Temperature gage Accuracy (Page 1/1)
Yellow-88 JUL 16, 12:12 PM
Hi

How accurate are Fiero temp gages?
Can they be calibrated?
olejoedad JUL 16, 12:42 PM
When calibrated, they are reasonably accurate.

Contact Forum member J Gunsett for Fiero gage restoration, repair and calibration.

He does an excellent job at a reasonable price.
theogre JUL 16, 02:25 PM

quote
Originally posted by olejoedad:
When calibrated, they are reasonably accurate.

Contact Forum member J Gunsett for Fiero gage restoration, repair and calibration.

He does an excellent job at a reasonable price.

Often this won't help for long or not at all.

Even if you have a "calibrated" temp gauge & Matching New Sender... They still often lies because of crap wiring & grounds anywhere in the car.
Example: see https://www.fiero.nl/forum/Forum2/HTML/146784.html Fuel, Engine Temp or Oil Pressure gauges have problems? & http://www.fiero.nl/forum/Forum2/HTML/138175.html Instrument panel bulb layout

This is a big reason Why GM & other brands Don't put Numbers on most temp gauges even now.

ECM's ECT & more are way more accurate because the wires are short connected directly to the ECM + Regulated to 5V. Not 12 to 15V the rest of car have to deal with on to of iffy grounds.

Even after taking the dash out to fix problems there & multiple senders saying 40-60Ω & clean all grounds & multiple gauges, the Oil Pressure still say nearly 80PSI.
Temp Gauge very rarely reads above 200°F but ECT on ECM scanner often reads 200-210° driving, rising when idle until finally rad fan turns on ~ 221°F (Factory program for DIS Dukes.)
The only thing close to accurate is the fuel gauge now that actually read full or bit above when station pump trip off or empty before the pump sucking air that wreck that. Even that should not be completely trusted in most vehicles including new models. Gauge Empty Does Not = Tank Empty so people buy fuel before the pump runs dry. 90+% of all types of pumps carrying anything will Fail & Fast when run dry.

------------------
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(Jurassic Park)

The Ogre's Fiero Cave

1985 Fiero GT JUL 16, 04:26 PM
I have the early gauges, and I found it to read very high, it would be past the red when the fan finally turned on. I fixed the pegging on startup, and that combined with grounding the sending wire about 10 times in a row (giving it time to get all the way over, wait a second, and go back, unlike the pegging at startup) got it "recentered", which gives me accurate readings. I have the Rodney lower temp fan switch, and the needle is at exactly 105*when the fan kicks on (which is the rating for Rodney's switch) so at least at max operating temp, it is very accurate. My fuel gauge also is perfectly accurate, I ran it dry purposely before replacing my pump, and it runs out at exactly empty, not before, and when I fill it, it is exactly full, not over, not under. My Fieros gauges are the most accurate gm gauges I've seen (all 6 others all had plenty of inaccuracy)
Yellow-88 JUL 16, 04:53 PM
Thank you.

Yeah .... They don't seem like precision instruments.

My wife's GT seems to run cool. It turns on the fan only when it should. It has a 200 Deg switch.
The temp gage runs always near the red line. About 240.

Idea; use a real temp gage for a number and bend the GM needle to match. ( ?? )
pmbrunelle JUL 16, 07:42 PM
Engine cold temperature reading is calibrated by pulling the needing off and pushing it on with a different angle.

Engine warm temperature reading is calibrated by changing the value of the resistor behind the gauge.

For warm temperature calibration, replace the original resistor with a potentiometer.
Warm up the engine, and measure its coolant temperature, such as with the ALDL datastream data.
Adjust the potentiometer such that the gauge reads the same as the known-good coolant temperature measurement.
Replace the potentiometer with a fixed resistor for service.

Look here for more pics/info:
https://www.fiero.nl/forum/Forum2/HTML/143983.html
Yellow-88 JUL 16, 10:14 PM

quote
Originally posted by pmbrunelle:

Engine cold temperature reading is calibrated by pulling the needing off and pushing it on with a different angle.

Engine warm temperature reading is calibrated by changing the value of the resistor behind the gauge.

For warm temperature calibration, replace the original resistor with a potentiometer.
Warm up the engine, and measure its coolant temperature, such as with the ALDL datastream data.
Adjust the potentiometer such that the gauge reads the same as the known-good coolant temperature measurement.
Replace the potentiometer with a fixed resistor for service.

Look here for more pics/info:
https://www.fiero.nl/forum/Forum2/HTML/143983.html



Thank you for the professional answer. Again thank you.
Matthew_Fiero JUL 17, 09:01 AM
I'm curious if this will work for us 3800 folk who don't get an accurate reading from the gauge.
theogre JUL 17, 09:10 AM

quote
Originally posted by 1985 Fiero GT:
I have the early gauges, and I found it to read very high, it would be past the red when the fan finally turned on. I fixed the pegging on startup, and that combined with grounding the sending wire about 10 times in a row (giving it time to get all the way over, wait a second, and go back, unlike the pegging at startup) got it "recentered", which gives me accurate readings. I have the Rodney lower temp fan switch, and the needle is at exactly 105*when the fan kicks on (which is the rating for Rodney's switch) so at least at max operating temp, it is very accurate. My fuel gauge also is perfectly accurate, I ran it dry purposely before replacing my pump, and it runs out at exactly empty, not before, and when I fill it, it is exactly full, not over, not under. My Fieros gauges are the most accurate gm gauges I've seen (all 6 others all had plenty of inaccuracy)

Fuel gauge on most vehicle w/ resistor senders are never that accurate.
Again, most OEM set the Gauge Empty sooner then Tank Empty for reasons above.
Plus tank design often makes it impossible to suck the tank dry even on level ground. In Fiero & some others, running < 1/8 - 1/4 tank can "run out of gas" driving in hill country because fuel flows to back or front of tank leaving the pump sucking air sooner. Part of why many including Fiero have baffles to help prevent that but only helps on short hills.

Most Senders "@ Full" isn't exact top Ω as publish but often bit more. Fiero etc Publish data say 0-90Ω but fact is often 0-91 to 0-93Ω some even higher & gauge display a bit above Full as show in https://www.fiero.nl/forum/...HTML/146784.html#p12

Even w/ new vehicles w/ BCM controlling most to all gauges, many still see a bit over full because many still have resistance senders & top value isn't exact to spec.

Often only the Station Pump may affect how Full the tank & gauge see as many nozzles, flow rate & Fill port plumbing will decide the Nozzle will trip off using Nozzle lock feature OR someone manually try to control the flow @ the nozzle.

Worse many try to "stuff" a tank to get "max" fuel in the car can cause big problems as liquid heats up & forces fuel up the vent line that can "poison" the EVAP system because carbon Hates Liquid then stops working. This is Why 87+ Fiero w/ "bigger tanks" have aux tank that may never see liquid but needs it to make sure EVAP Never sees Liquid. Often this can trip MIL & set "codes" as cause problems for PCM trying to test the EVAP system, about same problem w/ fuel cap loose on many newer models.