Perplexed - R90/6 Still No Electric Power

Solved - R90/6 No Electric Power

I posted a question here

http://forums.bmwmoa.org/showthread.php?t=48468

a couple of days ago asking for some help. Bike suddenly lost electric power. With the ignition switch on, no idiot lights, headlight, turn signals, or starter - nothing.

I appreciate the suggestions I received during the week.

Today I spent some time on it and could use some more advice. Here are updated details.

I traced 13v from the battery into the starter relay (red wire).

13v comes out of the starter relay on the second red wire.

So, starter relay not the cause of the problem?



12.9v comes into the ignition switch (red wires).

12.9v goes out of the ignition switch in the first on position (green wire)

12.9v goes out of the ignition switch in the park lights positions (gray wire)

12.9v goes out of the ignition switch in the headlight position (yellow-white wire).

So, ignition switch not the cause of the problem?


The two 8A fuses on the board behind the headlight are good (removed, checked good, replaced with new ones anyway).

If I remove the fuses, then I have the 12.9v coming from the iginition switch (green and gray) on the right hand side of the fuse holders.

As soon as I put the fuses into the circuit, voltage drops to zero.


What single point of failure for all the electrics is on the downstream side of the fuses?

I'm guessing it is something open/broken because if it was a short, the fuse(s) would have blown or the bike burned up?

But, again, whatever it is is causing total loss of power because nothing on the bike works. What the heck could that be?

Regards,

Barron

Barron_Williams said:

I posted a question here

http://forums.bmwmoa.org/showthread.php?t=48468

a couple of days ago asking for some help. Bike suddenly lost electric power. With the ignition switch on, no idiot lights, headlight, turn signals, or starter - nothing.

I appreciate the suggestions I received during the week.

Today I spent some time on it and could use some more advice. Here are updated details.

I traced 13v from the battery into the starter relay (red wire).

13v comes out of the starter relay on the second red wire.

So, starter relay not the cause of the problem?



12.9v comes into the ignition switch (red wires).

12.9v goes out of the ignition switch in the first on position (green wire)

12.9v goes out of the ignition switch in the park lights positions (gray wire)

12.9v goes out of the ignition switch in the headlight position (yellow-white wire).

So, ignition switch not the cause of the problem?


The two 8A fuses on the board behind the headlight are good (removed, checked good, replaced with new ones anyway).

If I remove the fuses, then I have the 12.9v coming from the iginition switch (green and gray) on the right hand side of the fuse holders.

As soon as I put the fuses into the circuit, voltage drops to zero.


What single point of failure for all the electrics is on the downstream side of the fuses?

I'm guessing it is something open/broken because if it was a short, the fuse(s) would have blown or the bike burned up?

But, again, whatever it is is causing total loss of power because nothing on the bike works. What the heck could that be?

Regards,

Barron

Click to expand...

Hi Barron,

Being totally new to the Beemer world, I am suspecting that all is wrong you have a grounding fault. This often happens the British Bike world and especially after the machine has been restored and nothing works after all is said and done. Check your wiring schematic looking to see if you have proper grounding points on the frame. Also check your wiring to insure at some point they are not frayed touching the frame somewhere.

Good Luck!

Rick MacPherson

Hi Barron

I had a similar problem with my '73 R75/5 last March. As it occured, the heavy red wire in the main wiring harness (of all wire) had been pinched by the fork lock ... crushed is a better word. It was really frustrating ... intermittent power loss, power to everything that needed it, but then nothing. On again, off again...

I finally found the problem when I pulled the harness off of the machine. It was, of course the original and brittle, so I replaced the whole thing with a new harness.

For the old /5 the main harness was only about $60US. The /6 harness is significantly more.. . .

"If I remove the fuses, then I have the 12.9v coming from the iginition switch (green and gray) on the right hand side of the fuse holders.

As soon as I put the fuses into the circuit, voltage drops to zero.


What single point of failure for all the electrics is on the downstream side of the fuses?"
=======================================

Try this test to shed more light on the problem

With the fuses removed apply a load such as a stop lite bulb
between the "12.9v coming from the iginition switch (green and gray)" and
ground. Watch the bulb - if it lights and stays lit then problem is probably
"downstream". If light does not lite or flicks out then there is a fault
upstream that will not pass any significant load.

Main current runs by heavier red wire from battery to diode board
then from diode board to starter relay
then from starter relay to ignition switch
then from ignition switch to fuses dep on position of ign switch

diode board connections are common - board does not act on
passage of current but connections must be clean at board
to make path continuous

starter relay does not act on passage of current to ign switch
but connections at relay must be clean to make path continuous

no fuse in circuit between battery and ignition switch - just
joints at diode board and at starter relay and at headlight
bucket board and at ignition switch

your bike's behavior sounds to me like there is a broken wire
or bad terminal connection between ignition switch and the
battery which passes only minimal current - enough to
measure but not enough to stand any significant load

I agree with the previous posts - if you don't have a hard short (that should be blowing fuses) downstream, then you've got a high resistance connection up stream between the battery and your measurement point. A mostly broken wire, severely corroded contact(s) or even severely corroded wire (have seen heavy white corrosions/oxidation occur under cable/wire jacket on battery ground wire, and also on the batter to starter/relay wire before) could cause several kilo-ohms of resistance that would drop your measured voltage down to almost '0' as soon as you try to draw a a few hundred milliamps through the circuit. You may have a combination of both these conditions - though I think that would be awfully unlucky..

Thanks! Progress - R90/6 No Electric Power

I really appreciate the feedback.

I think the break through for me was the idea that a wire could be broken, yet have enough voltage to measure, but not enough to stand any significant load. Bingo!

As a test, I ran a jumper wire from the positive battery pole to the red connectors on the board in the headlight bucket and all the lights came on like normal.

Now that I know where problem lies, I'll do some more work to narrow it down to a broken section of wire or high resistance at a connector.

However, I'm expecting it will be the above mentioned red wire in the main wiring harness between the starter relay and ignition switch.

Thanks again to all who offered advice and thoughts.

Cheers,

Barron

Check your neutral indicator light to see if the bulb is good. On some airhead BMW's if this bulb burns out it can affect the electrical circuit. It's one of the peculiarities of the design.

Resolved - R90/6 No Electric Power

I've got a couple of weeks riding and a few hundred miles on the fix (we have had great fall weather), so I want to add a couple of things -

First, I want to include the solution to this thread in case somebody else has the problem in the future. The problem turned out to be a damaged section of wire. Specifically, the red (power) wire between the starter relay and the ignition switch in the main harness. It was puzzling, because without a load, I could read 12.9v all the way to connector board in the headlight shell. But, the wire was damaged enough that although the digital meter would read the voltage, the wire couldn't power a load. I fixed it by building a new section of wire from the starter relay to the ignition switch and on to the connector board. I ran the new wire alongside the main harness and through the rubber grommet at the bottom of the headlight shell.

Second, as I asked for help solving the problem, I heard from three folks (two /5 owners and one early /6 owner) who had experienced the exact same failure. This is a heads up to you /5 and /6 folks to maybe ensure you have a yard long stretch of wire with connectors on each end in your onboard parts stash. This would have been a major problem if it had happened in the middle of a ride. But, with the knowledge I have now (and a yard of wire in the tool bag) would have been a simple fix on the road.

Third, thanks to all who offered advice and suggestions. Couldn't have fixed it ithout your help!

Regards,

Barron