Temp sender or alarm electronics?

by Alan
(UK)

Great website! I have a Volvo MD21b with a single temp sender. Temp gauge shows normal but alarm intermittently bleeps when normal engine temp is reached. Do you think could be the sender or the alarm electronics? Many thanks, Alan

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Jun 13, 2026
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starstarstarstarstar 		Temp sender follow up
by: Anonymous

Many thanks for your prompt reply John! There is only one sender for both the gauge and alarm. Removing the connector from the sender stops the alarm and isolates the temp gauge.

I will have a closer look to see where the alarm test button is and try it. Strangely there is no sign of an alarm in any of the MD21b wiring diagrams in the manuals that I have, only the gauges.

On the basis that you say it’s more likely to be the sender, I have ordered a replacement to do a test. It’s a third of the price that the Volvo dealer wanted but it should be good enough for this purpose.

Sorry to go off topic but I want to install solar panels on this 45 year old boat. Does your book cover this?


All the best, Alan




Jun 12, 2026
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starstarstarstarstar 		Always check the sender first
by: John

Hi Alan and thanks for the positive comments, and for being first to use the forum. I have a Volvo MD22L on my old Westerly so I would assume we have the same system. In most cases I would check out the sensor first. There is a separate sender for the gauge and for the alarm. Given it is intermittent first check that the connections are clean and tight as the alarm sender on these is a simple temperature switch, and engine vibration can cause this at the sender terminal. If it were the electronics it would probably be a constant alarm and faults often result in multiple alarms not working. Does the alarm activate both audible and visual when you use the test button? I do cover engine instrumentation in the Marine Electrical and Electronics Bible in some detail

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