[Zhagar]

Hello dear alfa users.

I wish to ask for your help about my beloved vehicle. I have an issue for quite some time (because I haven't found a way to fix it yet) and it's about the temperature in the car. It's an Alfa 156 1.9 JTD (8V) 2002. So my problem is as followed:

I don't get any reading of temperature on my temp gauge BUT my cooling is working corectly (i've measured it manually and it activates @ proper degrees).

These things have been exchanged: 1) termostat, 2) temp gauge itself (which did nothing), 3) temp sensor (the black thingy below the stat). I don't know what else to check? Do you have any ideas? Is there a electronic issue (cables, fuses, etc.)?

Another funny thing - the part of the temp gauge that is working is the RED light when the temp goes ''over the edge'' (also tested manually when disconnecting the temperature sensor). Also, when I plug the car on the OBD there are no anomalies. I honestly don't know what to do.

Thank you for all the anwsers/ideas in advance!

 

[dgmayhead]

Hmm. Not much else to investigate! I'll get the blue sky thinking going. Try reversing the polarity from the sender. Maybe the gauge is wired backwards.

 

[Zhagar]

Hmm. Not much else to investigate! I'll get the blue sky thinking going. Try reversing the polarity from the sender. Maybe the gauge is wired backwards.

The lighting and everything works and I've had this gauge from the start of the purchase. It just happened one day and the temp gauge never moved since. Some mate told me I should check the connectors (on the gauge).

 

[dgmayhead]

You could measure continuity to the gauge from the sender. This could help you understand how it works.
My understanding is the sender is a thermister device which receives a low current. The resistance of the sender changes with temperature and this changes the current. Changes in current move the gauge. Thus the temp gauge is effectively operating as an ammeter. The led will receive current separately so this will not indicate it's receiving a signal from the sender. On old fords and triumphs disconnecting the sender (stopping the current) would lead the gauge to fail safe and read high. Obviously Alfa like to do things differently but unless my logic is floored measuring the current or resistance from gauge to sender will help understanding.
Hope the rambling helps.

 

[jwq]

The fact that the warning light and fans work is normal, as 2 temp senders are used. One dedicated to temperature guage, and one for the ECU.....The ecu controls fans etc. In your case I guess there's a problem with the one for the guage. Either the sender, connections or gauge itsself.....
Depending on the car, it can be either a combined temp sender, mounted in the thermostat housing, whereby 2 senders are integrated in one sensor housing (can be seen by looking at number of electrical connections in the plug to the sender), or more rare, 2 completely seperate senders.

 

[Zhagar]

The fact that the warning light and fans work is normal, as 2 temp senders are used. One dedicated to temperature guage, and one for the ECU.....The ecu controls fans etc. In your case I guess there's a problem with the one for the guage. Either the sender, connections or gauge itsself.....
Depending on the car, it can be either a combined temp sender, mounted in the thermostat housing, whereby 2 senders are integrated in one sensor housing (can be seen by looking at number of electrical connections in the plug to the sender), or more rare, 2 completely seperate senders.

Hmm I've heard it before that 156 has a second temp sender but never really found it. Can you please tell me where it is located? One is below the termostat, that one is easy to locate, where would the second one be?

 

[jwq]

Hmm I've heard it before that 156 has a second temp sender but never really found it. Can you please tell me where it is located? One is below the termostat, that one is easy to locate, where would the second one be?

It's most likely the one below the thermostat.... It looks like 1 sender, but it actually contains 2 seperate sensors, with seperate electrical connections if you look at the electrical connection plug......
It could be that 1 of the sensors is defective (or someone at some point installed a wrong (single) sender ? .... )

from elearn:
The engine coolant temperature sensor and the combined sender unit for the temperature gauge and engine coolant overheating warning light are fitted on the thermostat and connected to the injection control unit ( See subassembly 1060 DIESEL FUEL INJECTION ).

 

[Henrik_45]

I have uploaded a wiring diagram for the temperature sensor unit in post #33 in the attached thread.

http://www.alfaowner.com/Forum/alfa-147-156-andamp-gt/195005-cabin-temperature-sensor-2.html

 

[Zhagar]

I will look into this - I will also post picture with this sender on them. Thank you so far!

 

[jwq]

I will look into this - I will also post picture with this sender on them. Thank you so far!

also check if the number of pins in the sender part corresponds to the number of wires in the connection plug to the sender......

 

[dgmayhead]

Last thermostat I bought had a different plug on the sender so I had to unscrew the old one frm the broken stat and put it in the new thermostat.
However, you say it was working but then stopped so the plug is likely to be correct. You have also bought a new gauge. I would have investigated this option last as it can't have been easy getting that out of the dash.
Anyway, a multimeter and a little logical thinking is what you'll be needing.

 

[Zhagar]

This is what my sensor looks like. Also, the gauge stopped working before I exchanged the sensor. I thought buying a new sensor would fix the thing, but it did nothing.

 

[Zhagar]

Last thermostat I bought had a different plug on the sender so I had to unscrew the old one frm the broken stat and put it in the new thermostat.
However, you say it was working but then stopped so the plug is likely to be correct. You have also bought a new gauge. I would have investigated this option last as it can't have been easy getting that out of the dash.
Anyway, a multimeter and a little logical thinking is what you'll be needing.

I only plugged in a different gauge hopefully diagnosing that the gauge itself was the problem. But on the different gauge there was no movement (and the engine was heated), so I plugged my old gauges inside the dash.

 

[Henrik_45]

I assume that you have three or four wires in the plug to the temperature sensor.
If you unplug the connector and measure the voltage from pin 1 to chassis (Ground) if you measure a voltage you can see if the connection from the gauge to the temperature sensor is ok.
If you don't measure any voltage the connection to the gauge is broken.
If you do measure a voltage on the pin 1 wire, then try to measure the resistance between pin 1 on the sensor to chassis. If I recall correct you should measure a resistance between 220 to 2200ohm depending on the water temperature (lower ohmic resistance at high temperature)

 

[dgmayhead]

I assume that you have three or four wires in the plug to the temperature sensor.
If you unplug the connector and measure the voltage from pin 1 to chassis (Ground) if you measure a voltage you can see if the connection from the gauge to the temperature sensor is ok.
If you don't measure any voltage the connection to the gauge is broken.
If you do measure a voltage on the pin 1 wire, then try to measure the resistance between pin 1 on the sensor to chassis. If I recall correct you should measure a resistance between 220 to 2200ohm depending on the water temperature (lower ohmic resistance at high temperature)

Yes, this is what i was trying to communicate unsuccessfully! However, i think low resistance at low temperature. This way unplugged gauges always fail safe and read hot.

The sensor needs 12V to it. Voltage=current x resisitance therefore when hot resistance is large and current is small and when cold current is large and resistance is small. It's this change that operates the gauge. Although I'd guess the gauge also needs power. The fact it has a working LED indicates it probably does.

I also would attempt to measure voltage across the sensor in addition to checking continuity to ground while the ignition is on and resistance directly through the sensor with the plug disconnected.

I would expect that shorting the plug should give low resistance and move the gauge to cold. A broken circuit will leave the gauge reading high.

Anyway I'm confusing myself now so might have a play at the weekend just to confirm that when the sensor is unplugged the gauge reads hot. If this is the case you may have a short circuit to gauges monitoring circuit which is giving a low resistance value and thus making it read cold.

 

[jwq]

Yes, this is what i was trying to communicate unsuccessfully! However, i think low resistance at low temperature. This way unplugged gauges always fail safe and read hot.
......

It's an NTC, ie negative temperature coeficient. So high resistance at low temperatures.
Example:

 

[jwq]

For what it's worth, this is the elearn procedure for faulty temp gauge :

 

[Zhagar]

Just checked earlier. It has three wires. What both of you are saying makes super sense to me and I thank you from the bottom of my heart for your effort. Shame I don't have the skills to do what you're saying. I am willing to learn though, I just don't know how to use the multimeter and where to put. I'm a total and complete idiot when it comes to electricity.

I remember another detail on how this started. Before the gauge died totaly, it had periods of suddently dropping to the bottom and then raising back to normal temp. It did that a couple of times mid-drive for about a couple months. Then one day it died completely.

@dgmayhead - If you unplug the sensor, the gauge goes bat**** crazy to the end and turns on its red light.

How does one fix a short circuit - what would be the cause of it?

 

[dgmayhead]

Ok. Unplugging the sensor makes the gauge move. This indicates the wiring to the gauge from the sensor is probably good and the gauge has power. Good news.
Multimeters are around £10 and can do lots of things. I wouldn't recommend playing with domestic mains but it's difficult to cause harm with car electrics.
Initially you can use your multimeter to just confirm if you have a complete circuit by touching either side of the sender circuit. All the other settings are really just an extension of this ability.

 

[dgmayhead]

I know the sender is new but the symptoms increasingly indicate it is providing too low resistance. Therefore, it is either the incorrect part, broken or has a faulty plug.
I'm not sure if it can be bought as a separate part new and the thermostat unit is a rip off and reasonably awkward to fit. I'd get a scrap yard sensor, plug it in but don't screw it in until you have put it in your cup of tea and seen what the gauge says.