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amplifying signal from crank position sensor

Discussion in 'Electronic Basics' started by kell, May 28, 2007.

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  1. kell

    kell Guest

    My 1989 Jeep Cherokee started having an intermittent problem where it
    dies sometimes when I'm driving and is getting harder to start..
    After reading this page:
    http://www.lunghd.com/Tech_Articles/Engine/Basic_Sensors_Diagnostics.htm
    I tested the crank position sensor (cps) with a voltmeter. It showed
    about .4 volts ac or a little less when cranking. That's below the
    acceptable voltage range of .5 to .8 volts stated in the article.
    The cps is on the bell housing, very difficult to get to. It's
    recommended that you lower the transmission a little. You need a
    floor jack, which I don't have.
    I got the idea of building a circuit to amplify the signal from the
    cps.
    Some basic facts... there are only two wires coming from the cps.
    With the car turned off, the cps is supposed to measure about 275 ohms
    (mine measures 230). As I indicated before, the cps generates a small
    signal (detectable with a voltmeter set on ac) when the flywheel is
    turning and the slots are flying past the sensor.
    While I had the connector unplugged, I also tested the pins on the
    wiring harness side of the plug. With the ignition key turned on both
    pins show 3.37 volts with respect to ground and zero volts across the
    pins.
    I need some kind of fix, even a temporary one, just to get the car
    running. I park on the street and have to move it periodically.
    I thought of using a non-inverting op amp circuit, powering it
    independently (just a couple of nine volt batteries for now) because
    I'm not sure how it would work using the vehicle's electrical system
    to supply the power. Can you use a single supply if the op amp is
    amplifying a small ac voltage several volts above ground? There
    wouldn't really be a fixed ground for the op amp to refer to.
     
  2. kell

    kell Guest

    Now that I think of it, the 3.37 volts is ground for the op amp. So
    maybe it would work to run it off the vehicle's battery...
    Should I do this?
     
  3. Jim Thompson

    Jim Thompson Guest

    Conduct another test first. While cranking are you getting a spark?

    Pull the _ignition_module_ and take it to a shop that can test it.

    ...Jim Thompson
     
  4. kell

    kell Guest

    Assuming the ignition module is that thing with the coil clipped into
    it. I pulled that out. Then I took the coil out of the ignition
    module, clipped a cap across the primary terminals and touched 12
    volts to it, got a phat spark. So the coil is ok. But there are
    apparently some electronics in the module, because it has more than
    just two wires going in and there's a cavity under it filled with some
    potting material. So now I have to go and find somebody who can test
    this module? Sheesh, why can't they make cars a guy can FIX.
     
  5. kell

    kell Guest

    Forgot to mention, no spark during crank.
     
  6. Jamie

    Jamie Guest

    Hmm, by looking at the specs i'm guess it's a reluctant type of
    pick up?
    since the ohm reading is low on yours, it can be assumed that
    a couple of turns are shorted there by reducing output voltage..
    how about using a step up transformer.
    something in the audio class should do it.
    you can use an pot on one side to shunt out over voltage.

    Using something like a BuckBoost configuration..
    Radio Shaft has an audio output transformer you could use
    by putting the primary leads across the sensor, then, connect
    one of the leads of the secondary to one of the primary leads.
    the other secondary lead will be a lead going to the computer.
    and remaining lead of sensor which is also connected to the
    primary of the transformer will go to the computer..
    in the old days, Picture tube boosters were like this.
    you will have to play with the phase polarity to get it to
    boost.
    The ratio of this transformer should be close enough to give you
    a slight raise in voltage.
    the primary has a CT so you can play a bit..
    You could also use the 1:1 audio xformer they have with the same
    config, that would double it. use a shunt R on one side to drain
    out over voltage.
    so in your case. 0.4 volts would be 0.8 in theory how ever, roughly
    thinking about loss from the sensor due to extra load and all it most
    likely will end up to be around .6 volts/
     
  7. kell

    kell Guest

    Question: I have in my box of magic junk a pre-wound pot core of a
    power ferrite material with a 1:1 ratio, almost an inch and a half in
    diameter. I think each winding has 860 uH inductance. Would it work?
     
  8. Jamie

    Jamie Guest

    Nope
    you need something with much higher L in it.
    You could try it but I don't think it'll work.
    something in the H/ high mh range..
     
  9. kell

    kell Guest

    I typed in "audio transformer" at the Radio Shack site and found one
    http://www.radioshack.com/search/in...dio transformer&origkw=audio transformer&sr=1
    but they don't seem to give the ratio? I'd like to use 1:1. Don't
    see how I could use a 600:1 tranny, which I think is the other typical
    ratio for an audio transformer, isn't it? I guess I'll have to go
    into the store and look at the package to find out.
     
  10. kell

    kell Guest

    Okay, I found a 1:1 transformer at RS, item 273-1374
    600-900 ohms
    300Hz to 5KHz response
     
  11. It is a Variable Reluctance Sensor.
    The amplifier will be a differential amplifier with
    about a few k of input resistance. There is probably
    an added dc bias current at the inputs that allows
    detection of open-circuit sensor, (and sometimes
    short-circuit).

    It is therefore not a case of simply providing an
    extra stage of amplification. Any extra amplifier
    will probably have to fake the floating dc resistance
    of the VRS. This will be a transformer output, padded
    up to about the same 240 ohms output resistance.
    In the event of a 'failed-sensor' detection, (real or
    false), the electronics might well cut the engine.

    Poor connections can often cause a false failed-sensor.

    So the first thing to try is to get rid of that plug
    and socket, and do a hardwire, (as suggested in the URL).
    Use a three-way screw terminal block.

    A VRS has an output impedance that looks like an inductor
    and resistor in series. In an industrial app I have seen
    the output increased with a capacitor across the output
    leads.... of the order of 0.1uF or less.

    Perhaps you might try your cranking experiment again,
    stabbing various cap values in parallel to see what
    the effect is.
     
  12. kell

    kell Guest

    I could be wrong about this, but it seems to me the meter should see a
    bit of voltage across the pins in that case, which it doesn't. This
    vehicle is from 1989 and may be a bit less modern in that respect.
    Well, yesterday I pushed it to get it across the street to a legal
    spot. Damn jeep.
    I tried pinching the connectors to make them tighter, but I didn't
    want to cut the connector off the wiring harness.
    That's a good tip.

    I had another idea of sticking a dime-store magnet on top of the cps
    to make the magnetic field stronger. I figure it probably failed
    because heat and vibration weakened the magnet. There's certainly
    plenty of both at the top of the bell housing, and that vehicle has
    200k on it.
    The new sensor I bought has the same resistance as the old one, so I
    doubt the old one has shorted turns.

    Thanks for the info Tony.
     
  13. Phil Hobbs

    Phil Hobbs Guest

    If you use a $5 Nd-Fe-Bo magnet, you can probably remagnetize the thing,
    but be careful you do it in the right direction, or you'll screw up the
    timing something awful.

    Cheers,

    Phil Hobbs
     
  14. Jim Thompson

    Jim Thompson Guest

    Uuuuh? Is there really a magnet there, or just a steel "tooth" going
    by a VR pickup that has current flowing thru it.

    I know for a fact that ignition (distributor) VR pickups have no
    magnets.

    ...Jim Thompson
     
  15. Phil Hobbs

    Phil Hobbs Guest

    Hmm, could be. The ones I've seen (not automotive, admittedly) used
    permanent magnets. That way they don't require power.

    Cheers,

    Phil Hobbs
     
  16. Jim Thompson

    Jim Thompson Guest

    I misspoke, there are no _rotating_ magnets, but there is one
    underneath the rotor assembly.

    But flywheel pickups don't use magnets, at least those I know of...
    there's no convenient "loop".

    ...Jim Thompson
     
  17. Phil Hobbs

    Phil Hobbs Guest

    Any magnetic sensor is going to have exactly the same problem...if you
    use a bar magnet with a field of 1 kG, and lots of turns, it doesn't
    take much to give you a nice big signal. Remagnetizing the magnet with
    the wrong polarity is likely to make the ignition fire half a tooth
    early or late.

    Cheers,

    Phil Hobbs
     
  18. Jim Thompson

    Jim Thompson Guest

    Might be worth measuring and see if there's current flow in the pickup
    coil.

    ...Jim Thompson
     
  19. kell

    kell Guest

    Okay Tony,

    The capacitor trick worked. I tried 0.1 uF first; car started ran
    bad. Bunged in a 0.01 uF and ALL'S WELL.

    Jim & Phil:
    The crankshaft position sensor definitely has a permanent mag. It
    attracts a piece of steel.
    A passive device like that ought to last the life of the car, by
    rights. However, there's a lot of heat and vibration at the sensor
    mount on top of the transmission bellhousing. Weakens the magnet as
    the engine hours pile up. This thing is a routine maintenance item,
    but if it fails out in the middle of nowhere, PITA because your car
    won't run without it.
    This has given me an idea to sell a little device containing a
    capacitor that can be hitched onto the wiring harness with those quick-
    snap connectors that pierce the insulation of the wires. Cheap
    insurance at a few bucks. I think the off-road crowd would buy them.
    You're on top of a mountain somewhere and your jeep won't start
    because your cps has 130k on it. Snap the magic doodad on your wires
    and away you go.
     
  20. Marra

    Marra Guest

    Buy a new sensor !
     
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