Need instrument panel gurus help!!

I guess you could do basic continuity checks of the wiring.

Here's a user-friendly diagram for the 2.7

Figure 5 ... Instruments (2.7)

There are 3 plugs to look at in the wiring:

The 12 pin plug labelled "41049" near the left rear shock absorber (signal wire is pin 5). i.e. red-white wire.
The next plug is inboard of the passenger footwell relay panel (labelled 41024). You may need to remove the leather "saddle" forward of the centre console. Look out for hidden screws (don't try to pull the saddle off until you have all the screws out, or you may break something. It's the upper white 20 pin plug (signal is on pin 18). On left hand drive cars, pin 18 is the third one down from the top left hand side of the plug. You still haven't filled in your profile page, so I don't know if you have a left hand or right hand drive car.

The next plug is on the back of the speedo itself in the instrument binnacle (but I assume you already know where that is because you've had the speedo checked). Again, it's the red-white wire.

If simple wiring continuity checks don't find anything.... worst case scenario, I guess, is the part inside the gearbox which triggers the speedo sensor.

Tell me about it. I've been trying to make sense of them (and redrawing them) for the last 4 years.

Regarding your previous post...

Is this your boss' car, too? (or were you referring to your wife?)

 

No need for an oscilloscope, just use a DVM and push your car slowly. You should see an alternate of high voltage (>7V) and low voltage (<1V).

 

Bad plug wires, wrong plug wires or badly routed plug wires on the 5/8 bank can interrupt the speedo signal.

 

There is nothing particularly unique about the speed sensing system on the F355. I vaguely recall, however, that one of the user systems (downstream of the speedo) has been known to cause problems. After the speed signal is sent to the speedo, the speedo divides the frequency by 2 and then sends this on to the Motronics ECU, the suspension ECU and the F1 gearbox ECU (if applicable).

Anyway, let's stick to the basics.

Gearbox reluctor >>> Speed Sensor >>> Plugs and Wiring >>> Speedo >>> other speed using systems.

Sorry, I don't know if we're teaching granny to suck eggs. Did you check the resistance of the red-white wire from the speedo plug to the sensor plug? Another loose pin/socket may be causing the low voltage. I don't know what ohms are to be expected. They can vary a lot depending on how good your engine-chassis grounds/earths are (maybe you could just use a long piece of copper wire to complete the test circuit, rather than using local grounds/earths). If the resistance is more than a few ohms (copper wire should be milli-ohms), then try breaking up the wiring into sections (The easiest plug to get to is the one behind left hand shock absorber actuator). Look for bent or corroded pins. Check your engine-chassis earths. Clean and tighten up as applicable.

You've been told the speedo is ok. You've also replaced the speed sensor, but there's no guarantee that the new sensor is working. Here's a detailed tutorial on how to test Veglia Borletti Type 1 & Type 2 speed sensors: http://www.dinoplex.org/instruments/vegliaspeedosensor/ (courtesy of dinoplex.org) Follow all the safety instructions (including the use of a resistor in series with the meter) or you might damage something. You can wave a bit of ferrous metal in front of the sensor to watch the voltage/current go up and down.

Thanks for your honesty. Some FChat members might object to offering advice to commercial operators, but in the interest of getting a fellow Ferrari owner's car back on the road ...

 

Excellent! Thanks for the follow-up and the tip. Yes, these sensors seem to be pretty fussy about clearance (especially the crankshaft sensors which have to be shimmed).

 

Nice