348 348 Speedo trouble

There should be a locking tab to press in. Look underneath the connector. Also be careful when pulling it off as the printed circuit can get stuck in the connector. There are threads on how to repair your speedo head if you search.

 

Sometimes the plugs can be tight. You have to be persistent, but be careful. The wiring harness is a ribbon type on the instrument side.

 

Is that an optical illusion, or is your wiring in bad condition?

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If that black earth wire is shorting to the white-yellow wire, it could stop the speed signal getting to the Motronics ECUs. Also, if the speedometer circuit board speed signal circuits are not isolated properly, these two wires shorting together could affect your speedometer reading.

P.S... I've used Italian colour codes for the wiring colours. I forgot to write RN (red-black) for the speedo sensor to speedo signal wire.

 

Can you confirm that the wire insulation on the white-yellow (bianco-giallo) and black wires (Negro) is damaged? All you may need to do is put insulation tape around the individual wires.

 

Common problem which typically results in needing the speedo repaired or replaced. But before that... have you tested the wiring? In other words, electrical troubleshooting? As a fyi, the speedo circuit goes through the connector in the left side of the engine bay.

Here's the basics:


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Lots of 348 speedometer threads as well. Do a search here to find them because not every problem has/had the same cause and they may help you save money from "just replacing parts".

 

Did you check the circular connector behind the left rear shock absorber? The pins sometimes get corrosion on them.

http://www.members.iinet.net.au/[email protected]/Ferrari348/F348_Fig5b_Instruments.GIF

Component number 115

Also check for 12 volts on the green & white wire at the speedometer with the ignition key on.

Sorry, I see Bob has already suggested this.

 

Correction.... Wade.

If you've found no power problems and the wiring from the sensor to the indicator is ok, look for dry solder joints or burnt components on the speedometer circuit board.

Do you have a "mousetrack" seat belt system?

 

It would help if you posted what year and model your 348 is. Too many variations throughout its run, and several different wiring diagrams as well.

But yes, carefully remove the whole unit first. Then the backshell.

 

Ian was talking about Connector 115, which is round (originally).

https://www.ferrarichat.com/forum/threads/348-quick-voltage-check-connector-115-pin-2.505341/

Looks like yours has been replaced. Here's mine.

 

Personally, I wouldn't go any further into the speedo. It's almost worth its weight in gold. So collateral damage would be costly. Best to verify the integrity of the system wiring first, in my opinion.

 

I didn't realise the newer 348's had 355 connectors. I'll amend my late model diagram.

Later model 348 Instrument Diagram "5b"

Thanks for the link, Wade.

Luigi, when you say there is no evidence of shorted or broken wiring, are you using a ohm meter? You can't rely on visual inspections. The wires on plug 115, for example, sometimes detach from the pins/sockets. Don't worry about fuses. All the instrument power wires are connected in series. If your tachometer and seat belt light works, the speedometer has power.

 

You may not have to remove the button if you can unplug and take out the green circuit board inside the speedo. If your wiring from the speed sensor to the speedo, as well as power supplies to the sensor and the speedo are good, and you already have a new speed sensor, the most likely failure is of one or more of the components (IC-s and/or the transistor) on the green circuit board. Some members managed to repair the green board by replacing some of the components (which are available on the internet). You should be able to find the posts on this repair which are very detailed and with many pictures. If I remember well, one of the members here also bought a number of extra components for the speedo repair and made repair kits, probably still available.