The Fuse Box Mysteries: Overloaded Circuit

A few days ago I had just left the house and happened to look down at the dash on my 1988 R100RS with 104,xxx miles and noticed that the V-meter was not registering a charge. As in < 8V. Upon activaing the turn signals, I then noticed that the front turn signals were not working, either on the dash or the lights on the fairing.

Getting home, I suspected a fuse. Lo and behold, the lower of the two fuses was blown.

I put in a new fuse and it, too, blew immediately. This leads me to think a short exists in the circuit.

Question to the group: How does one begin to track down a short in a circuit? The wires are all bundled up, mostly, except for the connectors. This is a stock wiring harness that has not been altered or repaired. Where do I begin?

I talked with Matt Parkhouse and he can't take it for a week and the weather is too nice here to wait a whole week. Any insights are appreciated.

Rick.

Wire up a bulb in a socket, something like a turn signal bulb or a brake light bulb. Remove the fuse from the fuse holder and then connect the two wires from the bulb to the two sides of the fuse holder. The bulb is basically replacing the fuse. Turn on the ignition. The bulb will either light up bright or very dim. If it is bright, then there’s a short and one side is grounded. If the bulb is dim, the circuit is operating properly.

Since the fuse blew as soon as you put it in, I would guess that one of the lighting wires has a cut in the insulation and is grounded. Check the parking light up front, or the taillight. With the bulb installed, replacing the fuse, as you start moving wires around, when you happen to push the bad wire the telltale bulb will change from bright to dim when you push the ground path off of the frame. That will point out the location of the problem. Keep the telltale installed until you find and fix the problem then keep pushing wires around with the bulb dim and staying dim showing the problem is really fixed. You might have to look at a wiring diagram to follow all of circuits that come off of that fuse. The telltale bulb will let you keep checking without blowing multiple fuses.

What is the ampere (A) number on the fuse? It should be a 8A fuse.

The parking light, the tail light and the tachometer are all the consumers connected to the first position of the ignition switch which connects power through the lower fuse. That's why you still could ride.

were the REAR signals working ? just wondering since you mentioned fronts, might get one partial blink that gives a clue

try to locate a chafe point or a fallen-off hot wire ( yeah have to take some parts off to see socket bases ect)

some tricks-

have someone power the circuit while you stick an ear of various parts, listen for the spark.

Cause the fuse blow in the dark while looking at various parts , you might see the spark at the source.

It can be risky to start disconnecting wires for a VOM check connecting both ends of run, and have to understand the wire diagram and how the loom is run to make sense of it. Its not a job for every one, easy to boof a connector , ect .

Get a box of fuses and have at it




added note;

lol heck yeah to -
'get a fire extinguisher"

.. and set it handy but away from the gasoline spill or hissing propane tanks , which shouldn't be there anyway
PG is being helpful this time though- if you DONT have a fire extinguisher handy near an exit or two in the garage,
today is a good time to put one there , but that wouldn't actually FIND the short.

But blowing fuses is proof they work;
Fuses prevent damage when they overheat & melt , before the permanent wiring heats & melts its insulation.
Thats all.
As long as you use the correct A rating ( or smaller) the circuit isn't stressed even by repeated blow outs.

Probably be a lot more elegant to get the VOM meter out and find connections that SHOULDN"t be there on some wire ( AKA a Short)
but that can require temporary disconnections which can be a PITA
plus a solid understanding of the wiring diagram for your bike.

One concept you have to keep in mind though ; ALL circuits are basically "Shorts" (as in 'paths to ground'),
except a working circuit does useful work between the (+) supply and (-) ground.
You are looking for unexpected grounds ( shortERs , I suppose)

If (?) ]the signals are involved, you might get a quick blip of light from one or several signals , which is a good clue
(those bulbs 'worked" , its the one that DIDNT flash where the problem may be)

Short of that step ( nyuk nyuk) , can you change switches setting ect so the short does NOT occur instantly ?
DE-powering the power wire should prevent the blow, until activated.



I've found an underdash short by spotting the spark in the dark and was only looking there cause I finally heard it for the general location.
Lucky I guess but it took some set up too.
Plus I had to have a friend helping, which may rule out this method for some commenters
Fixing the short was the Beast THAT time.

My fuses are buried in the head light. if I expected to blow a bunch . I'd consider make a jumper to make access easier.
You could even make the jumper wire enough to work further around bike, alone. Yeah complicated but just ideas

Dont really have enough info to add specifics to generic "short in the dark" kind of comments.

are you saying that the short happens only when signals activated?

gtgt_bangbang said:

Get a box of fuses and have it

Click to expand...

And a fire extinguisher.

I would go to the last work you did near any part of the wiring harness, especially if you moved it, and look for wear on insulation. Because I make those sort of mistakes. Then check the sockets/connections at all the things that don't work, turn signals and voltmeter, especially those exposed to weather.

heres a brief explaination of what I mean by

"ALL circuits are shorts" (I shold add; It just that some are shorter than others! )


Think of a simple light bulb circuit, working correctly;

If you stuck one probe of a VOM on the beginning of the SUPPLY ( +) side (the connector panel),
and the other probe on ground ( any ground , anywhere),

you should get a connection (seen as some level of resistance on the VOM)

If you pulled the bulb OUT of the circuit , you should get an OPEN ( infinite resistance, no number for R)

If you pulled the bulb OUT of the circuit , and you STILL get a connection,
you just found the shorted wire and can focus your further inspection on that run.


Next level , you have to understand the wiring diagram ,

to know if there are multiple paths to ground, for instance a 2-bulb light system (wired in parallel)
AKA a front / rear signal circuit ( after power 'departs' from the connector block.)
and on my bike there could be multiple parallel branches,
for instance the REAR signal bulb supply ALSO feeds the buzzer &or buzzer relay branch.
A short in any of those branches could be the culprit.


Now , true that a simple supply -> bulb -> ground circuit would be next to useless;
the bulb would sit there glowing " look at ME!" until the battery died

So there are switches and maybe relays
and sometimes flasher units ( which are basically 'fuses' , with a self re-setting aspect built in to the to them)
which complicate the search.

and if there are black boxes / brain units ( digital sort of equipement ) a simple VOM can be useless at some segments of circuit

to skip the high level hand waving when you just wanna fix your bike not get a lecture darnit

assuming the problem involves the signals?

remove each lens & bulb socket, checking to see if the hot side / power wire has slipped off/ got pinched/ ect and landed on a ground.

The bulb acts as a resistor that normally kept the circuit from over heating,
if it got 'removed' from circuit by being down stream of a short, the fuse is the next weakest link.

Pull off the handle bar switches and look for a wire ( chafed, separated, even 1 strand broke free ) that is touching the metal bar


When the fuse blows (in headlight shell? I dont know your bike , just mine) , it will give a little hot flash or light

Disconnect battery, put in a new fuse, stare inside the headlight shell in total darkness with dark-accustomed eyes, re-connect battery,
and look for a simultaneous 2nd flash .

several wires have to jump from one assembly to others ( the main harness leaping from frame to headlight is a great example)
Are any of the runs tight at any point , at any range of movement ?


Pull the tank where a wire could have got chafed or a connector fell off

"Pull the tank where a wire could have got chafed or a connector fell off "

+1

ebeeby said:

"Pull the tank where a wire could have got chafed or a connector fell off "

+1

Click to expand...

Everything is pretty well tucked under the top tube. No chafing, no jock itch.

Manitou_Commando said:

OKAY, I rigged up an 1157 bulb in a socket, tested it across the battery terminals (worked) and then connected it to the fuse holder. Nothing.
No dim, no bright with the ignition switch in the ON position. Am I to conclude that, since no energy is going to the 1157 bulb that the circuit, at this very moment, is A-OK?

Put an 8A fuse in the holder and with the ignition switch still in ON position, got no turn signals, F or R.
Horn works. Brake light works. Clock works. Voltmeter is either working.
When I turn the starter, the fuse blows and the voltmeter once again shows no electron going to the battery.

Question: is it running at a loss or is this compromised circuit an isolated thing that has no bearing on the charging system ?

My problem, in part at least, is that I don't know how to approach finding the culprit.

Rick

Click to expand...

So, my own experience was that the bulb glowed dimly when the circuit was functioning properly. But from your description here, it sounds like when you had the bulb/socket plugged in replacing the fuse the circuit was functioning. Your original post said that the replacement fuse blew as soon as you installed it. Now, it sounds like you're saying the fuse blew only when you tried the starter.

The charging system has no relation to the fused lighting circuit.

I would go back to removing the fuse and wiring in the bulb/socket. Turn on the ignition and check the lights that weren't working previously. If they are working, then the circuit is working. If you aren't sure, use a meter or test light to check the wires to and from the bulb to see if you have 12v and determine if it is wired into the circuit. If the circuit appears to be working, I would go from front to back, moving wire bundles to see if any movement causes the bulb to light up. If it lights up, you have found the short. It is POSSIBLE that the short was off the frame (not shorted) but when you pressed the starter it caused the wire to move/vibrate and touch the frame and short out. I don't think that the starter is fused and it surely isn't on the lighting circuit, so it shouldn't be blowing the fuse.

Could be a bulb or a bulb's socket causing this.

I would take the bulbs of the parking light and the tail light out and also disconnect the tachometer. Then try it again.

I gave up and took the bike to Matt Parkhouse for diagnosis and remedy.
You will all be updated on the cause once discovered.

There is an accessories connector connecting the tachometer and the voltmeter. On my R100GS this connector was in the headlight bulb. Don't know for the RT. The connector is round and white, connecting four wires:

brown to GND, black to the coil, green/black to the fuse, grey/black to the bulb in the tachometer.

Go find that inline connector and disconnect This disconnects the tachometer and the voltmeter. Chance that either instrument is causing the short or there is a loose/open wire hanging in the air.

OK, Here's The Skinny

Matt P diagnosed the culprit as a faulty turn signal relay.
Replaced the part and I am on my way.
There's a reason Airheads turn to him.

And he pracrically lives next door. Lucky me.

Manitou_Commando said:

Matt P diagnosed the culprit as a faulty turn signal relay.
Replaced the part and I am on my way.
There's a reason Airheads turn to him.

And he pracrically lives next door. Lucky me.

Click to expand...

Great news! But how did he do it??

ebeeby said:

Great news! But how did he do it??

Click to expand...

You might be able to read about that in the ON in a few months.

ebeeby said:

Great news! But how did he do it??

Click to expand...

I asked Matt about his process and in his usual understated way, he just said he started at the front and worked through all the circuits until he came to the turn signal relay.

PGlaves said:

You might be able to read about that in the ON in a few months.

Click to expand...

You know, I have throught the same thing! But he usually writes about far more interesting stuff.