Gauge Sensor Resistance Values

[Scott H]

I am contemplating changing my gauges to an aftermarket setup. I looked pretty much everywhere to get the specs on the sensors. I decided to just measure them all.

I used a variable resistor between the sensor wire and ground then adjusted it until the gauge registered. I looked for this a lot longer than it took me to just measure it :mad:.

DJ, this may be something to save as a file somewhere.

>Scott

Oil Gauge
Thread size - M14 - 1.5
Level Resistance
0 300 Ohm
1/4 mark 150 Ohm
55 (1/2) 120 Ohm
3/4 mark 105 Ohm
110 (max) 24 Ohm


Fuel Gauge
Level Resistance
0 334 Ohm
1/4 135 Ohm
1/2 90 Ohm
3/4 35 Ohm
4/4 0 Ohm


Temp Gauge
Thread size - M16 - 1.5
Level Resistance
120 (cold) 968 Ohm
1/4 mark 438 Ohm
190 (1/2) 165 Ohm
3/4 mark 67 Ohm
260 (hot) 38 Ohm


Speedometer
Thread on transaxle - M17.5 - 1.5
Most speedos use 1,000 revolutions/mile and I think the Scorpion does too. I can not confirm 100%.

[DJ]

Great info and good idea, Scott!

I've made this post sticky and will grab the data and compile it into a tech note when I get the time. (I have several of those I plan to do but they seem to keep getting pushed down the priority list.)

[Will]

FWIW Scott, I think you'll find all of the above have "ranges", with variation of up to about 15% possible unit-to-unit.

I believe your oil and fuel are 300 Ohm and your temp is a 10K Ohm NTC (Negative Temperature Coefficient) Thermistor.

Why a 10K Ohm you ask? Good question. You see, NTC Thermistors are pretty much all in the same range due to being made similarly. Here is an "official" chart from a Bosch unit as plagarized from Ben Watson's book on the Bosch system:

Deg F. ____________Ohms
14________________7K-12K
68________________2K-3K
122_______________700-1K
176_______________250-400

[Scott H]

FWIW Scott, I think you'll find all of the above have "ranges", with variation of up to about 15% possible unit-to-unit.

I believe your oil and fuel are 300 Ohm and your temp is a 10K Ohm NTC (Negative Temperature Coefficient) Thermistor.


There is quite a bit of hysteresis in these too.

I played around to try and get what were the best readings. Moved them a bit high and then a bit low to see what it took to get the needle stay on the respective mark. There is not a lot of accuracy when you start to look close.

>Scott

[Scott H]

I believe your oil and fuel are 300 Ohm and your temp is a 10K Ohm NTC (Negative Temperature Coefficient) Thermistor.



For what it is worth I was looking at VDO gauges and they show theirs as 10-180 Ohm. The web site I was looking at refered to a "US Ohm range" of 240-33.

[Will]

I'll go back and check but it looks like the temp sensor is 1k not 10k.


Look again at the chart in post #3.

Then check yours in the freezer at 14F, bet you at at 7-12K Ohm resistance. Your "coldest" reading was still in the middle of the range for NTC thermistors- room temperature is "cold" for an engine, but still middle of the range of the sensor output for many of these types of sensors.

Because your gauge is fairly linear in response and the thermistor is not, the gauge will have an error at one or both ends relative to the actual temperature. The useful range is typically way at one end, past the knee point of the graph, where the response flattens out. Googling "Steinhart equation" will likely give you a more comprehensive explanation than I can.
In fact, I know it will 'cause I'm gonna stop trying to explain now while I'm ahead LOL! :)