Temperature Gauge

[rossco]

I'm trying to put in an actual temperature ( thermometer) gauge in so as I can see what the actual temperature the vehicle is running at.

When I connect the new gauge to the actual sensor at the thermostat it give a reading of about 80c, when I connect it through the existing wiring it doubles the temperature.

Anyone running an additional gauge through the existing wiring or another way ?

 

[jims-terrano]

When you connect through exisiting wiring is that with the old guage still in circuit? if so it is the extra resistance from the old guage that is causing your problem.

Jim T

 

[rossco]

Yes it is with the old gauge on the circuit so is there anyway of getting round the resistance ?

 

[Toolbox]

You will have to run another wire to the sender and piggy back it on to the existing connection. Take a live from something that is powered when the key is on.

 

[Terranosaurus]

Surely your guage needs a specific sender anyway to be accurate - there no telling that its calibrated to be correct with the nissan sender. Even if you go right back to the sender and rewire the 2 guages in parallel will still effec each other you need 1 sender per guage.

 

[Toolbox]

Surely your guage needs a specific sender anyway to be accurate - there no telling that its calibrated to be correct with the nissan sender. Even if you go right back to the sender and rewire the 2 guages in parallel will still effec each other you need 1 sender per guage.

How will wiring the gauge to the same sender affect the other gauge when the sender is just a resistance direct to earth? Its not like they have a high power consumption.

 

[Terranosaurus]

First of the guages aren't resistive loads they ae inductive loads which complicates matters further but ignoring that lets assume they are resistive loads.

For arguements sake the sensor has a resistance of 100ohm and so does the std guage so the potential across the sensor will be nominal 6v and thepotential across the meter will be nominal 6v.

So now we plug our aftermarket guage on. Its resistance is 200ohm, so there will be a nominal 8v across the guage and 4 volt across the sensor.

Now we plug them both in together and both cant' be correct now can they we can't simultaneously have 4v and 6v across the sensor from the same 12v supply.

Thats because the guages act as resistors in parrallel. 100ohm in parrallel with 200ohm is an effective 66.6ohm

1/R = (1/R1) + (1/R2)

So the potentail across the guages would now be 66.6/166.6 x 12 = 4.8 and the potential across the sensor would thus be 7.2v


Realise thats a bit long winded and I hope people can follow it.


Just for completion I have said nominal because as I hope we all know cars don't actually operate on 12v but on somewhere between 13.8 and 14.2 volts

 

[rossco]

OK, if I wire to the same sender thats not a problem.
If I wire them parallel to the same sender then I get double reading.
I can't put in an extra sender as I would have to drill into the thermostat housing.

So I guess I'll just run the gauge on it's own and disconnect the factory one.

 

[zippy656]

Am i missing some thing, why do you want two any way?


Zippy

 

[rossco]

I don't trust the factory one and wanted to see how accurate by comparing them.
Had some strange readings from the factory gauge, so least I have a better idea of what's going on.

 

[zippy656]

ah...

mine just sits in the middle, no matter what im doing.. crawling about town,, 85 on the mway..

towing the caravan up hills .... still sits the the middle

:smile:


Zippy

 

[rossco]

I would be happy with that but it's all over the place enough to make you wonder what's going on.

 

[zippy656]

Yep, i can understand that, id want to know too



Zippy