Speedometer woes

Discussion in '348/355' started by krazykarguy, Jul 9, 2018.

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  1. krazykarguy

    krazykarguy Formula Junior
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    Apr 17, 2014
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    Matt
    Hi all-

    This year, the speedo in my parent's car has been flaky at best. It typically works when the car is first started and cold. Once warmed up, the speedo needle fluctuates, dropping mph and eventually stopping on the "11-mph-this-is-a-Ferrari-and-it's-never-not-moving" pin. It happened every time I drove the car.

    I removed the instrument binnacle and 'tightened' the pins on the black wire of the connector, as they were the loosest, and I assumed a bad ground behind the speedometer unit. This had no effect.

    I also disconnected the speedo sensor cable on the LH side of the car, to inspect the pins for corrosion. I found none. The inside of both ends of the connection were like new.

    My assumption at this point is that the sending unit on the transaxle has almost given up the ghost. Is there anything else that I can/should check before replacing the sender (and creating a custom 32mm removal socket)??

    TIA-
    Matt
     
  2. jjtjr

    jjtjr Karting

    Aug 29, 2016
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    I would test the speed sensor with a DVOM. You should have 12 volts at the plug, and should receive a sine wave from the sensor to the speedo/ECU. Are you getting a check engine light too?
     
  3. Carper

    Carper Karting

    Aug 13, 2017
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    Suffolk, UK
    What model and year is the car ? I had identical problems with my 97 355 .... Sent head to a company in UK to fix and it was a dry solder joint. Cost £60 to diagnose and fix.
     
  4. taz355

    taz355 F1 Rookie
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    I have also seen the sending unit get melted by the headers
     
  5. Qavion

    Qavion Formula 3

    Feb 20, 2015
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    Sending unit failure seem to be the most common cause. They are not a simple coil device (on which you can do resisitance checks), but have electronics inside which can break down. It's odd, though, that you're not getting any other symptoms. The speed signal is sent to the speedo and the speedo then sends a signal to the F1 system (if applicable), the Motronics ECU/s and the suspension system.

    I'm not sure how you could test the output of the sender unit with the car in motion. Disconnecting the plug/s will affect too many systems (and generate computer faults which may require resetting with Ferrari workshop tools)

    I would be testing my toolmaking skills ;)
     
  6. krazykarguy

    krazykarguy Formula Junior
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    The car does indeed have a check engine light illuminated. I don't think that I've been in it when it doesn't. I don't have a list of current code(s), but could have my dad pull them. The Brake and ABS lights are on initially upon startup and then turn off about 10-15 seconds after starting the car, I'm not sure if this is related or not.

    To taz355's point, the car was recently outfitted with blanketed FabSpeed headers. The sensor is down pretty low and near the heat shielding for the LH inner CV boot, I'd think that it's pretty well shielded. Only the wiring and the connector (which I inspected) was 'near' the header pipes, and was in good condition with no melting.

    On the way home from the airport on Saturday, the 'lift system' (the amber light with the arrows pointing up to the wheels was also illuminated - but just that one time. The suspension switch still turned on and off the comfort setting, though. o_O

    I was under the assumption that it was the sensor - mainly because it seems to fail/stop working once it gets heated up (from the gearbox casing and the oil the sensor contacts). In the past, I thought it was pretty common that failing electronic components would act this way, right?
     
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  8. Qavion

    Qavion Formula 3

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    The wiring diagrams show a speedo sensor signal going into the speedometer on one wire and then coming out again on another wire to the computers. It doesn't show how much processing goes on inside the speedometer before it sends the data to the other systems, so it's hard to say what could be causing your problem (speedo internals or speed sensor). The Motronics ECU should throw a P0500 code for a speed sensing system problem, but unless we know how much processing goes on inside the speedo, I don't think it will tell us much.
    I don't believe Teeves or Bosch ABS systems use the speedo sensor, so the lights are probably just normal startup check lights. The ABS system looks at individual wheel speeds (using sensors on each wheel).

    I would go with the percentages and change the speed sensor. All electronics wear out in time (the more complicated they are). Yes, you could do a basic voltage check on the sensor plug to see if 12 volts are there, but since the speedo is intermittent, checking the voltage might not tell you much. Also, lots of systems on the car use the same fuse as the sensor, so you'd be seeing all kinds of problems if you lost power (including oil temperature and fuel quantity indication problems, air conditioning problems...).
     
  9. Qavion

    Qavion Formula 3

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    Just for reference... Here's a wiring diagram for the 5.2 car (showing the speed sensor circuit)... You can tap into the wiring at various points, but just be aware that having power on the car with some plugs disconnected can generate latched messages in computers which can't always be reset by turning the battery on and off.

    Instrument Wiring Diagram (5.2)
     
  10. Dave rocks

    Dave rocks F1 World Champ
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    Ian - absolutely fabulous job on those diagrams - very well done, sir :)
     
  11. krazykarguy

    krazykarguy Formula Junior
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    So fabulous, in fact, that the diagram is from the future!
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  12. Qavion

    Qavion Formula 3

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    LOL... Good eye... I made a last minute adjustment just after midnight in Sydney, Australia (14 hours ahead of you, I believe)
     
  13. Qavion

    Qavion Formula 3

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    Some interesting stuff on checking speed sensors....

    http://www.dinoplex.org/instruments/vegliaspeedosensor/

    Although the date range for the "Type 2" Veglia 68 0517 speed sensor goes to 1999, the part number shown on the webpage is only applicable (up) to the 348. Perhaps the only difference is the plug type on the F355?

    With a 9 volt power supply, a healthy sensor seems to produce a voltage range of 3~8 volts dc (with a 560 ohm resistor in series). Note, however, that the output of a Type 2 sensor is actually a pulsed (square) wave.

    I see someone was looking into creating new circuit boards for the Type 2's.

    https://www.ferrarichat.com/forum/threads/speed-sensor-update.483022/

    I don't know how hard it is to get into the speed sensor or if it would be cost efficient fitting a new board.
     
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  15. Qavion

    Qavion Formula 3

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    .... And an excellent response on the Technical Forum regarding whether or not the speedometer can also cause problems with the F1 system (if applicable), the Motronics ECU/s and the suspension system.

    https://www.ferrarichat.com/forum/posts/146091188/
     
  16. Qavion

    Qavion Formula 3

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    Another great diagnostic tip I picked up from the other forums.... Check to see if the odometer still ticks over when your needle stops moving. If it still ticks over, the problem will be in the speedo.
     
  17. krazykarguy

    krazykarguy Formula Junior
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    Ian -

    Your help in this matter has been astonishing. I also noted your post in the Tech Q&A section, which I assume was spurred by this thread. Thanks!

    I sent a PM to Rich from the 'new circuit boards' thread to see if they are still retrofitting sensor bodies with new modern electronics. It would be perfect if they could retrofit our sensor this winter while the engine is out for the major.

    The odometer stops the moment the needle hits the 11 mph pin. Is this another positive sign that it's the sensor unit? We've been getting 'free mileage' on the F-Car since mid June. :p

    One thing - it was noted in the Tech post that the speedometer head does indeed pass the sensor signal along to other systems (i.e. ABS, Motronic, F1, etc.). We have a perfectly operating 355, with the only exception being the speedo functionality... Wouldn't a failed sensor also mess up the other systems getting the signal through the speedometer?
     
  18. Dave rocks

    Dave rocks F1 World Champ
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    Matt - I'm not sure about the ABS as there are wheel sensors for that. See attached SD2 screen shots. Nothing on ABS showing speed sensor.

    Yes, the Motronic ECU (and F1 TCU) gets the data from the speed sensor. So, if you have anyone nearby with an SD2 you could check the sensor operation that way.



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  19. krazykarguy

    krazykarguy Formula Junior
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    I said 'ABS', when I meant 'Suspension' - this is information from the Tech Q&A forum post by Ian (link in post #13 on this thread).

    Thanks, Dave, for clearing that up!
     
  20. Dave rocks

    Dave rocks F1 World Champ
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    Yep - suspension ECU sees the signal


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  21. Qavion

    Qavion Formula 3

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    #19 Qavion, Jul 11, 2018
    Last edited: Jul 11, 2018
    Unfortunately, it doesn't help pinpoint the problem. Only the odometer still working is helpful in diagnosis.

    You do have a Check Engine light, but until you plug in an OBD2 scanner, you won't know if that's due to the speed problem or not. According to the Workshop Manual, the suspension light should come on for a speed problem. You said you got the ABS and suspension mixed up.... Can you confirm that the suspension light works (at any time)? It should appear for about a minute after first selecting the ignition key to ON. If the light extinguishes after a minute, you know the system is getting a speed signal (according to the manual). If you can confirm the light goes out, I think the percentages will be starting to lean towards a speedometer problem rather than a sender problem.

    Speedo repair may be best left to the electronics expert (and according to "Carper" above, you may get away with a cheap fix if it's something like a dry solder joint on the speedo circuit board).

    (EDIT: I first thought you wouldn't get a signal from the speed sensor with the car not moving. Apparently this is not the case with this type of sensor).
     
  22. krazykarguy

    krazykarguy Formula Junior
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    I am not currently with the car - I'm in South Carolina, and it's 950 mile north of me, in Vermont. I will get a CEL code list when I can convince my dad to do it.

    If I recall correctly, after the car is started, the CEL light, the ABS light, and the 'BRAKE' light are all illuminated. The 'ABS' and 'BRAKE' lights go out after about 10-15 seconds.

    The suspension malfunction lamp (second light from the top on the right side of the binnacle) was only on for my first ride home from the airport - but not again in the other 6 times I drove the car last week. Working the suspension switch on the center console will turn the comfort suspension light on the binnacle on the left hand side.
     
  23. phrogs

    phrogs F1 Rookie

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    You all want to see inside a speedo?

    Let me know I have a spare one to take apart anyway.
     
  24. Qavion

    Qavion Formula 3

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    Assuming the light is still ok, then that's another one for the speedo (being faulty). Interesting.

    Does your father have an OBDII reader for the Motronics ECUs? If not, I wouldn't bother him.

    There may be already lots of photos on the forum, Johnny.

    Assuming the 348 and 355 are similar... Here's a message thread which includes some closeup shots of broken parts on the internal circuit board... and what can go wrong if you change the wrong parts

    https://www.ferrarichat.com/forum/threads/348-speedo-burned-internals.503511/
     

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