K75S Fan Stays On and on....

[moto949]

The otherwise pretty nice 1993 K75S ABS (25k miles) I bought recently has its fan running constantly, from start up to shut down. I rode it 700 miles home after a long distance craigslist purchase, and since then about 300 miles of short rides, and the fan always runs. I worry that the common problem of a burnt out fan will be my next problem. Now, I did purchase a coolant temp sensor and I understood this to be the cure, but before I dig into this hassle of a job (removing fairing, etc) I recently heard the cause could be a Temp sensing switch unit (in the fuse box area), and since this is easy to swap out, I wonder if anyone has experience with fixing this fan-never-off issue. I'll mention that the bike runs fine hot or cold otherwise, no stumbling at all. So is the fix the one minute $60 plug in switch or the use-up-my-Saturday-afternoon temp sensor? Thanks folks!

[K1300S]

it is actually easier to replace the temp sensor than the switch...

taking the lower fairing off is a 30 second evolution.  two bolts on bottom and slide off.

getting at the switch involves removing the airbox. 

[Laitch]

The otherwise pretty nice 1993 K75S ABS (25k miles) I bought recently has its fan running constantly, from start up to shut down. I rode it 700 miles home after a long distance craigslist purchase, and since then about 300 miles of short rides, and the fan always runs.

Did you check your coolant level?

[Inge K.]

it is actually easier to replace the temp sensor than the switch...

taking the lower fairing off is a 30 second evolution.  two bolts on bottom and slide off.

It's not as easy as that, the sensor is located in the standpipe behind the radiator.

getting at the switch involves removing the airbox.

The fan relay is inside the relay box, you need to lift the fuel tank in the rear end and remove the cover/lid.

[Inge K.]

A popular mod is to add a switch to override the relay for the fan......maybe the PO forgot to tell you.
A phone call could save you some ............

[Scott_]

A popular mod is to add a switch to override the relay for the fan......maybe the PO forgot to tell you.
A phone call could save you some ............

+1, that was my 1st thought, manual fan switch.

[moto949]

Thanks for the replies.
    Re: Coolant -- thanks, but the level looks okay. BTW, would low-level trigger the fan instantly, even on cold start-ups? Not sure if BMW built this in.
    Re: extra fan toggle switch. I'm hoping a new sensor or factory fuse box switch should solve this issue. Plus I don't have a temp gauge to help remind me to throw the switch. But I'll save your suggestion as a last resort, thanks. The PO (who I never met since his friend handled the sale) was not even a shade tree mechanic and had it serviced only by dealers (which won't touch any BMW from the 20th century any more). I figure this fan problem cropped up as the bike sat unridden much these last few years.
    BTW I gotta agree with Inge K about the involved sensor replacement - it looks all tucked up in there, facing inward and requiring draining the system (I know, it probably needs it anyway).
    What I believe I'll try is borrowing a friend's temp sensing switch (when he gets back next week). If that doesn't fix it, I'll just have to change the sensor. I think I read that replacing the sensor can also fix a rich running condition -- not sure if I have that problem (if I do it's minor), but I intend to tune the injection soon anyway so if I have to replace the sensor, at least I'll know I'll be tuning from a clean baseline.
     I'll check back in a week or so, but in the mean time, if anyone else had this same problem and knows a fix, let me know. Thanks! 
   

[Inge K.]

To test: remove the relay and measure the resistance between terminal 31 and E in the socket.
At ~20^oC you should read ~2500 ohms, if you read a lot lower (fan starts at ~150 ohm)
you have a short in the sensor or the related wiring.
If the resistance is OK it's the relay that is the problem.......you could try to give it a few gentle knocks,
could be welded contacts.

[K1300S]

We are getting the terms mixed up. 
There is a sensor (sometimes called a switch) behind the radiator on top front of engine.  Acessed by removing the air box.  No fairing bits need to be removed on an S model.

There is a relay (sometimes called a switch) in the fuse box.  Accessed by lifting tank.


The talk of a manual switch to override the computer is to tell you to look for one the PO may have already installed and not told you about.

[rbm]

The temperature sensor in the stand pipe on the left front of the block is not a switch with ON/OFF characteristics.  It is a pair of negative temperature coefficient (NTC) thermistors joined together, with a non-linear analog response to temperature change.  The unit consists of a threaded metal housing, with two terminals.  Each terminal is connected to a thermistor with the housing itself being the common connection point for the pair.  As water temperature changes, the resistance of the sensor changes -- goes higher with lower temperatures, lower with higher temperatures.

The temperature relay in the relay box is not a simple mechanical relay, but rather a complex electronic circuit with active integrated circuits like you'd find in your computer.  There is a small relay on the circuit card that handles the (relatively) large fan current.  Over time and with vibration, the solder joints on the circuit card can fracture, causing the relay to fail.  It is possible to open the relay's enclosure and remelt fractured joints using a soldering iron, restoring the relay's function.

Inge's advice is aimed at isolating the failing component.  We know the fan and wiring are intact and operational.  We also know there are no aftermarket modifications to the electrics.  The relay and temperature sensor are still suspect.

[moto949]

Thanks for the details re testing, Inge. I'll do so when I get the bike down to my buddy's shop next week (I just now tried the tapping bit, but no go. Oh well, worth a try).

Re 'terms', yes, I believe on other posts, members sometimes may say 'switch' for both the Coolant Temperature Sensor as well as the Temperature Sensing Switch Unit. Anyway, I just got my hands on a Haynes which says the radiator must be removed and drained to swap out the threaded Coolant Temp Sensor but I have a feeling you may be correct for a K75S if I wriggle the wrenches down there just right- I suppose I'll find out next week if I need to change it.

As for a hidden switch, as mentioned, this bike was only tended to by dealerships, so I doubt they rigged a switch... then again, this was the same dealership that way-back-when put the fork oil amount for an early K75 (I measured it as I drained it) into this same '93 bike's Showa forks causing crazy dive under braking. 

Oh, just rec'd rbm's comments. Thanks for the details! So, being as they're both suspect, I will try to swap (or open/inspect/solder) the temp sensing switch unit first since that's easier. Then if needed, replace the threaded temp sensor. I guess you're saying the temp sensor has one terminal feeding the idiot light (when it reaches whatever predetermined temp) and the other feeding its signal to power the fan (at, hopefully, a slightly lower temp). Just to add confusion, I just checked my dash lights with the key turned to 'on' and my temp warning light, um, doesn't. The battery and oil etc do light up. Normal? Or, more confusion, bulb failure?

[Inge K.]

I guess you're saying the temp sensor has one terminal feeding the idiot light (when it reaches whatever predetermined temp) and the other feeding its signal to power the fan (at, hopefully, a slightly lower temp). Just to add confusion, I just checked my dash lights with the key turned to 'on' and my temp warning light, um, doesn't. The battery and oil etc do light up. Normal? Or, more confusion, bulb failure?

One of the elements in the coolant temp sensor is connected to the injection ECU for calculating the fuel mix.
The other element is connected to the relay which take care of both the fan and the warning light.
It's normal that the overtemp warning lamp don't show when you switch on the ignition.

[TimTyler]

    Re: Coolant -- thanks, but the level looks okay.

Did you actually remove the fuel tank and check the coolant level by removing the coolant fill cap?

The coolant level in the coolant overflow tank is not an accurate measure of how much coolant is in the cooling system.

... fan running constantly, from start up to shut down

I guess if the fan is running as soon as you start the bike, it's probably not related to coolant level.

[K1300S]

here is your sensor location with airbox removed.  no need for fairing and radiator removal....

[Martin]

+ 1 Marshall It's the easiest way.

[moto949]

Great photo, mlytle! Thanks one and all. I'll be at it after this busy work week and I'll update results.

[moto949]

Update: I just swapped out the temperature sensing switch unit (the white, multi pronged, 2 inch box under the rear edge of the gas tank), and upon first start, the fan did not kick on and the bike idled fine - so far so good! Then something came up and I had to leave for awhile.

Anyway, the unit I swapped it out for is thought to be in working condition, though it is a used old part so I'll only know it's a 100% fix when I go on a ride and hear the fan come on in traffic as needed, fingers crossed (again, I wish BMW added an actual temp gauge along with the warning light so there's no guessing/worries). If the fan doesn't come on at all, that would mean this TSSU has a different issue, I suppose. Hopefully that long ride will be later today, time allowing, then I'll know for sure.

BTW, I did pry open my "suspect" temp sensing switch unit and got in real close with a jeweler's eye piece thingie but found no breaks in any soldering connections (it all looked pristine in there), so the failure had to be somewhere else, perhaps inside the tiny internal, black relay itself (?).

Thanks again, folks, and I'll update again when I can.

[Martin]

The only time my fan kicks in is when I'm doing the CO adjustment in my garage, so I fitted a override switch. It is comforting to be able to test the fan, and with the OEM fan they need to be spun or they seize up.
Regards Martin.

[moto949]

It works!  Finally got back to the bike to give it a proper test. It started right up and idled smoothly as usual, even on a February morning (... okay, it's Florida and 60 degrees)  Approx four and a half minutes later the fan kicked on. Good feeling. So thankfully, I didn't need to change the sensor in the coolant junction after all, but I do have a new back-up one which I may swap out next coolant flush as preventative maint' (and it's good to know it won't be as involved as I thought). Thanks to all those who chimed in!!

[Laitch]

At 60º, four minutes seems early for the bike's fan to kick in at idle. Riding it will tell the story. Good luck!

[moto949]

Know what, you're probably right. What I think is affecting it is the idle speed; it's far too high at the moment, and that is probably building the heat too fast. It's up to 1500 rpm, even after settling down on the lowest idle switch level.

What happened was, when I recently got the bike it was running very rough during low speed acceleration. I changed the fuel filter (zero difference) and new plugs (noticeable difference since the PO - dealer installed - put in resistor plugs, which I heard is a no-no with resistor wires - I put in the correct ones (the ones I took out looked just right in color, etc) and it certainly got smoother, but far from perfect. I then had a local shop tune the injection and it only helped a little. Faced with either cleaning the injectors or swapping them, I went for the four-hole aftermarket green injectors. Soooo much better, like a new quick bike, but the idle crept up to the high level it's at now.

I'll be tuning the injection myself this time (which I think needs a re-do what with the new injectors and a valve shim change I just did) at my friend's shop with carb stix, and I'll be doing the ignition advance too.  At that point I'll adjust the idle down. Hopefully I've covered all the bases.

Thanks!
 

[Laitch]

I'll be tuning the injection myself this time (which I think needs a re-do what with the new injectors and a valve shim change I just did) at my friend's shop with carb stix, and I'll be doing the ignition advance too.

If it were my bike I'd balance the throttle bodies, like you intend to do, adjust the idle and stay away from the ignition advance if it hadn't been touched by the local shop—and I doubt that it was. They probably just went over the bike with a whisk broom and cleaned off the spilled coffee. :giggles

Take it for a ride to somewhere like Shark Valley Loop or out along the Myakka or to Loxahatchee then decide the next move.

[TimTyler]

... I went for the four-hole aftermarket green injectors. Soooo much better, like a new quick bike, but the idle crept up to the high level it's at now.

When I recently upgraded to the 4-hole injectors, it whacked my air-fuel mixture.

Once your bike is idling warm on the center stand (and the throttle bodies are balanced at about 1100 rpm) depress the green start button on the right hand grip for a few seconds. The engine RPM should increase a tiny bit. If it does anything else you need to adjust your air-fuel mixture. Easy to do using this method. Basically, pop off the rubber plug in the top-front-right of the airbox, stick a short 5mm hex wrench into the hole and note the position. Turn clockwise to reduce the amount of air allowed into the idle mixture, turn counter-clockwise to add more air to the mixture. Adjust until depressing the green button acts as suggested above. (FYI - Turning the screw simply allows/restricts air to leak past the airflow meter.)

With my freshly installed 4-hole injectors, pressing the green start button slowed my idle considerably and almost stalled the bike. Too much fuel. I added air -- about 5 full turns of the screw.

AFAIK this adjustment only affects the idle / low-RPM mixture.

Next, adjust your throttle bodies again if altering the mixture changed the resting idle RPM.

I would not mess with the ignition advance now. Maybe in three months when you're sure everything else is perfect.

[Martin]

Tim it would be interesting to see what doing a plug chop at highway speed say 60-70 MPH looks like it. Are you doing any fuel comparisons. And I know you probably don't have a Dyno, but on gut feeling do you think the 4 holes are an improvement. I have been mulling changing over to them, but the lack of concrete data has me wary.
A curious Martin.

[TimTyler]

Like most I noticed improved smoothness and responsiveness with the new injectors. The upgrade happened about 300 miles ago.

I haven't put any real miles on the bike since figuring out the air-fuel mix discrepancy a week or so ago. The sun's coming out next week though so check with me later :)