Temp gauge issues

[floating_viking]

Hi all, I just recently got a boat with a 5.7 Mercruiser. It seems my temp gauge is not working. I checked the resistance in the temp sensor and it shows 700 ohms, which seems to be what the chart says it should be. When I turn on the ignition, the gauge needle actually goes slightly backwards from 0 position. Could it be that the previous owner had tried to fix it and switched polarity?! Unfortunately I did not have time(a lot of screws to remove) to take the gauge apart the past weekend, but that'll be my task the coming weekend.
Any suggestions?

 

[tpenfield]

The temp sender should change its resistance as the temperature of the coolant changes. Did you test that?

Also, what chart are you referring to? 700 ohms sounds a bit high for the sender.

 

[alldodge]

Hi all, I just recently got a boat with a 5.7 Mercruiser. It seems my temp gauge is not working. I checked the resistance in the temp sensor and it shows 700 ohms, which seems to be what the chart says it should be. When I turn on the ignition, the gauge needle actually goes slightly backwards from 0 position. Could it be that the previous owner had tried to fix it and switched polarity?! Unfortunately I did not have time(a lot of screws to remove) to take the gauge apart the past weekend, but that'll be my task the coming weekend.
Any suggestions?

Need to know what year your boat is and if you have digital or analog gauges.

 

[UncleWillie]

Check that the gauge has a 12v supply and the 700 ohms is still measured on the gauge end of the sensor wire.

 

[floating_viking]

Actually the resistance values were given to me by another boater at the marina. I'll get those numbers confirmed. It does change value when engine heats up, so it does seem to work. My engine is a 1988 Mercuiser 5.7, boat the same year. All gauges analog. My next step was to get access to the meter to confirm all the above. But as mentioned, I have to get take the entire instrument panel off. The previous owner seem to have had some issues understanding how electrical components work. I have already replaced all the wiring for cabin lights and navigation lights. But I knew this when I was getting the boat and priced it accordingly. I'll likely take apart the instrument panel this weekend to fix the temp gauge so I might as well replace all the switches, fuse panel and cabling at the same time. A couple of six-packs should help with the task...

 

[tpenfield]

Here is a link to the Chaparral Owners forum that shows the resistance range for oil and temp gauges. 700 ohms is fine for room temp. So you can see if it tracks for higher temps. I was thinking about fuel sender resistance ranges, which is about 240-33 ohms.

http://forum.chaparralboats.com/index.php?showtopic=18809

 

[floating_viking]

So, took the instrument panel apart and got access to the temp gauge and all the wiring. Re-set all the wires, cleaned the brass washers at the sender unit and got the gauge working. Now back to replacing all the electrical on my instrument panel. Should be done by tonight, and then some boating tomorrow.