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My plan is as mentioned above to run the car at about 212 degrees and see how the gauge responds.
Initial impression is that this sender would work ok for Normal Operating Temperature (NOT) of 175-180 degrees.
We will see how it tracks in the temperature range above Normal Operating Temp of 175 to 180 tomorrow (hopefully.).
Note: I had to get behind the dash to access the Willcox adjustable resistor since I had tucked it up there so it would not be flopping around too much. When I was making adjustments today I noticed that it gets fairly warm so I will not be tucking it back in so tightly. I am also leaving it where it is readily accessible via that little access panel on the passenger side console below the radio.
Last edited by carriljc; Jun 10, 2021 at 11:30 AM.
Ok. I did the Duralast TU66 Temp Sender testing for the temperature range from 180°F to ~210°F.
The sensor appears to deviate too much from the digitally indicated temperature.
If in a bind you could install this TU66 Temp Sender and dial it in, as I did, to the 180°F range for the Normal Operating Temperature; however it appears to deviate too much from digital trend to be considered a "good" temp sender subsitute.
I will leave my TU66 Temp Sender installed since it's already there and I recognize that if forced to use it, then it will only be accurate at the 175°F to 180°F Normal Operating Temperature.
CONCLUSION: ON my 68 the Duralast TU66 Temp Sender is NOT a good substitute.
If someone knows of a different 3/8" sensor they want me to test let me know. I'll do if the sender is relatively inexpensive.
I'll bite. The spec sheet for the Wells SU109 was easy to find a few years ago. I tested that (with an Arduino), and it was dead-on accurate. I think that may now be the WVE 5S1018. It needs a specific circuit to work, usually connected to an ECU, so it won't work directly with a C3. https://www.rockauto.com/en/moreinfo...pt=4748&jsn=19
This has a 2-pin circuit for the ECU, and a third pin that (presumably) provides a variable resistance path to chassis ground.
I have one, I have not played with it yet. It should let me measure the actual temp with an ECU (or an Arduino), and also support an analog gauge.
Do you have a table of resistance measurements for any of the senders you've tested? This seems to be the secret sauce of any high-dollar sender replacement sold by the Corvette vendors. I was never able to find a datasheet for the older senders. Perhaps no one makes a sender that properly matches the OEM ones from our C3s anymore, so they sell whatever they can stock, and hope for the best.
Anyone have a part # for a 3/8" NPT replacement temperature sending unit that is close to the original 1/2" unit? 1970 C3. Upgraded to aluminum heads and need a replacement sending unit.
kjr6306
I have NOT tested the SU109..... it's on my never-ending list of things to do.....
Originally Posted by kjr6306
Anyone have a part # for a 3/8" NPT replacement temperature sending unit that is close to the original 1/2" unit? 1970 C3. Upgraded to aluminum heads and need a replacement sending unit.
jr6306
OK.... tested the SU109 vs the stock temp sender wired up to my stock gauge.
I did additional testing. See below for more postings on additional tests. It seems to be that SU109 is an acceptable 3/8" substitute as long as you dial it in with the Willcox adjustable resistor (or something that works the same as wired in behind the gauge - It is wired in parallel across the posts).
It is NOT a good 3/8" substitute. I deviated so far so early in the warmup process that I did not even bother fiddling with that wonderful adjustable resistor.
Last edited by carriljc; Jun 1, 2025 at 11:55 AM.
Reason: deleted image and updated text
If somebody really wants me to go through the adjustable resistor exercise on the SU109 just say so.
I left it installed since it just replaced the TU66 that was in there. I really doubt that it has a range where it would track with the original sensor but I reckon I can go through that if somebody really wants the data.
See posts below.
And, YES, I also checked the temp with temperature gun.
If somebody really wants me to go through the adjustable resistor exercise on the SU109 just say so.
I left it installed since it just replaced the TU66 that was in there.
I really doubt that it has a range where it would track with the original sensor but I reckon I can go through that if somebody really wants the data.
And, YES, I also checked the temp with temperature gun.
I love data, but I don't want to waste your time. Did you find a datasheet? I may have one for the Wells version. I'll take a look this weekend.
There is a schematic included that shows how to make it work with an ECU or Arduino, and I found it to be super-accurate for that. Perhaps the chart of resistances is useful? Would your idea work if you added an inline resistor to the sender, in addition to the adjustable on the gauge?
I've still never seen a temp to resistance graph of the OEM sender. You might need that, too, to get in the ballpark.
I did not find a datasheet.......
I've had that SU109 for years. I initially bought it to use as an Intake Air Temp sensor when I was using a GM 730 ECM ..... forget why I picked that SU109, but I think it was recommended on one of those websites for burning your own chips; which I did for a while..... I think it worked ok for, if not rapidly like the IAT SU176 I later replaced it with (and still using)....that SU176 is what is providing the temp for EZ-EFI IAT temp you see there.
If you see something in that data sheet that suggests I could find a useful operating range (which I doubt from the way it responded today) then I'll get do more testing. It was acting quite normal as the car warmed up but then zoomed well above that 210°F mark ...and the real temp was down at 166°F.
I reckon it's easy enough and I could get a couple of temp readings since I have that digital readout.
Ok.... when I get ambitious I will hook it up again and tweak the adjustable resistor to make the gauge read about 175°F to 180°F at NOT (my Normal Operating Temperature), about the midmark on the gauge, and then adjust the elec fan start temp to about 212°....... same test I did earlier with the TU66..... If we're lucky it will work and track within that range...... and if not then at least we'll have some info.
My original temp sender works wonderfully and tracks with the EZ-EFI...... I am only testing 3/8" senders in hope of finding one that works. It's not like I need one.
Ok....here we go. I did additional testing of the SU109 sender.
IF you use the Wilcox adjustable resistor, then It seems to me that the SU109 Temp Sender is a workable solution ..... see info below and tell me what you think:
NOTES: Looking at the gauge the lowest indicated temp is 100°F. 12 o'clock is 210°F. From plotting on graph paper and old discussion with Willcox the MARK between them is about 175F-180°F. I have a 180°F thermostat and it is "drilled" to allow venting. I typically have Mark VIII fan Opertion setpoints set such that the radiator fan starts at 180° and stops at 175°.
Note that in the testing below I manipulate those setpoints to allow the engine temp to change and observe the gauge operation.
Last edited by carriljc; May 31, 2025 at 07:22 PM.
Here I am just showing that I have adjusted the fan start temp up to 215....... I later adjust the start temp to 210° such that when the EZ-EFI saw 210 the fan would start..... this would allow me to monitor gauge reaction a little better.
