Jetta 2 CSX 1988 model. Temperature gauge indicate normal but from time to time drop to the off position just to wake up some time later. Allready replaced the sensor on the engine without any luck.

An intermittent problem like this is usually caused by a wiring/connector problem, probably at the back of or near the instrument cluster (IC). The IC performance should not cause engine problems - it is mainly a information readout device, but does have a datalink connection to the ECM and if there is a wiring problem with the datalink, you will get the intermittent.

This may have started with the way your aftermarket sound system was wired in and the physical impact it may have on wiring nearby. You may have a damaged cluster, but the only thing I can suggest at this point is to carefully check your wiring in the dash, particularly how you have wired in the power feeds to the radio.

If your temp gauge has only the cold (C) or hot (H) letters then the operating range would be the width of the needle under 1/2 way between c and h. If it remains steady at slightly over 1/2 way that would be just the setting of the gauge. How to get to know where the needle would sit at a specific position in relation to water temp. Do the following
Remove the temp sender unit and attach an earth wire to the body of the unit to earth. Place the sender unit in a container of water up to the threads .Put a mercury thermometer in the container beside the sender unit Apply heat to the container and watch the gauge needle move in relation to the thermometer reading. Operating temp will be about 80 degrees C on the thermometer and that would be about 1/2 way on the gauge.

Sounds normal. If the gauge moved further over, it would indicate an overheating problem, wouldn't it? Check that engine is at normal operating temperature after it warms up. If it is running too cold, your thermostat might be stuck open.

This is normal. The gauge should sit at around the 1/2 mark. And it will go up at idle until the fans kick on then go back down some. Fans will kick on around 210 degress

hi.
sounds like something is out of alignment . check the position and and adjustment of the linkages again, and make sure there is no play on any ball joints.

I think the light you are seeing is for low coolant. There may have been a small air pocket in the system. So just check the coolant level. If it is low add some. It's not uncommon to add a small amount a day or two after work was done on the coolant system.

you should switch the temperature sensor and oil pressure sensor you had on your first engine,because your new engine must come with the light indicator dashboard so the sensor of your new engine are not compatible with your gauges....lipak!...

change the coolant temperature sensor.

The easiest way is to see a Opel dealer and read out fault codes with Tec II.

sounds like you need to replace the thermostat.