View Full Version : Temperature Gauge Not Working

04-23-2006, 08:07 PM
350/260HP V8. I grounded the wire from the sender and the gauge moved all the way over to hot. Does that mean the sender is bad? Could this also be a bad thermostat? How do I know which one to replace? Thanks

04-23-2006, 08:15 PM
How are you making sure there's water getting into the motor? Temperature senders don't work very well in air. How old is your impeller? Do you see any fluctuation in the gauge when you run it? You can use a hair drier to heat up the sensor and watch the gauge.

04-23-2006, 08:20 PM
"No fluctuation in the gauge at all. When I shut off power, the gauge moves a 1/16th of an inch. New impeller 2 summers ago. Is there a thermostat in a mercruiser?"

04-23-2006, 08:46 PM
"Thermostat is in the normal location or a housing above the normal location.

Have you run your boat on the trailer? Look for water output at the prop. 2 years is about the max I would receommend for an impeller. If one or more of the vanes breaks off, it can block water passages and cause all kinds of problems. This is one service item I would become familiar with. You need to remove the lower gear case (put it in drive first and remove the speedo tube) and the water pump is right at the base of the drive shaft. Gen II has a metal housing and a larger impeller which I prefer but the Gen I works fine, too. If you have a Gen I, get the water pump rebuild kit that has the upper and lower housings. It's worth it in terms of peace of mind. It's definitely possible for the thermostat to go bad but they usually don't stick shut but it may have. How cold is the water where you're running it?"

04-23-2006, 09:40 PM
"The water is still very cold, but shouldn't the engine warm up enough to move the gauge? I can put my hands on the manifolds after running for awhile and they are hot, but not so hot that they would burn my hands."

04-23-2006, 10:18 PM
"It should get warmer, but at this point, I would remove the sensor and see how it reacts to heat."

04-24-2006, 07:59 AM
did you change themostat and put aftermarket gasket in with no ground clip.If so your sender is not grounded

04-24-2006, 01:53 PM
Buy a laser-pointed thermometer and stop guessing. It will tell you what and where is wrong (or not) in an instant. $50