Merc 5.0 TKS Heat Issue

[tehlib01]

Hi All,

I have a Merc 5.0 TKS in my Maxum 1900 SR3 with an Alpha one. The boat originally had a 4.3 in it which was later replaced with the 5.0. I am getting an intermittent alarm going off which I believe is a heat issue but I am debating if it is electrical or something else. I have narrowed it down to the heat / electrical side and its not the oil pressure or gear oil as it seems to go off when I high idle for a few seconds and the temp gauge ever so slightly drops.

The running temp seems to be always riding just over the 175 degree mark, likely around the 180 to 185 range. I was pulling a tube and the alarm would go off. If I stopped and did a high idle the temp would drop ever so slightly and the alarm would go off. It never seems to go any higher than what I have described. Oil pressure is always about at what it is on the gauge.

I had the boat in for some service and they did a water pump kit on it (small rubber impeller), new t-stat and replaced the gimball bearing and bellows as there was a small leak. I asked them to check the heating issue and they couldn't figure anything out. They said the used an infrared gun and the temp would climb, then I feel the boat sat and they just never looked at it again so I went and got it back. They also said they ran it without a T-Stat and same issue. They also said there are no blockages in the risers etc.

I had it out again later in the year and I ran it fast and hard around lake and the alarm never went off once. However I shut it down, turned it on and the alarm would sound. I would then increase the RPM at idle a bit, the temp would have a quick drop in the gauge, alarm would go off and then the temp would come back up a bit and no alarm. This last time I took it out, the temperature outside was definitely a bit cooler which makes, where as the previous time it was a very arm summer day.

I feel the alarm is riding right on the edge of the threshold however I have no idea where to go from here. I am unsure if maybe electrical connections need to be cleaned? I asked the shop to replace the temp sensor and they didn't feel it was required as they saw the higher temp with an infrared gun. Only other issue I can think as well is an old battery maybe and low voltage causing issue?

Sorry for the long post, just trying to figure this out.

 

[Fun Times]

To start off with, it sounds like you may want to consider buying yourself an infrared thermometer gun and check the temp yourself compared to what the temp gauge and dealer is suggesting so you'd know more where to start.

You should have a 160 degree thermostat so in some cases it wouldn't be so unusual to see the temp "gauge" read anywhere between 160 - 178....But if the temp of the engine is truly reaching anywhere over 180 then it's time to find the issue causing the engine to get warm...If the engine stays around 160 - 170 max, then it'd sound like an electrical issue which it may be as well.

What year is the engine/s (new and old), The boat year and what does the (ICM) ignition control module sticker say/read engine size wise? 4.3 or 5.0?

 

[badrano]

In addition to Fun Times reply....

I had a 5.0 TKS. Never had any issues like that but I did some reading for my on knowledge. I'll offer my knowledge in case it may help diagnose the issue.
TKS engines have 3 different temperature sensors. One of them is a temp switch which is needed for the TKS circuit...not the issue. The second temp sensor feeds the temp gauge and that is all it does...no tie in to the alarm. The third sensor feeds in to the MCM/ECM and is most likely a resistance based sensor where the resistance changes with temperature. The ECM generates the alarm (the tan/blue wire) for temperature and what ever other alarms the ECM can generate (never figured that out). Electronic parts do wear out over time when exposed to high temps. Depending on what year your engine is, that sensor could be on its way out.
This temp sensor, I believe, is on the t-stat housing and has a yellow and black wire. I don't believe that sensor is too expensive so it may be worth a try.

 

[Scott06]

The alarm circut is triggered of a temp switch on the thermostat housing tan blue wire going to it. Its in parallel to oil pressure switch and drive lube sensor. Any of those three sensors go to ground to trip the alarm. Easy to test one by one. Unplug them when alarm goes off. Verify temp pressure or level on the one that is tripping the alarm, replace sensor or fix condition causing the issue .

 

[tehlib01]

Thanks, I will take a look and see if I can find it. That engine was put in in 2013, but I have no idea if the sensor was ever replaced or it came with the motor etc.