Temperature Gauge Reads High

[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.