used the starter wire again. Once it started that way I could shut it off and restart it with the key. It does not appear to be a starter problem as it works connecting directly to battery.

The only other thing different is this does have a Ford remote start module but even it wouldn't crank it.

Anyone else have any ideas?

 

Not sure why it would do that, but I would get some urgency in testing / replacing the batteries before you have a FICM on your list.

 

Low batteries in modern vehicles can quickly cause an expensive cascade of problems. So many items require a correct voltage to run properly and a standard alternator barely can keep up. Much less recharge dying batteries at the same time.
Save yourself an expensive lesson and get a couple fresh batteries before running it again. And check your alternator too.

 

Located the no crank issue. It has done it a couple of times and I can get it to crank if I lock and unlock the doors with the remote fob.

The weak battery issue was a bad ground strap from engine to frame on the right side. I started to check battery voltage on both batteries and got two different readings when they were hooked up. The right battery would read 12.9 while the left battery was reading 12.1. While running the right battery showed 13.9 volts while the left battery was showing 12.7 volts.

Since those batteries are in parallel they should have read the same while sitting and while being charged. If I ran a jumper wire from neg to neg on batteries I would get identical voltages. I ran a jumper from neg on left battery to engine and got the correct voltage readings. The left battery cable is bolted to the top of the frame so all I could figure was there was no ground to engine from frame. Again I confirmed that with a jumper wire from frame to engine.

Turns out that the ground strap that is part of the negative battery cable on the right battery had corroded in the crimp connector and basically was open. I replaced cable and both batteries are reading and charging correctly. It cranks fine now.

There is no visual indication that the ground strap is bad at all. It looks good but does not pass voltage.

I also checked to confirm the ground strap from engine to cowl on the right side was there and connected and it was. The truck was running on the right battery only and the left battery was just there taking up space.

If anyone else has weird electrical problems don't rule out that ground strap.

 

That's a good troubleshooting find.

Yep, the battery cables on our trucks were devolved from the 7.3 and have issues. Ford decided to save pennies, yes pennies, and use the frame assembly as a conductor across the batteries. Really poor idea, but any internal cable fault even without this design can be an issue.

I made a video about improving the stock layout for those that don't want to do a full bore replacement. Once I get my truck back up and running I'm planning on two more upgrade versions, but it has to wait.

Anyway, you might want to check this out.