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The dashboard temp gauge on my wife's Alfa 156 1.9jdt 16v (2005) is slow to respond and only goes up to ~70, so as part of ongoing investigations I am currently wondering if the readings are low due to a faulty sender unit, and if so would this also affect the engine management and potentially result in poorer fuel economy?

Heat comes from the heater even when the gauge needle has not moved, which might suggest a possible faulty sender unit, but I also haven't yet checked under the bonnet to see whether the thermostat is functioning correctly, and I'm not confident that just checking hose temperatures will suffice in that regard.

Pity we can't just remove the old 'stat and test off the car in hot water, like we used to be able to do!!

Any ideas appreciated.
 

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the 156 has 2 seperate dedicated temp.senders. One is for the temp.gauge, the other is for the engine ECU.
Low reading of the temp.gauge could of course be caused by a couple of things:
a) the gauge reading is correct, and the temp. is indeed low. This could be caused by a bad thermostat.
b) the temp.sender for the gauge is faulty
c) the gauge is faulty.
 

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There may not be anything amiss. The mark on the gauge that lies half way between 50 and 90 is actually 80! (You can confirm this by looking at the ecu’s sensor using mes.) So if the needle hovers around there all is well. The slow response is probably just the time it takes for the engine to warm up. Mine can take ten miles or so to get to temperature in slow traffic in winter. The ‘top hose cold till engine hot’ test still works on these.
 

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Temp gauae is not real temp, so first you should test from OBD or direct sensor (v, ohm etc).

Probably thermostat is old/open.
 

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Discussion Starter #5
Thanks for your input, jvq. I'd intended to follow it up today but didn't make it. Is the second sensor you mention built in to the sender unit on the thermostat, which apparently has 4 pins, as ePER doesn't show a second sensor in the cooling system?
 

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Discussion Starter #6
Thanks for that R.T - yours seems to be behaving very similarly to mine, so perhaps there isn't a problem after all. I'll be checking via OBD soon.
 

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Thanks for your input, jvq. I'd intended to follow it up today but didn't make it. Is the second sensor you mention built in to the sender unit on the thermostat, which apparently has 4 pins, as ePER doesn't show a second sensor in the cooling system?
The temp sensors are fysically located in one sensor unit, located in the thermostat housing. Though it looks like one sensor, there are actually 2 sensors with separate electrical connections present.

in picture: pin 2 goes to gauge, (sensor 1). Other side connected to ground
pins 1 and 3 go to engine management (sensor 2)
see:
 

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Discussion Starter #9
The temp sensors are fysically located in one sensor unit, located in the thermostat housing. Though it looks like one sensor, there are actually 2 sensors with separate electrical connections present.

in picture: pin 2 goes to gauge, (sensor 1). Other side connected to ground
pins 1 and 3 go to engine management (sensor 2)
see:
Thankyou for confirming my suspicion, and the other info you sent.
I have now found via OBD that the temperature reached whilst driving, with the gauge showing marginally above the mark between 50 and 90 , is 80.8C, and when stood it slowly increases due to no through-flow of cold air (max not recorded).
I now feel there is nothing to worry about, but may carefully experiment with restricting air flow through the rad in order to get quicker warm up, both of engine and in-cab!

THANKS TO ALL FOR YOUR RESPONSES - VERY HELPFUL INDEED!
 

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I just spent a few hours this weekend measuring the resistance vs temperature of my various 156 sender units. The units are all pretty much with 10% of each other on the ohm meter (50 to 100C), but are wildly different when hooked up to the dash gauge. The ECU and dash gauge use a similar NTC thermistor type, with two NTC units housed in a 3-pin sensor. The ECU feeds its NTC with 5V via a 980 OHM resistor, yet the dash gauge feeds in 12V via 100 OHM resistor. Therein lies the design error. The gauge is expecting an old school high power NTC, and the ECU will be okay with a cheaper one. So I guess they just put two of the cheaper ones in the sensor unit. The fix might be to follow the ECU circuit, and linearize the gauge with a little microcontoller, which is exaclty what the fuel gauge does. It seems an oversight not to get the fuel gauge microcontroller to linearize the temperature gauge as well. I feel a little weekend project coming along...
 

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I've read before that the gauge on the 156 is a bit hit-or-miss .... On some cars it's fairly accurate, on others it's off .....
You could also use a OBD gauge to display temp as measured by ECU, which is usually a lot more accurate ;)
 
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