Auto temperature sensor/gauge replacement & testing

[jabalv]

Still working to fix up my old tractor, fixed already so many things. Now from total crap it looks and drives much better.
Here is some picture of it in summer:

Still a lot of work to do, but at least it's not moving 


So.
I have temperature sender: https://vdo-webshop.nl/en/temperature-sensors/457-vdo-coolant-temperature-sender-120c-m14-4103590693433.html
2 Temperature gauges, just 1 spare: VDO 40-120degree (C) with 3 pins (+ / - / G) G -> one that goes to sender)
Tested gauge with 24V power supply:
a) Add + / -    -> gauge stays on 40  // OK[/li][/list]
b) Add + / - and G to -  (0 resistance) -> gauge moves to max 120 // OK
c) Add + / - and G to sender, sender ground to - gauge pin // BAD - sender shows 80C degree when in room temperature (20C), expected to show 40
d) Testing sender resistance - shows ~230-280 it varies. But according to sender manufactured information: https://vdo-webshop.nl/9575-large_default/vdo-coolant-temperature-sender-120c-m14.jpg[/li][/list]
including tolerance that should be fine when it's 280, it should show on gauges 40(C), but when connected it shows 80(C).

a,b,c, steps repeated with 2 gauges and same result.
Obviously same 80(C) shows when cold engine and when it warms it's max, which is obviously wrong.

1) Which part is faulty? Maybe I'm doing something wrong? 
I thought that faulty is sender, but now I'm confused, cause not clear indication or I missed something?

2) I still kind of thing that sender is faulty. So as far as I understand resistance is important to pair those 2 together. So could I replace that sender with another which is in same 40-120(c) degree interval with 300-22 resistance accordingly?
Will that work? And maybe some tweaking can be done with resistors in the middle?

Can you guys please advice? 

[jabalv]

[floobydust]

Is the gauge 12V or 24V ? Check the datasheet. I don't know its part number.

[Rerouter]

It should be a 12V gauge for that generation of gauge, the 24V ones had a ceramic resistor mounted to that black plate on the rear

If the sender is measuring ~280 and the engine being about 40C is reasonable, then he is reading correctly, The way to test these is remove them from the engine and drop them in a coffee cup or similar with boiling water, or as the quick and dirty point a small butane torch at the tip for ~20 seconds
The resistance should smoothly increase, coffee cup will usually max out around 80C (water is cooled by sender) if the torch, then less clear where you end up, but you expect a smooth increase somewhere near 100C,

For the gauge, once you know atleast 2 points on the range of the sensor, see what it reads for those points on the gauge,

Better yet there will be a part number on the side, if I know that, I might be able to give you the exact specs,

[jabalv]

Gauge is 24V

[jabalv]

It should be a 12V gauge for that generation of gauge, the 24V ones had a ceramic resistor mounted to that black plate on the rear

If the sender is measuring ~280 and the engine being about 40C is reasonable, then he is reading correctly, The way to test these is remove them from the engine and drop them in a coffee cup or similar with boiling water, or as the quick and dirty point a small butane torch at the tip for ~20 seconds
The resistance should smoothly increase, coffee cup will usually max out around 80C (water is cooled by sender) if the torch, then less clear where you end up, but you expect a smooth increase somewhere near 100C,

For the gauge, once you know atleast 2 points on the range of the sensor, see what it reads for those points on the gauge,

Better yet there will be a part number on the side, if I know that, I might be able to give you the exact specs,

Hi, I removed gauge and sender from engine and did tests. It shows 80 degree (celsius) , so putting in it boiling water to test further I don't see much point as it already shows wrong when not in engine at all, just in house room temperature.
Part number of gauge is: 310 475/3 10 24V 8.80 VDO and part number for sender in that link I provided 1st post.

[Rerouter]

Yep, and now I feel silly, should have noticed the older light holder,

The modern equivalent for australia would be a 310040002, your region might be a little different,

The plugs should be the same, but the light fitting has changed to one that clips in. and will match the sender you linked to,

You have me thinking way back now, what has probably failed is the 150 ohm resistor inside or one of the solder joints... trying to remember the meter type, either way would need to remove the bezel to fix, which not many people have a ring press to put back on cleanly.

[jabalv]

Hi. So that you actually thing that problem is in gauge? So basically all 2 gauges are faulty?

Should that bezel pop off easy?

[jabalv]

That bezel removal can be quite tricky as I'm googling now. And tried to pull it off, it holds strong there.

https://www.davebarton.com/pdf/52mmfaceinstall.pdf

I believe just need to use 1st method in mentioned url and slowly get rid of it. But I have a feeling that it involves need to buy new one afterwards.

[jabalv]

I did carefully removed bezel. But I probably need some wrench to unscrew whole mechanism from back (in picture I posted before). Will check later today because don't have any wrenches available at the moment.

[floobydust]

I've never been able to pull off the bezel without wrecking things, it's a crimp all the way around. Then what? You can't fix much inside. Wirewound resistors do not decrease in value.
You can measure the gauge two coil resistances from GND to S and GND to +V, before taking it apart.
It looks like a custom VDO part for Volvo? 310-475-003 plus some other options. Perhaps they have a 12V supply in the original 24V system.

[jabalv]

I actually managed to remove bezel quite easy and I think I will be able to fit it back as well.
Inside looks following:

[jabalv]

I've never been able to pull off the bezel without wrecking things, it's a crimp all the way around. Then what? You can't fix much inside. Wirewound resistors do not decrease in value.
You can measure the gauge two coil resistances from GND to S and GND to +V, before taking it apart.
It looks like a custom VDO part for Volvo? 310-475-003 plus some other options. Perhaps they have a 12V supply in the original 24V system.

Replace wire wound resistor?
I did measure GND to S etc. I noted resistance, but I didn't found any manual that specifies how much there should be.
Yes, it's Volvo BM/Valmet, so could be something VDO custom for them. This is 24V system.

[Rerouter]

The white power resistor is the power to the gauge, the wire wound only needs to be 1/2W, and is the signal resistor, it scales the range of motion of the gauge,

The power resistor in almost all cases does not need to change unless the value has shifted off the printed value