I've had an engine "overheat" issue that we think we have finally isolated.  After installing a manual temp gauge after the thermostat and before the sending units we discovered that the engine is not overheating. It ran at 190 at full load and never changed. Also the infrared gun showed normal on various parts of the engine. Good news! The gauge goes over 200 then the alarm buzzer goes off.  

Now that we know this my first thought is to clean up the connections and see if it is a grounding issue. As you may know or may not know, the Total Command System was a bit ahead of its time and the complicated with a 71 page manual.  If that doesn't solve the issue I'll look into replacing the sending units and gauge.  A couple asks.

  1. How do I identify the proper sending unit?  There are two and should I assume one is for the alarm?
  2. Anyone have a temp gauge laying around?

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Hi Mike - I am dealing with the same problem on my '72 41 with twin 427s and total command system.  I found my port water temp gauge to be reading 10 degrees above my aftermarket mechanicals mounted in the engine room.  My starboard temp gauge starts at zero but after a few minutes into warmup, it pegs the 220 mark, with the mechanical reading normal.

I asked the same question about the sending unit part number and I couldn't find any cross-reference between the old Chris numbers for the sending units to commercially available units of today.

After much digging in super disk, I found that AC Delco made these gauges and they require and AC Delco flavor of sending unit.  After more digging, I found what appears to be the off the shelf equivalent from O'Reilly's: https://www.oreillyauto.com/detail/b/standard-ignition-5753/air-con...

Note that the replacement has a button head connector vs a threaded connector so if you went this route you'd have to change the existing ring terminal connector on the boat side.

Now, don't take my word for it just yet as I have the replacement in my hand and will be installing it on Friday evening.  I will post back my results.

Mike,

Your description of installing the mechanical temperature probe after the thermostat and before the sending units confuses me. It sounds like you are actually sensing the temperature of the coolant at two different points of the coolant loop.

I suspect that you actually are sensing a mild engine over temperature issue. There is the electrical engine temperature sensor and there is the over temperature switch. The two sensors are on different circuits so if the over temp alarm is actually going off and you have an elevated temp gauge reading, I would say that the coolant temperature is actually going above the trip point. A bad ground will make the gauge go to full scale but will also prevent the alarm from going off.

On your engine, VT8-370, the temperature sensors are in the coolant return line from the exhaust manifold and going into the heat exchanger housing. I don't see the mechanical temperature probe in your picture of the sensors so I suspect that you are actually sensing the coolant temperature on the cold side of the heat exchanger.

This all leads me to suspect that your heat exchanger is getting plugged and not removing enough heat at full power to maintain proper engine temperature.

Ron,

Thank you so much for your response.  You have so much knowledge.  I've attached pics and will try to respond to a couple of your comments.

  1. The manual sender is just past the thermostat as best I can tell.
  2. We checked with an infrared.
  3. The heat exchanger was boiled in 2017 after I bought the boat and I have continued to have the issue.  The impeller is new, and the water pumps rebuilt.  Cooling%20sensors.pdf

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