1971 R/75 starter button

[stevenrankin]

Thanks for the write up and pictures! Yeah, I use a plastic bag for switches as well. LOL, learned the hard way. St.

 

[olironrider]

How is your battery? Have you monitored the battery voltage at the terminals when you hit the starter button? If the voltage drops dramatically, then the battery is the problem. If the voltage stays at nominal, say 12.5v or so, then it's something else.

That something could be the starter relay under the tank. The relay can develop corrosion on the terminals. Locate the relay, and unplug/replug a few times...that might help. The relay is a black plastic cube about 1" square on the left side of the frame backbone.

The Slash 5 Series have an aluminum “can” Starter Relay; also mounted on the Main Frame “backbone”; left side, just aft of the Headstock…



Slash 6 & 7 went to the cubes, which are harder to open and “fuss” with for field expedient repairs.

The Starter Relay is another area where frequent maintenance is required: pulling all the spade connectors apart to clean and re-treat with dielectric paste “periodically”. Additionally, I have pried the “can” lower, crimped flange away from the circuit board; found that the “master” relay actuates fine (minimal amperage use) however, the “slave” relay contact points are arched/carbon fouled due to the amperage passing through to the Starter Solenoid. Little points file work there; good contact cleaner flush; reinstall the can lid and crimp closed. A dab of dielectric paste around the crimp-to-board joint… good to go !

I’m pretty sure I’ve got a picture of the open “can of snakes”… somewhere ?

 

[melville]

Even better than having to dick with the insides of a $100 relay that's a BMW specific part is to replace it with a $30 relay that can be bought anywhere. The big can relay continued into the /6 and here's my adventure with one:

Build thread blah blah blah R75/6 "Ernst"

i've been enjoying your build. i find a seal puller is a great help when getting the springs on the side and center stand. i use a cheap one from...

www.advrider.com

Note that the basics of this one were outlined by Matt Parkhouse in ON right around the time I posted that.

 

[olironrider]

A’yup, as I went from the Slash 5 Series to the Slash 7 Series (then back to the Slash 5s), you’re right about the Starter Relay in a can continuing into the Slash 6 Series.

Thinking about the FLAPS work-around for the BMW branded Starter Interlock Relay, “8”…



… the PNP transistor dis-allowing starter engagement while the engine is running, producing charging voltage, is not part of the after-market relay ?

Therefore, the starter thumb button is “hot” whenever powered; will engage the starter when actuated… could be trouble ?

 

[RPGR90s]

I don't need to know if the relay clicks, I need to know if the switch clicks. Is the switch operational?

What makes you think the switch should click? Does the horn button click? As long as the switches move in/out, they make contact.

 

[olironrider]

What makes you think the switch should click? Does the horn button click? As long as the switches move in/out, they make contact.

… the thumb switch does not made a sound when depressing it for starting. If you look closely at the second picture in post #19; all the start function does is push the brass washer in the button; a few millimeters towards the two(2) raised points; slightly compressing the “rubber baby buggy bumper” in the base; making electrical contact.

Switching the turn signal L- R function on the same, throttle side perch; may have a tangible “feel” but, an audible “click” is unlikely.

As RPGR90S correctly stated, the thumb switch on the clutch lever side does not make a sound neither. Like the starter switch, when you push the horn button towards the base; the brass washer in the button is pushed a few millimeters towards the two(2) raised points; slightly compressing the “rubber baby buggy bumper” in the base; making electrical contact.

The Switching the head light function: LO - HI - FLASH, may have a tangible “feel” but, an audible “click” is unlikely.

Whereas the starter switch is connected into the system by two(2) wires, the horn button is connected into the system by one(1) wire and then, grounded to the switch mount and handlebar, chassis (pictured in post #20).

Is you feel confident with a “hot-stick” probe, check the electrical path of the start switch internal circuit ?

You have to do this at the Starter Interlock Relay; under the fuel tank; left side, just aft of the front down tube/headstock; an aluminum can with several wires out the bottom: post #22. Look for a BROWN with BLACK STRIPE (BR-SW) wire under the can: post 31b. Pull that female spade connector off the post. In the same area, look for a GREEN (GN) wire at post 15. Pull that female spade connector off the post.

Connect a test light, hot-stick, between the two(2) wires… preferably using an insulated alligator clip on the GREEN (GN) lead !

The GN wire is system voltage; the BR-SW is switched ground.

Turn ON the ignition; depress the starter thumb switch to simulate a START… the test light should light ? Not ? Reference the first picture in post #19 !

Turn the ignition OFF; reconnect the wires: BR-SW to terminal 31b; GN to terminal 15… after insuring they are bright-n-shiny clean for good conductivity ️️️

 

[melville]

A’yup, as I went from the Slash 5 Series to the Slash 7 Series (then back to the Slash 5s), you’re right about the Starter Relay in a can continuing into the Slash 6 Series.

Thinking about the FLAPS work-around for the BMW branded Starter Interlock Relay, “8”…

View attachment 97164

… the transistor dis-allowing starter engagement while the engine is running, producing charging voltage, is not part of the after-market relay ?

Therefore, the starter thumb button is “hot” whenever powered; will engage the starter when actuated… could be trouble ?

BMW eliminated that transistor in the /6 relay. It didn't prove to be a problem. Seems we Airheadists have more sense than they originally figured.

 

[olironrider]

BMW eliminated that transistor in the /6 relay. It didn't prove to be a problem. Seems we Airheadists have more sense than they originally figured.

 

[olironrider]

… continuing onwards with your starter switch help: here’s the tool I mentioned as a “hot-stick”:



The larger one has been a faithful partner for decades; I like it best for the insulated clip; 6 - 12 VDC.

The smaller unit, I purchased from MAXBMW quite awhile ago; stays in the motorrad’s onboard tool box; 6 - 24 VDC.

Both add a light bulb for “load” during testing, even for setting points timing.

Check back with us, as we’re interested in the many issues you’re trying to fix !