<VV> Thermister Swap?

[Frank DuVal]

Let's see where to start, OK, an article on troubleshooting a Corsa gives:

With the engine cold, the reading should be 5000+ ohms. An engine 
running at normal temps can range from 500 (300F) to 750 (225F) in Ohms. 
Lower than 500 ohms simply means your engine is getting hotter than 
normal. Running the engine for a good 20-30 min usually hits 300F or so, 
but is stable at that temp. A lot of stop and go driving, heavy traffic 
congestion may force it higher. Running the engine for only 15 min will 
barely get the temp to 200F (1000 ohms) and your gauge may not even 
reflect it. Ohm readings of 310 or less indicate the engine is reaching 
400F or over. Less than 200 Ohms indicates engine temp is 450F +. The 
range of the gauge is from 200F (1000 ohms) to 600F (100 ohms). At 5000 
Ohms, the temp is 70F. At 575F, the alarm will go off.

And I found this info on  robinsonheli.com   for CHT probe 3080-38:

200 °F is 745 ohms
475 °F is 38 ohms
500 °F is 32 ohms

So, it is a thermistor, but the range looks really off at higher 
temperatures. Somewhat close around 200 °F. At normal driving 
temperatures, the gauge will read over temperature.

Frank DuVal

On 8/29/2014 10:28 PM, John Beck via VirtualVairs wrote:
> Wow, it does look like a thermistor rather than a thermocouple.  I saw 
> photos but no data sheet.  I wonder if the curve is anything like the 
> Corvairs.  I thought about rebuilding some DOS (dead old stock) ones 
> about ten years ago and couldn't find any bare, off the shelf 
> thermistors that were close to the original curve. Building additional 
> electronics to match the curve seemed like way to much work so I lost 
> interest.   This unit looks like they had the Corvair sensor in mind 
> when they designed it.  --J.B.
>
> Patrick via VirtualVairs wrote:
>> To all, I have discovered that the connecting shaft or tip  on my
>> thermister spins freely. I cannot tighten the small nut on the wire. 
>> In addition, I
>> have no reading at all on the gauge and I am  unable to gain any 
>> resistance
>> when I do an Ohm test on the unit. I am thinking  that whatever 
>> wiring the
>> tip is connected to beneath the porcelain has  broken resulting in a 
>> failed
>> thermister.
>>
>> Now, I have located used original AC ones (tested) for around $80, and
>> there is a NOS thermister on eBay now for $400. Interestingly, as  I 
>> researched
>> even further I have found info on a Rochester 3080-38 TEMPERATURE PROBE
>> referenced from a FlyCorvair.com article on head temps.  In the aircraft
>> catalog mentioned in the article their  Rochester thermister/temp 
>> probe has a
>> temp rating from 250F - 550F  degrees, and it has 3/8 24 thread size, 
>> the same
>> as the one I recovered from my  Spyder. Also, it appears DC because 
>> it is
>> used in light aircraft  perhaps Corvair powered too. Cost of the new 
>> Rochester
>> temperature  probe is $80.
>>
>> Question: Will this thermister work in my Spyder, is it compatible  with
>> the factory gauge and CHT opening on the head?
>>
>> (I do not have the measurements of the Rochester temp probe but it looks
>> very close to our AC unit from the picture in the catalog)
>>
>> Patrick
>> 64 Spyder Coupe
>> Metro Detroit, MI
>>
>>
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