Temperature Gauge Reads High

[john4153]

My tractor is a 2003/2004 Case DX40 with 550 hours, which is about identical to the TC40DA.

Yesterday, temp gauge went into the red while mowing. Air temp was 70°F +/-. It appeared to be a thermostat that failed closed. Replaced that. Also replaced sensor. Both removed parts tested OK on the bench. I did a resistance/temperature profile on the removed sensor. FWIW, here it is (the 210°F is an estimated value):



Does anyone know what the resistance is supposed to be? More important, what should the voltage at the wire to the sensor relative to ground be? Is it 5V, as suggested by automotive sensors? If so, I am concerned the voltage regulator on the PCB may have failed. Is there any way to test the gauge easily before replacing it. That is, given a certain resistance to ground, what should it read? What is the usual operating temperature of the coolant? My local dealer doesn't know.

Thanks,

John

 

[JC-jetro]

John,

one piece of information that is important to know is actual temperature. You need a non-contact infra red thermometer to do so. It just might be every thing is calibrated to the right or what your gauge shows is not representative of the actual temp. I have resistance info for my rig that might be completely different than yours. You might look the thread below just for some additional info.

JC,


http://www.tractorbynet.com/forums/...179639-how-much-temp-too-much.html?highlight=

 

[john4153]

I used a non-contact thermometer on the thermostat housing. It read below 82°C (thermostat setting is 82°C). After awhile, the radiator started to warm up. Engine block never exceed 80°C at any spot tested. Temp gauge was well into the red. My IR thermometer is from Harbor Freight and is, of course, affected by the emission characteristics of the target. It also has a laser guide, but I don't trust that completely. That is why I did the temperature/resistance chart using decent quality instruments (Fluka thermocouple, Beckman DVM, stirred water bath). The resistances I measured at the sensor on the tractor were consistent with the chart results.

Of course, the dealer is happy to sell me a new meter, which are currently back-ordered by CNH, but I learned long ago that after a few blind attempts to fix something, it is far better to put forth the effort to actually diagnose the problem.

John

Edit: Your cold resistance of 690 ohms was a bit higher than mine at 450 ohms or so.

 

[Welshman]

Please don't be insulted, but are you sure your radiator screen AND the radiator are clean of debris? I have to clean the screen every time I mow, and pull it and clean the radiator fins about three times a season.

 

[john4153]

The DX 40 does not have a radiator screen. I back clean it with air and water, and it is not obstructed, except by the hydraulic radiator that is in front. Air screens to the motor compartment are free of debris. In addition, the radiator stays relatively cool by IR temperature.

John

 

[jinman]

John, here are the specs on your temperature sensor:

35 C (96 F) - 670 plus/minus 80 ohms
80 C (176 F) - 118 plus/minus 6 ohms
105 C (221 F) - 54.5 plus/minus 2.7 ohms
115 C (239 F) - 42 plus/minus 2.5 ohms

As you can tell from the data above, the thermistor in the sending unit is not linear. I think you are far more likely to have a defective temperature gage on your instrument panel. The gage is replaceable, but quite expensive. I'm giving you a link to my thread on testing and replacing that gage. Read it all the way through. You'll have to click on the photo links since TBN didn't have thumbnails in 2006. I suspect the gage is twice as expensive now or even more, but it will be far less than a new instrument panel.

I just looked up the parts for you:

Sending unit is about $63 p/n SBA385720101
Gage for inst. pnl is about $62 p/n 87300471

Both these parts are at Messicks.com, a TBN sponsor dealer.

 

[JC-jetro]

80 to 85 C is not indicative of overheating. The thermistor has no moving part and see it difficult for it too damage. As jim said I think your issue might be with the gauge calibration.

JC,


Jim, Is the needle adjustable ? is there any possibility to bench calibrate the gauge?

 

[jinman]

Jim, Is the needle adjustable ? is there any possibility to bench calibrate the gauge?

You could put a dog-leg bend in the needle. That's the only adjustment. It's either go or no-go.

 

[JC-jetro]

You could put a dog-leg bend in the needle. That's the only adjustment. It's either go or no-go.

That works for me :thumbsup: So you can spin out or remove the glass. I'd probably run the engine so it would be fully warmed up, take the temp reading while warming up and at about 180-185 of deg F of the T-housing bend the needle right in the middle and call it good. That's about as fine of adjustment I would need.

JC,


Ps. By the way I had a a couple of situations that the needle actually partially spunned and was lose. In one case the needle just came off that I had to put it back with a drop of crazy glue. may be that's the case here.

 

[john4153]

@jinman and JC-jetro

Thanks for the replies.

The resistance specs you have are close, but do not agree with what I found. Since my values are lower, it is not a poor contact question on my side. My meter is calibrated against precision resistors.

Mowed for about 2.5 h today with my ohmmeter attached to the temp sensor. Never went above 183°F (i.e., was never less than 43.4 ohm). Usual temp was about 181° (44.2 ohm).

Did notice one thing this morning that may point to the problem. The gauge basically measures the current of a fixed voltage through the unknown resistance of the sensor. If that "fixed" voltage is too high, then the current is falsely high and the temp reads high. I attached my voltmeter to the gauge lead and ground. Voltage started at 9.34 with switch on, but engine not running. I expected 5V, but any stable voltage would do. After a minute or so, voltage jumped to 11.24. With engine running voltage varied approximately between those values. The temp needle was also unsteady when the gauge is in the circuit. First thing to do is to check the grounding of the instrument module. Second thing will be to look for the voltage regulator and test it out. The usual suspects at this point are the ground, the capacitors on the regulator, poor solder joints (lead free solder problems? Tractor is 10 years old.), or bad regulator. Fortunately, I have parts that will probably do to replace anything I find.

BTW, I did read the excellent link you (jinman) posted on testing the meter before starting this thread.

John

 

[jinman]

John, the meters usually fail low instead of high like yours. You can ground the wire to the sending unit and the gage won't go all the way to maximum as it should. Yours should peg out if it is high. If you have a calibrated resistor or a potentiometer set to the right value using your meter, you could substitute that to ground and see if the meter responds correctly. The meter circuit is a bridge with the sender being one leg of the bridge. However, there is no schematic for the instrument panel circuits.

 

[jinman]

I actually posted bad info. The temperature gage for the TC40DA was the number I posted. The number for the earlier TC40D and similar others is 86540855 and runs around $88 to $90.

Also, I think this is a similar schematic to the actual gage based on what it looks like internally. This is from an International-Harvester Firetruck, but I'd bet the circuits are similar.

 

[john4153]

Thanks for the schematic. Don't understand why they use coils for a DC circuit that changes slowly, but will tear into it tomorrow. At least, I am comforted by knowing the engine-related components (water pump, thermostat and sensor) are OK. I am a bit more comfortable dealing with the electronics.

The gauge at my local CNH dealer is around $60, but is back ordered at the distributor in Indianapolis. No telling what the actual price will be when it becomes available.

John

 

[jinman]

The reason they use coils is that they are electromagnets that control the needle movement. The needle is not shown in the schematic because it is not in the circuit, but its position is controlled by the magnetic flux in the coils.

 

[john4153]

Again, thanks for the clarification. I had assumed the meter was a simple D'Arsonval moving coil type.

John

 

[jinman]

I think it's more of an electrodynamometer or wattmeter movement. It's not like any D'Arsonval movement I ever worked on.

Did you see this photo of the gage innards?



Though not high current, the meter seems to be represented by this diagram.



I have another photo from above with the needle removed. I'll post that tomorrow morning.

Found the photo:

 

[john4153]

Well, after much testing, including setting up a test bench with a power supply and a stirred, heated water bath, I could not isolate the problem. The meter read high both when it was plugged into the display and when it was isolated by itself. All resistances were about the same as reported above. Changing the supplied voltage to it from 10V to 15V made no difference. So, I bought a new meter ($100), and it seems to read OK. It is a bit cold here, and the tractor was not under load, so the needle stays just a bit below the center mark.

Here is a picture of the innards. That fixed resistor is labeled 56 Ω and that is what it tested to be. All solder joints were shiny. One note, when I drilled out the rivets (1/16"), the needle just fell off. My thought now is that the real problem may just have been a loose needle that slipped about 1/2" (arc length) from where it was supposed to be.



Maybe the best solution would have been to follow JC-jetro's and jinman's advice (posts #7 and 8) and "bend" (i.e., reset) the needle.

Thanks all for the help.

John

 

[JC-jetro]

John,

I think the needle trick would have worked just fine. May be you could have rotated left or right to be right at the middle with engine normal hot and put a drop of epoxy on the needle so it would stay.

JC,

 

[jinman]

After tearing into that gage and finding it consistently off, you could also repaint the meter face to correspond to tha reading of actual temperature with a non-contact IR thermometer. Still, after doing all that tear-down and testing, why not spend the $100 and have a working gage reading as it should? It's the solution I would have chosen too. I feel like if I can afford to own a DX40, TC40DA machine, I should be able to spend the money. I know dollars are tight, but Iwasted a lot more money than that when on liberty ashore in the US Navy.