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Old 27-09-2021, 13:58   #1
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Coolant gauge fluctuations up to 120degC

Hi all,

My family and I live and cruise onboard our yacht, Providence.

Been chasing this problem for a little while now and had a chat to local service shops resulting in still no success.

Before I flood the details for context - my questions are - what are the two circled components in the photo? And does anyone have a part number for the correct part for A?

Engine is a yanmar 4JH4-TE (2019), installation was done by authorised mechanic before I purchased the boat.

Engine temp gauge fluctuates between 90 and 120 degrees Celsius when at cruise 1800-2000rpm.
Alarm buzzer and warning light are working fine during test phase on the panel and don’t active when the gauge reads 120......

Engine doesn’t appear to be overheating by any other indications.
Thinking so far is that it’s a problem with the gauge, wires, or sender.

The current sender has been mutilated in the past and it looks like someone was trying to drill and solder an earth wire onto the brass body. Local shop quoted me $384 dollars for a new VDO coolant temp sender, part only. Seems a bit steep at any rate.

I figure the A and B in the photo are both sensor/senders of some description for the engine coolant. But exactly what? I’m not confident the correct part is in A and need it replaced because of the mutilation at any rate. As I don’t have much faith in the local service on the issue I am sort of left in the dark on what to do.

Thanks for any and all help offered.

Best regards, Dan
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Old 27-09-2021, 14:14   #2
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Re: Coolant gauge fluctuations up to 120degC

Sounds like your local vendor has misplaced a decimal.

https://www.summitracing.com/search/...rature-senders
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Old 27-09-2021, 14:29   #3
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Re: Coolant gauge fluctuations up to 120degC

The sealant on the threads could possibly prevent the sender from getting a proper ground connection and the mutilation could be someone's attempt at a better connection.

It is normal to have some fluctuation as the thermostat cycles open and closed. Observing this can verify that the thermostat is actually working. Normally this fluctuation is barely discernable. If you have an IR (laser) thermometer you may be able to see if your temperature is actually fluctuating. If it actually has that amount of fluctuation (+/- 30 degrees) then I would suspect a sticky thermostat (it should only fluctuate by a few degrees). A new VDO sender should cost around 30 dollars. You may have separate senders for the gauge and alarm/light.
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Old 27-09-2021, 15:13   #4
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Re: Coolant gauge fluctuations up to 120degC

Just get an of the shelf sender unit with the correct thread should be about 10USD
One is your temp sender the other will be your overheating switch.

You can test the gauge by putting something like a 100ohm resistor from earth to sender connection and should indicate 160f or at least stable reading !
If its a one wire sender there shouldn't be any thread tape on it !
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Old 27-09-2021, 23:52   #5
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Re: Coolant gauge fluctuations up to 120degC

Quote:
Originally Posted by Bill Seal View Post
Sounds like your local vendor has misplaced a decimal.

https://www.summitracing.com/search/...rature-senders
Fair difference!!!
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Old 27-09-2021, 23:53   #6
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Re: Coolant gauge fluctuations up to 120degC

Quote:
Originally Posted by Emmalina View Post
Just get an of the shelf sender unit with the correct thread should be about 10USD
One is your temp sender the other will be your overheating switch.

You can test the gauge by putting something like a 100ohm resistor from earth to sender connection and should indicate 160f or at least stable reading !
If its a one wire sender there shouldn't be any thread tape on it !
Thanks for this. I didn’t think about testing the gauge but this makes sense.
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Old 27-09-2021, 23:55   #7
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Re: Coolant gauge fluctuations up to 120degC

Quote:
Originally Posted by Thumbs Up View Post
The sealant on the threads could possibly prevent the sender from getting a proper ground connection and the mutilation could be someone's attempt at a better connection.

It is normal to have some fluctuation as the thermostat cycles open and closed. Observing this can verify that the thermostat is actually working. Normally this fluctuation is barely discernable. If you have an IR (laser) thermometer you may be able to see if your temperature is actually fluctuating. If it actually has that amount of fluctuation (+/- 30 degrees) then I would suspect a sticky thermostat (it should only fluctuate by a few degrees). A new VDO sender should cost around 30 dollars. You may have separate senders for the gauge and alarm/light.
Thanks Thumbs Up - a few things for me to follow up on there in the morning
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Old 28-09-2021, 03:39   #8
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Re: Coolant gauge fluctuations up to 120degC

Temperature Senders are often Bi-Metalic Thermo-Couple devices, whose resistance decreases as Temperature rises; resulting in a higher reference voltage at the gauge.

All Gauges:

1. Remove Sender Wire from “S” Terminal on Gauge, and Turn ON Instruments Circuit.

2. Test Voltage between “I” Terminal (External Power Supply) and Ground Terminal “G”.
Tested voltage must match nominal voltage of the Gauge (ie: 12V Gauge must test between 10 - 16VDC), and the Battery Voltage.
If no voltage - check Ignition or Instrument Switch(s) and/or Fuses.
If tests less than battery voltage, repair or replace (Violet) wire from Ignition or Instrument Switch.

If a Sender fails “Open” (Infinite Resistance), one or both (if Dual Station) Gauges will read Zero Scale.
If a Sender Fails “Closed” (shorted - Zero Resistance), one or both Gauges will read Full Scale.

Other confusing Ohmmeter results, such as readings lower than expected, could indicate a poor Ground Connection at the Sender Body (Sender to Engine Block). Check for pipe dope or tape on Sender Threads.

Temperature Gauges:

1. Disconnect Tan Sender Wire from Gauge (Terminal “S”) and turn Instrument Power ON.
The Temperature Gauge should indicate ZERO Scale (< Low Temperature).

2. Short the Gauge Sender Terminal “S” to Ground Terminal “G”.
The Gauge should deflect to FULL Scale (> High Temperature).

3. Measure Resistance from Tan Sender Wire (disconnected from Gauge) to Ground.
American Temp. Senders will read: 450 Ohms (Engine Cold @ 1000F)
or 29.6 Ohms (Engine Hot @ 2500F )
European Temp. Senders will read: 281 Ohms (Cold @ 400C) and 22 Ohms (Hot @ 1200C)

4. Disconnect Tan Wire from Sender on Engine Block.

5. Test continuity of Tan Wire.
Zero Ohms from Block End to Gauge End - If not, repair or replace Wire.

6. Measure Resistance from Sender Terminal (on Block) to Ground on Block.
Should read as per (3) above - If not, replace Sender.

Excerpted from:
Basic Engine Gauge Theory and Testing
https://www.cruisersforum.com/forums....html#post2597
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Old 28-09-2021, 09:09   #9
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Re: Coolant gauge fluctuations up to 120degC

Aren’t there some decimals missing in the 3rd resistance test? 1000F is pretty hot for a cold engine.
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Old 28-09-2021, 09:19   #10
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Re: Coolant gauge fluctuations up to 120degC

Get a hand held "point and test" thermometer and do some checking. They are cheap now.
Yanmar seems to have a lot of issues with their senders though. I fought an overheating problem once for a month, only to discover it wasn't overheating, it was the sender!

Less likely: Does your engine run too cool? Maybe the Thermostat is opening and closing?
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