You need to start at the batteries and the connections to the posts. Most likely you have very dirty cable clamps and/or corrosion inside the insulation. Put you meter on the battery post and watch the voltage there when you try to start. If it's at least 12.5 V and stays there that confirms bad connections. A 7.3L will start on one battery assuming everything is clean and tight. A bad battery will also drag down the good one, so you should probably replace both.
Trying to turn the engine from the flex plate gear teeth is an exercise in futility as far as starting it. Unless you suspect the engine is locked up.

 

Thanks Mr. Hansen. Just came back from measuring the battery, it didnt go down to 0, I had it connected to the house charger , so it was showing 14.31, when it clicked it was showing 13.2 - 13.3 at the battery. The fender relay clicked pretty loud also. What should I do next? I hope I don't have to replace the big power cables?

 

Next measure between the battery post (not the clamp) and the clamp. Should start out as zero voltage and stay relatively low when you put a load on it by engaging the starter. If it rises to even 10V that indicates a high resistance connection between the post and the clamp. Do the same thing from the battery clamp to the fender relay. Starts at low voltage and increases if you have a high resistance somewhere between the battery clamp and the relay. Also do the same across the relay and then between the output terminal of the relay and the starter. What you're looking for is a high resistance somewhere along the line that's robbing the starter of voltage it needs to turn the engine.

 

Okay I finally had a chance to go back to troubleshooting yesterday. I have ruled out the starter relay on the fender, the starter tested good, battery tested good. I did a bench test on the starter off the transmission and it turned just fine with the existing fender relay. I even tested it with another brand new relay in place with no result, just started solenoid making a loud click. Is there a starting diagram somewhere so I can understand where to measure voltage drops? I'm not seeing any code (like P0705) for the neutral sensor

 

With the charger connected, it won't show the true state of the battery. Try doing the voltage drop test without the charger connected and see what happens.
As far as a diagram, the starting system is pretty simple. Attached is a sample (may not be EXACTLY like your van)
What you want to do is measure across every connection (battery post to battery cable clamp, etc.) component (starter relay, etc.)

 

I tested the battery at advance auto parts, so pretty positive that its not the battery. Now I;m wondering if the high torque starter i’m using requires two batteries to start. Maybe it’s that silly of a problem. It was starting and running fine the day before it quit. Youre right , the wiring to the starter motor itself is pretty simple, but it doesnt show the wiring to the alternator? Could the diode on one of the alternators have shorted and caused this problem?

 

These vehicles came with two batteries. On the vans one was mounted where it is easy to access and from what I understand the second one was mounted under the vehicle.

Between the glow plugs and the starter these engines need a lot of amps and that is the reason behind two batteries. Your symptoms appear like there is a lack of amps to run both the glow plugs and starter at the same time.

 

Mine is an ambulance so the second battery is in a compartment by itself, also easy to access. But I did notice on a 70º day where the glow plug did not turn on it also just clicked the solenoid. I may just have to buy the second battery now.

 

Try taking a second vehicle and hooking up some jumper cables and see what happens.

On the glow plugs you need to check the voltage on the large post to see if the relay is coming on or not. The WTS light on the dash is just a idiot light and has nothing to do with the glow plug relay.

Have you looked at your existing battery to see what the cold cranking amps is CCA?

 

The cca on both batteries is 850, but the second battery tested weak at 650, my topdon says 50% charge and "REPLACE", and I don’t have any other car with more than 650 cca battery. Maybe I’ll try my rogue battery anyway and see what happens. I dont really understand what you mean by checking the voltage on the large post , can you elaborate?

 

The glow plug relay is controlled by the PCM, the WTS light is also controlled by the PCM but has nothing to do with the relay other than getting you to wait a bit before trying to start. So the relay can be on even if the WTS light isn't on.

On checking the post for voltage, the glow plug relay has two large post. One has voltage all the time even with the key off. The other post will only have voltage when the relay in on. One of the relays small post will have voltage all the time that the key is on, the other post gets a ground signal from the PCM to turn it on and off. So if when you first turn the key to on and even if the WTS light isn't on and the glow plug relay has voltage on both large post then it is on and can stay on for up to 90 seconds until the PCM removes the ground to turn it off.

 

ahh, i see. thank you. I do notice that the voltage indicator in the dash goes way down to just above 8 when i turn the key to acc, and the WTS turns on. Then it comes back up to around 12 and thats when i turn the key to start the engine. If the glow plug relay is stuck connected i’d expect the dash voltage indicator will hang around low voltage for 90 seconds, which it isn’t. But i will verify it, it’s just that the gp relay is hard to get to in the van. Everything is hard to get to in the van, lol.

 

You can just do what you said, watch the volt gauge in the dash. When it drops down low the relay is on, the plugs themselves draw upwards of 190 amps so that puts a huge drain on the battery.

Do this, put a volt meter onto the battery when it is fully charged and before you turn the key on to start and see what the voltage is reading. Then watch it if you can when you turn the key to on and see how far the voltage drops, then check the voltage when you turn the key to start.

Record these readings for reference. From my memory when I first turn the key on in my truck the voltage on the batteries will drop from 12.7 down to 11.4 or around there. Then when I hit the starter it will drop a bit more. Also from my memory you need at least 10+ volts to wake up the PCM and start the truck

 

Your voltage dropping that far on the dash gauge is a sign that you have a high resistance connection somewhere between the batteries and the dash (and everything else.)
Start by cleaning the battery posts and clamps (both hot and ground side) and reconnecting them. Use a battery post brush like this:

If that doesn't help. Clean the ground cable connections at the engine block.
Also check the connection at the starter relay. There should be a connection with two fusible links that head to the fuse box. If that's not clean and tight, then voltage to the PCM and instruments will be reduced.

Also check the battery cables at the batteries for corrosion under the insulation. If there's green goo in there, it doesn't conduct electricity worth a darn.