Temperature gauge and sensors

[Gatorman]

Cold beer for whoever figures this out!
I’m having some odd temperature gauge behavior on my 1982 FI, and a search of every gauge and sensor related post didn’t give me any clear answers. Fan switch is new, heater core bypassed and system has been burped with Mark’s throttle body heater.
Gauge was reading about 190 driving but a good bit higher than actual when idling at operating temperature (fan cycles on at about 230-plus indicated on gauge whille IR readings appeared normal.) I disconnected the high temp switch and gauge dropped quite a bit but still reading high (210-220). Installed new sensors, cut out some brittle wiring and replaced spade connectors, but now it reads very low (halfway between 120-190 when fan comes on). Also, now if I disconnect the high temp switch the gauge dies altogether. (First time it went right to 190, then bounced around before it died, but now it just dies). Temperature gauge resistor appears OK and I cleaned the ends before reinstalling, but gauge behaves the same.
Finally, another mystery to me - there appears to be a very small resistor soldered into the high temperature switch wiring, just ahead of the sensor. Never saw anything similar mentioned so maybe it was another one of my late father’s jury rigs, but not sure what the point was since the resistor under the dash appears to be OK.

Any clues?
Thanks

[RRoller123]

That resistor may be a telltale. Possibly not original gauge? And the resistor was added to "calibrate" it?

[Gatorman]

That resistor may be a telltale. Possibly not original gauge? And the resistor was added to "calibrate" it?

I suspected that, assuming it wasn’t original, but wanted to make sure it wasn’t part of stock system before I try running without it. Would gauge read lower with more resistance?

[RRoller123]

Don't know, that depends upon its design. Someone here I am sure knows if the tank float increases or decreases resistance as the tank is filled.

[Gatorman]

Don't know, that depends upon its design. Someone here I am sure knows if the tank float increases or decreases resistance as the tank is filled.

Just to clarify - we’re talking about temperature gauge not fuel gauge

[Gatorman]

That resistor may be a telltale. Possibly not original gauge? And the resistor was added to "calibrate" it?

I suspected that, assuming it wasn’t original, but wanted to make sure it wasn’t part of stock system before I try running without it. Would gauge read lower with more resistance?

Mystery (mostly) solved - cold beer for me!
Removed the resistor and now fan kicks on at the 190 zero. Still don’t understand why gauge died when I removed the high temperature connector (or why gauge reads higher with apparently less resistance in the circuit) but I guess I won’t worry about that.

[RRoller123]

ooops. my mistake, need to read these more closely.

[gchocevar]

Gatorman,

I had the same problem and the same question on my 79. When I disconnected the hi-temp switch the gauge read normally. I traced through the 79 schematic and found that the hi-temp switch shorts out the gauge across the dropping resistor in series with the switch.

I concluded that the intent was for the gauge to work normally, but if the high temperature switch is made, it pegs the temperature gauge. I also found out that my switch was bad. I added a a Hi-temp warning light and wired it to the switch, keep it completely out of the gauge circuit.

Either you have a bad switch also, or a pocket of air in the head causing the gauge to peg.

Greg

[Gatorman]

ooops. my mistake, need to read these more closely.

No worries - thanks RRoller. You were on the right track with the resistor.

[Gatorman]

Gatorman,

I had the same problem and the same question on my 79. When I disconnected the hi-temp switch the gauge read normally. I traced through the 79 schematic and found that the hi-temp switch shorts out the gauge across the dropping resistor in series with the switch.

I concluded that the intent was for the gauge to work normally, but if the high temperature switch is made, it pegs the temperature gauge. I also found out that my switch was bad. I added a a Hi-temp warning light and wired it to the switch, keep it completely out of the gauge circuit.

Either you have a bad switch also, or a pocket of air in the head causing the gauge to peg.

Greg

Thanks Greg - I knew how it was supposed to work and it never pegged, just read hotter than it should. I suspected a bad high temp sensor, but the odd behavior continued after I replaced both sensors. I have it sorted now that I’ve removed the extra resistor.