1999 mercury cranks slowly with key

[watershoes]

I recently bought a used 16ft jon boat with a 25hp 1999 2 cycle mercury on it. The boat cranks up just fine with the pull rope but when i try and crank it with the key start, it barely has enough power to turn over once. I know the starter is fine because when i connect it straight to the positive wire from the battery it cranks right up. Just bought a new key switch too, not sure if the solenoid is bad, if i have it wired wrong (the previous owner had all of the wires disconnected) or what. please help! i just wanna be able to start this thing from the console.
pictures of the solenoid and starter side of the motor.


if you cant tell by the terrible pictures, the top post on the solenoid is connected to the positive wire from the battery and positive for the key. The right post is connected to the positive to the starter, a yellow wire to the key, and another yellow wire that goes in the switchbox somewhere i think. the left post just has two ground wires on it,

 

[Barnacle_Bill]

Temporarily substitute the battery cables with jumper cables. Also a voltage drop test can tell you exactly where the problem is.

 

[Texasmark]

The wiring to the solenoid small terminals should consist of a yellow wire with a red stripe or red wire with a yellow stripe which is 12v from the key switch in the start position. The other small terminal is a black wire and attaches to the block or something metallic and works it's way back to where the battery - terminal is connected.....usually the engine block or in some instances the starter case and the case is in electrical contact with the engine block, either of which complete the circuit.

When the key is switched to start, you should hear a very definite click coming from the solenoid. If not, either your 12v is not getting there or your ground wire is not getting electrically back to the batt -, or the solenoid coil is open.

I assume you took the large red wire from the battery off the input to the solenoid large terminal (3/8") and touched it to the output red wire when the starter started. Great, that eliminates a lot of ghosts to chase.

So, if the energizing circuit testing didn't find a problem then get a new solenoid as the high current copper contacts internally are pitted badly and can't handle the current necessary to roll the starter.

 

[watershoes]

thanks for the advice. I did a voltage drop test and everything is 12v except for the terminal where the wire from the ignition switch comes from, which was sitting around 3v when i tried cranking it.
Also, i removed the problem part where all the wires were connected and it looks alot more like a rectifier than a starter solenoid to me.

is it possible for a motor to try and crank with everything just hooked up to the rectifier? Or did the guy who sold me this boat just take the solenoid out and not tell me, because i cant find anything that looks like a starter solenoid anywhere else on the motor. There is a spot between the switch box and the starter where it looks like there used to be two bolts mounting something.
sorry if these are dumb questions but this is my first boat and it has turned into a bit of a project

 

[Texasmark]

Go to the battery + terminal and follow the big red wire into the engine till it terminates. Usually this termination is at the 3/8" solenoid high current terminal. This terminal is also used as a junction point......convenient place to distribute 12v. Your picture is a rectifer and the output red wire would connect to the same solenoid input terminal as the battery wire.....aka, that't how you recharge the battery.

Go to the starter. Usually there is only one terminal on the starter which is the power input. Usually there is a large red wire, similar to the battery lead that connects this terminal to the output terminal of the starter solenoid. The return for the starter to the battery - is through the case of the starter, through the bracket and to the engine block where the battery - large black cable usually connects. The solenoid is a zinc plated cup with mounting ears, and a top hat, 3/8" terminals on either side and 2 small ones 90 degrees away from the big ones and near the bottom of the unit. You may be right.......you don't have one.

 

[Barnacle_Bill]

The picture you posted is the rectifier. It sounds like you just did a voltage test and not a voltage drop test. When you do a voltage drop test if the wire or connector being tested is good it will read less than 1 VDC. There are examples of this test on You Tube.