348 Speedometer Schematic

http://ferrarichat.com/forum/showthread.php?t=77024&highlight=116987

 

If you're looking for a schematic of the speedo head itself, you're never going to find one.

If you're looking for a wiring diagram of the system, it's in section L, figure 5 or 5b.

#40 is the speedo, #88 is the sending unit, #115 is the connector by the shock.

 

Just got your message. It's almost impossible to read the wiring diagrams when they are scanned and posted. I'll sketch it out and post it for you in larger size a little later today, OK? It'll be much more legible.

 

After studying the diagram and comparing it against my 348, I've made up this chart. It's somewhat simplified, but I don't think any of the other wiring is really involved in making the speedometer work.

12 Volt battery power goes into the fuse panel on the right kick panel. When the ignition switch is turned, relay "e" closes and sends power to fuse #6. The power from fuse 6 is sent to the speedo head through one green/white wire, and to the speedo sensor on the transmission through another green/white wire (with a few stops along the way). This wire goes through the connector by the left rear shock absorber, and through the connector right next to the sensor.

A red/black wire comes out of the speedo head and runs to the fuse panel on the right kick panel. It runs through the panel, but not through any fuses or relays. When it comes out, it is now a red/white wire which runs through the connector by the shock, through the connector by the sensor, and into the sensor.

The way I read it, the sensor takes its 12volt input signal and sends out a variable signal, depending on the speed it reads on the transmission gear. This signal goes to the speedo head, which compares it to another 12volt signal, and gives a reading on the needle.

The other wires on the speedo head connector are power sources for warning lights and voltage supply for the speedo illumination. I don't know what the white wire at the speedo plug is for. It shows as a blank in the diagrams.

My guess would be that you snipped the wires when the ignition was turned on and ran full 12 volts through the red/white wire when the cutter contacted the wires. This must have fried something in the head.

With my test light, I got power at the green/white wire on the speedo head and at the green/white wire on the sensor when I turned on the ignition.

I hope this helps.
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Cool, Miltonian!

A voltmeter connected to the Red/Black wire should be able to show our speed, then (if you know how to translate).

 

I would assume that your description is correct. A voltmeter reading on the red/white or red/black wire, as the speed of the transmission gear increases, should vary in a linear manner that can be read by the electronics in the speedo head.

I think there are speedometer shops in all major urban areas that have the test equipment to analyze internal faults in the head. This diagram just shows what's involved in the external wiring.

 

Good to know. Sounds like we'd need to use a $0.60 frequency counter chip to translate our speed then (instead of a voltmeter).