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R75 died for no apparent reason

D

dodge1chevy2

New member
Title should read R75 died for no apparent reason

Just purchased a 1972 R750/5, Bike appears to be original and is in pretty decent shape. Started easily and came to a decent idle. I was checking the electrical and found a single problem. The left front turn signal did not light. I pulled the lens and jiggled the bulb. The light would come on only intermittently. Replaced with a new bulb. Same response. Sometime during this process, I decided to turn on the ignition switch. This was obviously a mistake. The starter played dead and the only light that was faintly visible was the instrument case. I failed to mention that I attached a mini charger directly to the battery. It took only 30 min. for the green light to come on. Someone suggested that it could be a fuse. I'm not thinking that the battery is a problem. The only fuse that I could easily locate runs to an outlet plug mounted on the small luggage rack on the back of the bike.

I would appreciate any suggestions on how to proceed.

rickt
 
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The early /5s were not fused...not sure if they started doing that by the 1972 models. So what do you hear, if anything, when you hit the start button? Nothing at all, or a faint clicking under the tank? Be sure and check that the ground connection at the battery terminal and at the ground connection at the speedometer connection area is tight...not crank on it, but just make it's clean and tight.

I wouldn't be so quick to dismiss the battery. How old is the battery? Try hooking up a voltmeter across the battery terminals and hit the start button. What does the battery voltage say before you hit the button and while you hold the button down? If when trying to start, the battery voltage drops to below 9v, you have a bad cell in the battery.

Another thing that should be checked is the starter relay which is a 1x1" cube on the left of the frame under the tank. The pins can get corroded in the socket, so finding the relay, pulling it out and replug it a few times should help clear the corrosion.

Let us know what you find.
 
Follow up

Thanks for the recommendations. I turned on the ignition key on top or the headlight and the headlight came on. The high beam flasher also works. All turn signals did not lite. Pushing the starter button provided nothing, no starter noise, no clicking. The taillight came on, but the stop lite did not come on with either the hand or foot petal.


I will check the other items tomorrow.


Thanks again for the suggestions.


Rick T
 
You're going to find folks on this board with way more mechanical experience than I, but when I had similar issues (died for no apparent reason) with my R100RS, close inspection of all wiring showed that it was old, insulation was rotten, and I had much of the wiring harness replaced.

Try looking at the wiring, one section at a time, and see what looks as if it may be bad. If it is in fact all original, it could be some old, failing insulation on one or more wires that is causing a short somewhere.
 
Starter not cranking

I will examine the wires. The test proposed earlier was performed by attaching my voltmeter to the battery and pressing the starter button. The voltage read 13.5 volts before turning on the ignition, 12.5 after turning on the ignition key, and no change when pushing the starter button with no sound from the starter. It would appear to me, and I am certainly no expert, that the problem lies with the starter or wiring to the starter. So, I will investigate further. I wonder if the bike would start with the kick starter?

Rick T

I noticed in the Clymer manual in the wiring diagrams section for R50/5, R60/5, and R75/5 (with fuses) that two fuses are shown. The first connects the ignition and light switch block with the marker light, the tach light, and the tail light/brake light. The second fuse is shown to connect ignition/light switch block with the ignition coil, the ignition and light switch block the neutral indicator light and the horn. With my limited knowledge it is not possible to know if the bike has fuses and where they would be located.

Any suggestions?
 
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It might start with the kicker. Suggests to me that if you're not hearing anything, that the starter relay is not latching. That needs to happen in order for the heavy current to be routed to the starter.
 
When they finally added a couple of fuses (and I'm not saying this includes your bike), they were inside the headlight. Being "old stye", they were about 3/4" long, white cartridge fuses with the contacts on the ends and the fusing element visible if the assembler took a little care.
Some people would add their own fuse setup to older bikes, so also look near the battery, under the seat, and under the gas tank.

re: your measured voltages - 13.5 at key Off is good; 12.5 with key On is reasonable (any lights? early bikes didn't require the headlight to be on, but it may been rewired; may also be affected by whether the points are open or closed; or there may be an unwanted "drain" somewhere). No change when you push the button means either a bad button or associated wiring, a bad starter relay (common) or associated wiring, bad high-current cables (the heavy ones from the battery to the starter/selenoid and the ground to the transmission; they may rot internally, so scrutinize for corrosion or greenish crap at the edges of the insulation), or maybe a bad starter (forward of the airbox).

Sometimes a mini-charger requires an overnight connection, in spite of the green light coming on... You need to "fill" those electrons, not just top it off.

If you're familiar with the kick routine, definitely give it the ol' college try.
 
Follow up

I was about ready to remove the tank. I would like to be educated. Can you describe the Kick Routine.

Thanks
Rick T
 
Pauls1150 said:
When they finally added a couple of fuses (and I'm not saying this includes your bike), they were inside the headlight. Being "old stye", they were about 3/4" long, white cartridge fuses with the contacts on the ends and the fusing element visible if the assembler took a little care.
Some people would add their own fuse setup to older bikes, so also look near the battery, under the seat, and under the gas tank.

re: your measured voltages - 13.5 at key Off is good; 12.5 with key On is reasonable (any lights? early bikes didn't require the headlight to be on, but it may been rewired; may also be affected by whether the points are open or closed; or there may be an unwanted "drain" somewhere). No change when you push the button means either a bad button or associated wiring, a bad starter relay (common) or associated wiring, bad high-current cables (the heavy ones from the battery to the starter/selenoid and the ground to the transmission; they may rot internally, so scrutinize for corrosion or greenish crap at the edges of the insulation), or maybe a bad starter (forward of the airbox).

Sometimes a mini-charger requires an overnight connection, in spite of the green light coming on... You need to "fill" those electrons, not just top it off.

If you're familiar with the kick routine, definitely give it the ol' college try.
Click to expand...

Officially, all the 1972 models should have the two fuses in the headlight bucket. However, BMW is famous for running through all the old parts before using the new ones, so you might have a pre-1972 wiring harness. Also, it was common in many states in those days for the dealer to title the bike the year they sold it. If it's a true 1972 the frame should have two tabs welded to the rear sub-frame down tube just ahead of the rear fender which hold on the chrome side covers.
 
As for the kick start...your bike should have the CV carbs so it's a little different than older slide carbs, such as those on /2s or even those on the R50/5 and R60/5. You should have the lever on the side of the air filter housing...you should turn that to the full choke position. With ignition off, I would kick the engine maybe 4-5 times with the throttle closed. Then, ignition on, throttle cracked maybe 1/8 to 1/4, move the kicker around a few times until you feel the compression stiffening it up. At that point, you should give it a quick kick to spin the engine...hopefully it will sputter to life. With the kicker, you should be aware of situations where the engagement gear of the kick start and the transmission gears might hit each other top of teeth. If that happens, the kick start lever will absolutely lock up...if you jump all over it, it can do damage. To clear this, you need to pull in the clutch handle and kick through...that will reposition everything inside the transmission. Repeat the process to start. So be ready to "feel" if it's ready to lock up...otherwise you should be OK to kick start the bike.
 
The kick routine really isn't much different from the other European bikes shown in your profile.
Generally:
Gas on, give it a moment to flow down and fill both float bowls (clear in-line fuel filters are a boon here); key on; if it's cold, apply the choke (push down on the lever on the left side of the airbox); maybe just a little throttle; push the engine over slowly as Kurt describes to find a compression point; let the kicker come back up; give it a good shove all the way down to spin the engine over (and keep your foot down to the peg until it's obvious that the engine either started or not).

If you decide to try bump-starting, second gear is sometimes better than first, and be aware of the potential torque reaction (the bike trying to rotate around the crankshaft) as you drop the clutch.

Of course, this all assumes that nothing is wrong in there somewhere...
 
Near the bottom of this page, Snowbum has his recommendations for bump starting or push starting...he suggests higher than 2nd, typically 3rd. As Paul says you can get some high reverse torque in the lower gears and can brake loose the rear flange taper connection in the transmission...could create some damage.

https://bmwmotorcycletech.info/drvshftboltstoolstorque.htm
 
Two things to check are the relay under the gas tank on the left side, if memory serves me correct there is a relay with about 6 wires, and the other one is the kill switch. I've seen them go bad and then we just crossed the wires to disable the kill switch and the bike started fine. The kill switch was the problem. The wires are in the headlight.
 
There is no kill switch on a /5. Same for 1974, /6 models.
With respect to your starter relay, the /5 starter relay is unique. There is a circuit inside that will prevent the starter being engaged if the motor is running and the alternator is producing sufficient current to extinguish the gen light in the speedometer. A side effect of this protection circuit is that the starter will be disabled if the alternator rotor goes open.
Also, the /5 is hand wired "point to point" at the starter relay. There is no combination plug that bundles the wires and places them on their respective spade connectors. You can pull the blue wire on the underside of the starter relay to defeat the lock out circuit for the purpose of testing. However, the best test would be to pull the front cover and test the rotor continuity by isolating the brushes and using a test meter on the slip rings of the rotor. You will be checking for continuity in the rotors windings using the "ohms" setting of your meter. An open rotor will display no value or infinite resistance. Use a business card in between the brushes and the slip rings, and be sure to pull the ground cable at the battery before pulling the front cover

All of this assumes an original, un-modified starter relay scheme.

This dropping of the starter by the circumstances detailed above has happened on 2 of the 3 R75/5 bikes I have owned. In both cases, I was able to use the kick starter to get home and the fault was the rotor gone "open".

And another thing,....
Fuses on a /5 that is equipped with them are inside the headlight bucket and are associated with the peripheral lighting circuits.
 
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Troubledhooting a silent starter

OK. So let's start with the tank off and you found the starter relay. Pull the black wire that goes to the starter and jump it to the positive post on the battery. You have eliminated the relay and starter button. If the starter turns over then you have eliminated the starter as a problem!
Reconnect that wire and pull the brown/black wire.
turn on the ignition and run a ground wire to the terminal on the relay that held the brown/black wire. If the engine turns over and you have eliminated the starter as a problem then the relay is good as well.
Open up the headlight and look for the terminal block where all the wires are connected to different harnesses. With the ignition on, locate the brown/black wire on the terminal block and ground it. If the engine turns over you have eliminated all the wiring up to that terminal block. that leaves the handlebar switch as the problem.
Check all turn signal bulbs out of their sockets and inspect the wiring connects at the sockets. Put all the turn signal parts back together with known good bulbs, cleaning off all corrosion. Go back inside and look for the blue/red and blue/black wire connectors. Run a hot lead to each of these in turn to test both left and right sides respectively. If one single side does not work, front and rear, then jiggle the switch. If the lights come on momentarily then the switch is the problem. This may be a long shot but the same switch operates both the turn signals and the starter so I've led you in this direction to make sure that you get to the switch as a culprit. Still just a guess but testing everything going in this direction will actually verify which part of the system is not right.
 
Pushing the starter button provided nothing, no starter noise, no clicking. The taillight came on, but the stop lite did not come on with either the hand or foot petal.
Click to expand...

Looking at an electrical diagram for the R75/5 which also says 'starting '69 models' shows 2 fuses in the headlight shell. One connects the flasher, the starter relay, the horn, the break light and the high beam flasher.

/Guenther
 
Guenther said:
Looking at an electrical diagram for the R75/5 which also says 'starting '69 models' shows 2 fuses in the headlight shell. One connects the flasher, the starter relay, the horn, the break light and the high beam flasher.

/Guenther
Click to expand...

Right. So if the brake light doesn't work then check the horn as well. The green/black wire is powered on by the main switch so if you go back to the brake light switches (front and rear) with the ignition on, then a test light should show power as well as the green/black wire to the back of the horn and the left coil.
 
Excellent reply!

1074 said:
OK. So let's start with the tank off and you found the starter relay. Pull the black wire that goes to the starter and jump it to the positive post on the battery. You have eliminated the relay and starter button. If the starter turns over then you have eliminated the starter as a problem!
Reconnect that wire and pull the brown/black wire.
turn on the ignition and run a ground wire to the terminal on the relay that held the brown/black wire. If the engine turns over and you have eliminated the starter as a problem then the relay is good as well.
Open up the headlight and look for the terminal block where all the wires are connected to different harnesses. With the ignition on, locate the brown/black wire on the terminal block and ground it. If the engine turns over you have eliminated all the wiring up to that terminal block. that leaves the handlebar switch as the problem.
Check all turn signal bulbs out of their sockets and inspect the wiring connects at the sockets. Put all the turn signal parts back together with known good bulbs, cleaning off all corrosion. Go back inside and look for the blue/red and blue/black wire connectors. Run a hot lead to each of these in turn to test both left and right sides respectively. If one single side does not work, front and rear, then jiggle the switch. If the lights come on momentarily then the switch is the problem. This may be a long shot but the same switch operates both the turn signals and the starter so I've led you in this direction to make sure that you get to the switch as a culprit. Still just a guess but testing everything going in this direction will actually verify which part of the system is not right.
Click to expand...


Excellent reply on troubleshooting the /5. I really could have used this last year. I'm saving this for future use! Very clear. Thank you.
 
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