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Old 01-06-2024, 05:33   #1
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MD-17C Warning Buzzer Control Box

My engine control panel buzzer is going off continually after changing my gauges. The alternator is charging, the oil pressure is normal, and the engine isn't over heating.

The schematic does not show the hermetically sealed blue box that the buzzer is controlled/triggered by, but I wanted to see if anyone has that device part number or a schematic, or knows it's principle of operation?

It's likely that the buzzer simply grounds-out through the the oil pressure sending unit, and the alternator, when it's not charging, but for the over-temperature warning, I'm thinking there is must be a transistor voltage threshold that has to be met, for the buzzer to go off during an over-heating condition.
The charging and oil pressure lamps are working normally, that is, that when the engine control panel is first turned on, they are illuminated, and when the engine starts, they turn off. The temperature gauge seems to work normally as well, although, I replaced the Volvo gauge with a generic gauge, but they have the same connections according to the schematic below.

This is the only schematic in the shop manual, and doesn't include the small blue, buzzer control box.

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Old 01-06-2024, 05:58   #2
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Re: MD-17C Warning Buzzer Control Box

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....... but for the over-temperature warning, I'm thinking there is must be a transistor voltage threshold that has to be met, for the buzzer to go off during an over-heating condition. ........
The over temperature warning is activated by the over temperature sender in the engine cooling system. The engine sender is simply a switch that closes when the target temperature is exceeded. The switch closes to ground (battery -ve).

It is the principle as the low oil pressure alarm and sender.
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Old 01-06-2024, 06:13   #3
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Re: MD-17C Warning Buzzer Control Box

More info https://shoptransmitter.com/blog/tem...e%20(%20source).
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Old 01-06-2024, 06:15   #4
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Re: MD-17C Warning Buzzer Control Box

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The over temperature warning is activated by the over temperature sender in the engine cooling system. The engine sender is simply a switch that closes when the target temperature is exceeded. The switch closes to ground (battery -ve).
I don't believe this engine has a temperature switch. If it did it couldn't drive the temperature gauge. Binary (off or on) temperature switches are used on smaller simpler engines. Ones that will never be hooked up to a temperature gauge. It either shows a warning light or doesn't.

The OP engine temperature sender changes in resistance with temperature so the 12V supplied is dropped by a variable amount as temperature changes which produces a continual voltage range that can drive the gauge.
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Old 01-06-2024, 06:22   #5
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Re: MD-17C Warning Buzzer Control Box

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I don't believe this engine has a temperature switch. If it did it couldn't drive the temperature gauge. Binary (off or on) temperature switches are used on smaller simpler engines. Ones that will never be hooked up to a temperature gauge. It either shows a warning light or doesn't.

The OP engine temperature sender increases in resistance with temperature so the 12V applied is dropped in voltage as temperature rises.
I'll defer to your knowledge of the MD-17C as I would have thought it would have had both a temp switch (binary) and a temp sensor (resistive) like many other marine diesel engines.
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Old 01-06-2024, 09:32   #6
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Re: MD-17C Warning Buzzer Control Box

@Statistical is correct- There are no other sensors on the engine, and I don't think I missed any wiring connections. I drew out the connections and took pictures before swapping gauges.
If I can't figure it out, I'll just put a N.O. temperature switch in and toss the blue control box. That box is really not needed for three wires waiting to ground out... I might just put a diode on each just so current can only flow one-way.
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Old 01-06-2024, 16:08   #7
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Re: MD-17C Warning Buzzer Control Box

Is there a over-temp light?
How many wires are connected to seperate terminals on the single temp sensor? 1,2, or 3.

If there are 2 or 3 wires, it may be a combined sensor / switch i.e one wire for the resistive temp component and one for the over temp binary switch plus (sometimes) one for the ground. Fairly easy to check with an ohmmeter, especially if you remove it and heat it in water on the stove.

Another possibility if there are 3 wires, it could be 0-5V sensor.
5V supplied by the harness (1 wire +ve, 1 wire -ve) and varying voltage (normally 0.5 to 4.5V) on the third wire going back to the gauge unit. Again, easy to check with a voltmeter.
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Old 02-06-2024, 06:40   #8
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Re: MD-17C Warning Buzzer Control Box

@Wotname there is no over temp light or switch, and the sensor is a single wire unit, that must be a thermistor that completes its circuit through "grounding" to the engine block (see schematic in my first post).
I'm going to guess that the new gauge has an internal resistance that differs enough from the original Volvo gauge, and that is causing the alarm. This might also mean that my new gauge isn't reading correctly, although my temperature readings are the same as I was getting before the swap.

When I have a chance, I'm going to check the resistance of the old Volvo gauge, and use a variable resistor to observe and record the range needed for the gauge, and compare that to the new gauge.
So, I might find that I need to add a resistor to the "over temp" lead of the blue buzzer box, as long as it's not in series with the gauge lead, or causes the gauge to read incorrectly.
It will take some testing when I have time.

Otherwise, a standalone N.O. temp switch would be the easiest solution, and then I can get rid of the blue buzzer control box.
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