Thermistor or Gauge Malfunction?

[monza66mo]

I just got a new to me '65 Corsa 180. I've only driven it for the past 4 days and put about 150 miles on it. The "normal" operating temp seems to be 400 degrees at highway speeds in 75-degree weather. Today, as I was headed home, the needle jumped way past the 600 degree mark after only a few minutes on the highway.

I freaked right out and pulled off the highway ASAP. The engine was running fine, the belt was not too loose, the fan was spinning nicely and both air doors were wide open.

So... What do you think? Is the gauge malfunction or the thermister? Or something entirely different?

The thermo-coupler that Clarks sells seems tempting to me but I'd like to hear from folks that have used them.

[Frank DuVal]

Did the engine "smell" hot or was it pinging more than normal? If not, probably not reading right.

Lates (Corsas) seem to run about 375 on the gauge, while earlies (Spyders) tend to run 400. Either if they reach a real 600 should be pinging like crazy and the red light should have come on (it is set for 525?)

Read here for some troubleshooting advice:

viewtopic.php?t=12113&start=16

[caraholic4life]

Frank provided good sound advice but there is also a chance your car may have mouse nests blocking the air flow.
Ifthe car you purchased had been sitting for a while before you got it there is a chance that there could be blockages between the cylinders that keep the air from moving through to cool them.

If the car was kept in a place where mice could have been lurking, there is a chance there could be mouse nests under the top cover and or in the lower shrouds.
There could also be mouse nests in the Heater Duct hoses or other related heater components.

Taking the top cover off of you engine to inspect for obstructions does take a good amount of time and effort but could be worth it if there is actually something blocking the air flow.
A quicker method would be to look in the lower shrouds for signs of debris there, the next place would be the heater hoses and duct work. Even if these areas are free of debris, it does not guarantee that there is nothing between the cylinders.

Something else you could potentially do is purchase an inexpensive Infrared Laser Thermometer to determine the engine temperature. I bought one from my local Harbor Freight store for about $25.00 last year. https://www.harborfreight.com/121-infra ... 63985.html

I did a quick search on this forum and found several Mouse Nest related topics:
In all, there were 23 different topics that resulted...Here are a couple of the more recent ones:

Cleaning ductwork May 12, 2024 http://www.corvairforum.com/forum/viewt ... st#p138526

Odd noise from heater fan.... Nov 23, 2020 http://www.corvairforum.com/forum/viewt ... st#p115290

'66 turbo head temp switch / part 2 Jul 24, 2019 http://www.corvairforum.com/forum/viewt ... st#p107256

Re: '66 turbo head temp switch / part 2

66vairguy » Fri Jul 26, 2019 11:27 pm
Keep in mind the temperature snap switch for the light and buzzer is on one side of the engine and the temp gauge sender is on the other side.

It is possible, although unlikely, that the side of the engine with the snap switch is running hotter. Typically this only happens when debris like a rag or mouse nest is resting over the cylinders.

More likely the snap switch is bad and is closing at too low a temperature, not uncommon. Clark's sell the snap switch. Two different ones, make sure you order the correct one.

[Nashfan]

I just got a new to me '65 Corsa 180. I've only driven it for the past 4 days and put about 150 miles on it. The "normal" operating temp seems to be 400 degrees at highway speeds in 75-degree weather. Today, as I was headed home, the needle jumped way past the 600 degree mark after only a few minutes on the highway.

I freaked right out and pulled off the highway ASAP. The engine was running fine, the belt was not too loose, the fan was spinning nicely and both air doors were wide open.

So... What do you think? Is the gauge malfunction or the thermister? Or something entirely different?

The thermo-coupler that Clarks sells seems tempting to me but I'd like to hear from folks that have used them.

I had the same exact thing happen on my Spyder and it turned out to be that the wire to the thermistor had some worn insulation on it and was grounding on the head.Once I fixed that screw up it was fine. Later I bought the Vairtrix sender to get rid of the thermistor... those things are NICE! What they do is simulate a perfectly calibrated on spec thermistor. You will know how far off or on the gauge is during the "self test" that it goes through when powered up. If the gauge is correct, it will go to 400 degrees during the test, and if the gauge is out of wack it will either read higher or lower than 400 during the test. These senders are very easy to install and work well.

[bbodie52]

There are two electromagnetic coils inside the gauge. The one depicted on the left side acts like an electromagnetic return "spring" that provides a constant pull of the needle to the left side — whenever the ignition key is ON and voltage (nominal 12 VDC) is provided through the coil to ground.

The coil on the right is installed to provide a variable magnetic field that gets stronger as current through the coil increases. Because it has more turns of wire in the coil, it has the potential of providing a magnetic field that is much stronger than the coil on the left. The strength of the magnetic field on the right-side coil is determined by how much electrical current is passing through the coil. The variable resistor (thermistor) that is mounted in the DRIVER'S SIDE (left-side) Corvair cylinder head decreases internal resistance (measured in OHMS)(Ω) to current flow as the head temperature increases.

Both gauge coils receive the same 12VDC source voltage from the ignition switch. The coil on the right has more turns of wire, so it has the potential of being "stronger" than the coil on the left. It is a variable magnetic field coil that has an increasing magnetic field that grows stronger as the resistance provided through the thermistor in the left cylinder head decreases. The thermistor resistance value changes with increasing heat from the cylinder head. When the two coils inside the gauge produce an equal magnetic field, the pull on each side would be equal, and the gauge needle would point straight up (400°). As engine heat increases, the thermistor resistance decreases and the magnetic field on the right coil becomes stronger — pulling the needle further to the right. A dead short to ground or maximum heat/minimum resistance to ground through the thermistor would increase the variable coil on the right to maximum, which would "peg" the needle all the way to the right side.

BATTERY(+) — THERMISTOR — GROUND(-)

To troubleshoot, turn the key ON and disconnect the wire from the thermistor. This effectively removes the path to ground through the thermistor to the cylinder head. If the needle moves to the left side with the wire disconnected from the thermistor, this would be an indication of a dead short to ground through the thermistor. If the needle remains pegged to the right even when the wire is has been disconnected from the thermistor, you would likely have an indication of an electrical short to Ground at some point along the wire path between the gauge and the engine location. Sometimes the wire can become "pinched" where it passes through some sheet metal. If the wire insulation is cut, the wire could form a connection to chassis ground (an electrical short) that would cause the coil inside the gauge to each a maximum current flow and an associated maximum electromagnetic field — pegging the needle to the right.


ORIGINAL AC THERMISTOR (LEFT) and AFTERMARKET REPLACEMENT THERMISTOR (by VairTRIX) (RIGHT)



https://ssl.corvair.com/user-cgi/catalo ... ow_page=51

Part number C1718: 65-66 TURBO & 140 CYL HEAD TEMP SENSOR *THERMO COUPLER - REPLACEMENT FOR THERMISTOR

Weight: 0 lbs 4 oz
Catalog Page(s): 51
Price: $ 192.00



Part number C1717: 62-64 TURBO CYL HEAD TEMP SENSOR *THERMO COUPLER - REPLACEMENT FOR THERMISTOR

Weight: 0 lbs 4 oz
Catalog Page(s): 51

VairTRIX Logo.jpg

https://www.vairtrix.com/#!/1/featured/ ... sories/159

Thermister wire routes from the lower shroud up thru the heater duct on the left side and into the engine compartment and hooks to a single connector spade plug hanging out the main harness that goes to the front of the car.

Left (driver's side) cylinder head

[Frank DuVal]

Good point Andria, those non-contact temperature guns are GREAT! for diagnosing heating or cooling issues. A must have tool. I even use mine to see how hot my cast iron frying pan is.

[66vairguy]

I didn't notice anybody mentioning the OVERTEMP warning light sensor is on the passenger side head, and the thermistor for the gauge is on the drivers side head and it sits right above the exhaust manifold so careful routing of the high temperature sending wire is required.

It is possible for just one side of the engine to overheat as some mentioned with a critter nest or shop rag on one side.

The thermistors do fail, check for cracked or broken porcelain. Also possible is the wire lead shorted to the metal shroud, (check the rubber grommet on the heater shroud elbow).

If you do install the Vairtex sender keep in mind they require the "bulge" (electronics?) in the wire lead to be as far as possible from the engine. Some have let the wire "bulge" contact hot surfaces and reported the Vairtex sender failed.

[66vairguy]

BTW - the original wire that attached to the thermistor had a high temperature woven cloth covering. Now Clark's sells one with a high temperature plastic insulation.

The standard plastic insulation used on automotive wiring will not stand up to the heat near the thermistor.

[monza66mo]

They mystery continues... Today, I took off the left lower shroud to get a look in there and see the condition of the thermister wire. Everything looked OK and no sign of grounding or fraying. I took the right lower shroud off just to see up in there.

While the shrouds were off, I did a road test on the highway for 30 minutes in 80 degree ambient temp and the gauge seems to be working fine now.

Maybe it actually did get too hot the other day and the shrouds being off help with cooling? Maybe the thermister is intermittingly failing? Maybe the gauge failed due to poor ground? Who knows? I'll keep an eye on it.

[66vairguy]

They mystery continues... Today, I took off the left lower shroud to get a look in there and see the condition of the thermister wire. Everything looked OK and no sign of grounding or fraying. I took the right lower shroud off just to see up in there.

While the shrouds were off, I did a road test on the highway for 30 minutes in 80 degree ambient temp and the gauge seems to be working fine now.

Maybe it actually did get too hot the other day and the shrouds being off help with cooling? Maybe the thermister is intermittingly failing? Maybe the gauge failed due to poor ground? Who knows? I'll keep an eye on it.

Good luck. Yes there is a wire wound resistor inside the gauge and the contacts can corrode. And the ground is important. I've cleaned up a few gauges to get them working.