Cylinder Head Temp Gauge Question

[Jesus]

A mate of mine has just bought a cylinder head temp gauge and is now struggling to get it to work, for some reason he thought I'd know how to fix it, but I don't so was hoping one of you could help shed some light on the issue,

It's installed as per the diagram below:



and is number 7 on this page:

http://www.vwheritage.com/vwh/VW-Beetle-Parts/Interior/VDO-Gauges/?srchPart=V310901

Now the main question is how does it work?

There is a positive and negative from the sender to the gauge and then a seperate positive and ground for the gauge to light up, so how does the sender work/ get power?

It seems to be wired in correctly but isn't reading anything?

I know this is a bit basic, but I always turn to someone else for help when it comes to electrics as it confuses me!

[Andy]

If he has wired in the gauge, make sure he does not have the red wire from the sender going to '+' on the gauge, I did that and it didn't work, switch it round as per the diagram and it worked, for me.

I think it works similarly to a 'bi-metallic strip' the two metals produce a current, with change in temperature, the current also changes so the gauge can display a reading. No need for an external power source, apart for the light.

[Fastbrit]

It's not measuring current, so much as resistance, which changes with temperature.

[Andy]

It's not measuring current, so much as resistance, which changes with temperature.

I had a feeling I didn't have it exactly right 

[Peter]

I think its a thermocouple,
its a join between two metals and when it heats up,
the join will create a very small voltage

[Jesus]

Thanks Kindly for all the responses,

My mate Daren went and switched the wires around and now it's working perfectly.

He has just asked if anyone knows what kind of range the head temp should be?

Thanks Kindly

Alex

[Phil West]

Hi Alex,

340F is hot,  240F is cool.  Obviously depends on conditions and what you're doing but 260F would be a decent enough number to look for as a starting point.  Hope that helps!

Cheers
Phil

[69Stu]

Also, I believe ( and maybe somone can confirm or dismiss this ) that these guages are factory calibrated at 24 deg C, so if it is 14 deg C ambient temperature then the gauge will read 10 Deg C hotter.

Another point to watch out for. I checked my guage by placing the thermocouple into a kettly of boiling water with a thermometer, and the gauge read 230 deg F, 18 deg F higher than the boiling point of water.

Hope this is of some use.

[Jesus]

Thanks for all the swift replies!

Appreciated Alex