temperature relay diagnostics - puzzling result & my mechanic may suck

[Eville Rich]

So in the process of chasing a no-start issue, I decided to go through the cooling system to see what does or doesn't work.  I'd replaced a frozen fan earlier in the summer and haven't heard it come one, but I'm not sure if that was due to actually not needing to (no light popped on the dash) or something else.  I ran the diagnostics described in the repair guidance link.  I did the jumper from A2 to 45 (15 on my temp relay, but whatever). Fan turned on.  I did the jumper of A3 to ground and the light popped on.  Lovely.  Then I (my mechanic), managed to touch socket 45 against the ground socket (31).  Heard a pop.  Uh oh.  I went through the fuses and found that the horn fuse (bottom one - Doppel - Horn Lufter) had blown.  Replaced it with a good one.  Went back through the diagnostics and could get the overheat light to come on, but not the fan.  Crap.  I then connected the fan to the battery and it ran.  So I conclude that something between the relay sockets and the fan may be damaged.

I see from the diagram that the load shed relay is connected to socket 45 (15) of the temp switch relay.  But I also see the switch between pin 45 and A2. That circuit is the one you jumper to force the fan.  A2 appears to be connected to the load shed relay via some sort of switch, but I'm not quite sure what the function is there.

My hope is that this is an issue with the load shed relay and nothing more.  I did swap it with the horn relay (the horn works with either relay) and got the same result - jumping socket 45 to A2 does not cause the fan to spin.  I'm a little clear on what may have been damaged.

Any thoughts or guidance are appreciated. 

Eville Rich
1987 K75s

[rbm]

You shorted switched power (pin 15) to ground.  As you discovered, that popped fuse #7.  Since replacing fuse #7 restored switched power to the module (i.e. you got the overheat light to illuminate), then the short might have damaged the electronics inside the temperature sensing module, and you might need to swap with a working unit.

If you have a switch between A2 and the load shed relay, that sounds like it is not OEM.  Did a previous owner fiddle with the electrics?

[Eville Rich]

The switch with A2 was a reference to the relay diagram.  Not a separate switch. Sorry for the terminology.  Not the issue.

What is the temperature sensing module?  Where does it sit?  I've had this bike since the beginning of June, so still getting oriented on the parts and connections.

Thanks
Eville Rich
1987 K75S

[Eville Rich]

Is the temperature sensing module the temperature sensor itself or something else?  Just trying to make I understand the terminology. 

Thanks,
Eville Rich
1987 K75S

[rbm]

That what you call a "Temp sense relay" is actually a complex electronic module.  Being mostly solid state electronics, it is sensitive to voltage spikes on its supply lines.  It is possible that the short between 15 and ground caused a spike that might have fried something inside the module.  I'm only speculating at the moment.

[Eville Rich]

The various testing did plus the short to the #7 fuse all occurred with the temperature relay/module removed.  While I haven't confirmed the functioning of the relay/module, I don't think I damaged it.  I think I'm looking at an issue between the A2 terminal and the fan. I'll trace the wires. But if there is anything between the fan and A2 terminal to know about before I drive in, that would be great to know. The wiring diagrams I've looked at so far haven't suggested anything, but also seem a little incomplete.

Thanks again,
Eville Rich

[Laitch]

I'd replaced a frozen fan earlier in the summer and haven't heard it come one, but I'm not sure if that was due to actually not needing to (no light popped on the dash). . .

In a correctly working cooling system, the fan starts before the light is lit. The light indicates overheating; fan activation is designed to prevent overheating. If the fan doesn't start until after the overheating light is lit, the system is faulty.

The wiring diagrams I've looked at so far haven't suggested anything, but also seem a little incomplete.

Which diagrams are you using? Do you have online links to them? What about them seems incomplete?

The fan motor is connected to a plug by two wires. That plug receives two other wires. One is a brown ground wire; the other is a wire that connects directly to temperature sensing unit terminal A2, unless the wiring has been modified. Both Haynes and Clymer wiring diagrams show that wire as a Violet/yellow wire.

What is showing up at your fan's plug?

[Eville Rich]

Thanks for the help looking at this. I was looking at the wiring diagrams that are on repair guidance part of the website (relay layout). My comment about completeness was related to seeing how the larger system went together. Basically I was paranoid about all the potential interactions the short might have caused. But the problem is actually solved.  I need to remember to keep it simple.

I got back from work and redid the testing. Same result.  Decided to just try a different fuse in the #7 slot. The one I'd used showed continuity. But with a different fuse, the problem was solved.  Go figure. My first replacement fuse still shows continuity, but something about it wasn't working.  I'm not even going to guess.  Maybe the terminals. When I jumper A2 to terminal 15, my fan now runs.  Stupid problem leading to worst case complicated thinking.  I should know better.  Now I'm going to continue my journey down the no-start diagnostics. Might be able to motobrick yet this weekend.

Thanks again,
Eville Rich

[Laitch]

I'm not even going to guess.  Maybe the terminals.

You guessed.  :nono

[Eville Rich]

I've been known to contradict myself within five seconds. Not really, I'm very steady. Biggly steady. Yeah, no.

The wife will confirm I'm naturally contrarian.

Rich