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Proper Temp Gauge Reading

Old 05-20-2013, 08:53 AM
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The photograph in post #11 is from my digital cluster that was originally fitted to my 1990 S4 Auto which was a Hong Kong delivered car.

I just opened up this cluster and confirm there is a high temperature warning light as well, that is activated when the temperature is over 120 degrees C. This means that with the digital clusters fitted from MY 1989 onwards there are two warning systems for over temperature, the light and the digital readout. Unfortunately I cannot determine whether there are two temperature sensors or one sensor as we have no circuit diagrams for the PCBs fitted in the digital dash, however we only have one pin reference in plug 4 for the temperature sensor input?????

In addition the digital cluster (GTS model cluster from a Japanese Car) now fitted to my car, also has degrees C numeral on the temperature gauge facia.

From all of the above I would not like to define what type of temperature gauge facia was fitted to which digital cluster on what model car with what option number.
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Old 04-03-2018, 01:25 PM
Michael Benno
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Default 1988 S4 Proper Temp.

From the Owners Manual for 1987 and 1988 model years:

"Needle in center field - normal Under normal driving conditions, needle should remain in center field. The needle may reach the upper field, especially at high engine loads, but should return to "normal" when engine load is reduced.

The cooling fans are electrically driven. They are switched on automatically when the coolant reaches 80░ C/176F. Even when the engine is turned off, the fans will continue running, until the coolant temperature has dropped to 75░ C/ 167░ F and until the engine itself has cooled down sufficiently. Therefore, never touch the fan blades as they will rotate spontaneously when the thermoswitch turns the fans on."

For reference, I tested my 1988 S4 last night and let it run until the coolant fans came on. I monitored the temps of the components using a infrared laser probe. Sure enough, the fans came on at 79-degress - measured at the radiator temp switch and that the temp sensor on the top of the water bridge that feeds the gauge (item 4 in pic below) and got the same reading once.

Item #4 is the temp sender for the dash gauge.

I compared the reading with what was displayed on the dash gauge. The needle was dead center of the middle field (see below).

If I would have been smart I would have noted the temp at the first hash marks. I'll keep my probe in the car and try to get some readings when I hit the various marks. Hope this was useful for you all.
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Old 04-03-2018, 10:38 PM
dr bob
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The S4 fans care little about the water bridge temperature. The fan controller reads the temp sender in the bottom front left (US driver's) side of the radiator, and the AC pressure transducer on the AC drier. There is an auxiliary function that runs the fans to cool the intake manifold if it is too hot after shutdown, and/or the auto transmission when fluid temp is high (>110║C per switch markings) at the cooler port on the transmission. Bottom line -- listening for fan turn-on as a way to "calibrate" the gauge needle is not really an accurate way to do it. Follow Michael's method: The thermocouple probe on the water bridge or a shot with the IR gun (get closer than 6" away) will be a better way.

The cooling system depends on the fans to deliver cool-enough water to the block, then depends on the thermostat to do the final coolant flow adjustments needed to stabilize engine temps under the wildly varying load and ambient conditions the cars can endure.
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