Oil pressure light on...

Discussion in 'Airheads' started by headtube, Nov 22, 2012.

  1. headtube

    headtube 6 mesas de invierno!

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    ... my idle is about 800-900 rpm. (79 RS) The oil pressure light comes on at every stop. When I give her some throttle it disappears. If I set the idle above 1000, no red idiot light. I must admit though, that I prefer a low idle but I don't like the drop in pressure.

    Do all Bimmers do this? Perhaps a faulty OP switch? What does the forum recommend for an idle speed?
    #1
  2. Airhead Wrangler

    Airhead Wrangler Long timer

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    First question that everyone will ask:
    are you sure (REALLY SURE) that you aren't looking at the charge light? It will do exactly what you described when working correctly.

    It's a very common mistake. The oil pressure light should NEVER be on with the engine running regardless of idle speed unless someone changed out your pressure switch for one with a much higher switching pressure.
    #2
  3. headtube

    headtube 6 mesas de invierno!

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    You are correct sir! Thanks for pointing this out. Phew, a novice mistake. It's only when I took a good look that I noticed the order of idiot lights. The names are under the lights on the dash, not on top. So, what does that mean exactly when the GEN light comes on below 1000 rpm? No charging?
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  4. Airhead Wrangler

    Airhead Wrangler Long timer

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    Right, it means you aren't charging - you're using more electricity than you're making. It's operating at a loss usually up to about 2000 rpm. The light will go out around 1000 or 1100 when all's functioning correctly, but the stock system doesn't REALLY charge until around 2000 rpm. Too much idling in traffic can drain your battery if you really try hard. It doesn't become a real issue until you're trying to run electric clothes, extra lighting, or other large electrical load though.
    #4
  5. mark1305

    mark1305 Old Enough To Know Better

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    And once you've answered that question, the next best step is to put a real gauge to the motor and check the pressure at cold idle and hot idle (after 30+ minutes riding).

    The hardest part is finding an adapter for the threads of your oil pressure sensor. Don't know what year they changed from the tapered threads, but my 83 R80 has M12 X 1.5 threads. I made an adapter from 1/2" OD X 1/16" thickness aluminum tubing threaded with the M12 external on one end and 1/8 NPT internal at the other to use a simple pressure gauge from any pool suplly store or Home Depot to check actual pressure. Stick a flex hose from a grease gun in there between the adapter and the gauge if you want to get fancy.

    Edit: BTW, I have had an oil light scare the crap out of me on my R 80 just within the neighborhood. One of those things that makes you nervous for a long time, even after you find the culprit
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  6. DoktorT

    DoktorT BigBrowedNeandereer

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    Check your idle spec. My 79RS says 950-1050 rpm at idle. I set mine typically 1050 to 1100. Gen light out at idle.
    #6
  7. mark1305

    mark1305 Old Enough To Know Better

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    And better cam oiling at that rpm too.
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  8. khale

    khale ride dirty

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    My oil pressure light (yes, the amber light below the red charging indicator light) is always on, as soon as I start the bike and as I ride. Oil level is good.
    Do I have a bad sensor? What could be wrong?
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  9. Airhead Wrangler

    Airhead Wrangler Long timer

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    You better hope so. Get an oil pressure gauge and check it. If it's good, get a new sensor.
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  10. Big Bamboo

    Big Bamboo Aircooled & Sunbaked

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    Are you talking about the R75/5? First of all, I wouldn't ride it until I figured out what was wrong. For it to be on all the time without some major pressure loss would tend to indicate the wire is shorting to ground somewhere. I suppose it's possible for it to get stuck on, but I've never heard of that. Replacement senders are cheap, swap it out and see what happens. After checking for shorts etc first.
    #10
  11. Bill Harris

    Bill Harris Confirmed Curmudgeon

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    Ditto on what they said. Oil is the lifebood of any engine. :deal

    Quick and dirty way to test? Turn the ignition on. The oil light should be ON. Pull the connector from the oil pressure switch, the light should go OFF. Touch the connector to ground, the light should go ON. Depending on how it acts, you either have a good warning system, a bad bulb, or a bad sensor.

    --Bill
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  12. dm635

    dm635 I Roll

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    I was able to take my R80 home for a couple days before I commited to buy it. Main reason was because the oil light was on constantly. Where the crash bar was connected to the frame, it had pinched the wire to the oil pressure sensor. I then bought the bike.
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  13. bmwhacker

    bmwhacker Still on 3 wheels

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    Life in the Airhead BMW lane...you'll get used to it.:lol3
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  14. khale

    khale ride dirty

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    Thanks, Bill. I did as you said. Ignition on, oil light is ON. I pulled the connector on the oil pressure switch and the light remained ON on my gauge. Does this mean bad sensor or a ghost in my electrical system? (man, I hope not). I've already got another oil pressure switch on the way anyways.

    When I replace the sensor, do I need to drain the oil, or is it OK to just do a swap? Looking at the positioning of it, it looks like it's above the oil level and sump, so oil shouldn't come out when I pull it right?

    Also, I recently replaced my gauge incandescent bulbs with LED bulbs. Before swapping with LEDs, the oil pressure light (incandescent bulb) would be ON when the key is in ignition, but the light would turn OFF when the bike was cranked. I wonder if it was coincidence that my oil sensor went bad right around this time, or there maybe a deeper issue.
    #14
  15. Garbln

    Garbln Been here awhile

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    On my 76 R90/6 I had a flickering oil light at times. I changed the switch because it was leaking, that stopped the leak but not the flickering. The oil pan gasket was also seeping so I decided to change that also. When I pulled all the bolts out and dropped the pan I was surprised to find the round oil pickup laying in the bottom of the pan. The two bolts holding the pick up on to the pick up tube had unscrewed and let the pick up and the spacer/extender fall off. This meant that the oil pick up was actually about 3/4 inch shorter than it should have been! As long as the oil level was on the full mark I was OK, Fortunately I had not owned the bike very long and the last owner was very picky and always kept the oil level to the full mark. When I re assembled the pick up I locktighted the bolts and safety wired them in place, that ain't gonna happen again! Just something to be aware of if you get a flickering oil light when your oil is a little low and you make a fast stop.
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  16. disston

    disston ShadeTreeExpert

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    You better hope the new switch works. If it is Ghosts special procedures are required and they sometimes do more harm than good it seems.

    [​IMG]
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  17. NC BMW Rider

    NC BMW Rider Been here awhile

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    Sounds like the wire leading from the sensor is grounded somewhere between the sensor and the light. If you moved a bunch of wires around when you converted to LED's some insulation may have broken and now you are grounding somewhere along the wire.

    Good luck.
    #17
  18. khale

    khale ride dirty

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    thats a very good point. I'll investigate
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  19. khale

    khale ride dirty

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    You were exactly right. There was no insulation on the connector for the oil pressure switch bulb and a brown wire was in close contact with the wire, causing it to ground out.
    Thanks for everyone's help. :D:clap
    I'm also very glad I wasn't actually subjecting my engine to low oil pressure as well.
    #19