Problems with Speedo in Montreal

when I first saw this i thought you had a problem with an Alfa Montreal speedo!

So an auto S4? Not so sure you have a speedo cable on that one (someone know for sure?)

AngloSpider, a member in the UK had problems with his auto S4 speedo. The auto speedo sender is different to the manual sender, and you cannot buy it anymore....never seen them s/h either to be honest!
What you have is an impulse generator sender screwed directly into the right hand side of the gearbox, that sends a pulse to the speedo. On an auto spider you have in any case to drop the auto transmission down somewhat (as if you were going to replace the rear gearbox mount) to replace it.
Might be worth getting under there and at least checking the wires are intact and not chafing somewhere.
On the manual spider which also has a speed pulse generator fed by a speedo cable from the gearbox, then wires from th sender go directly to the Speedometer itself, going through fuse #7 (7th fuse from the left in the pull down fuse box)
I would check that in case the auto sensor runs through the same fuse (don't see why it shouldn't)....pull the fuse and clean the contact or replace.
Photo from the manual shows the location.
This link shows what it looks like as well as what it looks like inside, when Anglo got through gently opening it
http://www.alfabb.com/bb/forums/spi...ulse-generator-s4-automatic-transmission.html

Just to confirm, SS4 is right, there is no cable or mechanical drive, just the wires straight from the sensor in the gearbox.

Given that it starts to work at 120 I would guess it is vibration that is making it work and would try at the speedo head end first. The connections to the circuit board can get a bit loose, there are three prongs on the back of the head that fit into sockets on the board. You may need to squeeze these to get a tighter fit.
I've just re-done mine after I got fed up having to smack the pod to get the speedo to function.

If you want to check the sensor sender you can do the following:
It needs a power supply so check the pink and black wire to the sensor. It should be live when the ignition is on.
There is no earth/ground wire as it grounds though the body of the sensor.
The other, grey, wire is for the signal. The signal is strong enough to light a 12v led. So you can test by putting rear of car on axle stands, under the axle itself, hooking up an led +ve lead to the connector for signal, and the -ve to any ground point on the car. Then start the car. The led should be lit up - the pulses will be coming so fast that you will not see the light flashing.