7/17/2019 Alfa Romeo 164 Coolant Temperature Gauge Correction Alfa Romeo 164 Coolant Temperature Gauge Correction

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Alfa Romeo 164 Coolant Temperature Gauge Correction

by John Stewart, aka “Roadtrip” AlfaBB.comAugust 2014

General Background

The 164 coolant temperature instrument panel gauge is notorious for giving inaccurateindications. For a few minutes work and about two dollars in materials, you can make your dashindications accurate, instead of guessing that the coolant temperature is ok. The commonmisconception is that the Alfa V6 engine runs “hotter” than other cars, which is not true. Ifanything, they run cooler. The reason for this misconception is that the dash gauge typicallyreads half-way between the 175F and 250F index lines during normal running, which wouldmake one think the coolant temperature is about 215-220F. In reality, there is a mismatchbetween the ohm value of the sending unit (located on the hot side of the coolant beneath thethermostat) and the dash gauge itself. This error is typically reading 15-20F higher than actual.

If the cooling system is healthy and doing its job, the temperature of the coolant during normalrunning at speed should stay about 185-195F, the opening value of the thermostat (depending onbrand, etc.) It is important that the engine fully warm-up in order to send the correct signal to theengine ECU to ensure that cold run enrichment is completely taken out. This increasesperformance and enhances fuel efficiency.

That said, it should be noted that coolant temperature does not stay constant throughout thesystem and depending on where you measure it, it changes. The Coolant Temp Gauge SendingUnit is located on the hot side of the thermostat. The Radiator Fan Switch is located on the upperleft (cool side) tank of the radiator. The factory spec for tripping on the radiator fan is LowSpeed , 198F and High Speed, 206F. The OEM factory value for the thermostat is opening startsat 188F (+/- 4F) and is completely open by 210F. And keep in mind that the thermostat onlymoves a third of an inch from full closed to full open. Aftermarket replacement thermostats mayhave slightly different values from OEM, but typically open at about 190-195F.

Methodology

The ohm value of the instrument panel gauge was ascertained using a variable potentiometerwith a range of 0-1000 ohms. The brown wire from the instrument panel gauge to the sender wasdisconnected and the Potentiometer connected to it and a ground.

The ohm values at the various indicated temperatures were as follows:

Instr Panel indication Measured Ohms Factory Repair Manual120F …………………...1000 ohms ………….900-1400 ohms140F …………………… 450 ohms ………… 470 - 600 ohms160F …………………….330 ohms…………..235 – 300 ohms175F …………………….260 ohms ………….175 – 215 ohms190F …………………….210 ohms …………..not published215F …………………….150 ohms ………… 135 – 165 ohms230F …………………….120 ohms ………… not published250F ……………………. 90 ohms…………. 80 – 100 ohms

7/17/2019 Alfa Romeo 164 Coolant Temperature Gauge Correction Alfa Romeo 164 Coolant Temperature Gauge Correction

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Fig 1. Ohm values of various gauge indications. Needle position at normal running temperature.

The potentiometer was then set to the various gauge index marks and the resistance measured.As you can see, the ohm values are not linear. At very low values, the range is large, while in thenormal working temperature range of the engine (about 175-215F), the values settle out at about30 ohms per 10 deg change. This is the range, and specifically about 195F, that we’re mostconcerned with getting accurate indications.

The sending unit values were measured with an ohmmeter attached to the sending unit, theengine started, then as it warmed-up, IR readings were taken along with the resistance values forthose temperatures. These are the results:

Sending unit measured ohm values correlated with temp and instrument panel gauge

Temp Sender Gauge Delta80F ………1900 ohms ………. none ………….

100F ……… 450 ohms . ……… 980 ohms ……. 530 ohms140F ……… 420 ohms ………. 250 ohms ……. 170 ohms160F ……… 330 ohms ………. 210 ohms ……. 120 ohms175F ……… 180 ohms ………. 270 ohms ……. 90 ohms190F ……… 140 ohms ………. 210 ohms ……. 70 ohms215F ……… 100 ohms ………. 160 ohms ……. 60 ohms230F ……… not taken ………. 140 ohms …….250F ……… not taken ………. 90 ohms

Max TemperatureWarning Light on

7/17/2019 Alfa Romeo 164 Coolant Temperature Gauge Correction Alfa Romeo 164 Coolant Temperature Gauge Correction

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Conclusion

The Alfa 164 Water Temperature Gauge and Sending unit are mismatched causing the gauge toread about 15-20F higher than the actual coolant temperature at normal running temperature.Further, the mismatch is not linear. It should be remembered that the coolant temperaturereadings were taken with an IR temp gun on the outside metal of the lower thermostat housing.While maybe not exactly matching the inside coolant temperature, it should be very close.

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Accurizing the Instrument Panel Coolant Temperature Indications

Since the mismatch between the gauge and sender is not linear, we have to decide on what rangewe want the indications to be the most accurate. This should be the normal running temperaturerange, i.e. about 190F. The ohm delta between the gauge and sender is 70 ohms, so theoreticallya 70 ohm resistor in the line should correct the gauge.

As a practical test, I made up resistor inserts of 27 ohms, 47 ohms, and 62 ohms and installedthem with the engine at about 180-190F (as measured with the IR gun).

The 27 ohm resistor made the instrument panel gauge read 7 degrees hotter than actual.

The 47 ohm resistor insert made the panel gauge read dead-on at 185F

The 62 ohm resistor insert made the panel gauge read about 5 degrees cooler than actual.

Keep in mind that interpolating the index marks on the instrument panel gauge is pretty difficultanyway, so don’t try and split hairs on the accuracy issue here. All three resistors did a pretty

good job making the indications accurate enough to trust. I chose to use the 47 ohm resistor asbeing the most accurate overall.

Making the Resistor Insert

The Resistor Insert is simply a wire made up of a female spade terminal and a male spadeterminal at the ends, with a line resistor in the middle. The female end connects to the sender andthe male end into the car’s wire harness. A stiff piece of paperclip wire makes a nice stiffener toprevent wire breakage. Solder the resistor between the terminals, wrap with bit of electrical tape,then a piece of shrink wrap with the stiffener underneath. Resistors are about a dollar for a packof six at Radio Shack.

END