Take wire off sensor and touch it to ground with ignition on gauge should read all way over red hot if good be sensor. If need new sensor they have match gauge.

The last post is a great way to test the gauge. Keep in mind that on those engines, if you are not mowing the gauge seem to move very little if at all since you are not building any heat (no engine load).

On my 445 JD the temp gauge needle comes up to just touching the green in the gauge, stays there unless I'm mowing really heavy stuff and then it may move another needles width up. Not a lot of movement but it does move.

One detail that should not be overlooked on the subject of "what temperature the gauge is indicating": If the gauge pointer just barely moves off the peg (cold side) when the engine is running with no load on a cool day, but moves more to the right when the load or ambient air temperature increases.......it may indicate the thermostat is stuck open (or is absent altogether...was removed ). If the pointer does not move off the peg at all: The sender unit wire is disconnected/broken, or the sender is defective, or the gauge is defective, or the gauge has no power input (from ignition switch output). If you touch the wire that attaches to the sender unit to a ground and the pointer moves all the way to the right (max temp), the sender is defective.If you touch the wire to ground and the pointer does not move all the way to the right, you need to check that the gauge feed wire is delivering 12VDC to the gauge before you arbitrarily replace the gauge. These gauges utilize an internal (usually internal to the gauge itself) "constant voltage regulator" to assure that the displayed information does not fluctuate with minor changes on battery (system) voltage in normal operation of the machine. What that translates to is this: Using a DC volt meter, the gauge input wire (the wire from ignition switch) will show battery voltage but the gauge output wire (the wire that runs to the sender)will show something less than battery voltage (like 8V or 6V). Make sure the wire is not connected to sender nor grounded before testing input & output voltage of gauge (that would cause invalid readings). Never rule out the chance of having a broken wire going from gauge to sender. If the gauge shows good input and output voltage readings but the guage will not max out when the sender wire is grounded, try using a "jumper wire" to test for broken wiring. Touch the exposed end of a short wire to the output of the gauge (the side with less than battery voltage) and the other end of the wire to a ground. If the gauge now shows max temp, the wire from gauge to sender is faulty. If a fuse "pops", you were touching the wrong side of the gauge wiring. Chances are good that simply grounding the sender wire will reveal a defective sender unit but in case that turns out wrong, you have been given the advanced test procedure as well.

On my 445 the needle will rise to just short of half way and then noticeably drop to a 1/4 of the gauge when the thermostat opens, then rises to just short of 1/2 way and drops again, indicating a working thermostat. It works this way regardless of outside temps.

It would be interesting to hear from other owners (whose machines have a temperature gauge) whether they too observe the range of fluctuation of the gauge pointer as described by jeffinia (whether JD, 445, or other brand). This much drop and rise in engine temp seems to conflict with the premise of a thermostat being able to "maintain" a pre-determined "minimum operating temperature". This much variance would seem to indicate a thermostat that was still working, but slow to open and then slow to close again. A properly working thermostat (in a system where the components are correctly sized for the engine and the task) should allow the engine to reach the opening temp rating of the thermostat (minimum operating temperature) and then maintain that temp without much fluctuation. (thermostats only determine the minimum operating temperature, MAXIMUM system temperature is determined by radiator size (capacity), coolant flow rate, how hard the engine is working, and ambient air temperature.

I`d like To hear from others too, Ive had a new cam and water pump put in last year and new thermostat at that time(deere dealer did the work) And temp gauge runs same as before work was done, Have just short of 800 hrs now and it runs great!!!

On my 425 once warmed up, the temperature gauge would never move higher than the very bottom of the green. At least for the first ~ 500 hrs. when the thermostat went on me. When I replaced it with the hotter thermostat Deere went to, it would cycle to the higher side and drop back to the lower range. The tractor ran like crap. The dealer, and here is the importance of the dealer, went out of his way to get the tractor running right. It took several visits to the dealership, and finally with a new carburetor designed to work with the hotter range thermostat, the tractor ran close to the original. Soon after that, though, I went back to a diesel CUT.

Cheers

JDFANATIC

I'd like to correct my previous post now that more talk on this has refreshed my memory. It's actually normal to see the needle fluctuate as the thermostat opens and closes if the engine isn't under a lot of load. This is because once the thermostat opens, the radiator cools things down quickly. The 425/445 both use the same radiator as the 455 diesel, so it's oversized for the gas engines. The engine isn't actually fluctuating as much it might seem as the sender is located right at the thermostat, exaggerating the change in reading. If you run one hard, the fluctuation tends to go away as the thermostat stays open.

If your temp gauge is registering a fluctuation (as described by jeffinia), your engine IS experiencing cooling and warming. The sensor is located near the radiator hose fitting that delivers heated coolant LEAVING the engine headed for the top of the radiator. There are actually 2 types of plumbing arrangements in use on today's engines. One type is the old standard way of configuring the thermostat to modulate the flow of coolant on the OUTLET side of the engine (thermostat meters coolant from the engine into the upper radiator hose) and the return (lower hose) is uncontrolled. The other configuration places the thermostat to modulate the flow of coolant returning from radiator (lower hose) while the outlet hose is uncontrolled. With this type, the thermostat bulb SENSES by being in contact with the HOT coolant, but the thermostat valve controls the cooled, return coolant on it's way to the water pump. Regardless of which type is used, the temperature sensor (be it for gauge or warning light/alarm) is always located near the upper hose outlet. If your gauge is fluctuating, your engine temp is fluctuating. This could be due to one of the scenarios I mentioned earlier in this thread. Is it cause for concern? Probably not (unless it fluctuates from extreme overheat to stone cold). I can understand that the size of the radiator and flow rate of water pump being adequate to cool a larger engine might lead to some fluctuation if the same components were used on a smaller engine. Another thing is that the engines on OPE really do have a broader range of operating loads than your typical family car and so don't produce a constant heat load (as noted by several posters).

My 445 gauge works identical to what jeffinia describes. It's up over 2000 hours now with no problems so at least in my case it doesn't seem to be detrimental. I once had the bright idea of reversing the fan direction so in the winter it would blow the heated radiator air into the cab area. Fortunately I checked it out further and found out that most of the time it blew cold air.

A slight fluctuation of the gauge is considered normal. If you don't find a bad wire or sensor, you prob have a bad IP.