I would think that the base resistance of a temp sender (out of the box internal reading) would affect non linear readings once the sender is installed and is heated. Is there a base-line out of the box ohms reading that is published? Could this base-line reading help determine an accurate sender?

 

Out of the box cold seems to be all over the place. I've been checking them in boiling water for comparisons.

 

I don't have a BWD to check right now but I have a new Delco with the correct pin from Paragon , an LL and an old 1970 delco that I took apart and cleaned up and now it works perfectly.
The new Delco when cold reads 951 and in boiling water reads 111
The LL when cold read 1120 and in boiling water 102
The old Delco when cold reads 394 and in boiling water reads 78 - perfect reading

You could be right that the out of the box reading might be some indication of the senders accuracy or compatibility.
If you think about it the highest reading needed is 220 ohms which will give a reading of 100 degrees and 51 ohms will read 250 degrees.
So 220 ohms to 51 ohms is all that is needed for the gauge to go full scale.

 

That's what I am thinking. I have an old spade type for a 72 vette, at garage temp reads 650 ohms. A new wt203 that reads 471 ohms, and another new wt203 that reads 432 - these two are out of the box readings taken in the heat of the garage. I will repeat my readings in the morning to allow all three units to reach a cooler room temp (in the kitchen). I will also be boiling them in the morning and taking measurements. I'll post what I find.

 

That design of temp sender is only linear in a narrow temperature band...probably 50 degrees or so and around the boiling point of water. So, I imagine these gizmos are either calibrated, or selected, based on that temperature. Thus, the resistance reading at ambient temperature probably means nothing.

Your thinking is good. And if the device was less crude, it would be useful. Unfortunately, it is what it is. Now, if you could bring a pot of boiling water with you.....

 

I boiled two sensors today, the LL product and the WT203.
Temp readings for the LL:
100 = 375 OHMS
120 = 293
140 = 220
160 = 168
180 = 126
200 = 99
210 = 93
Temp readings for the WT203:
100 = 321 Ohms
120 = 268
140 = 227
160 = 189
180 = 160
200 = 133
210 = 124
With my temp gauge the LL reads 1/4 scale at full operating temperature (with heat gun, about 180 degrees.) During a hard run, the meter creeps up to near the 210 mark, then upon slowing down and cooling settles back to 1/4. At engine shut down, the gauge stays at the last reading and does not fall to 100 (off).
My temp gauge with the WT203 reads 210 and full operating temp, then during a hard run the meter moves to the second mark (between 210 and 250, then settles back to the 210 mark upon cooling down a little. Again, upon engine shut down, the gauge remains at the last reading. One other measurement, I disconnected the ohm wire at the sender and measured to ground. I obtained a 77 Ohm reading and I am "assuming" that reading is the 90 ohm resistor mounted on the rear of the gauge. I'm sticking to the LL unit for now.

 

Jim are you sure the above readings aren't reversed , LL verses 203 ?
The set of ohms readings on the top will produce a higher gauge reading than the ones on the bottom.

On engine shut down all the gauges I've seen freeze , needle stays put.

 

What if the gauge's 90 ohm resistor is faulty? I think I'm fighting a gauge issue.