Hello everyone, I'm hoping I can pick your brains as I'm at a bit of a loss.

Last year my fuel pump packed in, I replaced it with a Mapco one and all was going fine. Strangely, it may have continued to run after the killswitch had been engaged on a handful of occasions, which may be relevant here.

Long story short, I ran the tank dry and a few miles down the road after topping it up the bike died. It'd crank, but not run. I suspected the fuel pump might've had it again.

I've got the pump out and on the bench and if you give it 12v it spins. So that's fine. 

I've checked the relay and it clicks, so I'm guessing that's fine. There's 12v at fuse 6, so all's well there and when I crank the starter I get 12v (or thereabouts) to the green and white wire on the connector that goes into the tank. So power is making its way to where it should be but the pump definitely doesn't run when connected to its wires in the tank. Should voltage go to any of the other wires on this connector whilst the starter is cranking?

I see from the wiring diagram that the feed and earth to the pump appear to come out of the level sender in the tank, so would I be right to assume that the sender is faulty? Is this a common issue or anything any if you have experienced before, and what am I looking for when troubleshooting it?

Any help would be hugely appreciated, the bike is a 1985 K100 RT.

Cheers!

Are those electricity thingies that make the pump run getting through the wires in the tank? 

You say the pump works on the bench, and you have 12v going into the tank, but the pump doesn't run in the tank.   Sounds to me like the wires in the tank are bad.

Be aware that the tank is a dangerous place to be poking around with electrical stuff.

Take the wires off the pump and pull them out through the filler and connect your meter.  Is there 12v? 

Can you connect the pump to the wires when they are outside the tank?  Is there 12v?  Will the pump run?

Hello, thanks for your reply.

Power goes to the green and white wire on the loom side of the connector plug that goes into the tank whilst cranking but no power reaches the terminals that connect to the pump. Don't worry, I've not a death wish and have removed the wirIng from the tank for testing. No power whilst cranking. I've also already connected longer wires to reattach the pump outside of the tank and it doesn't run whilst cranking (no power). Hook it straight up to a 12v feed though and it spins up fine.

So presumably there is an issue with the in-tank wiring but what I'm hoping to find out is whether or not there are any known or common faults with the in-tank wiring that occur and what to check for. Specifically if any of these faults would cause the pump to continue to run on occasion after the engine has been switched off. Just so that I can get an idea of what I might be looking for if I remove the entire sender unit from the tank.

Many thanks,
Russ

My K100 is a '92 so I don't know if the tank connector is the same, but I have had several problems with mine, specifically with loose connections and corrosion/dirt in the connections. 

At least several owners have replaced the tank connector with a 4 pin SAE trailer connector.

Can you read any voltage on the ends of the wires in the tank where they connect to the pump?

If you're getting power to the tank connector but no power inside the tank to the pump and the pump runs on the bench, then it could be either the tank connector is not making proper contact or the wiring for the pump has disconnected at the sender. I've had both these issues and it can be an easy fix.
The connector issue is a common problem. Your machine is over 30 years old and corroded electrical contacts would be the first thing I would look at. Push and pull the connector in and out a few times to dislodge any corrosion. Use a good squirt of electrical contact cleaner like isopropyl alcohol (brake cleaner is the same and cheaper). Or try some DeoxIT which is made specially for this purpose.
If you're not careful, removing and installing a new pump can stress the pump wiring where it connects to the sender. The connection can just break off. I just replaced my sender with a second hand unit. Be aware that there is an O-ring under the sender plate that may need replacing. They get old and brittle and cracked and can sometimes not seal again.

I have had problems with that connector on several occasions and have ended up replacing the whole connector with a standard automotive 4 pin spade terminal plug and no longer have any problems.

That's a good strategy Rick. The location of the plug and the fact that it gets unplugged and replugged more than any other connector on the bike.... isn't it any wonder it gives us grief after 20 odd years with wear and tear and corrosion. I bought some waterproof connectors a while back off ebay but like a lot of other things, I haven't got around to sorting that yet. But a good clean and a regular squirt with Innox and I've not had any further issues... so far. Now that I've bought  new connectors I probably won't need them. 

One thing that causes the plug to fail is that over the years the tank often gets removed without unplugging the connector (very easy to do) and that strains it and eventually causes the female part of the plug to open a bit so that a good connection cant happen. It  is easy to squeeze the connectors to close them a tad but very easy to squeeze too far (don't ask).

Rick, or anyone else who has done it, can you post a picture of the replacement plug you used?  Specifically I'm interested in whether there is a compatible female side that screws into the tank (in place of the old '85 and earlier thermoresister type fuel level sender).  I'm thinking about adding an in-tank float sensor (without switching to a post-85 tank, and deleting the existing female side that screws into the tank as I won't need the therrmoresisters for that).  Sorry for the minor thread hijack.

For the early type (85 or earlier) the mic plug for a 27meg CB Radio or in the relay box is a spare connector that is for the OEM alarm.

RicK G wrote:For the early type (85 or earlier) the mic plug for a 27meg CB Radio or in the relay box is a spare connector that is for the OEM alarm.

The one Rick is referring to is the older type connector for the 7L and 4L warning lights for fuel, front left of the tank.

The newer type has the wire coming from a plate closer to the back right of the tank, with a plug and socket under the right side cover. That sender has a fuel level lever and float that can drive a fuel gauge on some models, as well as a single fuel warning light on the dash.

My two 1985 models have the later type, but depending on when in '85 it was produced and for what market, some may have the earlier type.

Can you confirm which type you have on your bike?

Either way, with the sender unit which also houses the fuel pump power connections, there should be continuity from the outside connector right through to the terminals that attach to the pump. Unless the part where the conductor goes through the tank fitting has an internal discontinuity, a soldering iron should deal with any issues found. As mentioned and you confirmed your awareness and lack of death wish, keep away from fuel and its vapour.

Bill

My 92 LT had a similar issue and a previous owner modified it. As this was the later tank it had the plug connector on the rear right of the tank. The wires are individually soldered into the plug on the tank and about 300mm from the tank in a very accessible location was a set of bullet connectors, all correctly colour coded which allowed the tank to lift clear for access to relay box/rad filler but which could be easily disconnected.

You know, I feel slightly ashamed as I'd totally overlooked the connector itself as I'd suspected it was to blame last time when in reality it was the pump (but possibly a combination of the two, truth be told)! 

It looks fine, the pins aren't corroded and look consistently sized which is why I'd not paid it too much attention at first but I've cleaned it out with brake cleaner (which I had to hand, great tip - thanks for that) before deciding to stuff a wire into the green/white cable's plug on the loom side and onto a small spade connector to connect it to the tank side and got power to the lead in that lives in the tank! So hopefully that rules out a broken connection anywhere other than the plug. The pump doesn't run when connected in this manner but that's presumably because one of the other leads provides an earth, I didn't have time to check the wiring diagram before work.

I'm going to go home at lunchtime, chop the pesky connector off and replace all of them with bullet connectors for now to see if that solves the issue. As I'm feeling woefully optimistic I've already ordered a couple of waterproof 4 pin plugs as it's always handy to have that kind of thing handy.

Thank you everyone for your responses and taking the time to help, this is a great little forum - I'm just sorry that this thread may end up being filed as 'just another faulty connector'. Oops. I'll report back in case I discover anything else of use to someone in the future.

Cheers!

I'd check the internal connection for the pump at the sender before "chopping the pesky connector off" needlessly. The tank connector may be perfectly fine. EDIT: I didn't properly read your post

RussK100 wrote:....
It looks fine, the pins aren't corroded and look consistently sized which is why I'd not paid it too much attention at first but I've cleaned it out with brake cleaner (which I had to hand, great tip - thanks for that) before deciding to stuff a wire into the green/white cable's plug on the loom side and onto a small spade connector to connect it to the tank side and got power to the lead in that lives in the tank! So hopefully that rules out a broken connection anywhere other than the plug. The pump doesn't run when connected in this manner but that's presumably because one of the other leads provides an earth, I didn't have time to check the wiring diagram before work.

Just use a DMM to do the same test for continuity on the brown wire

indian036 wrote:

RicK G wrote:For the early type (85 or earlier) the mic plug for a 27meg CB Radio or in the relay box is a spare connector that is for the OEM alarm.

The one Rick is referring to is the older type connector for the 7L and 4L warning lights for fuel, front left of the tank.

That's the one I have (7L and 4L, front left of tank).  Rick, thanks for the steer towards the connector.  I think what you are referring to is the side that connects to the wiring harness.  Is there one on the side that screws into the tank that is available as an aftermarket item?  Probably a long shot, but thought I'd ask before spending too many hours trolling the internet for something that might not exist.

Problem diagnosed (but not solved... yet)!

I've traced it to a break in the earth wiring somewhere inside the tank.

There's continuity up to the plug between the brown wire and frame but not to the -ve connector that attaches to the pump. So to confirm this I've run a cable from the -ve side of the pump and earthed it onto the block whilst cranking the bike and roar, she runs! 

So now I'll remove the sender unit and check / fix it as appropriate. A bit of a pain as there's half a tank of fuel and fairly fresh paint on the bike but needs must! 

Thanks again everyone!

Cheers,
Russ.

  Is there one on the side that screws into the tank that is available as an aftermarket item? 

Not as far as I know

RicK G wrote:For the early type (85 or earlier) the mic plug for a 27meg CB Radio or in the relay box is a spare connector that is for the OEM alarm.

Just a heads-up on the mic plug for early tank connectors.

LeonW has just bought one from Jaycar (Aussie electronic hobbyist chain) and found that the pins on the tank fitting are slightly too small to reliably contact the metal tubes on the mic plug. 

Rick or other experts, are there minor variations of that type of mic plug? I've never had occasion to use anything but the one that came with the CB or marine radio, so no compatibility issues have arisen, let alone connecting to a BMW tank!

Bill

I was relying on what has been reported here on the forum. It is possible that other pin sizes do exist but I was not aware there was any difference. The plug for the alarm in the relay box fits AFAIK.

RicK G wrote:

  Is there one on the side that screws into the tank that is available as an aftermarket item? 

Not as far as I know

Thanks.  I was afraid that was going to be the answer. Will have to try to employ some creativity on this.

RicK G wrote:......The plug for the alarm in the relay box fits AFAIK.

Just to confirm this has been done.... https://www.k100-forum.com/t8631-frustrated-1985-k100rs-owner#116886

Justin, just a guess but I don't think what you want to do is possible. The tank connector for the pre'85 tank which is fixed to the tank front/left also incorporates the fuel level sensor. This is not swappable with the later float fuel level sensor. You could look at swapping to a later tank but I think you will find other compatibility issues. I'm not sure if others have found a workaround for that.
However, this 4 pin chassis mount socket and plug is available on ebay and it looks very close to the early K100 tank connection. Hopefully this is the sort of thing you're after. Its certainly cheap enough to just check it out.

Cheers

Hollister, that looks very promising, thanks. I bet I can find a way to attach that to a compatible nut and thread it into the tank that way.  I want to use the original plug on my harness, which is in good shape.  But I want to build something that will allow for different wiring inside the tank to power the fuel pump and put in a float sensor.

I have an Acewell Speedo (with fuel gauge integrated) installed on my bike.  I found an aftermarket float sensor that has the right resistance to be compatible with the Acewell.  I think I've figured out a way to mount it inside the tank by attaching it to one of the internal braces with a screw or a dab of JB Weld (or both).  I wanted to build something new on the tank side, rather than cutting into the unit that threads to the tank, because I've heard that finding a replacement for that is hard (i.e., I don't want to take the chance of destroying a perfectly good, rare, vintage part on an experiment that may not work).

I know I could avoid all this by getting a post-86 tank. But this is more interesting.  And, if it works, quite a bit cheaper.  If it doesn't work. I'm only out a few dollars worth of parts.  And I can go to the newer tank as plan B instead.

Acewell speedo changes the dynamic a bit, and the float sender for the incorporated fuel gauge sounds like a goer.

Perhaps it would be worthwhile putting in a request in the "Parts Needed" section for a known unserviceable older type fuel sensor to cannibalise for the screw attachment to the tank, with a plug/socket such as the one Holister suggested somehow incorporated.

With the rarity of functioning or new senders, I'm with you on not wanting to destroy a good one. 

Bill

agree Justin

there are quite a few what you would call pipe tubular type senders (vdo have a good range ) that would be mountable in the tank and suitable for the acewell of all different sorts of lengths
I agree don't hack the tank wiring connectors ...but you could pull the old sensor out and rewire it to the new mounted in tank tubular sensor

have fun

charlie99 wrote:"but you could pull the old sensor out and rewire it to the new mounted in tank tubular sensor"

I'm nervous about trying this, given the rarity (as I understand it) of the part.  I've seen pictures of the innards of sensor unit and it looks pretty fragile. And its also not clear to me whether the unit can be pried apart or if cutting of the plastic is required.  (I'm several hours away from my bike right now and can't pull it out to look). Either way, I'm not sure I have the skills to do it without catastrophe. 

I like Bill's idea of tying to find someone who has a non-operational one to experiment on.  I'll give that a try.  I wouldn't feel bad about breaking something that was non-operational to start with.

Justin, I have a feeling that the 4 pin chassis mount socket I pointed you to on ebay is possibly not impervious to liquids but at $6 its worth taking a look. You may be able to seal the back of it with Permatex/JB Weld after you've attached the wiring. But I think Charlie's suggestion is best, but use and old unserviceable unit as suggested by Bill. Preserve your working sensor. Interesting little project. Cheers

indian036 wrote:

Rick or other experts, are there minor variations of that type of mic plug? I've never had occasion to use anything but the one that came with the CB or marine radio, so no compatibility issues have arisen, let alone connecting to a BMW tank!

Variations in the pins. My guess is that Leon bought Molex 0.093" series pins (OEM for most Japanese bikes) and yes, they will fit with a serious push and yes, they are marginally too big. Squash them together with a small screwdriver and they'll do the job. The OEM type are available from UK suppliers like Silicon14 (Farnell) but they're ridiculously expensive. I've used the Molex-and-screwdriver approach on three bikes now with no problems.

The sockets in the connector that I bought were too big, but the socket contacts can easily be removed and the then 'modified' to fit the pins in the tank sender unit.



If you're really fussy, you can cut a 16mm x 1mm pitch thread on the sender to make a positive connection.



In this picture, you can see that the two halves of the body are clipped and glued together, so it's not difficult to get at the internal wiring if necessary.

Paul.

Paul, that picture is extremely helpful.  Much better than pictures I looked at before.  How hard is it to get this thing open? Do you just pry it open (carefully) with a utility knife or similar.  Is it liquid-tight? If so, what precautions are advised for keeping it that way upon reassembly? Or maybe it doesn't matter?

I'm starting to rethink modifying my existing unit.  But I am still very wary of messing it up!

Just prise it apart.

It doesn't need to be liquid tight as the thermistors need to be immersed in fuel to sense the temperature difference. You'll see two tiny holes, one at the top and one at the bottom of the plastic body.

Paul.