Ugh... Confusing. I assumed (when I added the colours to my earlier diagram) that the sensor connection marked "+" was +12 volt power from the car and the signal wire was "-". This also made some sense from a colour matching perspective: "green to green" and "brown-ish to red-white". However, you're saying that the sensor connection marked "+" is the signal side. (with green and orange as possible colours). In online pictures, I've found brown & black, brown and yellow, green and black-ish, green and brown... (colours in no specific order) I think I'll just put two black wires on my diagram otherwise the colours will just confuse people. Fig5_Instruments_Handbrake_ABS_5.2(1997~)
I did not look at the wiring diagram earlier so I assumed the speedo sensor has 3 wires (Hall Effect). It actually has 2 wires so it is not Hall Effect but a kind of induction sensor, having an induction coil and some electronics. The Hall Effect sensor testing method therefore does not apply here. I also found my saved file (attached) containing good info on the Ferrari speed sensors, including testing. The sensors on the 348/355 should be the "Type 2" referred to in the document.
Interesting. I don't know where I got the idea it was a Hall Effect sensor (perhaps on the forum or perhaps because it had electronics inside it with a square wave output). The tutorial graph shows the waveform varying from 5~11 volts dc. Anyway, I'll remove the reference to "Hall Sensor" in my diagram.
It appears that the pickup coil/capacitor/transistor form a local oscillator at 1.47 Mhz which, while running, produces the "high" output or "on" state (+6.8 V). When a steel object is brought to near the coil, it causes the oscillator to stop oscillating (the coil's inductivity is brought completely out of resonance with the oscillator circuit) and this results in "low" output or "off" state (-0.68 V actually), according to the output signal graph below. The "square" wave output is somewhat rounded at the corners and it is not zero to 6.8 V but -0.68 V to +6.8 V. It is interesting that the oscilloscope graph is stated to be for 40 Km/h and the square wave frequency is 41 Hz, i.e. it seems that the speed indicated on the speedometer is directly proportional to the Hz of the square wave (easy for the speedometer calibration). In this case, a square wave from a signal generator with amplitude of 6.8 V (5 V amplitude will also do, I think) and duty cycle of 74% (duty cycle of the "high" phase) will be good for testing the speedometer circuitry. The square wave from a signal generator will not have the "low" phase as negative (-0.68 V) but it will be zero. This would probably not be an issue when doing a test just to see if the speedometer is responding. I guess the more expensive signal generators have an option to move the "low" phase of the wave to below zero. Image Unavailable, Please Login
Hmmm... Not sure if it's worth the risk, hooking up that cheap signal generator to my car. Anyway, I'll hook the signal generator up to my oscilloscope and see how close I can get the waveform to that graph. I wonder why the speedo divides the frequency by two before sending it to the other systems. P.S. I must be blind, I didn't see the Vp +/- values before... and I still can't see the 40kph
I added the 40 Km/h to the graph as, when I downloaded the graph picture, its original file name stated that it is at 40 Km/h.
Ah, I see. I tried to extract the picture from the pdf to see if was named, but found I couldn't. Just checked the wiring on my car and the red-white wire is matched to the brown on the sensor... and the green-white (which looked strangely yellow) matched the green on the sensor (which is how I thought it should be). Anyway, I'll draw a plug on my diagram to show where the wires should be (in relation to the clip on the plug). Add colours as required... F355 Instrument Wiring Diagram My curiosity had some rather beneficial side-effects. I found a missing nut on a P clip, a loose engine lid latch hook, an oxygen sensor cable sitting on the exhaust....
Just go to the website shown at the beginning of the document I posted and you can download separately all images (and see 40 Km/h as the file name of the square wave graph).
Just back from a 30 min run. First of the season. All is well and running correctly. Thanks for all the help guys. God these cars drive like a dream.
