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Speedometer and Odometer Not Working

Discussion in '308/328' started by Lawrence Coppari, Nov 2, 2020.

  1. Lawrence Coppari

    Lawrence Coppari Formula 3

    Apr 29, 2002
    1,968
    Kingsport, TN
    Full Name:
    Lawrence A. Coppari
    Yesterday both my speedometer and odometer did not work when driving my '87 328GTS. I have the car on my lift at the moment have access to the sensor. I unplugged it but found no voltage at the leads with the ignition on but engine not running. Should I have 12V to ground on either of the leads? Tachometer and all other gauges work fine.
     
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  3. Steve Magnusson

    Steve Magnusson Two Time F1 World Champ
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    Jan 11, 2001
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    #2 Steve Magnusson, Nov 2, 2020
    Last edited: Nov 2, 2020
    Yes, the VB, green-white, wire at the 116987 speed sensor should be +12V relative to ground with the key "on". Based on your location, please confirm it is a US version 328.

    Edit: Your tach working confirms that a lot of upstream stuff supplying this +12V power is OK. If you don't have +12V power on the VB wire with key "on" (but the tach is working) = check/unplug/reseat the connector in the passenger footwell and/or the connector in the engine compartment.
     
  4. Lawrence Coppari

    Lawrence Coppari Formula 3

    Apr 29, 2002
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    Kingsport, TN
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    Lawrence A. Coppari
    Steve, yes it is a US version. The fuel gauge works with the key on. Attached is a piece of a schematic for an 89 model year. Is the connector block for the engine wiring harness (169) in the passenger footwell so I can check voltage there?
     

    Attached Files:

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  5. Steve Magnusson

    Steve Magnusson Two Time F1 World Champ
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    Jan 11, 2001
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    Connector 169 is in the engine bay (so do check there). Connector 135 is in the passenger footwell so check there if connector 169 is dead.
     
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  6. Lawrence Coppari

    Lawrence Coppari Formula 3

    Apr 29, 2002
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    OK, I'll raise the vehicle again and look for connector 169. I visited 135 a number of years ago for something else.
     
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  8. Steve Magnusson

    Steve Magnusson Two Time F1 World Champ
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  9. Lawrence Coppari

    Lawrence Coppari Formula 3

    Apr 29, 2002
    1,968
    Kingsport, TN
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    Lawrence A. Coppari
    Yes 169 is right by the oil cooler. I pulled it apart and put it back an now have 12V at the sensor. I sprayed both parts with contact cleaner before reconnecting. However, the speedometer and the odometer still do not work. I was sure they would work because before I had zero volts at the sensor and now I have 12V. I checked the resistance of the sensor. One way it is 9.5 Mohms; the other way is 17 Mohms.
     
  10. Lawrence Coppari

    Lawrence Coppari Formula 3

    Apr 29, 2002
    1,968
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    And that seems high to me.
     
  11. Steve Magnusson

    Steve Magnusson Two Time F1 World Champ
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    Can't help you with that. You might do a search on the speed sensor part number "116987" to get prior threads. I know someone has done some pretty serious design work to rebuild/re-engineer this sensor, but can't exactly remember who offhand (Adrian?) -- maybe he knows if there's some measurements that can be made on the stock sensor itself that could either confirm goodness or confirm badness.
     
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  13. Lawrence Coppari

    Lawrence Coppari Formula 3

    Apr 29, 2002
    1,968
    Kingsport, TN
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    Lawrence A. Coppari
    Steve, thanks for the help.

    As my background is not electrical I have a question. I have two Fluke multimeters. One of them reads Hertz both in AC and DC. In DC, if I attach a lead to a battery pack then quickly repeatedly tap the other lead on the other side of the battery pack, the meter tells me the cycles per second of my tapping. So my question is can I put the multimeter in parallel with the sensor and determine if there is any output from the sensor when the rear wheels turn with the rear of the vehicle raised.
     
  14. Steve Magnusson

    Steve Magnusson Two Time F1 World Champ
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    Yes, I believe the voltage signal in the RN (red-black) wire to ground should be some sort of square wave that would give an AC, or Hertz, result on a multi-meter when the wheels are turning (and the VB wire is at +12V) if it was working.
     
  15. Lawrence Coppari

    Lawrence Coppari Formula 3

    Apr 29, 2002
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    I'll give it a try by checking the RN lead to ground.
     
  16. Steve Magnusson

    Steve Magnusson Two Time F1 World Champ
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    Forgot to add that this test needs to be done with everything plugged in (not at a partial unplugged speed sensor connector) -- i.e., probe the RN wire terminal at the plugged in 169 connector.
     
  17. Lawrence Coppari

    Lawrence Coppari Formula 3

    Apr 29, 2002
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    Kingsport, TN
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    The wires to the 169 connector run through a conduit so I could not get at them at the back of the 169 connector. The plug has a 33 year old rubber boot over it and I'm afraid that I will tear the boot trying to slide it down the conduit in order to get at the back of the connector to insert a probe. The connector at the sensor has two staggered prongs, one on the sensor side and the other on the lead in side. I opened the connector and wrapped a thin braided CU wire around the male RN lead, then closed the connector. I got a signal from red-black to ground whose value kept time with the spinning rear wheels. This causes me to feel the sensor is working.

    I assume the signal goes through one of the connectors that are in the passenger floorboard area. I plan on checking there next.
     
  18. Lawrence Coppari

    Lawrence Coppari Formula 3

    Apr 29, 2002
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    Kingsport, TN
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    Located the red-black wire under the footwell from the conduit that comes from the rear of the car. The signal is present there. Incidentally, 109 HZ is about 30 mph judging from the tach. So now this leaves me with three possibilities. The signal gets lost between the connector and the speedometer/odometer unit, or the speedometer/odometer is bad, or there is a bad ground at the speedometer. Speedometer needle can't be stuck because odometer does not work.
     
  19. Lawrence Coppari

    Lawrence Coppari Formula 3

    Apr 29, 2002
    1,968
    Kingsport, TN
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    Lawrence A. Coppari
    This morning I got to the back of the speedometer and checked the three wires that produce the speed reading/distance traveled. They go to a large red male/female connector behind the unit. The green registered 12V, the black was ground, and the red-black produced frequencies with the rear of the 328 raised and wheels turning. Therefore I must conclude the speedometer unit is on the fritz. Six years or so ago I sent my clock off to D&M Restoration in S. Carolina. They fixed it and it still works perfectly. They also repair speedometers and tachometers. Has anyone had experience with them regarding speedometers? Many mention the shop in Palo Alto but I'd rather use one on this side of the country.

    I was able to trace wires and get readings at the back of the unit at the red connector by removing the bracket that holds the unit in place. Doing that enabled the unit to be pulled forward out of its hole in the metal plate. The steering wheel was lowered first so the metal plate cleared the steering column enabling me to get my hands behind the unit to remove the two round nuts that hold the unit in place.
     
  20. ducowti

    ducowti Formula 3

    Jan 27, 2008
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    David
    Paola Alto Speedometer is the go-to for these Ferrari/Veglia units. IIRC a bench test is/was about $75.
    I assume you checked the ground? I'm not a fraction as knowledgeable w.matters of electronics as is Steve, and probably you, but in my experience across boats, motorcycles, and my 328, aside from a loose connector 169 being the culprit in my 328 speedo's problems, the problem has almost always been a bad groun; Occam's razor!
     
  21. Lawrence Coppari

    Lawrence Coppari Formula 3

    Apr 29, 2002
    1,968
    Kingsport, TN
    Full Name:
    Lawrence A. Coppari
    "The green registered 12V, the black was ground, and the red-black produced frequencies with the rear of the 328 raised and wheels turning." Perhaps I should have stated it differently but my intent was to say that in addition to the unit getting 12V and a pulse signal, the unit also had a proper ground.
     

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