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Old 01-26-2016, 07:29 AM   #32
High Wire
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City: Cape May, NJ
Vessel Name: Irish Lady
Vessel Model: Monk 36
Join Date: Dec 2007
Posts: 4,966
Before running down this rabbit hole you want to verify the tachs are still reading correctly with a strobe tach.
If these are analog temp gauges there are plenty of things that will cause a variation of a reading. Just a slight change of resistance in the connections of the harness wiring could cause what you are seeing. Here's what I would do to check the gauges. Since you need to compare Granny Smith apples to Granny Smith apples, do everything identical on each engine. Always WRITE DOWN all results.

1) Engine not running. Disconnect the lead at the temp sender. Put a 47 ohm resistor between the harness lead and block ground with short length test leads. Read the gauge as accurately as you can. Write the value down. Repeat on the other engine. Next, add a second 47 ohm resistor in parallel to the first. This will move the gauge needle higher or lower on the scale for a second reference point. Record readings for both engines. If they read the same, great and move to #2. If not, start cleaning connections.

2) Swap senders. The temp senders should not have thread sealant on them. That will affect the resistance seen by the gauges. You can measure ohms between the hex head of the sender and a good clean engine block ground should be < 1 ohm. Run the boat as before. Did the problem move to the other engine?

If you still believe its a gauge problem...

3) Engine running. NEED TO BE VERY CAREFUL to not get anything caught in moving belts!!! Disconnect temp sender and measure resistance of senders with a digital VOM. While the engine is warming up (depending on the gauge manufacturer) the resistance will rise or fall as temp rises. These readings will be a moving target so a second person will help. Record IR temp and ohm readings every 10 degrees. Compare engine data. Do the ohms readings agree?
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Archie
Irish Lady
1984 Monk 36 Hull #46
Currently in Cape May, NJ
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