Hi Guys,

So I'm sitting looking at my coolant overflow tank trying to decide what it's purpose is other than to act as a holding tank if there's too much pressure to the coolant. What I don't understand is there doesn't seem to be a outlet pipe feeding the coolant back in to the radiator?

I've been considering moving my coolant overflow from the usual position next to the battery to just below the coolant cap, would that do any harm?

Same hose flows both ways as needed. If the radiator cap blows then it just makes a mess out the top and all over below it.

I don't think your bike will know if you move it or not.

duck wrote:Same hose flows both ways as needed. If the radiator cap blows then it just makes a mess out the top and all over below it.

I don't think your bike will know if you move it or not.

Thanks for the reply. I didn't really explain my question very well earlier as I wrote it from my phone.. What I mean to say was it seems to just be a gravity feed allowing excess coolant to flow down into the overflow tank - so what provides the pressure for the coolant in the overflow tank to flow back into the radiator? 

As for the question about moving my coolant tank.. I'll be moving it pretty much directly next to the cap. I'm just worried that the overflow tank being so close to the cap/pipe will cause some sort of problem if there ever is an issue with pressure build up?

rxbbie wrote:Hi Guys,

So I'm sitting looking at my coolant overflow tank trying to decide what it's purpose is other than to act as a holding tank if there's too much pressure to the coolant. What I don't understand is there doesn't seem to be a outlet pipe feeding the coolant back in to the radiator?

I've been considering moving my coolant overflow from the usual position next to the battery to just below the coolant cap, would that do any harm?

Like Duck says - the hose allows flow in either direction. The clever bit is the radiator cap...see some detail at the following link:
https://www.k100-forum.com/t8299-radiator-pipe-suckage#97525

Heats up: Pressure increases, cap outlet valve opens and vents excess water to expansion tank
Cools down: Pressure drops, cap inlet valve opens (under slight vacuum) and allows system to suck water back from the expansion tank.

If the radiator cap is working properly (valves not fouled and/or seals not swollen and blocking the ports), you will notice very little flow to or from the expansion tank in normal operation. 

If you are riding in hot weather, especially in slow moving heavy traffic and/or have a problem with the fan not working, then you are likely to get a flow to the expansion tank, which is OK as long as the expansion tank does not overflow and the cap is working properly to allow water to flow back to the radiator as the system cools down again.

I guess the main reason for the expansion tank being where it is normally located is so that you can easily see that the water is at the right level pre-ride and it is easily accessible to be able to top it up, if required. I don't see any reason why you couldn't move it just below the radiator cap - but there's not a lot of space there (the cap is already pretty inaccessible under the edge of the fuel tank). However, if you do go for a bit of a water carnival and end up overflowing the expansion tank (there's a little vent hole in its rubber cap), you will end up with hot water and steam blowing up in front of you (and probably soaking the all the electrics under the fuel tank) which is probably not very desirable while riding. In its current location, such an overflow event squirts the water and steam up under the seat and down the back of your right leg. Not good, but better than in your face! It doesn't happen very often, but I have experienced it once and I doubt if I am the only one to have had such an experience.

Born Again Eccentric wrote:

rxbbie wrote:Hi Guys,

So I'm sitting looking at my coolant overflow tank trying to decide what it's purpose is other than to act as a holding tank if there's too much pressure to the coolant. What I don't understand is there doesn't seem to be a outlet pipe feeding the coolant back in to the radiator?

I've been considering moving my coolant overflow from the usual position next to the battery to just below the coolant cap, would that do any harm?

Like Duck says - the hose allows flow in either direction. The clever bit is the radiator cap...see some detail at the following link:
https://www.k100-forum.com/t8299-radiator-pipe-suckage#97525

Heats up: Pressure increases, cap outlet valve opens and vents excess water to expansion tank
Cools down: Pressure drops, cap inlet valve opens (under slight vacuum) and allows system to suck water back from the expansion tank.

If the radiator cap is working properly (valves not fouled and/or seals not swollen and blocking the ports), you will notice very little flow to or from the expansion tank in normal operation. 

If you are riding in hot weather, especially in slow moving heavy traffic and/or have a problem with the fan not working, then you are likely to get a flow to the expansion tank, which is OK as long as the expansion tank does not overflow and the cap is working properly to allow water to flow back to the radiator as the system cools down again.

I guess the main reason for the expansion tank being where it is normally located is so that you can easily see that the water is at the right level pre-ride and it is easily accessible to be able to top it up, if required. I don't see any reason why you couldn't move it just below the radiator cap - but there's not a lot of space there (the cap is already pretty inaccessible under the edge of the fuel tank). However, if you do go for a bit of a water carnival and end up overflowing the expansion tank (there's a little vent hole in its rubber cap), you will end up with hot water and steam blowing up in front of you (and probably soaking the all the electrics under the fuel tank) which is probably not very desirable while riding. In its current location, such an overflow event squirts the water and steam up under the seat and down the back of your right leg. Not good, but better than in your face! It doesn't happen very often, but I have experienced it once and I doubt if I am the only one to have had such an experience.

Thank you, that clears everything up. The reason I'm trying to mount the overflow tank next to the cap is to create the minimalist look I'm going for with my cafe conversion. Definitely need to consider your point about it spilling on to the electrics under the tank though as the wiring is already a mess under there. 

When I took the battery and coolant over flow tank out I noticed this pipe and cable lying loose. Do you have any ideas what they are? I've tried to show where they connect to in the photo attached. Cheers, Rob.

Rob,

The hose possibly used to be connected to the battery vent - most modern maintenance free batteries don't need a vent hose because they are sealed. 

I can't quite see where the wire you have highlighted in your photo goes to. The wire which goes through the grommet under the battery tray is the wire to the Gear Position Indicator switch (GPI) - and it doesn't look like it is part of that.

And while we're on the topic, radiator caps do go bad eventually. (Which I experienced on my 86 K75 turbo which caused several unnecessary and messy cap blows.)  The radiator cap is only about $25 new from BMW so it is not a bad item to replace as regular maintenance after 30 years.

Even cheaper is to just replace the radiator cap washer.... or just turn it over.

There's a pressure activated spring inside the cap that can wear out.  Next time you have it off to change the coolant take a look.

Didn't mean to imply that the cap didn't fail Duck, just that often it can be just a simple washer. I ordered a new washer for my 'Old Girl' and turned the old one around to get me by till the new one arrived (a suggestion back then by Olaf). 40k later and the old washer is still doing its job just fine.
Cheers

The indicated black wire is the GPI switch lead. Should end in a four-pin connector - white for pre-86 bikes (tank connector front-left) and black for post-85 bikes (tank connector on a tail at the rear). Dates approximate - someone will be along to correct me soon

Dai wrote:The indicated black wire is the GPI switch lead. Should end in a four-pin connector - white for pre-86 bikes (tank connector front-left) and black for post-85 bikes (tank connector on a tail at the rear). Dates approximate - someone will be along to correct me soon

Dai - that's what I first thought...then I edited my answer (talking tosh) because, if you zoom in on the pic you can see that the mystery wire continues forward (and under) of the battery tray The bit you see under the tray and the loose end with missing connector appear to be the same colour...and a different colour & thickness to the GPI wire which goes through the grommet. It looks like the GPI wire curls around and goes into that blob of black masking tape and off to the the right hand side of the bike.

So it does.

Sticking my neck out here... 

I rekon it's the lead for the accessory (merit) plug that sits on top of the cover for the coils.

Maybe?

It's a possibility?

Perhaps?

Could be?

Might be?

If the mystery black tube/cable at the top of the photo is in fact part of the electrical system it might be useful to trim back a tiny bit of the outer covering to expose the colour coding on the cores. This will then be able to be identified from a coloured wiring diagram for the bike. The outer covering removed can be replaced by heat shrink when it is restored to it's proper function (or removed if the project requires).

I agree that the lower tube is battery vent pipe used on wet cell batteries.

Cheers