temperature gauge escalates to max reading

[bbodie52]

It would seem that somewhere along the line you learned a lot about electronics, troubleshooting and fault isolation, corrected the problem, and are happy with the outcome. I would bet that a lot of us in "the audience" also learned a lot during this process — which is the nature of the Corvair Forum!

[BobWitt]

Great to hear you have recovered a reasonable reading. Do you have your lower shrouds on your engine with working thermostats? I ask as mine will only get up to ~250 at idle, but I have no lower shrouds...

I've read these gauges are not real accurate, and you will learn what is 'normal' for your car, and will react when it goes out of normal...

In the end, I think Brad 'nailed' the dart to 600 - you were loosing ground through the coil that pulls the needle low while retaining current flow through the thermistor which drives it high..

Enjoy the Show!

[Jerry Whitt]

The engine does have the lower shrouds and the stock thermostats. Car runs very good. Once again, thank you all for the help!!!

[Charlie]

Jerry Whitt and I worked on his 1965 Corsa's head temperature gauge that was "pegged" at 600 degrees.
Jerry figured this was either a gauge problem ($) or a thermistor problem ($$$), so we tested both.

Hopefully our diagnosis was correct that it was neither, and possibly a ground problem, since the gauge now works.
The actual values that Jerry and I got on the gauge are about 30% off, but follow the basic "curve" so at least the gauge and thermistor work?

NOTE : Resistance values on charts are approximate, and our results DID vary by about 33%)

Bob corrected the coils inside the gauge, so I corrected the diagrams per his updates (below)
~ Thanks Bob!

This is the basic circuit for the Corsa head temp thermistor and gauge:
(A Monza with a 140 may not have a head temp thermistor OR the related gauge ~ Please advise?)

(A Monza with a 140 may not have a head temp thermistor OR the related gauge?)

Head Temp Thermistor.jpg (11.9 KiB) Viewed 660 times

This is the basic circuit for testing the Corsa head temp gauge:
Simulate the thermistor by using a rheostat, potentiometer (or individual resistors) that should generate (approximately) the readings shown:

Test the thermistor by measuring the resitance which should show (approximately) the readings shown depending on the current engine temp.

Head Temp Gage~Testing Thermistor.jpg (14.42 KiB) Viewed 660 times

This is the basic circuit for testing the Corsa head temp thermistor (on left cylinder head):
With the engine hot, the thermistor’s resistance should be very low, about 100 to 150 ohms or so?
With the engine cold, the thermistor’s resistance should be very high, even "infinite")

The thermistor’s resistance should be very low when hot, and very high when cold, even "infinite"

Head Temp Gage~Testing Gage.jpg (17.65 KiB) Viewed 660 times

We would appreciate any input/proof reading that anyone may have here, to make this more useful?

[SteveH]

Woo Hoo!!! Great teamwork folks!

[BobWitt]

The only thing I think that Charlie's schematics fail to represent is the TWO coils within the Gauge. The "Low" coil is grounded via the gauge cluster and the "High" coil is grounded through the thermistor...

The "Low" or left coil is always trying to pull the needle low while the "High" or right coil is trying to pull it high.

So with a cold engine the resistance in the thermistor reduces the current through the "High" coil and the needle stays "low"... Conversely, with a hot engine, the thermistor flows a lot of current, pulling the needle high... So, far the discussion is how it is supposed to work...

In this case, the "Low" coil had a reduced current flow (zero flow?) due to poor case ground and therefore it did not counter the force generated by the "High" coil. Result is the needle goes full "High".

The following is my spin on Charlie's schematics, and a photo of what the gauge looks like inside with the two coils..

This photo is of a fuel gauge, but the temp is very similar!

Good write up Charlie!

[Charlie]

Even better, Bob,

I over-simplified it for testing purposes but yours could help with a gauge problem!

Thanks for the added info!
Charlie

[Jerry Whitt]

PS. Got an award at the local car show. Made this a better week!!!!!!!

[cad-kid]

[6T5VAIR]

So if one has a "pegged" gauge, what is the process to troubleshoot?

[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 (32.39 KiB) Viewed 67 times

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