Mercruiser Electrical Issue- Low power. Help pelase!

[Trudy]

Hi Everyone, I have a 2007 Mercruiser 5.7L TKS (carbureted)

Here is the timeline of events..

-Last week at sea, the engine died. Found the 15 amp fuse on the engine harness was blown. Replaced it, immediately blew. Repeated this one more time and then called sea tow

-Using the electrical schematic, i checked the integrity of all the connections on the circuit. Everything looked OK.

-Removed the 12v power from the Fuel and Oil pressure gauges. Jumped the purple 12v wires to get powback to the engine. She started and ran PERFECT for about 10 minutes, idling about 600-700 rpm. I then shut the engine off

A short time later, I restarted the engine and it is very low on power/ running rough. Idling at only about 350 RPM. Still dont have fuel/oil pressure gauges connected. Fuses and all connections look good.

Now im lost…..

 

[Trudy]

Ignition coil test: 3 ohms on secondary and 11,000 ohms on primary.

Alternator is definitely running hot. Noticed battery was a little low, but charged it up with an external charger. Same issue immediately upon restart.

 

[achris]

Likely you have a shorted TKS diode. The battery drained due to your jumpering.. Replace the diode (DON'T buy one from Merc. It's just a simple diode you can get from Tandy or Radio Shack), recharge the batteries and you should be good to go.

Chris...

 

[alldodge]

Jumped the purple 12v wires to get power back to the engine.

Which wires were jumped?

Check your ignition fuse, the one that is on the red/purple going to the key switch. If its blown then something in the TKS system is shorted. Most likely the water temp stitch or TKS module, along with the diode.

 

[Trudy]

Which wires were jumped?

Check your ignition fuse, the one that is on the red/purple going to the key switch. If its blown then something in the TKS system is shorted. Most likely the water temp stitch or TKS module, along with the diode.

I removed the power to the all the gauge’s (connected the purple wire from ignition to purple wire running back to engine from gauges). Both of these purple wires are normally terminated to the oil pressure gauge.

Again, the first time i did this, she started and ran great for ~10 min. I checked the diode with my meter and it seems to be OK. The 20A fuse on the ignition is also OK. And the 15A fuse on the engine was OK.


I appreciate the help guys!! You are the best!

 

[alldodge]

So its not blowing fuses and its running ok with gauges disconnected?

 

[Trudy]

So its not blowing fuses and its running ok with gauges disconnected?

It WAS running ok when gauges were first disconnected… But when i started it a 2nd time, that’s when the low power issue started. Very weird.

 

[Trudy]

It WAS running ok when gauges were first disconnected… But when i started it a 2nd time, that’s when the low power issue started. Very weird.

 

[alldodge]

That is weird, unless something happened to the ALT maybe. Measure the voltage at the ALT with it running

 

[Trudy]

That is weird, unless something happened to the ALT maybe. Measure the voltage at the ALT with it running

The alternator was EXTREMELY hot, but it was putting out about 13.5 volts. I figured the heat was due to the load on the ALT since the battery was close to dead. I’m leaning towards replacing it either way

 

[alldodge]

It would get hot if the battery was low like it was or if it had a shorted cell.

Is the Bat now between 12.5 and 12.7V?

 

[Trudy]

@alldodge charged the battery up with external battery charger. Started and ran great for a few minutes, then she started idling all over the place. From 200-950 RPM. Alternator temperature was good. Now im thinking its either a distributor rotor/ spark plug wire/ clogged fuel filter, or something like that. View attachment trim.B519F18D-914C-43A9-ACE2-AA5B4D8B6FB7.MOV

 

[alldodge]

From what I saw it rev'd up and then settled back down. I would look at the carb

 

[Trudy]

@alldodge this entire escapade started with the 15A fuse blowing on the ignition circuit.. I’m still quite confident it’s electrical. It runs GREAT now until its warm. Then the idling issues start. Could this be a TKS module issue?

 

[Trudy]

@alldodge also worth mentioning the alternator is no longer overheating since the battery is properly charged.

 

[alldodge]

Yes, I understand the fuse issue

The TKS module works very slowly, open when cold and closed when warm. If power was disconnected it would not move until it started getting cold which would take a while.

After the module closes the carb works the same as all other 2bbl carbs. If float level changes, something gets stuck other things, it will change performance

It may be electrical, but just using what I think it might be. If something starts drawing more current, this could change spark if it went low enough, but for it to come right back doesn't cause me to lean in that direction