With the playfield up, turn on the machine. Drop a coin. What does the coin relay do?

I'm looking at the schematic. I don't see a coin chute relay. Do you mean the coin relay in the bottom of the cabinet? The credit relay? What do you mean by "coin bar"? Are you talking about the little switch with the wire?

When you drop a coin in the coin chute or manually activate a coin switch, does the credit unit reel advance? How is the machine set up? Does the machine automatically reset when you drop a coin in the coin chute, or do you have to drop a coin in the coin chute and then press the credit/start button?

You'll have to look at the circuit that includes the coin chute switches... Check the 2nd coin chute relay switch and the 3rd coin relay switch with the brown wire and black/white wire. Make sure the switch contacts are clean and adjusted properly...

Does the Score Motor rotate a half-truths if you manually start to rotate it? Or, place a ball into the kickoff hole even in Game Over mode?

With 10 credits on the credit wheel, will the machine start a game when the start/credit button is pressed?... It might be that the credit unit is set for 10 credits maximum. You can manually deduct some credits by pressing the credit unit reset solenoid plunger. Get the credits down to 8 or less. Then put a coin in the coin chute to see if it will add credits to the credit unit...

First thing, the coil on the coin door is known as the coin lockout coil, and this is not your problem. As you have credit on the credit reel, than all that is need is a press of the start button.
So when you turn on the machine and press the start button, does the Credit Relay energise? If this is happening, then check to see if the Coin RE is enegising. Report back.

Try to manually rotate one of the bakelite cams on the score motor assembly clockwise (when viewed from the front of the machine) with the game powered on and lit up (by pressing the left Flipper button).

Does the m9tor kick in and rotate the cam assembly further electrically? If not, figure out why. Use standard electrical troubleshooting techniques along with the game schematic.

Very carefully check the wiring in the coin door going to the coin switches, especially those yellow wires. It won't start if one is broken. Don't ask me how I know this.

You can check the circuit that includes the start/credit button. Do you have a multimeter? Check to see if there is voltage at the start/credit button switch. You can also manually press in the credit relay to see if it will start the reset process...

With that, you have a broken wire on the coin door, or a problem with the coin door Jones Plug, or a wiring/switch problem on the Replay Unit or a Jones Plug issue in the backbox.

OK, be methodical and you can solve this. The key is understanding the startup process and the schematic.Here is an excerpt from the schematic that shows some of the stuff associated with starting a game. The credit button you are pressing to start a game is colored green.

A simple thing to check first is the credit unit zero switch which is highlighted in red. This will be found on the credit unit and it will have a W-O wire and a GRAY-Y wire on it. That switch has to be closed for the game to start. You can see it is open (NO) in the schematic. This would be the state with zero credits on the wheel. Once it goes to 1 credit or more, that switch has to close. You can also short that out with a jumper or however and see if that helps.

The coin switches are highlighted in yellow. The circuit to complete the startup passes through both of those. If there are any issues there, it won't start. In my case, everything looked good but then when I inspected it closely I found one of the tabs on a swtich was broken. The circuit was cut off, and it wouldn't start. So do make sure those are good. To see exactly what that looked like, here is a link directly to the post where I described what I found. This was on Bon Voyage but Monte Carlo is very similar vintage and works the same (I have both).

(I saw Mrbally's post just as I was uploading this, and I agree. That is the yellow area above. Besides the coin switches like he said it could be a problem at the connector somewhere. Also sorry it is a WHITE wire on the Monte Carlo, on the Bon Voyage it is yellow).

https://pinside.com/pinball/forum/topic/bally-em-bon-voyage-wont-start#post-2541170

If everything looks OK with those, then we can go from there, it is a matter of working through that one circuit and figuring out what isn't right.

Bally_1973_Monte_Carlo_Schematic_Diagram_startup (resized).jpg

First a warning, remember we don’t know your level of expertise here. There is Mains voltage exposed with in the cabinet of a Monte Carlo. You need to be careful while probing with the power on. Have a read here about Mains power. The first post is a Monte Carlo. https://pinside.com/pinball/forum/topic/mains-power-exposed-parts

Next, as pointed out by xsvToys, we need to be methodical from here.

So to get stared, with the machine unplugged from the wall, DMM set to Continuity or Ohms. Connect one end of the DMM to the yellow wire on the transformer. The other to the Blue / Orange wire of the start button. You should get a beep or read zero ohms (or close too). If this result is present then do the same to the other side of the start button, connect one end of the DMM to the Grey/yellow wire of the Start SW the other to the Brown/Red wire of the Credit RE. This wire will be connected to the coil of the Credit RE. You should get a beep or read zero.

What we are doing is check for where the fault is, which side, if any, of the start SW.
The other thing to keep in mind when follow a schematic, this schematic is drawn at a point in time. That point is – Game reset > Single player game >ball one in the shooter lane and power off.