Overheating convertible. Urgent help please!!

I don't get it…
The car runs to hot, the fan will not engage.

I checked the fan motor by bridging the wires from the thermocontact at the radiator.
No problem it runs just fine then.

So i assumed the thermocontact at the bottom of the radiator was broken. Bought a new one and installed it. Fan still not engaging…

I checked for the ground by testing with a multimeter by "shorting" between the ground on the back right off the cylinder head right up to the ground pin on the fan. It's a perfect ground.

This is a '85 convertible 1.8 with a single speed fan, so to my knowledge there is also no relay in between?

This should be reasonably straightforward no? Power up to the thermocontact, thermocontact gives power, fan enables?

What am i overlooking here?

Do you have 12V from one side of the Thermoswitch to the Battery?

Is there a good Ground on the pin of the Motor (brown wire) to frame?

Is there continuity from the other side of the thermoswitch to the other pin on the motor?

If you don't have 12V Battery on one side of the switch, the that is possibly your issue.

If no 12V then check your fuses F1

You can run a 15 amp fused wire from the battery to the radiator fan.

Your Radiator Fan switch should be rated the same as your t-stat.

Switches go bad, have you removed your thermoswitch, place a dvom reading OHMS on the 2 pins of the switch then heat the switch with a ciggie lighter to hear it click and read 0 ohms.

You also may want to flush and back flush your cooling system.

Briano1234  what would the community begin without your speedy and clear help!! Many thanks again!

I test the ground with the beep funct of my meter. It's a very clear signal, whether that's good enough i don't know.
When i bridge the wires the motor runs good so i guess the ground must be ok?

 the cable to switch gives 13v so that should be fine too

I did an ohm reading on the old switch with a cigarette lighter as you told. But no click and ohm reading stays the same.
So i guess the old one is really broken.

About the new one, I don't want to remove it as it is all new coolant and don't want to replace for the third time..

By I guess no choice here. Waiting for the coolant to cool down and will remove the new switch to do the same test.

I "hope" it's broken, otherwise I don't have a clue about what's going on…

Ah just another quick remark. The temp gauge goes all the way up (as normal) but I thought that also the LED should start blinking when to hot. And that's not the case.

I had a temp meter in the expansion barrel and measured until the water was about 96 degrees Celsius.  The water in the barrel started cooking out of it.
Then i shut down the engine.

Things are getting a bit fussy, I must be overlooking something.

Tested the 2 switches in almost boiling water with a digital thermostat.
The new one opens at about 96C Celcius
And the old one opens at about 94C Celcius… So it's not broken after all.

But then why is the fan not initiating. Is the water not hot enough? Am i too precocious maybe?

Problem is that i cannot meassure the temp when the water circuit is closed. All I see is the temp gauge rising above half.

And i know that is not the case in my other two mk1's. There the needle goes to the middle and stays there.

This car has no MFA so i cannot check oil temp for example

Is there an other ways of telling the engine is going to hot?

Is it normal the water boils over when you leave the cap off the expansion barrel?

Well yes and no to the expansion tank.  If it is over filled then when the coolant gets hot, then it can "boil out".

When was the last time that you changed your t-stat?  Always test them as well.

You have an option for a couple of things.

On my Earlier Rabbit 81.  The Fan was directly Tied through a fusible link to the radiator switch, then from the switch to the power side of the fan.  the other side was tied to ground. You can do the same thing by wiring it directly to the Positive cable and use a in-line weather proof fuse.  Solder and heat shrink all connections.  You would just splice into the power lead of your radiator fan.

Have you replaced the Main earth, from the battery to the frame and the frame to the engine?  If still OEM I would swap that out in a heartbeat as this will resolve a bunch of flakiness of your electrics.

Did you look at your fuses?  

I drain the AF, in a tin foil disposable container (large aluminum roasting pan).  Then I strain it through a fine mesh strainer and re-use it if it is less than a year or two old.  I rinse out the old a/f jug and fill it from the pan through the strainer and a funnel.

It is not a problem for the temperature guage to indicate above half way. That was normal for my Scirocco. The reason the system is pressurised is to raise the boiling point, so running up to temperature with the expansion cap off is not advisable.

A sure fire way to test the system is functional is to leave the engine idling for half an hour. Without the movement of air over the radiator, once the whole system is fully hot, the fan will eventually cycle on and off every few minutes to stop the engine overheating. To speed up the test, hold the throttle at around 2,000 rpm while the car is stationary

Ok, it's obvious… I've been to quick with making my assumption the engine is overheating and having no fan.

I took it for a longer drive and got it good on temperature and all looks good in the end.

On my cabby and gti the temp indicator stays nice in the middle. This one rises over the middle and goes up and down a bit (more precies actually) so that got me panicking. Sorry for all the fuss! But actually learned a few neat things again. So thanks for the much appreciated help!!

The issue with the older mk1's was that as you drive the car (If you have the Radiator Cars affixed Top one is most important.)  the air flow over the matrix keeps the car at normal temps,  After tooling around or getting to a stop the airflow isn't over the radiator so the temp spikes, and will usually turn on the Radiator fan at stop.  This could be what you are seeing.