Hi all, got my 1991 4V K1 running today for there first time since I started the restoration over 12 months ago. When I bought it I suspected the fan was not running at temp and sure enough it doesn't. I did run the temp up to about a needle width shy of the red zone before I chickened out and went for a run up the street to cool it down. I have searched and read heaps here. I have fed power to the connector under the seat and the fan runs. I have been fastidious in the cleaning of all connectors and the temperature sensor below the radiator was cleaned including the thread until it looked like a new one. New washer too. Advice on checking the relay is to ground pin 16 of the Motronic. Fine, but which is pin 16? I have searched and searched for diagrams but can't find it and obviously don't want to guess... Can someone please post a pic of the Motronic cover identifying the pin sequence. Once this is sorted I can look at the sensor if needed. Thanks very much.

don't forget that some of these things have 2 power supplies fed to the fan controller  (well on the older non motronic systems any how )

not sure about the switching on the 16 valve jobs

good luck

Thanks Inge, once again you have come to the fore with much knowledge. I will investigate and report progress

I have jumped the Motronic and the fan runs strong. I have also swapped over the fan relay with the fuel pump relay and the fuel pump still works so no issue there. As both the fan and dash temperature light did not come on during the road test I think the next stage is to pull the temp sensor below the radiator. Even though I am confident all the connections are solid, I will check and clean again to be sure. Inge, I know you said that based on these tests so far the Motronic is toast, but is there not a possibility the temp sensor is at fault?

is there a chance you could probe the motronic connector whilst attached to the motronic - on the temp sensor to see if it does indeed change values  whilst running ?

most of the referred tests are not dynamic but passive ..ie should measure x ohms at x temp

I'm sure that that converts to volts whilst active on the computer and running

just  thought '

good luck

Before taking things apart here's what I'd do:

Start the bike and let it idle.

If it overheats then the warning lamp in the cluster should come on.

If something is amiss and the engine gets hot and the warning lamp doesn't come on then the radiator cap will blow. (Just like a car.) If that happens then shut the bike down.

This will not hurt the engine.  That's why the radiator cap is there - as a mechanical safeguard if the electronics fail.

Duck, I guess I could try running it up to a higher temperature, as it may not have reached fan cut in temperature although to be honest my other K's all cut in well below the red. I don't mind taking it apart and re checking everything and will do some static tests on the sensor. I may well replace the sensor anyway as they are not an expensive item. The Motronic is able to send a signal to the fan via the relay and activate the fan. Issue seems to be that no signal is being received from the sensor to tell the Motronic what to do. Combined with all this, when it is hot the motor is reluctant to hold a steady idle and a blip of the throttle will result in a stall. The sensor on 4V motors, as I understand it, also affects the fuelling as it warms so a couple of indicators that there is something amiss with the sensor or a connection. The coolant stub was removed during the refurb and a new O ring and stainless screws fitted.

KiwiK100 wrote:Inge, I know you said that based on these tests so far the Motronic is toast, but is there not a possibility the temp sensor is at fault?

Given the descriptions of test results on the link that Inge supplied—and from your own tests—wouldn't an effective strategy at the moment be temporarily substituting a working motronic from one of your cohort's bikes to determine if that remedies the condition?

Laitch, I do not know anyone else with a 4V motor. My K100 is an '87 and does not have Motronic. I have pulled the radiator this evening and removed the sensor which I will test when time allows. Inge is suggesting that the Motronic is toast and I fear he is probably right but I will test the sensor anyway. There is continuity from the sensor plug to the Motronic, and when the Motronic is jumped the fan runs so wiring seems to be all good. When I took it for a brief ride down the street over the weekend I noticed that when the engine was hot it would not idle and stalled easily, but was fine when cold. The temperature sensor also feeds info to the fuelling so could this idle issue indicate a faulty sensor?

KiwiK100 wrote:Inge is suggesting that the Motronic is toast and I fear he is probably right but

The Motronic in the link is toast since the warning light still funtion as it should,
as I understand it you haven't tested if your warning lamp get any signal from
the Motronic...yet.

Inge K. wrote:

KiwiK100 wrote:Inge is suggesting that the Motronic is toast and I fear he is probably right but

The Motronic in the link is toast since the warning light still funtion as it should,
as I understand it you haven't tested if your warning lamp get any signal from
the Motronic...yet.

I tested again to be sure. The warning light does not come on when grounding pin 16. Of course the bulb may have blown so I will check that out this evening when I conduct a resistance check on the sensor

KiwiK100 wrote: I tested again to be sure. The warning light does not come on when grounding pin 16.

By grounding #16 in the connector you are testing the signal path to the fan relay.
To test the warning lamp and its related wiring, ground #20.

I tested the resistance on the sensor and it is open circuit. Toast! So a new one will be ordered directly and hopefully this will rectify the problem. I also checked the circuit to the fuel warning lamp by grounding pin 20 and all was well. Hopefully this is where is thread ends.... 
Thanks for everybody's help.

KiwiK100 wrote: Hopefully this is where is thread ends.... 

I hope by that post you mean you'll post the results of your future remedies for this condition in this thread, too.

Laitch wrote:

KiwiK100 wrote: Hopefully this is where is thread ends.... 

I hope by that post you mean you'll post the results of your future remedies for this condition in this thread, too.

That's a given Laitch. I have every intention of posting the results. New sensor is on the way from the good folks at Motobins and couldn't resist the opportunity to put a few other bits in the order too.

New sensor arrived today and I was straight into it after work. I set up both old and new sensors over a pot of warm water and started to take some readings comparing them to the graph found on the forum - thanks CF. The new sensor was spot on the money every time, but the old sensor was out by miles. At the critical end of the scale, at 100ºC  the new sensor was reading 215 ohms while the old sensor was reading 512. Clearly the old sensor was probably never going to get to a low enough resistance (175 ohms) to start the fans. At the other end of the scale, at 1.7ºC the new sensor was 5640 ohms which is correct while the old sensor read a massive 12420 ohms. 
This weekend it will be new sensor back in, a thorough check (again) of all the connections, everything back together and a test run to be sure all is working before starting to fit up all the freshly painted panels. All 18 of them! Will post results as they happen

Well Friday night is officially the weekend, so I decided to forego the Friday night drinks with my mates and get amongst it in the garage. Fitted up the new sensor, put the radiator back in and topped up with coolant. Fired her up and put a stopwatch on to see how long it would take, if and when the fan kicked in. Ten minutes, 30 seconds was all it took when that wonderful sound of the fan doing its business kicked in. Temperature gauge was sitting half way between the mid mark and the 3/4 mark, and cooled down to about the same spot on the low side of the mid mark before shutting off. I'm very happy and off for a couple of well earned beers! Thanks for the advice from all that contributed. Luckily this ended well for the Motronic.