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'91 R100RT: Parasitic Current Draw

banzaibob

New member
The background: Bought this RT last fall. Got her up and running about a month ago.....WOW, what a great running bike! The Fairing is long gone, torn up by the previous owner and not even on the bike when I got it. It had a sort of butchered electrical system. Since the electrics normally included in the fairing were gone also, I had to sort of "clean up" the wiring harness. I re-did the wiring harness and was pretty thorough in finding faults and isolating circuits that were no longer used. I was able to find that the main "hot" wire to the key switch suffered some thermal damage. I had to unravel the wiring harness and replace that wire. (From memory) it appeared that at one time there was a problem with the headlight relay and the main hot wire melted some insulation along the length of the wire. Anyway, I pieced everything back together.

I've put about 800 miles on it, like I said, it runs great. The problem:

Haven't ridden it in a while because of the cold snap that everyone is going through. Came out to it two weeks ago, dead battery. Since I didn't know when the previous owner installed it, I went out and bought a new one. Rode it around...no problems. Came out to it a couple of days ago, dead battery. Not only that, it wouldn't recharge...deader than a stone. Bought a new battery today. Hooked up the positive and left the negative off. I checked the current draw and sure enough, it was drawing about 400 mili-amps with the key off.

I am going to dig into it fully tomorrow or the next day. I don't have any problems with checking the individual circuits. I know that the various lighting circuits draw about 5 or so amp and the ignition maybe about that much. What I'm wondering; does that 1/2 amp of parasitic current draw jog anyone's memory? Granted, it could be a funky wire with bad insulation but does anyone have an idea of a specific component that draws specifically that much current? How about the various relay's? I'm sure that I'll find it but it would be helpful if someone knew of something that drew that much current off of the top of your head. Thanks!
 
As you said, pull the fuses to see which circuit makes the draw go away.

On one note that comes to mind, not specific to your bike but I have seen diodes in the rectifier bridge go "weak" or "lazy" if you will but you still have a great charging system.

About the only time I have ever been able to confirm this is with an oscilloscope but on the road, a powered test light has gotten me through to a really good guess.
 
Some electric areas:

0.5A * 12V = 6W

In that range are some bulbs:
Parking Light = 4W
Rear Light = 5W
Instr. Lights = 3W (each)

0.5A -> 0.5Ah which would drain a fully charged standard 25Ah battery in 50 hours.

Typical fuses for the late model airheads:
1 = Parking and rear lights, speedo lighting (on in Park)
2 = Horn, flasher, brake light, high beam (on in Ignition)
3 = Special equipment (always on, usually nothing connected, but maybe on RT)

If all is wired correctly and no crossover of the supply wires to these bulbs and fuses they all should be disconnected when the ignition key is off.

Since you mentioned some "modifications" to the RT I would check for extra wires connected directly to the battery. Also connections to the special equipment plug in the headlight compartment.

To disconnect the diode board from the battery disconnect the spade/wire at B+(the thick red wire with the white plastic on the right):

View attachment 48765
Be careful! This wire is HOT all the time. You could even plug your ampmeter right in between.

I assume you checked directly at the battery poles that the battery is dead.

/Guenther
 
Guenther suggestions are right on... also suspect are relay coils in this current range. if it were my bike, i would put the ammeter in series with the battery, key off, and just unplug assemblies watching for the parasitic to drop. voltage regulator, headlight relay, starter relay, diode board, dash unit. and before any of that, i'd look at the bike in the pitch darkness to see if there is a dash bulb or some other glow.

-dan
 
As others have said, .5 amp is too much. With only two fuses, pulling them will not do much as far as troubleshooting. Typical current draw on an airhead with RT/RS fairing (clock installed) is about 60 to 110 ma. I would also start by unhooking one wire at a time to see when the current drops. My bet is the diode board, which is where I would start. Remember to remove the battery ground before taking off the front cover, one spark there could fry some expensive stuff.
 
I hooked up the volt meter in between the negative terminal and negative cable, set on the 10A fused setting. On the positive terminal was the thick 4 or 6 gauge wire to the starter and another 12 or 14 gauge wire that went to the key switch. I measured.....it was drawing 0.514 amps, bad. I unplugged the key switch and the main hot wire, no change. I then disconnected the larger starter cable. It read 0.000 amps draw. I reconnected the starter cable then removed the top engine cover and noticed that it was still drawing 0.000 amps. I then noticed that the starter cable looked a bit flattened out and there was what appeared to be a stamped BMW logo on the flattened out part. I then noticed that on the inside of the top engine cover was a logo of the same size cast into the metal right above the area of the slightly flattened cable.

So.....the question is: Do you think that the top cover mashed the logo into the insulation just far enough to cause a little bit of current leakage? Too bizarre to be true? See photos....
 

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Could be a red herring. Can you mash on that spot on the cable with your fingers and change the reading? What's the reading when you reassembled everything? Still 0.000 or back to 0.5? You made a change to a main cable, taking it off and reattaching it. Could be that you "correct" an issue by just doing that. :dunno
 
A red herring was the first thing I thought of. However, it would be the very simplest explanation, simplest usually being the most likely. Anyone who has seen spark plug wire leakage at night knows that insulation can fail in varying degrees. Keeping my fingers crossed.
 
Mashing the conductor does not give the current a path to follow. If perchance the insulation was broken and current could flow I would expect a dead short to ground by way of the engine case. In this case you would have sparks, melted copper and/or aluminum, puddled insulation, and the smoke would have escaped.
 
Nope! We talk about wire sheathing (insulator) and a hot copper wire inside. Either it insulates or you do have a short to cover/ground. BAM!

But it seems you are in the right area.

Do you still have the solenoids for the SLS (the emission thingy) system in use? I just checked on mine they draw ca. 0.6A. If you have these two solenoids still in use then disconnect them and check the current again.

/Guenther

Edit: While doing my little test Paul already replied. So I confirmed what Paul said.
 
Mashing the conductor does not give the current a path to follow.

I guess my point was that whenever you have something intermittent like this, typically one begins to move wires around in order to see if that physical motion causes any changes. Certainly in this case, I'd be putting everything back together the way it was...if the problem returns, then you're definitely onto something. If not, it's something else.
 
Looking a bit closer into the electrical diagram my idea about the solenoids won't work. Too many more visible problems would have lighted up.

Connected to the starter terminal is the red wire going to the diode board's B+. To eliminate the diode board you have to put the top cover back on and verify that you get the 0.5A draw again. Then go to the diode board and disconnect the fat red wire as shown in an earlier picture.

/Guenther
 
I love things that fix their selves. Problem is, seldom are these fixes permanent. I would look at that smashed cable again. But, test the diodes on the diode board for leakage. This is usually where I find problems like yours.
 
So far, the mashed cable narrative is holding up. Checked and re-checked. I re-routed the cable so where it is not going over the solenoid but approaching the terminal from the bottom. Re-installed the top engine cover and rechecked the currant draw......0.000 amps. I'm sure that here are probably enough musicians here that have experienced 60hz. buzz or RF interference from leaky or weakened insulation around a guitar cord. This explanation is not beyond the realm of possibilities.

I'm still skeptical however. If I come out to it tomorrow and the battery is dead, I'll know.
 
It's possible, not likely, you found the problem.

In a few, very few, couple in forty years that come to mind, you can have discharge through the insulation from the conductor to ground. In every case though that I can think of, the primary culprit was insulation that had been soaked in oil/grease/slime for many years. No visible breaks in the insulation but cleaning them off didn't help. The insulation was too soaked.

These were also very high current capacity batteries on 24V circuits.
 
It has been two days. Re-checked current draw this morning, 0.000 amps. Voltage was 12.6v. Fired right up. I too, was skeptical about the whole "current leaking through the insulation" narrative." But what I am looking at right now would leave me to believe that it is quite possible. Only a long term look will be conclusive though.
 
It has been two days. Re-checked current draw this morning, 0.000 amps. Voltage was 12.6v. Fired right up. I too, was skeptical about the whole "current leaking through the insulation" narrative." But what I am looking at right now would leave me to believe that it is quite possible. Only a long term look will be conclusive though.

:)
 
it's really highly unlikely that you were consistently getting 1/2 amp current flow thru the insulation. more likely, the pressure of the cable on top of the solenoid distorted the solenoid, bringing the continously hot terminal in contact close to the solenoid coil winding terminal thru the blackened paste of burned copper vaporized off the main contacts. the clearances are tight inside the solenoid contact chamber and it is a very dirty, conductive environment. might even have a crack somewhere in the solenoid but most likely distortion only.

wrap the pinched spot with eletrical tape or heat shrink tubing regardless as it is suspect. but your routing the cable properly has solved the issue whether is was insulation bleed or solenoid pressure. nice catch!

-dan
 
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