308 Speedo, Odo, Sender System Fix | Page 3 | FerrariChat

308 Speedo, Odo, Sender System Fix

Discussion in 'Technical Q&A' started by RJay, Apr 8, 2006.

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  1. Paul_308

    Paul_308 Formula 3

    Mar 12, 2004
    2,345
    #51 Paul_308, Dec 24, 2010
    Last edited by a moderator: Sep 7, 2017
    Steve, never having held an Impulse Generator I've assumed the it is like the 3x8 TDC sensors, or everyday auto Crankshaft Position Sensors with no moving parts, just a metalic snoot extending near the internal rotating magnetic part (left photo from JagsThatRun.com). I'm starting to wonder if the Impulse Generator has a rotating element which engages into the gearbox (right photo with speedometer cable input to gearbox). The Impulse Generator innard photos above, while offering a great for the electronics, provide no clues about the business end of the sensor. Can you clarify? IMHO the coil on the pc board looks to be no where close (1"+) to what would be necessary for a variable reluctance type, normally 1/4" or less.

    Slight change of topic, there was a thread about the Impulse Generator mounting upward down until a certain year then mounting changed to facing downward. I wonder if change may have been made because of oil leaks into the part causing failures. In fact there is currently a thread about 'how much gear oil leaks out when the generator is removed' suggesting oil intentionally bathing the snout of the generator.

    For the record, my immediate interest involves introducing a speed correction module (SpeedoDRD) between Impulse Generator and Speedometer. They have furnished an electronic buffer between Generator and their DRD and another between their DRD and the speedometer. Before I do this I need to fully understand both buffers need, functionality and long term effects, to fully protect my speedometer.
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  2. Steve Magnusson

    Steve Magnusson Two Time F1 World Champ
    Lifetime Rossa

    Jan 11, 2001
    25,041
    30°30'40" N 97°35'41" W (Texas)
    Full Name:
    Steve Magnusson
    Yes, I also believe they both operate on the variable reluctance principle, but

    the rotating part need not be a magnet -- it just has to be a piece of magnetic material that can carry magnetic flux and change the amount of flux passing thru the coil. On the F flywheel sensors, the magnet (and the coil) is built into the sensor itself, and the rotating part is just a piece of iron (either the teeth on the ring gear or the pins in the flywheel). When the rotating part is not in front of the sensor, the magnetic flux passing thru the coil is low; when the rotating part is in front of the sensor, this better completes the magnetic circuit, and the magnetic flux passing thru the coil is high. The voltage generated across the coil is proportional to the change in flux with time -- i.e., V = K*d(flux)/dt (that is, the coil generates a voltage pulse when there is a change in the flux. If the flux is just constant, whether high or low, the voltage generated is zero.)

    Agree with you that whatever is on the rotor that causes the change in magnetic flux has to be fairly close to the coil and the thing carrying the flux thru the coil, but I believe that that is the case as described by this jpeg:
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    Steven -- Can you indicate whether the flat piece with 4 wide blades rotates or the flat piece with 3 narrow blades rotates?
     
  3. steven_ew

    steven_ew Karting

    Apr 3, 2009
    107
    Yeh, that's right-

    first of all- all the muck =oil/dirt/grime etc. The car did have a significant leak from the front crankshaft oil seal (now replaced) and I expect after years it all just seeped in, and probably didn't do wonders for the electronics... Now cleaned up nicely in ultrasonic bath.

    The part with the arrow - the 'pillar' is a tiny little coil of wire that fits between the four-pronged mount and comes close to the rotating three-pronged rotor below; so yes does come into fairly close proximity.

    Presumably this is a reluctor device that is similar to how electronic ignition is triggered then?

    As you say those photos are the 'wrong way up' -> that is exactly how the sender came apart. I will post some more pics of cleaned up circuit board together with an attempt at the schematic in the next couple of days. When first tested as is (like this) it didn't produce anything, but after replacing the two transistors it came to life at a low voltage

    Merry Christmas!
    Steven
     
  4. Steve Magnusson

    Steve Magnusson Two Time F1 World Champ
    Lifetime Rossa

    Jan 11, 2001
    25,041
    30°30'40" N 97°35'41" W (Texas)
    Full Name:
    Steve Magnusson
    #54 Steve Magnusson, Dec 24, 2010
    Last edited: Dec 24, 2010
    Exactly. Please do post the schematic if you can -- the fact that you are getting output pulses now is a good sign that the voltage pulses from the coil are being generated and detected; however, the low amplitude output signal seems more like something is wrong in the electronics (i.e., the voltage pulses from the coil usually aren't sent to the speedometer directly, rather they trigger something else to send more constant amplitude pulses to the speedometer as the voltage pulses from the coil will vary greatly with rotor speed).
     
  5. stephenofkanza

    stephenofkanza Formula Junior

    Mar 5, 2005
    542
    Kansas
    Full Name:
    Stephen LeRoy Sherma
    I have an 81 308 that the sender is mounted on the top.
    Most 81's and onward the sender is mounted on the bottom - where there is oil is the oil loss. The change coincided when the transaxle got a pump.

    Stephen



     
  6. steven_ew

    steven_ew Karting

    Apr 3, 2009
    107
    Hi Paul - that's right; what happens is the speedo drive (on my 79 car) is screwed onto a boss mounted on top of the gearbox. There is a small (~?3mm) rotating square section that mates with a drive coming out of the gearbox... this drives the reluctor(?) blades that you see in the 'top down' photo and come close to the coil etc.

    Should be allowed free at some point this afternoon for some more work on it ;-) ;-)
     
  7. Paul_308

    Paul_308 Formula 3

    Mar 12, 2004
    2,345
    Thanks Steven, that clears up many things for me, including the coil in the photo is actually at the moving vane instead of some distance away, changing it's role from choke to pickup coil. I wanted to believe these impulse generator were made like the TDC sensors at the flywheel, picking up a flywheel signal but that confirms otherwise that the entire mechanical and electronic circuitry are integral to the impulse generator.

    In all probability, the Veglia electronic speedometer design is an adaption to earlier mechanical cables systems, such as my right pic above whereas the TDC sensor design is like the left pic. Thus requiring no modification to the gearbox. I'm getting more confident that these things are repairable although most would be happy to plunk down the price for a new part.

    Can't say why I'm not just satisfied to enjoy my car from the driver's seat but also derive satisfaction understanding how it's made. I am eager to see the 'cleaned up' photos to get closer to the circuit.
     
  8. steven_ew

    steven_ew Karting

    Apr 3, 2009
    107
    I know - new part in UK is ~£90 - but I want the challenge to fix it!! Working on circuit diagram just now and will post shortly.

    One correction though - I posted about the vain from memory the other night - looking at it again now it's actually the FOUR eared part that turns.

    S
     
  9. steven_ew

    steven_ew Karting

    Apr 3, 2009
    107
    #59 steven_ew, Dec 27, 2010
    Last edited by a moderator: Sep 7, 2017
    Hi!


    Right, so, here it is! My attempt at writing out the schematic. I apologise for my lack of electrical knowledge, so it's not necessarily written out correctly!

    I don't understand how it works either.. I was expecting the coil to be connected to at least one of the transistor bases...

    In the photos the long new red and blue wires are the wires that the kind instrument technician used to connect up to the test bed/oscilloscope.

    I know previous posters have said the body of the sender is ground = but looking at it closely now the circuit board is completely isolated from ground and so it appears to me that the red/black wire (which sends the pulse to the speedo) must go through the speedo and be connected to ground there?

    047J100 - the yellow box - I think is a capacitor?#

    Interestingly the little coil seems to consist of two coils, which seem to be connected together at one end.

    It was quite tricky to write out so I might have made some mistakes and I'll be very interested to hear if someone can understand how this circuit will work??!?

    THanks

    Steven
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  10. steven_ew

    steven_ew Karting

    Apr 3, 2009
    107
    Also, the emitter of T2 does not seem to be connected to anything!>?!
     
  11. Steve Magnusson

    Steve Magnusson Two Time F1 World Champ
    Lifetime Rossa

    Jan 11, 2001
    25,041
    30°30'40" N 97°35'41" W (Texas)
    Full Name:
    Steve Magnusson
    Thanks for the correction -- that would make sense as the distance from the rotating vane to the coil end needs to be as small as practical.

    Can you tell if the two coils are wound in the same directions or different directions? Anything visual to tell if one coil has a lot more turns than the other or are they about the same # of turns? Is the center post of the coil a small permanet magnet or just a piece of iron (might be hard to tell, because, if only iron, it could be in a magnetized state)?
     
  12. steven_ew

    steven_ew Karting

    Apr 3, 2009
    107
    Well, it is tiny.. But

    1. I think there are two coils. On the reverse side there are certainly three solder joints and I'm very sure one of them has two wires from the coil...
    2. Bearing that in mind - looking at the coil there ia one complete 'layer' and one half layer - so I wonder if one coil is half the size of the other.
    3. It's hard to know because it's so small if they go in different directions- but I think they might be wound in opposite directions
    4. The core seems to be magnetic a little - it deflects a magnetic common pin and minutely directs a compass. So I think it may well only have been iron and is now a tiny hit magnetic!

    Does that help!!

    S
     
  13. Steve Magnusson

    Steve Magnusson Two Time F1 World Champ
    Lifetime Rossa

    Jan 11, 2001
    25,041
    30°30'40" N 97°35'41" W (Texas)
    Full Name:
    Steve Magnusson
    If you can individually follow the two wires that are connected to the same place -- if they go to the same side of the post, they are wound in the same direction; if they go to opposite sides of the post, they are wound in opposite directions.

    Every little bit helps! ;) It is a complication to understanding that there is no ground in the pulse generatoir itself -- might require understanding what's inside the speedometer to see how it all works. Having that one emitter not used is sort of a curiosity...
     
  14. Paul_308

    Paul_308 Formula 3

    Mar 12, 2004
    2,345
    #64 Paul_308, Dec 27, 2010
    Last edited by a moderator: Sep 7, 2017
    I will have several corrections to the circuit later...my eye is quite problematic right now. But I can post the photoshopped image of what I will work from. Someone asked about the lack of emitter connection, well it is just bad copper which has come off, quite obvious from the picture (can no longer call it a photo).
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  15. Paul_308

    Paul_308 Formula 3

    Mar 12, 2004
    2,345
    #65 Paul_308, Dec 27, 2010
    Last edited by a moderator: Sep 7, 2017
    Actually, it had to be connected to the copper trace just below it for a complete current path through both transistors. Given presence of vibration, having an unsupported transistor would have caused significant flex to the board especially at the small emitter pad , so the pad would break away easily and eventually fracture from the main trace. A better layout shown at bottom, keep this in mind for the fix.

    Be interesting to know how many sensors lost their life this way. Once you're completely satisfied everything is ok, place a dab of soft epoxy (encapsulating type) on the transistors and coil capturing them to the pcb to prevent danger of vibration causing a future problem.

    Your circuit drawing shows R1 between transistor collectors and in fact it is between T1 collector and T2 emitter (the large 'C' trace below). Also, the diode is a zener (drawn with wings on the cathode).

    I'm anxious for you to re-establish the connection using a piece of #22 solid copper wire (standard method of strengthening a pcb trace when copper adhesion is an issue) and test it out again for voltage.
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  16. steven_ew

    steven_ew Karting

    Apr 3, 2009
    107
    #66 steven_ew, Dec 28, 2010
    Last edited by a moderator: Sep 7, 2017
    The easiest way was to take a photo and zoom in - which seems to reveal the two connected wires to go to the left of the coil - so I guess they are wound in the same direction.
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  17. steven_ew

    steven_ew Karting

    Apr 3, 2009
    107
    But; I don't know that for definite because it could be that the two non-connected wires go to the same side - it's just so hard to tell because of the gunk on the board, and I'm not very keen to clean it more and risk damaging the coil~!!
     
  18. steven_ew

    steven_ew Karting

    Apr 3, 2009
    107
    #68 steven_ew, Dec 28, 2010
    Last edited by a moderator: Sep 7, 2017
    That's very interesting, and would certainly solve the problem of a transistor with only two connections..
    Looking at the board again (see the zoom below) I can definitely see an area where there would have been a round copper, but I'm not sure it was connected because the solder on the 'C' section has a very sharp edge?
    I guess there's not much to be lost from trying though!!

    SW
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  19. steven_ew

    steven_ew Karting

    Apr 3, 2009
    107
    #69 steven_ew, Jan 10, 2011
    Last edited by a moderator: Sep 7, 2017
    Right, the plot thickens!

    Here is a photo of the back of the sender, before unsoldering the transistors.

    I think it's quite clear that one of the transistor legs was in fact never connected on the circuit?

    Very strange

    Steven
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  20. Paul_308

    Paul_308 Formula 3

    Mar 12, 2004
    2,345
    Thanks for posting that last photo Steve, it's very compelling. So I give, for now. That transistor is being used as a diode, either because they got a better deal on buying twice as many transistors vs cost of a diode...or most likely...it was mandatory to exactly match junction characteristics of the primary transistor. Some dual transistors are even made for this purpose. Both are valid reasons I've seen done before.

    I still plan on breadboarding the circuit this spring, guessing on the coil turns and using a drill to turn a steel bar. Magent placement will be figured out when it all comes together as I don't know where to find one small enough to fit into the small dia coil core. Hope I won't need to take one of my guitar pickups apart. Just have to know what's going on.
     
  21. karabil

    karabil Karting

    Feb 22, 2010
    112
    Australia
    Help--my 328 started clocking miles while stopped at lights and now turns over at about 80 MPH with the engine idling in the garage! If I stop the engine and restart the speedo and odo will be normal (o reading) for about 1 or 2 mins. I have disconnected the sensor but it still does the same thing. Who is going to believbe me when it comes time to sell that the miles were clocked up in the garage???!!!
    Can some one say if I am right in assuming that the speedo must be getting a signal from somewhere. I have asked a number of mechanics and they say that they have never experienced the problem.
     
  22. Paul_308

    Paul_308 Formula 3

    Mar 12, 2004
    2,345
    Normally, extraneous electrical impulses from outside the speedometer would affect BOTH speedo and odo, not just one. The speedometer circuit contains a NLA integrated circuit which sends the processed impulse generator signal to both the speedometer and the odometer. Lurid details lurk in the beginnings of this very never-ending thread.

    These electronic problems can be fixed by specialty shops such as Palo Alto Speedometer in the Bay Area (CA) and others. Another shop was recommended recently but I can't find it quickly. Note that not everyone loves PAS unconditionally because of slowness and ??.
    Repair costs appear to approach $200.

    Removal of the speedometer is addressed by posts such as:
    http://www.ferrarichat.com/forum/showthread.php?t=218109
    http://www.ferrarichat.com/forum/showpost.php?p=65461&postcount=2
    http://www.ferrarichat.com/forum/showpost.php?p=135801940&postcount=3
    Also, someone suggested using dental floss to remove the 4 plastic corner lights
     
  23. karabil

    karabil Karting

    Feb 22, 2010
    112
    Australia
    Sorry Paul , I may not have been clear but yes both the odo and speedo start running together. I can start the car and all is fine for maybe 2-3 minuites then suddenly the speedo reads around 80 mph and the odo starts turning ant around a 10th of a mile per second or two. I am actually in Australia so USA guys are probably not relevent.
    Any way thanks for the info and if you have further ideas i would love to hear them.
    Thanks Bill
     
  24. Paul_308

    Paul_308 Formula 3

    Mar 12, 2004
    2,345
    Appreciate the clarification for meaning of 'turns over at 80'...now I understand the symptoms which do not point to a bad speedometer, indeed sounds like an outside signal. There is some chance that the alternator is putting out spikes causing this. Can you disconnect the alternator, taping it off safely?
     
  25. Paul_308

    Paul_308 Formula 3

    Mar 12, 2004
    2,345
    I am in the process of installing a speedometer correction unit and they (12oclock.com) caution that noise from spark sources can cause spurious readings as well. These units are made for motorcycles so conditions are different yet principles and circuitry are the same. So, have a look at your engine running in an midnight unlit garage looking for an errant spark wire.
     

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