K75S cranks, won't start. Getting confused. Help? - K-Bikes.com - Excellence in Motion
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post #1 of 11 (permalink) Old Oct 23rd, 2010, 4:54 pm Thread Starter
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K75S cranks, won't start. Getting confused. Help?

So I bought my first BMW bike (have had many of the cars) this summer. It's a '94 Mystik Red K75S. My girlfriend and I both have K75S's now. Hers is a '91. The two of us sound like a pack of bees riding down the road. It's quite funny watching people's heads turn when they hear the natural whirring of 2 K-bikes coming down the street. I've put about 3000 amazing miles on it and have loved every minute! But a few days ago, I started it up and within a block of my place it stalled. Cranks, won't start. I've done some checks but am getting stumped. Need help please!

I have spark. No fuel.

Pulled pump out and put 12 volts to it. It works.

I have 12 volts at the #6 fuse when cranking.

The red/constant power wire at the fuel pump relay connector has 12 volts.

When cranking, the yellow wires coming out of the fuel pump relay connector don't have any voltage. So I thought the fuel pump relay was bad. Changed it. No go.

I jumped the fuel pump relay connector , pump still doesn't run.

The harness connector for the fuel pump / sender is secure.

What am I missing? A ground? Something more involved with other components?
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post #2 of 11 (permalink) Old Oct 24th, 2010, 9:42 am
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The connector for the fuel tank is a known weak spot.
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post #3 of 11 (permalink) Old Oct 24th, 2010, 12:40 pm
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A ground Yes....maybe! I have done a lot of searching on that kind of stuff and come across a few posts on similar problems. So if all your connections check out, then I can say it again.......Dirty Starter.....Maybe!

Benelli 50cc at 14
Yamaha RD 200 at 16
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BMW K100RS at 27
BMW R100GS at 34
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post #4 of 11 (permalink) Old Oct 24th, 2010, 3:06 pm
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always do the easy stuff first..

Two connections to check - as mentioned first check the connector going into the tank - known problem area - remove and reseat. A problem here will prevent the fuel pump from working.

In the earlier model Ks this connector was - I think - a bad design. I don't know the particulars of your model year. In the early years there were pins on the tank side mating to female connectors in the harness side - eventually the female connectors loosten up and fail to grip the pins, causing a connection problem and no power to the pump, even though the plug appears to be firmly in place.

Second - check and reseat the computer connection. Might be a good time to clean these contacts.

If the connector under the tank is the problem the problem will return if not fixed.

"What follows is based on actual occurrences. Although much has been changed for rhetorical purposes, it must be regarded in its essence as fact. However, it should in no way be associated with that great body of factual information relating to orthodox Zen Buddhist practice. It's not very factual on motorcycles, either."

Robert Persig - Zen and the Art of Motorcycle Maintenance


1985 K100RS (Sold)
2000 323i (Sold)
2003 1150GS (Sold)
2009 528Xi
2004 K1200GT

Last edited by Beemer01; Oct 24th, 2010 at 9:10 pm.
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post #5 of 11 (permalink) Old Oct 26th, 2010, 10:42 pm Thread Starter
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So I replaced the fuel gauge/sender harness (thought I had an open in it) but it didn't fix my issue.

Further testing has found a lack of ground at the fuel pump relay connector.

I apply a ground manually to the connector and voila! the fuel pump relay works now.

I noticed in the wiring diagram the fuel pump relay ground is supplied by the ignition control unit. I have also verified the continuity of the ground wire between the ignition control unit and the fuel pump relay connector.

So is it safe to assume the ignition control unit is $%#[email protected]! 'd up?

I know the control unit itself has a good ground because I have spark.

Any thoughts?
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post #6 of 11 (permalink) Old Oct 26th, 2010, 11:32 pm
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Have you seen this page:

http://hrsbstaff.ednet.ns.ca/bvogel/...ont-start1.htm

There are several conditions that must be met for the fuel pump to come on and I think one of them is that the hall effect sensor has to send a signal to the brain that then allows both fuel injector pulse and to keep the fuel pump relay pulled in.

I wouldn't count the control unit out before checking all it's inputs first.

Tony G

'87 K100LT "Her Teutonic Majesty"
'79 XS750 "The Triple"
'72 A65T "The T-Bolt"
'68 B25 "The Blue Bike"
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post #7 of 11 (permalink) Old Oct 27th, 2010, 10:33 am Thread Starter
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I really appreciate the link to that troubleshooting page. I hadn't found that in my searches.

I'll keep posting my findings. Hopefully someone else can make use of this ordeal on their own bike...
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post #8 of 11 (permalink) Old Oct 27th, 2010, 10:57 am
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I've had two random things lose ground which prevented the bike from starting this summer.

The Hall Sensor and the starter itself.

Caused me a lot of grief as everything seems interconnected in bizarre ways. The electric system makes for some difficult trouble shooting!

Considering you have a spare bike right there, why not swap ICU and see if it works. Nice to have a reliable testbed right there.

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post #9 of 11 (permalink) Old Oct 28th, 2010, 11:12 pm Thread Starter
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Well, it appears I may have more than one issue. Or I've goofed somewhere in my diagnosis.

Although the ground connection at the transmission case looked fine, I cleaned it.

I then had power coming out of the fuel pump relay when cranking.

So after a lot of diagnostic time, I figured I had fixed it via cleaning the ground that I should have cleaned right at the beginning of my diagnostics!

So I put the tank back on and assembled the bike. It started but barely ran. Then stalled. Now I was even more confused.

I was intrigued to reapply 12 volts to the pump to make sure it would run. I unhooked the tank harness connector and applied voltage to it. The pump didn't run. As I had mentioned in my first post, the pump ran when I applied 12 volts to it.

I gently tapped on the pump and it ran. So I have now determined I need a pump as well! What's even more crazy is the pump was new at the beginning of this season.

I'm not sure if a weak ground could have caused my new pump to deteriorate so fast? Nonetheless, I have a new pump on the way.

I may have been chasing my tail all this time... But I'm not sure. On a positive note, I've learned a TON about the bike.

If it doesn't run after installing the new pump, I'll probably invite all you guys over for a BMW bike-burning / cookout. Don't forget to bring marshmallows.
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post #10 of 11 (permalink) Old Oct 29th, 2010, 4:57 pm
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Quote:
Originally Posted by teutonic9

I'm not sure if a weak ground could have caused my new pump to deteriorate so fast? Nonetheless, I have a new pump on the way.

I may have been chasing my tail all this time... But I'm not sure. On a positive note, I've learned a TON about the bike.

If it doesn't run after installing the new pump, I'll probably invite all you guys over for a BMW bike-burning / cookout. Don't forget to bring marshmallows.
Don't mention the bike burning.........someone I know well did that with a 650 Yamaha with fueling issues many years back, that is after emptying a Browning 12 ga. into it first.Yep....I was there. But the next morning he did sell what was left for $35.00.

Anyway , that is what electric motors do when they don't get full current.....they burn out. Which brings me back to the starter. You should clean it, the load shed relay and probably most of the other ones are grounded through the starter as weird as it may sound. A dirty starter will cause all sorts of electrical gremlins on BMWs. A pretty simple job on them bikes, and a brush kit, if required , is a lot cheaper than a starter.

Benelli 50cc at 14
Yamaha RD 200 at 16
Yamaha RD 350 at 17
Honda CB 750 F at 18
Honda V45 Sabre at 24
BMW K100RS at 27
BMW R100GS at 34
BMW K1200RS at 53
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