NOTE: This could all be because Im misreading the temperature gauge because it doesnt have real temperature numbers.

Hi all, for a while i was having issues with the radiator fan not working correctly, sometimes it turns on properly when needed and sometimes doesnt at all even when the temperature rises up.
When it doesnt work I shorten the 2 wires that connect to the thermoswitch and the fan starts working so the fan is ok.
I replaced the thermoswitch (82c), flushed the engine, replaced the thermostat and added new coolant but still have the same issue.
The thermostat opens below the green line and the fan usually turns on just above the green line but when it doesnt the temperature goes up to the yellow line and even higher (I live in the middle east with hot climates).
On highway speeds the temperature is steady on the green line and most of the time when I get off the highway the fan doesnt turn on and the temperature reaches the yellow line and even higher before I shorten the thermoswitch wires.

Now, can it be because of the different locations of the temp gauge and the thermoswitch?
The temp gauge is located right after the hot coolent leaves the engine at their hottest temp, and the thermoswitch is located at the bottom of the radiator where the coolant is at its lowest temp.
So maybe the thermoswitch is working properly and the temp gauge shows very different temperature than the thermoswitch?

Again, this could all be because Im misreading the temp gauge and the yellow line is a safe operating temperature, but because I own this car for a couple of years and Im used to the temp being on the green line in normal driving (city and highway) Im a little bit concerned.

Edited 14 June, 202514 Jun by OmerGarabagi

The temperature gauge didn't have figures to save unnecessary worry and concern to driver/owners.

Normally the needle I believe would be at (above) the yellow line, possibly higher in hotter climates, how accurate you gauge is and your coolant thermostat, and fan switch are only you can tell by testing them but if your car was previously running fine wherever the needle pointed that is was normal for your car and gauge. Variation from this normal is to be investigated particularly if the engine is running hot (or cold) regardless of where the gauge needle points.

I don't know if your coolant thermostat opening at the green line is correct or not but I would expect the fan to switch on above the yellow line on a different car to yours but I am not a 1993 Favorit owner or driver (it has been 30+ years since my wife owned a Favorit).

Shortening the two wires may have improved electrical contact, if so it is a good thing.

The 82c thermoswitch has to be correct for your model and year and be a good quality part (many modern made parts are not good quality and some are very poor quality or worse). If I am correct I believe with other Škoda models of that period at least you need to get the correct colour plastic band on the thermoswitch.

Good that you flushed the engine, did you also flush the radiator and if possible or appropriate flush the heater matrix too.

At motorway speeds in other cars the needle would be above the yellow line let alone the green line but if the fan comes in when it should and you have no signs of overheating then I suspect it is as you think and the gauge reads low.

I had a car where the thermostat was almost right next to the thermoswitch and neither agreed with the other about temperature, the level of accuracy and tolerance to acceptable margins could be wide. Plus by the 1990s coolant temperate gauges could be biased to go to a certain showing and stay there regardless of real life fluctuations of actual readings.

Combined (mechanical) oil pressure and coolant temperature gauge for a 1960s British car.

Coolant (electric sender and gauge) for a 1970s British car, markings left off to save drivers' worry (not that the gauges were very accurate anyway).

Edited 14 June, 202514 Jun by nta16
typo

10 hours ago, nta16 said:

The temperature gauge didn't have figures to save unnecessary worry and concern to driver/owners.

Normally the needle I believe would be at (above) the yellow line, possibly higher in hotter climates, how accurate you gauge is and your coolant thermostat, and fan switch are only you can tell by testing them but if your car was previously running fine wherever the needle pointed that is was normal for your car and gauge. Variation from this normal is to be investigated particularly if the engine is running hot (or cold) regardless of where the gauge needle points.

I don't know if your coolant thermostat opening at the green line is correct or not but I would expect the fan to switch on above the yellow line on a different car to yours but I am not a 1993 Favorit owner or driver (it has been 30+ years since my wife owned a Favorit).

Shortening the two wires may have improved electrical contact, if so it is a good thing.

The 82c thermoswitch has to be correct for your model and year and be a good quality part (many modern made parts are not good quality and some are very poor quality or worse). If I am correct I believe with other Škoda models of that period at least you need to get the correct colour plastic band on the thermoswitch.

Good that you flushed the engine, did you also flush the radiator and if possible or appropriate flush the heater matrix too.

At motorway speeds in other cars the needle would be above the yellow line let alone the green line but if the fan comes in when it should and you have no signs of overheating then I suspect it is as you think and the gauge reads low.

I had a car where the thermostat was almost right next to the thermoswitch and neither agreed with the other about temperature, the level of accuracy and tolerance to acceptable margins could be wide. Plus by the 1990s coolant temperate gauges could be biased to go to a certain showing and stay there regardless of real life fluctuations of actual readings.

Combined (mechanical) oil pressure and coolant temperature gauge for a 1960s British car.

Coolant (electric sender and gauge) for a 1970s British car, markings left off to save drivers' worry (not that the gauges were very accurate anyway).

Thank you for replying.
I drove this car almost 10000km and most of the time in normal driving (highway and city) the needle would stay at the green line and the fan would turn on just above it so I consider it as normal operating temperature.
Maybe I should try testing the temp sensor with boiling water to see how accurate it is and where is the needle pointing at a specific water temperature.
I did flush the radiator but not the heater core, the water that came out of the engine and radiator did look quite good so I guess the heater core is fine.
After installing the new thermoswitch I tested it at home by letting the car run (when parked, didnt drove it) and seeing when the fan will kick on, and it did between the green and yellow line as it should, but maybe because the car was stationary and the radiator wasnt being cooled by moving air the temp gauge and the thermoswitch were reading the same temperature (because the hot coolant wasnt being cooled in the radiator).

I drove it a few hours later and when I got off the highway the temp gauge showed that the temperature was rising (as expected, to the yellow line) and the fan didnt start working even until a arrived to my destination a few minutes later, I suspect its because the radiator was still "cold" after the highway driving and was still cooling the coolent inside it, so coolent temperature at the thermoswitch was colder than temp at the temp gauge and thats why the fan stayed off.
Maybe I should let the temperature to go even higher above the yellow line to see if the fan starts working because every time I would shorten the 2 wires of the thermoswitch because I didnt want the engine to overheat.
So basically I need to test the temp gauge and let the coolent to got hotter to see if the fan turns on at high temperatures.

Edited 15 June, 202515 Jun by OmerGarabagi

Maybe I misunderstand what you mean by shorten the two wires, I thought you meant you were cutting the wires back to get to clean solid cores to make good electrical connection to the connector (rather than any temperature or other adjustment).

By all means test the sensor at boiling water it will confirm at one temperature point at least how this translates to the gauge is a different matter. Switches tend to be faster reaction than the old metal thermostats and the old metal thermostats are built with an allowable tolerance of range, so one 82C old metal thermostat may begin to open and close and be fully open and closed at slightly different temperatures to another 82C old metal thermostat. The rating on a metal thermostat is a nominal temperature at which it starts to open (not when fully open). The sensor will also have a range of tolerance, much less usually than the old metal thermostats. So don't expect them to be too close in temperature, plus where they are located and surroundings make a difference.

I should have also asked if you flushed the coolant system and then put a coolant system cleaner in, drained that out and flushed, back-flushed and flushed again, the radiator, engine block and heater matrix. Any internal coolant system debris needs to be loosened off and fully flushed out to stop in going around the coolant system and setting elsewhere and perhaps potentially partially or fully blocking an area. Some times the water comes out cleanish because it's just running past the debris, hence the cleaning and back-flushing. Clearing out the engine block and heater matrix are very often overlooked because of concentrating on the radiator. As well as internally the radiator needs to be clear externally, gentle brushing to remove any external debris and the radiator need a clear path for the fresh air to get to it, even putting badges, plaques, spotlights, etc. in the way can cause restrictions and lessen the cooling to the area.

Even if the car is stationary with the engine running the radiator is helping with the cooling by dispersing some heat, obviously the lack of air movement from the car moving does mean there is less cooling effect but the water pump is circulating the coolant around the system and the electric fan will switch on when required to provide air movement.

If you are in a hot country you may want wider margins for cooling the engine rather than lessening the margins by delaying the operation of the fan - what exactly do you mean by shorten the two wires?

Oil is also an engine coolant and in hotter (and/or colder) weather temperature countries a better quality (usually but not always fully synthetic) engine oil (and gearbox oil) can give greater protection by wider margins and longer lasting margins.

I am not sure why you need to worry too much about your gauge you are used to its readings so know when the engine coolant temperature at its usual running reading. Nice to have it more accurate but perhaps it is just the connections between the sensor and gauge that just needs the wires and connections cleaning and perhaps securing.

IIRC there are threads and posts here that cover the coolant temperature gauge working.

I see you have been here before, sorry I forget, so I might have covered engine oil before (do I now remember yours was your dad's car(?)).

Here is a thread on wrong sensor showing low gauge readings which might also be the resolve to your low readings or on the same theme at least and there are other threads and posts about which colour ring on the sensor for different years (models?) too which you can search for.

Hope this helps.

On 14/06/2025 at 12:20, OmerGarabagi said:

The thermostat opens below the green line and the fan usually turns on just above the green line but when it doesnt the temperature goes up to the yellow line and even higher (I live in the middle east with hot climates).
On highway speeds the temperature is steady on the green line and most of the time when I get off the highway the fan doesnt turn on and the temperature reaches the yellow line and even higher before I shorten the thermoswitch wires.

What thermostat do you have? With a 90°C thermostat, the gauge should be around the yellow line. If you have a 80°C thermostat, which was common in warmer climates, the observed behaviour would be correct. The engine heats up by around 5-10° before hot water reaches the thermoswitch.

If it reliably turns on bellow the red line, it should be fine. If it's still unreliable, the cause could be gas bubbles caused by HG leak.

On 14/06/2025 at 13:20, OmerGarabagi said:

can it be because of the different locations of the temp gauge and the thermoswitch?

Different position and different tasks.

The gauge on your panel takes signal from the thermostat switch and the radiator switch gives commands to the radiator fan.

A dirty or with corrosion in pins or with broken plug-cables or faulty thermostat switch will give wrong commands to your gauge and the needle will make you crazy with it's indications.

A faulty radiator switch will not give proper commands to your fan or can make it not start at all.

You should remove properly any air bubbles from your system,i live in Greece,not so hot as in the Middle east but we have similar problems.

On 16/06/2025 at 19:23, D.FYLAKTOS said:

Different position and different tasks.

The gauge on your panel takes signal from the thermostat switch and the radiator switch gives commands to the radiator fan.

A dirty or with corrosion in pins or with broken plug-cables or faulty thermostat switch will give wrong commands to your gauge and the needle will make you crazy with it's indications.

A faulty radiator switch will not give proper commands to your fan or can make it not start at all.

You should remove properly any air bubbles from your system,i live in Greece,not so hot as in the Middle east but we have similar problems.

I replaced the thermoswitch a few months ago, and sometimes it works and sometimes doesnt (maybe I bought a faulty one from ebay).

I even bought a laser thermometer and tested it when the temp guage was in the middle (yellow line in the first message in the post) and at the temp sensor it was about 105-109c and at the thermoswitch it was about 88-90c which i think proves even the new one is faulty.

Eventually I installed a manual switch for the fan inside the cabin (the yellow thing) which is a perfect fit instead of the plug where the cigarette lighter should be that the original owner didnt spec

1 hour ago, OmerGarabagi said:

I replaced the thermoswitch a few months ago, and sometimes it works and sometimes doesnt (maybe I bought a faulty one from ebay).

I even bought a laser thermometer and tested it when the temp guage was in the middle (yellow line in the first message in the post) and at the temp sensor it was about 105-109c and at the thermoswitch it was about 88-90c which i think proves even the new one is faulty.

Eventually I installed a manual switch for the fan inside the cabin (the yellow thing) which is a perfect fit instead of the plug where the cigarette lighter should be that the original owner didnt spec

When the temperature of the fan cut-in point is inconsistent, especially at IDLE it's a 100% certain sign head gasket is starting to fail!

Bubbles in the coolant will cause that, if you slightly elevate the idling speed, the cut-in points will become more consistent!

Replace the HEAD GASKET now, along with checking for liner protrusion before its too late.

It is possible you bought an unreliable new car part but it might also be a good functioning part Did you check (and if required clean) all the wiring connectors and wiring for all sensors, switches, the gauge, earth and power points and all other connections including earthing points. Check that all wiring in good condition along the wires and at connectors and that the wiring is secured and grommeted as required.

Another test you could try is to bypass the radiator switch by making up a suitable connector wire (and ensure it is secure and doesn't swing or flap around) that way the fan should remain (with ignition) and drive like this and see if the fan cuts out at any time of driving with the conditions from driving (heat, bumps, vibrations, etc.) that should help decide if the new part might be faulty. It all may depend on how you have wired the manual switch.

ETA: the photo shows what looks like a speaker cable exposed and unsecured at that area at least, if this is a permanent thing then this is not the way to do any wiring and whilst it does not really matter on things like a radio if you have such in the engine bay it can matter.

Edited 5 December, 20255 Dec by nta16
ETA:

18 minutes ago, nta16 said:

It is possible you bought an unreliable new car part but it might also be a good functioning part Did you check (and if required clean) all the wiring connectors and wiring for all sensors, switches, the gauge, earth and power points and all other connections including earthing points. Check that all wiring in good condition along the wires and at connectors and that the wiring is secured and grommeted as required.

Another test you could try is to bypass the radiator switch by making up a suitable connector wire (and ensure it is secure and doesn't swing or flap around) that way the fan should remain (with ignition) and drive like this and see if the fan cuts out at any time of driving with the conditions from driving (heat, bumps, vibrations, etc.) that should help decide if the new part might be faulty. It all may depend on how you have wired the manual switch.

ETA: the photo shows what looks like a speaker cable exposed and unsecured at that area at least, if this is a permanent thing then this is not the way to do any wiring and whilst it does not really matter on things like a radio if you have such in the engine bay it can matter.

Now that I have installed this button that goes parallel to the switch I can truly check if its the sensor or the wires because the button is connected to the wires going to the thermoswitch (these are not speaker wires they are thicker)

Also my electrical work isnt the best but it is safe, I also used A heavy duty 12v switch from old military jeeps)

I still have the old sensor, maybe I will try to replace it back (even though it was also problematic)

Please perform a BTB test!!!!!

I must admit I didn't think of, or realise, that red, black wire(s) was from the fan swich but thought it was a speaker cable from the radio, even worse that it is just hanging under the steering column.

It is up to you what you do with your car and how you test it, and sorting intermittent issues is usually more difficult, but I would remove the fan switch connections and bypass the radiator fan switch joining together the original wires and drive as I put before. Hopefully the fan switch and wiring were a temporary installation so easy to disconnect and return to factory.

34 minutes ago, nta16 said:

I must admit I didn't think of, or realise, that red, black wire(s) was from the fan swich but thought it was a speaker cable from the radio, even worse that it is just hanging under the steering column.

It is up to you what you do with your car and how you test it, and sorting intermittent issues is usually more difficult, but I would remove the fan switch connections and bypass the radiator fan switch joining together the original wires and drive as I put before. Hopefully the fan switch and wiring were a temporary installation so easy to disconnect and return to factory.

They are not just hanging under the dash, its a picture I took right after I installed the switch and before I did any cable management to the engine bay so the cables were not attached to anything and were just sitting in the footwell

37 minutes ago, Thefeliciahacker said:

Please perform a BTB test!!!!!

What is BTB?

59 minutes ago, OmerGarabagi said:

What is BTB?

bromothymol blue

ETA: I missed the reply to BTB? but below still stands.

56 minutes ago, OmerGarabagi said:

They are not just hanging under the dash, its a picture I took right after I installed the switch and before I did any cable management to the engine bay so the cables were not attached to anything and were just sitting in the footwell

Fair enough, and the connections and connectors at the least look untidy but if they are fully secure and sufficiently protected then it just looks untidy.

59 minutes ago, OmerGarabagi said:

What is BTB?

I forget what it stands for but it is a test for head gasket failure, sometimes called a "sniffer test" IIRC. Many use leak tester kits, which IIRC could possibly give(?) a false negative(?) but a good easy test if it is positive, or you could do a compression test across all the cylinders.

One example of a test kit. - https://www.ebay.co.uk/itm/396859738655

Edited 5 December, 20255 Dec by nta16
ETA:

1 hour ago, nta16 said:

but a good easy test if it is positive, or you could do a compression test across all the cylinders.

I will someday write about the OHV HGF and all the subsequent tests and their issues limitations etc

13 hours ago, OmerGarabagi said:

What is BTB?

Here is how you can do the test.

https://www.briskoda.net/forums/topic/511704-glitters-inside-my-coolant-expansion-tank/page/7/