Re: how do I test a starter solenoid?
1) Battery Load tested and charged to 12.65 volts?
2) With that battery out of the boat it is a good time to polish those connections. Don’t just look at them, make them shine! Also inspect the wiring thoroughally, buldges in the insulation or crunching as you flex it are a sure sign that the wiring is corroded internally. Replace as needed.
3) With the cables clean and tight and a good battery, you can now see if the problem occurs. If it doesn't you are golden. If it still doesn't work or works sporadically, check and clean the remaining cable connections on the solenoid and the starter. Once again, don’t just look at them, make them shine.
4) If it now works you are ready to go fishing. If not, use a short jumper wire to short the small terminal on the solenoid to the large terminal on the solenoid (the one that has the POSITIVE battery cable connected to it). Be prepared for a spark. You are really doing the same thing that the ignition key does in the START position. If the engine spins fine every time, you either fixed the problem, or if not, the problem is in the harness between the engine and ignition switch.
5) Disconnect and inspect the large red harness plug in the cowl. Clean it as best you can with electrical contact cleaner, smear some grease around the contact edges and reconnect. Now use the ignition switch to try and start the engine. If it works every time the connector was the culprit. If not, you now need your trusty voltmeter.
6) Put the POS voltmeter lead on the small terminal on the solenoid. The NEG lead goes to ground. Have someone turn the ignition key to START. If you don't see 12V troubleshoot the harness between the engine and control box. If 12V is present every time, the switch is good.
7) Measure the voltage at the starter (+) terminal while somebody turns the key over to "start" - if you get within half a volt or so of the battery voltage, the starter is faulty.
Bad voltage at starter (with clean tight connections), measure the voltage at the solenoid small terminal with the yellow/red wire with the key turned over to "start" you should read within a half-volt of battery voltage, otherwise you have a wiring fault between the keyswitch & solenoid. Good voltage at solenoid but bad voltage at starter means faulty solenoid.
8) Now connect a single jumper cable between the POS battery terminal and directly to the large terminal on the starter. If the starter spins each time you do that, the solenoid is likely the culprit.
9) Connect another jumper cable to the NEG battery terminal and a good ground (bare bolt head or bracket) on the engine. Now do the same test you did in step 8. If the starter now goes ok, you still have a negative battery cable problem or the solenoid is bad.
How is shorting the terminals of the solenoid going to test it to see if it's good or bad. Crossing the solenoid shouldn't be a normal, although it has been done by even me but it isn't healthy for the solenoid. Have you considered the ignition switch?