Odometer and Speedo problem 89 328GTS

I have problem with my Speedometer and Odometer. Original problem was that the odometer will keep running fast with the speedo pegged out, the odo will keep running while the car standing still and the engine is in idle. I have replaced the Impulse Generator twice but the problem came back after few hundred miles. Does anyone Know how can I test the line or what is the voltage suppose to be from the Impulse Generator?

 

Do a search and you'll find plenty of discussion. I had a couple threads several months back so maybe search for 'speedo' , 'speedometer' keywords and my username and you'll see some good info incl. voltages. Sorry, I didn't retain all that info so can't answer directly.

 

Most likely your problem is in the speedometer electronics themselves (i.e., it's very unlikely that a sensor, or wiring, problem can cause a false maximum reading -- problems in the sensor or the wiring usually result in a "dead" speedometer as ducowti had).

But to strictly answer your question about what the signals should measure like at the sensor:

The green/white (VB) wire at the sensor should be +12V (relative to ground) with the key "on".

When the rear wheels are stationary, the red/black (RN) wire should measure 0 VAC (relative to ground) -- i.e. a constant value voltage signal.

When the rear wheels are rotating, the red/black (RN) wire should measure "something" VAC (relative to ground) -- i.e., a changing voltage signal.

PS You can access these wires at the 14~15? pin C169 connector near the coolant expansion tank as ducowti showed in this thread:

http://ferrarichat.com/forum/showthread.php?t=197624

 

Thanks for your replies! The thread was very good and helpful. I wish I would have asked earlier, I had this problem over a year. Now I have my work ahead of me. I will not go under the car with the floor jack so I have to wait to find a shop so I can do the troubleshooting.
What is PAS? Also any tip for accessing the speedo? i.e. how to start opening the control panel ? I have never touched that area.
Again thanks for your reply!

 

Fairly common and well known problem happened to me also just sent them to the shop for repair, no big issue.