456gt speedometer sender | FerrariChat
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456gt speedometer sender

Discussion in '456/550/575' started by uberlink, Oct 25, 2025 at 8:34 AM.

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  1. uberlink

    uberlink Formula Junior

    Apr 23, 2012
    780
    Twin Cities, MN
    Full Name:
    Mark Johnson
    #1 uberlink, Oct 25, 2025 at 8:34 AM
    I have a 95 456gt with a manual. I’ve had a number of dash lights coming on — abs, spoiler, airbag, suspension — and now my speedometer has started failing intermittently. Coupled with this, when the speedo dies, the steering and suspension go loose.

    It all seems to point a one or more bad speed sensors.

    Googling around, I suspect the sensor on the transmission that feeds info to the speedometer is failing.

    Is that accessible to a normal person without a lift? How? If so, I’m tempted to pull it and clean the contacts (possibly try to test it somehow with a voltmeter) to see if I can figure it out.

    I may also take a look at the wheel speed sensors; those I can see. Any downside to unplugging them and cleaning contacts, to see if that helps?

    Thanks!
     
  2. Qavion

    Qavion F1 World Champ
    Silver Subscribed

    Feb 20, 2015
    14,634
    Sydney
    Full Name:
    Ian Riddell
    #2 Qavion, Oct 25, 2025 at 7:10 PM
    With so many faults, I would expect a common fault. I think the 456GT ABS system is independent of the gearbox speed sensor (because it has its own wheel speed sensors and the WSM shows no ECU speed input from a non-wheel speed sensor). Not sure about the Airbag system.

    Low battery voltage? Does your car start normally?

    Unfortunately we have no wiring diagrams for the 456GT. Typically, cars of this era would send speed data from the gearbox sensor to the speedo.. and the speedo would then send speed data to other systems (suspension, engine, etc). Typically, the instruments, speed sensor, suspension, airbag, etc, share a common fuse (not sure about the ABS). Try reseating the Instrument fuse. Make sure the contacts aren't corroded. Sometimes relay panel circuit boards fail. Try wiggling the fuse with the car running, to see if anything changes.

    On the F355, there is a circuit in the speedo which divides the speed signal frequency in two, then distributes the speed signal to other systems. So even if the speedo is working, the divider may not be.
    Usually the wire colours have faded on the gearbox speed sensor and you may not be able to identify the the 12 volt power wire (possibly green/white). I don't know how hard it is to access on the 456GT, but it's a pain to change on the F355. Your best access may be through the wheel arch? Experts? If you can get access to the rear of the speedo, also check for loose connections.
     
  3. uberlink

    uberlink Formula Junior

    Apr 23, 2012
    780
    Twin Cities, MN
    Full Name:
    Mark Johnson
    #3 uberlink, Oct 25, 2025 at 9:52 PM
    I’ve had the abs light on and off since I’ve owned the car—10+ years. So I think that’s its own thing, and I think likely a wheel speed sensor.

    The airbag light has occasionally lit up over the years. Maybe unrelated.

    The Speedo failure, the steering feeling light / floaty, and the suspension light are all new. I’ll have to mess with stuff a bit. I appreciate the suggestions very much!
     
    Qavion likes this.
  4. uberlink

    uberlink Formula Junior

    Apr 23, 2012
    780
    Twin Cities, MN
    Full Name:
    Mark Johnson
    #4 uberlink, Oct 25, 2025 at 10:20 PM
    Update - pulled the instrument fuse. All looked good. No evidence of corrosion or other issues. Plugged it back in. No change.

    I feel like I need to get my hands on the sender on the transmission to clean the contacts or possibly just replace it. Just no idea how…
     
    Qavion likes this.

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