2005 4.3 liter Mercruiser ignition trouble

[kepruno]

For the ground I’ve been using the black wire the goes to the cigarette lighter up by the dash and in the engine compartment the black lead stays clamped right on the battery post. Not sure how else I could be doing it wrong. Can see copper on wires by the cannon plug so I know I was making good contact there.

 

[nola mike]

I’m sure. Had meter in the middle of the connection on the jumper wire, 12v down to 9 while cranking. A splice in the purple wire you mean correct? That’s really all I’ve been checking this afternoon.

IF you spliced into the tach, and if you're getting 9v there while cranking (you should), and if you're getting 4v at the boat side of the plug, then the only things that could cause your drop are a frayed wire doing something funny or a poor splice/connection under the helm in the purple wire (or still a ground). You need to trace the purple from the tach where you're getting 9v until it leaves the helm. I'd also use the same ground point under the helm for all of the measurements.

 

[kepruno]

I’m assuming it’s the oil pressure switch located directly about the oil filter? Not sure what else it could be. Purple wire in purple/yellow out. When I disconnected it I get 10 volts at the coil.

 

[kepruno]

Bypassing the pressure switch by jumping across purple/purple yellow caused 4 volts at coil with key on and the fuse in the dash blew before I tried to crank it. Tomorrow I will look for short in purple yellow wire

 

[JustJason]

Did you run your jumper wire from the keyswitch to the coil yet?

 

[kepruno]

It’s this thing

 

[alldodge]

View attachment 410849
It’s this thing

That is the TKS diode and has nothing to do with current issue

 

[Fun Times]

View attachment 410849
It’s this thing

How did you come to the conclusion it’s a TKS diode issue?

https://yanmarshop.com/catalog/media/F53DEL3GOJSW4Y3INVQXE2LOMUXW2ZLENFQS6YTNMUXUOQKZIRBVQQSRI5JE6RCFJVBFCR22J5CUWVCTG5DUCMSBFZYGIZQ

 

[kepruno]

How did you come to the conclusion it’s a TKS diode issue?

https://yanmarshop.com/catalog/media/F53DEL3GOJSW4Y3INVQXE2LOMUXW2ZLENFQS6YTNMUXUOQKZIRBVQQSRI5JE6RCFJVBFCR22J5CUWVCTG5DUCMSBFZYGIZQ

Because I unplugged it and the boat started. That was my first clue. Plugged it back it and problem came back.

 

[kepruno]

Because I unplugged it and the boat started. That was my first clue. Plugged it back it and problem came back

How did you come to the conclusion it’s a TKS diode issue?

https://yanmarshop.com/catalog/media/F53DEL3GOJSW4Y3INVQXE2LOMUXW2ZLENFQS6YTNMUXUOQKZIRBVQQSRI5JE6RCFJVBFCR22J5CUWVCTG5DUCMSBFZYGIZQ

I also checked continuity and found that it was shorted. (Obvious because it was freaking hot) after reading your link I checked it the other way and found continuity in both directions.

 

[Fun Times]

Because I unplugged it and the boat started. That was my first clue. Plugged it back it and problem came back.

Seemingly seems a bit unusual giving issues being the engine is cold but odder things do tend to happen unexpectedly being a boat.

Glad you stayed with it to find the solution…

 

[kepruno]

Still may not be the root cause but unplugging the oil pressure sensor isolated that circuit. After that I was just looking for things the purple/purple yellow wire went to. But if the diode runs the TKS sensor sounds like I should check into that as well. I can trace the wire but if you guys know where it is that would help.

 

[alldodge]

If the diode is shorted the motor would start but not be able to be turned off. Now if the diode being shorted and something like the oil pressure switch is also shorted then the fuse should blow

 

[nola mike]

If the diode is shorted the motor would start but not be able to be turned off.

only when warm.

Now if the diode being shorted and something like the oil pressure switch is also shorted then the fuse should blow

but the fuse didn't blow until ignition on and op switch jumped, before cranking. I was thinking high resistance somewhere on the yell/pur side could draw down the voltage. agree that I don't see a way that the diode is the only issue. also didn't realize this was a tks. goofy system, weird gremlins.

 

[kepruno]

Temp switch has a short.

Something I noticed that may have contributed to the confusion the diode in alldodges diagram shows a purple wire and a
white wire. My boat seems to match everything else except the diode has a purple/yellow and a white wire.

 

[kd4pbs]

Total system voltage will of course be lower while cranking. This is due to the draw from the starter. I would think it would be more than 9 V if your batteries are in good shape if you get 9 V on the purple wire at the switch, but only 4 V on the coil positive then your problem for sure is in the purple wire going to the engine connector. The dead man switch is in line with that purple wire going to the engine. It sounds like you may have found the issue. Or at least one of them. Take one lead from your meter and clip it to the red connector on the back of your switch. Take the other lead and clip it to the purple wire connected to the I terminal. When you crank the boat, your meter should read just millivolts at most. Any more would indicate that either the switch is bad or for some reason the I terminal does not get power when cranking.

 

[kd4pbs]

I just happened to think, if the coil has a ballast resistor in line with the positive feed, then you could very likely get low voltage on the coil positive lead. So the best thing to do would be like others said and check how much voltage you’re getting at the purple wire on the engine connector when cranking.