Temp.gauge help.

kenacp · May 18, 2008

I have a 1998 JD 770 tractor and looks like the temp. gauge is not working. Can someone explain how it works and what is the best way to troubleshoot it.Broken wire,gauge, sensor,fuse? Any ideas? Thanks, Kenny

CATMAN · May 18, 2008

On the John Deere part catalog website, the sending unit looks to be a single wire electrical sending unit style. The sending unit it located on the left hand side of the water pump(by the fan). The easiest way to T/S this style of electrical, single-wire sender is to remove that wire from the sender and touch that wire connector to a good ground(no paint, bare metal) and with the ignition key on ("in the run position"). {NOTE:The engine doesn't need to be running.} When you touch that wire to a good ground and you watch the temperature gauge, the gauge needle should "peg" all the way over to the HOT side of the gauge. If it does this then the temp sender is the culprit, It means your gauge and wire are ok. These single wire sending units control electrical resistance to ground=lots of resistance when cold; and lesser resistance as the coolant warms up whichs allows the needle to move towards the HOT side of the gauge.

You know there is a possibility that your engine's thermostat is stuck in the "open" position and your coolant temperature is constantly staying too cool to register on your gauge. Are you not seeing any temperature gauge needle movement even when you are working the tractor hard?

kenacp · May 19, 2008

Thanks for the info Catman. I will check it out next week when I get back to the farm. I did about 2hours of bush hogging and the gauge did not budge off of cold.Just don't like the idea of not knowing if it's running hot or not. Thanks again for your help.

crashz · May 22, 2008

My 770 would not warm up either. I started with the thermostat, then checked the guage, then replaced the sensor along with the hoses. The sensor was a bit spendy, but I feel confident now that I won't have any cooling issues and if I do, at least I'll know about them.

kenacp · May 22, 2008

How much did the sensor run you? I'll check mine out next weekend.

milkman · May 27, 2008

I would think that the temp sensor is a common item, and I would take it out and take it to a parts house to match it, unless you just want a genuine high dollar JD part.

CATMAN · May 27, 2008

milkman said:
I would think that the temp sensor is a common item, and I would take it out and take it to a parts house to match it, unless you just want a genuine high dollar JD part.

I would caution people on doing that. If it was only a temp "switch" for an idiot light, then one can get away with a generic temp "switch". For a "sending unit" , I reccommend one buys from the OEM/dealer. There are differences in sendings units that look alike. Those differences will throw off the gauge. I have seen a school bus maintanence dept. put in generic sending units on a rear engine CAT 3208 and their gauge always read about 15-20 F too hot. We back-ordered the correct sending units from the OEM (Bluebird) and the dash gauges then read correctly.

crashz · May 28, 2008

I think the sensor was close to $60. Make sure they give you the copper gasket as well.

RobJ · May 28, 2008

crashz said:
I think the sensor was close to $60. Make sure they give you the copper gasket as well.

Ouch!!

crashz · May 28, 2008

Its a little steep, but luckily there aren't may sensors on these beasties.

For what its worth, the cooling system in my machine had plenty of corrosion damage and sediment in it, even though the coolant looked clean and green. I think that may have had something to due with the sensor failure.

I may add some more ground straps, even though I haven't had any noticable electrical problems. Could be grounding issue through the cooling system.