'78 R100RS - Oil sensor flickering at low RPM

Discussion in 'Airheads' started by shadeobrady, Jul 31, 2018.

  1. shadeobrady

    shadeobrady Adventurer

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    Hey folks - post title says it all (and it's the amber oil sensor, not the red generator light). It just started and it seems to happen infrequently. Bike is in pretty good condition and at around 56,5XX miles.

    I did notice today at the end of my ride it happening only on idle with the light flickering instead of a constant "on" state. As soon as I flicked the throttle the light would immediately go off. I figure I should add that the bike already had an oil temp gauge installed in place of the original dashboard clock/timer incase that matters at all (I've been riding it a while like this so I'm guessing/hoping not).

    Leaving the bike parked for now. What's the typical troubleshooting run-through for this knowing that my oil is already filled to level?
    #1
  2. B_C_Ries

    B_C_Ries Long timer

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    what speed is your idle???
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  3. shadeobrady

    shadeobrady Adventurer

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    Usually between 1,000 and 1,200 depending on how warm it is. It's been cooler mornings still lately so it'll start a little lower, but by the end of my ride I'm above 1,000.
    #3
  4. bmwrench

    bmwrench Long timer

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    There are two things that most often cause flickering of the oil light: failure of the switch, or incorrect assembly of the oil filter o-ring and cover. There are other possibilities, of course: worn bearings, excessively thin oils, or really unlikely, a stuck oil pressure relief valve.
    #4
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  5. shadeobrady

    shadeobrady Adventurer

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    I may end up just doing an oil change this weekend then. Since I've been riding it a bit since the last one (which was done by the previous owner), that seems like it would have been happening right from the start if they did an improper install. Any insight on that? I wonder if the switch is failing which would be a cheap and easy fix... may just grab a new one anyways.

    Also - haven't had to do an oil change on one of these in a bit and I remember the shims being pretty funky (and catastrophic sometimes if installed correctly). Know of any good image/video guides to this? Otherwise my manual and http://bmwmotorcycletech.info/oil.htm may be all I have to go on.
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  6. bmwrench

    bmwrench Long timer

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    Anton Largiader has an excellent piece on filter installion on his website, and you won't have to wade through the 'bum's copy.

    I would swap in a new switch if you don't have access to an oil pressure gauge.
    #6
  7. patrkbukly

    patrkbukly Life at 10,000 feet Supporter

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    Just my experience and I know this will sound ridiculous but;

    My 79 RT did that once after an oil change.
    I felt confident I measured the oil in precisely so I disassembled my filter, checked depth and all components then reinstalled and again checked amount. Same result.

    It would also happen as I began to slow to a stop, idle dropping to shy of 1,000.

    I went nut's pulling it all apart again, pulling the pan and inspecting and removing the screen, attaching an oil pressure gauge, replacing the pressure switch and changing the oil, filter, shim again and measuring the 2.2 quarts or whatever the book said with the same results.

    I then added 1/2 a quart more which put my level slightly over the line on my dipstick (without screwing it in) and the light never reappeared.

    That was many years ago and I have no logical explanation but thats at least what I went through once.

    I have since checked compression and ridden her 10's of thousands of miles so don't know what to say other than thats what happened to me.
    #7
  8. shadeobrady

    shadeobrady Adventurer

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    My oil is currently just shy of the 2 Ltr. max mark (maybe a little over 3/4 of the way there). Is it worth adding more? What's too much on an airhead?

    Also, if I'm already that close to the 'top off' line, wouldn't it be strange for the oil light to be coming on? I've read over 2 quarts the bike will often burn some of it off right away and settle you back down toward the 3/4 line.
    #8
  9. patrkbukly

    patrkbukly Life at 10,000 feet Supporter

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    Yes that would be the reasonable logic but I say try it. I defer to BMWWRENCH and the many other knowledgable folks on the site for real science and logic.
    I recognize what I am describing does not make sense but it happend to me just like that.
    WRENCH or many others here, can answer with real knowledge on the matter as opposed to my peculiar resolution.
    #9
  10. Warin

    Warin Retired

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    I'd give it an oil change. You don't know how long it has been in there.

    I'd also change the other fluids at the same time .. same reason.
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  11. MJH89

    MJH89 Adventurer

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    I’ve had the oil pressure light begin to flicker at idle on two separate occasions on two different airheads. Both times it was the oil pressure switch beginning to fail. A new switch and a couple minutes later and everything was back to normal.

    If you aren’t experiencing any other symptoms (change in engine sound, temp, idle speed, performance, etc.) then I’d be willing to bet it’s the switch itself.

    I think I remember paying around $12 for a new switch from a local auto parts store.
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  12. shadeobrady

    shadeobrady Adventurer

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    Definitely going to grab one of these then. Looks like it's part #61 31 1 243 414. Looks like it comes with a washer. They mention you can order a hollow washer instead (#PN 07 11 9 963 132) - not sure what that's about.

    Do you know if I should order that exact switch part number above? $12 sounds more like something from NAPA... I'm assuming you found a generic alternative?
    #12
  13. greentreephrog

    greentreephrog Been here awhile

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    I had it happen once also. Again, 'wrench mentions what I found - a friend had changed the oil filter and the steel shim had moved out of place as he located the filter cover. It was bent, so preventing the o-ring from sealing correctly. A simple fix, and now I always 'glue' the parts in place with a small dab of grease when I replace.
    #13
  14. hardwaregrrl

    hardwaregrrl Can't shoot straight Supporter

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    you could also measure oil pressure a the switch using a standard gauge. My pressure measured fine until it didn't. I've also had switch failures and just a dirty connection. Maybe pull the wire and cleaning it and the switch.
    #14
  15. MJH89

    MJH89 Adventurer

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    I'm in Europe, so it wasn't actually NAPA where I picked up the switch, but the same type of store. The only reason I ended up going with a generic part was because I didn't want to wait the 2 or 3 days that it would have taken to get one from BMW. If I had had the time to wait for the BMW part (which is probably Bosch anyways) I probably would have spent the couple extra dollars and ordered it from them.

    The oil pressure switch is not specific to BMW, it also fits many European car engines (Citroen, Peugeot, Fiat, Mercedes, BMW and VW all used this switch on various models). For example, I believe the BMW E30 3 series cars with the 1.8L SOHC engine (1982-1987) used the same switch. If you bring the old switch with you to the auto parts store it should be clear right away if what they have is the same.

    I dug up some of my notes from a couple years ago when I last had to replace an airhead oil pressure switch. Please double check for yourself regarding compatibility, but here are my notes for your reference:

    The airhead oil pressure switch has 12x1.5mm threads. (Some of the aftermarket parts manufacturers offer the same switch with either coarse straight threads or fine tapered threads.
    Make sure you get the one with the coarse straight threads and a washer.)
    • NAPA Oil Pressure Switch - Part # OP6065 - switches between 3.5-7.5 psi
    • AutoZone - Duralast Oil Pressure Switch - Part # PS486
    • Bosch - Part # 0 986 344 040 or Part # 0 344 101 040 (I think the second part number is an updated version of the same switch)
    • Hella - Part # 6ZL 008 280-061 - switches between 0.3-0.8 bar
    • Vemo - Part # V20-73-0122-1
    • Metzger - Part # 0910079
    Depending on the manufacturer the switch may be triggered at a slightly different pressure range, but they are all in the same ball park. They all seem to give a range rather than an exact pressure, so I assume they aren't really a precision switch to begin with. I remember being surprised at how low of a pressure it took for the switches to actually trigger and briefly searching for a switch with a higher trigger pressure before giving up on that idea.
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  16. Big Bamboo

    Big Bamboo Aircooled & Sunbaked Supporter

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    I have to ask. When you check the oil level with the dipstick, do you screw it in, or just touch it to the case...?
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  17. shadeobrady

    shadeobrady Adventurer

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    Thanks so much for this - will be looking for the part this weekend!
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  18. shadeobrady

    shadeobrady Adventurer

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    I unscrew, wipe, push down till it’s flush at the top, then check. I do not screw it in :)
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  19. tlub

    tlub Long timer Supporter

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    I'll add to those that had this happen. I had put together, from boxes of parts, a bitsa R75/5 for my sister in law. The oil light flickered at idle. Since I didn't know the history of the engine, and I had another, I assumed the lower end was toast. I swapped out the engine for one I knew was good, and spent about a week doing it. That engine was indeed good, and she has ridden it ever since. The old engine? Oh that. It was a bad sensor, and the lower end was just fine. It's now my spare.
    So-- I'd change the sensor first.
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  20. shadeobrady

    shadeobrady Adventurer

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    Does anyone know what the hell kind of oil switch this is? It looks completely different than the R75 or R100S bikes I've worked with in the past.

    Is this what is reading the oil temperature for the gauge the previous owner put in place of the dash clock? I suppose I could tear it off and put on an original, although it looks like I need to put a new terminal on the wire to work with the standard switches (I think they're called a Fully Insulated Spade?).

    Hopefully my easy part switch-out didn't just turn into something a lot more difficult...

    [​IMG]
    #20