Temperature gauge

[Mark S]

Hi all.

I noticed a month ago when first starting the engine on the hard that the temperature gauge was not working. I wasn't particularly worried about it at the time because everything else was working fine and plenty of water was exiting the exhaust with the by-pass both open and closed. When we went down the Danvers River, the first time the engine was under load, the gauge didn't budge. Now we're done with all the other chores and it's time to get the gauges working reliably. I don't like the idea of not having a reliably working temperature gauge.

Tonight I ran a jumper from the sending unit to the gauge sending input terminal to see if it were a wiring problem. No change. I ran another jumper from the ground terminal to the engine. No change. I'm sure there is power because the other gauges are working, although I'm not sure how to use the multimeter to check that without blowing its fuse.

My deduction is the sending unit is bad. Am I missing anything? How best do I determine whether the gauge is getting power?

Thanks in advance.

Mark S

[Baltimore Sailor]

If I remember correctly, the temperature sending unit has a variable resistor in it that changes resistance based on (duh) temperature. Unfortunately, that bare fact is all I know.

Hopefully someone else will be able to tell you how to hook up an ohmmeter to determine if the sending unit is responding to increasing heat as it should.

Good luck!

[jhwelch]

I'm not on my boat now so can't check this idea -- have you tried to
measure the voltage between the tip of the sending unit and ground?
It seems that would tell you if it is getting power.

I had a sending unit go bad on me once. It failed "open" so my temperature
gauge was reading 240 (max. scale).

-jonathan

[rigspelt]

Learning to use a mulimeter is a very handy skill on a boat. Saves a lot of grief. You won't likely hurt anything while checking boat DC voltages (caveat emptor) with a multimeter if:

1. It is set to DC voltage, two digit range (normal voltages 12-14 DC). If the range is wrong, good meters will just show an over-range symbol, or a number that makes no sense.
2. Touch only DC components, not AC.
3. Don't let the tips short across a power to ground element like the terminals on the back of the gauge; clip or touch directly to the components. Get in a comfortable position and set the meter so it is viisble before touching components with the leads.

• If you reverse the multimeter leads, voltage should read negative.
• If you touch both leads to positive sources, I don't think anything will happen (right? should just read zero since there is no potential difference).
• If you touch both leads to negative (ground) sources, the multimeter will just read zero.

[rigspelt]

Don Moyer posted a temp gauge/sender troubleshooter here: http://www.moyermarine.com/forums/sh...erature+sender

[Mark S]

Thanks, everyone, for the good suggestions. I love this forum!

I sat bolt upright in bed last night when I realized I had confused the GND post with the PWR post on the gauge ("Doh!") so I haven't been able to rule out a bad ground as of yet. When I disconnected the power to the gauge (thinking it was the ground) my wife noticed that the oil pressure needle on the next gauge pinned at maximum, a good indication that there is power to the temperature gauge because the power gets to the oil pressure gauge in parallel from the temperature gauge post. I'll check the power with the multimeter anyway, and the resistance of the sensor as well.

The info from Don that Rigsy provided is a good primer. I'm going to bookmark it.

Mark S