So I recently installed a horn on my 85 K100RT.  It doesn't have any guages, turn signals or anything so I wanted to add a horn to get it inspected.  I took the gas tank off and used a multimeter to find the wire that originally went to the horn, clipped it off and ran a new wire to the horn and it worked fine.  Put it all back together and now the bike won't start.  After messing with it I noticed the TPS no longer clicks when you turn the throttle.  I took the connection off the top and put the red multimeter probe to the positive terminal on the battery and used the black probe to check all 3 connections and each read roughly 12 volts which isn't right.  Then I connected the black probe to the negative terminal and used the red probe on each connection and all 3 read zero volts.  I'm much more mechanically inclined than electrical so I could use a little help here.  I can't imagine what I could have done when connecting the horn that would have impacted the TPS

Hi mcquinn

We can't keep calling you that. 

I spose you took the seat off before the tank,  and maybe the tray to get to the battery. Make sure the connector to the EFI computer is fully pushed in until you hear a click there too.

The click you hear from the TPS is mechanical, not electrical.  Try loosening the two screws that mount it and repositioning it.

The horn wires are green/gray.

daveyson wrote:Hi mcquinn

We can't keep calling you that. 

I spose you took the seat off before the tank,  and maybe the tray to get to the battery. Make sure the connector to the EFI computer is fully pushed in until you hear a click there too.

So I checked the EFI and it was fully pushed in.  And just for fun I unplugged it and tested the 3 throttle position sensor connections.  With it unplugged and the red probe attached to the positive terminal the middle TPS connection shows 12 volts and the left and right one show 0.  Once I plug in the EFI all 3 show 12 volts.  I'm thinking that's not a good sign

By "won't start" do you mean it won't crank, or that it will crank but won't run? Have you tried Duck's suggestion in post #3. Have you disconnected the throttle position switch entirely then tried to start the engine?

If it cranks but won't run:
Check fuse #6—the fuel pump fuse. If it isn't blown, clean its terminals and the sockets that receive it, then re-insert it. If that doesn't work, replace it anyway.

Removing the fuel tank requires disconnecting the four-pin electrical plug that powers the fuel pump. On your moto it is beneath the tank and might be on the left side. On later models it's beneath the tank on the right side. Clean both parts of that plug, reconnect it and be certain it is tightly connected.

It cranks and will fire up if I use starting fluid and then dies.  Going to rent a fuel pressure gauge and check the pressure but I can hear the pump run when I hit the starter button. I'm just focusing on the TPS because of the unusual readings but it could easily be a completely different issue.  It was running great until I clipped the green horn wire and ran it to the horn.  The horn worked fine when the button was pressed, I can't imagine how extending that wire that could have effected anything else with the bike.  I'll check the fuel pressure and go from there.

I did check that fuse and replaced it.  The back screw for the TPS is stripped, I can sorta adjust it by loosening just the front one.  And I did clean the 4 pin connector.

When you put the tank back on after removing it, did you attach the correct fuel hose to the correct spigot on the tank? It's a common mistake to accidentally swap them.

Have you got the hoses the right way around?

Yup, hoses are correct, just double checked them

Open the filler cap,  can you see fuel returning to the tank?

So the rear spigot of the tank connects with the front spigot of the fuel rail?
Can you blow air into the tank's front spigot using a hose?

McQuinn wrote:... It was running great until I clipped the green horn wire and ran it to the horn.  ...

As Duck pointed out above, the horn is Green/Grey, not Green. Are you absolutely positive you modified the correct wiring? The wire you cut SHOULD trace back to the horn relay. If you trace it back to the Load Shed relay, that would be problematic. Question, does the headlight come on with the ignition key, and turn off when you press the start button?

robmack wrote:

McQuinn wrote:... It was running great until I clipped the green horn wire and ran it to the horn.  ...

As Duck pointed out above, the horn is Green/Grey, not Green.  Are you absolutely positive you modified the correct wiring?  The wire you cut SHOULD trace back to the horn relay.  If you trace it back to the Load Shed relay, that would be problematic.  Question, does the headlight come on with the ignition key, and turn off when you press the start button?

I'll have to check this.  I found the wire that provided voltage on the multimeter when I hit the horn button and routed that wire to the horn.  That wire was green but I've since disconnected it.  I'll pull the tank back off and check. I'll send a pic of the connection when I take a look at it again.

Headlight does come on with the key on and then off while engine is cranking

I appreciate all the help and suggestions.  It's definitely a fuel issue, it just fired up and quickly died, I think it was just leftover vapor from the last time I sprayed starting spray in it.

daveyson wrote:So the rear spigot of the tank connects with the front spigot of the fuel rail?
Can you blow air into the tank's front spigot using a hose?

I haven't tried blowing air through the front spigot but I'm 100% sure the hoses are connected to the right spigots

The horn wiring terminates in a pair of wires with 1/4" spade connectors right at the front of the bike. There should never be a need to clip any wires under the tank to make the horn work. At worst case, you'd have to clip off the 1/4" spade connectors on the wire at the front of the bike to fit a different connector compatible with your aftermarket horn. I suspect you've clipped a wire vital to the fueling.

There were no wires running to the front.  It's a cafe racer conversion, it has no gauges, speedometer, tachometer or anything.  Under the gas tank is where the other half of the connections that were removed are.  The wire I snipped didn't go anywhere, it ended with the half of the connector that had been disconnected.  I'll post a pic after dinner so you see what I mean

Ok, got the tank off and the wire is green and grey, I didn't notice the grey until I got some light on it.  So all I did was extend that wire to the horn I bought and grounded the horn to the frame.  I thought it'd be harmless because the wires weren't connected to anything to begin with but I must have done something wrong

McQuinn wrote:I'm just focusing on the TPS because of the unusual readings . . .

The TPS could be disconnected entirely and the engine would still run acceptably. Some owner prefer how it runs without the TPS involved.

Can you see fuel returning to the tank?

Ok, I feel really stupid but I did find the problem.  There was a connection under the gas tank that had been inadvertently disconnected.  I must have bumped it while messing with the horn connection.  And it was hard to see because there are so many disconnected wires under the tank.  Sorry to waste everyones time, but I do appreciate the help and suggestions, they did help me to find a few other issues I wouldn't have otherwise come across.  She's running like a beast again and I'm about to go get her inspected, cheers guys!

McQuinn wrote:Ok, I feel really stupid but I did find the problem.  There was a connection under the gas tank that had been inadvertently disconnected.

It would be helpful for you to describe that connector and what was function was disconnected.

Don't feel stupid, there is nothing stupid about missing a small detail, and besides that we have all done stupid things in the past sometimes, if you have never done anything stupid there must be something wrong with you.

I for one appreciate honesty, especially uncomfortable honesty, and that is the quickest way to solve the problem.

Laitch wrote:

McQuinn wrote:Ok, I feel really stupid but I did find the problem.  There was a connection under the gas tank that had been inadvertently disconnected.

It would be helpful for you to describe that connector and what was function was disconnected.

This is my first BMW and first bike with fuel injectors, electronic ignition, an ECM and all that so I'm not entirely sure what function wasn't working.  The fuel pump was definitely cycling on and it had spark so I'm guessing the fuel injectors were disabled.  It was a 5 pin connector but I didn't make note of the wire colors, I just rushed to connect it and put it back together to see if it worked

Sounds like it could have been the connector that joins the injector harness to the main harness. Was it circular and black, and on the left side near the rear?