Starter won't engage

I'm sure by "coil" you mean "solenoid".

The correct solenoid for the 24 Volt 4020 is a 24 Volt solenoid. So, there MUST be 24 Volts at the two small terminals on lower part of the solenoid itself when the tractor is supposed to crank.

Here's what I think you should check:

Have someone activate the starter while you measure the voltage across the two small solenoid terminals. (Someone may have screwed up the wiring so the solenoid is only getting 12 Volts.)

Also, check the voltage at each battery while someone is trying to crank it, to rule out a weak cell or two that are causing low voltage.

Also, did the shop check the engagment of the Bendix when the solenoid is activated to rule out wear or mis-adjustment in the solenoid plunger and engagement fork mechanism?

Check those things out, and report back, and we'll go from there!

How do you expect it to work with batteries that weak?????

Charge up the batteries, and load test them.

This thing ain't gonna work without good batteries, properly charged!

The battery charging voltage will be over 28 Volts, the battery voltage after sitting overnight will be a bit over 25 Volts, and you need to be seeing like 18 to 20 Volts while it's cranking over.

When you get the batteries "up to par", check again, for the better part of 24 Volts at the solenoid coil terminals while cranking, just to be sure the wiring is correct there!