my one buddy has a cub cadet tractor that has a 3 cylinder diesel engine. [i can't see anything on any tags who made this engine]
the problem is the starter will not crank over. battery is good, and all wiring from the key to starter/glow plug relay checks good, plus all grounds, starter, and starter battery cables check out as well with no voltage drop.
attached, is a diagram showing the glow plug timer preheat module. it has 12v on the outer top row terminals in run and start.
the center terminal shows zero volts in run, but only 5.22v in the start position. if 12 volts is applied to the center terminal, which goes to the starter solenoid [almost identical to a mini-starter], the starter spins over as it should, and the engine starts.
is this module bad ?
the wire going to the glow plugs [from the relay] has 12v when key is turned on, and after xx seconds, the relay kicks, voltage to the plugs turns to zero, and the dash light for the plugs goes out [as it should ?].
i know nothing about diesel starting systems. TIA !

See if you can jump the starter to get it to crank.. you could have a bad ign switch

Does it have any safety switches like a riding lawnmower?

There are forums for most brands of tractors out there, they might be able to give you more detailed info.
https://www.mytractorforum.com/threads/cub-cadet-diesels.97148/

I've used them for info when working on my 1954 Ford NAA tractor.
Lots of good useful info on the tractor forums.

When I bought my ford diesel tractor I bought all the manuals for it at that time.. same thing
when I bought my crawler loader

Not knowing what model your tractor is I can only tell you what I know about my Kubota. Most small tractors under 40 hp. have a system called OPC. Operator presence control. I had issues and it was all related to a seat switch. The kubota has several switches mainly on the seat, pto, and hydro pedal. They all can cause no start and shutting down. I would do some checking to see what system his tractor has. Also check the fuses as when mine was acting up I accidentally blew the OPC fuse and then it would not crank. Replaced the fuse and then diagnosed it to the one seat switch that was not registering some one in the seat. Mine also had a second seat switch so if you flipped the seat up over the wheel the switch would allow the pto to run with out an operator in the cab. Rear pto implements like a rear 3 point log splitter or chipper. Do some research on that system.

On my Cub, not a diesel, if the brake pedal isn't pushed all the way down it won't crank, if the PTO switch is "ON" it won't crank, if you're not sitting in the seat it won't crank.

Thats the way my zero turn is... I jumped 1 of those switches

thanks for the responses guys !
i forgot to mention, when the key is turned to start, the solenoid clicks and you can see the starter gear jump toward the ring gear, but the solenoid wire at the starter is only receiving 5.22v, thus acting like a dead battery, just enough volts to excite the starter, but not enough voltage to turn the starter motor over,
when a full 12v is applied to the solenoid wire, either by jumping the hot lead at the starter connection to the solenoid, or applying 12v to the solenoid wire at the timer module [where the 5.22v originates], the starter works as it should.
that leads me to believe the tractor battery is good and the safety switch[es] are working. i would think if any of the safety switches were bad, when the key switch is turned to start, there would not be any voltage [no matter how low] sent to the starter ? i have the starter switch hanging, and it puts out 12v in both the run AND start positions, while in the run position, the start position has NO voltage present.
i did a quick check of all the fuses at the beginning of this ordeal, but i will go back over them to see if dumazz me missed something.
i also have jumper wires made up to bypass the safety switches.
as i have said, this is my first go at a diesel tractor, so i know nothing about it.
i hate to spend anything on a factory FSM to never use it again. if it were my tractor, i would have bought one as soon, or shortly after, i bought the tractor.

thanks again for the suggestions guys ! please keep 'em coming !

What model is it?

Forget what you think you know when it comes to these tractors with computers.

A place I used to work at had Kubota tractors that used to do the same thing. On them it was corrosion in the clutch pedal switch.

You need to go upstream with a voltmeter until you find your issue. It will either be a corroded switch or section of wire.

How does this little controller interact with the glow plugs? It sounds like the glow plug relay/light are completely separate from it? Is this box supposed to prevent cranking unless the glow plugs are warm?

Is this box big enough to hold a relay? If you're only getting 5v out, I would assume that it's bad...

7264

the box only measures 1 1/4" wide x 1 1/2" long x 1/2" thick [not counting the area for the plug. it also has some kind of potting material on the back, sort of rubbery consistency, or i would have taken it apart by now to see what's inside.
the tractor has no computer that i know of, looking at the wiring diagrams i have.
as to how it interacts with the glow plug relay, that relay supplies the two outer [top row] terminals with battery voltage at all times [in both run and start].
in the start position, at anytime, the center terminal [going to the starter solenoid] you can get a voltage reading regardless if the glow plugs are warmed up or not. this voltage is enough to kick the starter drive gear toward the flywheel, but not enough to get the starter motor operating. yet when you apply battery voltage to the center terminal, the starter motor works as it should !
there are no other wiring sources connected to either the glow plug relay or the timer gizmo that i can see from the diagrams.

i'll have to look closely at the clutch/brake switch the next time i get out there.

this gotta be something simple. i just have to find it.

It sounds like the glow plug sensor is bad.. I think if it doesnt read a certain temp it wont let
the engine to crank and fire up

I'm begging to think there's something bad in that box you pictured

I'm just curious why it would have two pins that take the same Run/Start signal

me too, but they are from the glow plug relay, which "seems" to be functioning correctly.
i'm almost ready to just install a pushbutton starter switch run through a relay.
that box was quoted by the local cub cadet dealer at $165.80 !!
i did a quick search on line [the bag] and couldn't find anything remotely close to that box at all.

A pushbutton is starting to sound pretty good

My tractor has a box to trigger a solenoid to cut off the fuel and turn it off when you turn the key off. It died, and a tractor junkyard wanted ~$250. So now I have a string that goes to the injector pump to pull a lever to cut the fuel off

The small wire you are applying 12 volts to make the starter crank is it Orange with a white tracer.