I did some more data gathering on this, and I think I had a bit of a breakthrough. Here's the TLDR: Although the OEM water temp sender was slightly closer the FSM spec compared to the CityRacer aftermarket sender, they were both in the right ballpark. The issue in my case seems to be more about how high the sender is sitting (I don't believe it was fully-immersed in coolant) due to a janky union. Read on for more details...
Since I still had my old sender, the new CityRacer aftermarket sender and the OEM sender, perfect time for a test. I took a bit of scrap metal (a simpson structural tie I had leftover from a reno), a stepper drill bit to embiggen the holes, and some scrap pieces of rubber fuel hose to keep the senders tight against the metal strap for continuity. I set em into a pan of water on the stove. Here's the setup:
Took the meat thermometer and taped it so it was immersed (it looks super-professional):
Anyways, here was my testing methodology:
- Verify continuity / no resistance between the metal strap and the body of each sender
- Check temperature at FSM Values (140F, 176F, 212F, 221F)
- Bring temperature 2 degrees higher than target temp, turn stove off (not sure if induction will interfere or not)
- Let temp come down to target temp, take Ohm readings
I did this for temps up to 176F, but ran into a problem at boiling, because of course I did. The hottest I could keep water temp with the stove off was 207F, beyond that it the stove needed to be on for a roiling boil. I did not have enough granularity to consistently hit 212 or 221, so I decided to just use the data I had and go to a spreadsheet. If I were to do this again, I'd use cooking oil and that would allow me to hit the temps I needed.
Here's the spreadsheet results:
I think it's fair to say that the OEM sender most-closely matches the electrical resistance called out in the FSM, but the CityRacer is pretty close. The old sender which was on the truck when I bought it was way out of tolerance, showing much higher resistance compared to everything else.
That said, this would mean that:
- I should have been getting accurate readings from the CityRacer sender, but they were showing high compared to what the thermostat / output hose read when using the laser thermometer
- The OEM sender would give me an even higher temp in the same circumstances, whereas the original sender would have been giving an artificially lower temp on the gauge.
This was a real head-scratcher, but I decided to re-assemble the truck and have a think on it later. I used permatex high-temp thread sealant on the new OEM sender, but as I had used teflon tape on the CityRacer one, I wanted to clean the threads out of the union, so I used a wire brush on a drill to clean up the threads. Re-installed everything, and I have a leak on the temp sender.
Did some research, figured I didn't knead/mix the tube of thread sealer enough, or let it cure long enough. So I made a new mess, took the sender out, cleaned everything up again, put a more thorough amount of thread sealer on,
waited 24 hours, and it still leaks!
. At this point I'm kinda like eff-it, back to the teflon tape. So I pull the sender again, clean the sealant off the threads, wrap it with a healthy amount of tape, and it still leaks
... At this point I'm starting to notice the sender is sitting lower in the union compared to the prior one, but I decide to try one more time (pull sender / clean threads / re-wrap in teflon tape). This time it still leaks, but only a teeny bit, so I run the engine up to operating temp, and.... The gauge is reading on the "B" marker when engine is around 190-200F. It works!
So... the OEM sender should have read hotter, but Im getting a lower temp on the gauge. I think its the position of the sender. See the before / after:
I'm not sure I have the right union on there. If I were to not use thread sealer / teflon tape, I would be able to finger-tighten that sender until it is touching the top of the union. When I used the wire brush to clean out the threads, I think I also cleaned out whatever thread sealer the prior owner had used to stop the leak as well.
New union on-order from CruiserTeq (queue sounds of loading the parts cannon)... I'll update the thread for a final comparison shot to see if Im right about the union not being right when I get the new part installed. Either way, I'm happy that the temp gauge is reading correctly now, but I need to once again go and pull that sender and wrap it with all the teflon tape I have in my possession to get me through the weekend...
