328 speedometer

Your 328 uses an electronic (Hall Effect) speed sensor on the RH side of the gearbox (and many of us have reported them going bad) -- item 76 in this jpeg (now PN 197181; was PN 116987):

(PS You should go thru the hassle of getting registered on the www.owners.ferrari.com site so you can get to these SPC drawings for your 328 on the FerrariUK site.)
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I should have added that the 328 Speedometer itself has been reported more than once as needing repair too, so don't just replace the sender (impulse generator) without doing some investigation (although it's usually wacky needle behavior when the Speedometer itself is bad rather than "dead" behavior).

The way it works is:

1. One wire going to the sender should always be at +12V (relative to chassis ground) with the key "on", and

2. When the rear wheels are rotating forward together (with the key "on"), the other wire should be a square wave going from ground to a positive voltage (relative to chassis ground). I don't think that you'll need an oscilloscope -- i.e., just using a multimeter on the DC or AC volt scale should be able to show something if the square wave is present.

 

Sage advice from Steve once again. It is most likely the sender....the speedos can fail, but rare...the tachometer is another story though....on the 328 they are notorious for suddenly reading low...but they can be repaired by most speedometer shops.

 

After bouncing speedometer needle behavior, it quit working altogether. Also not working is the odometer. Will have to follow troubleshooting tips.

Question: Does a bad 116987 sender unit cause both speedometer and odometer to stop working?

 

Yes (and, likewise, if one is working, and the other is not working = probably not caused by a problem with 116987).

 

Never had to check this one, but the sender unit is well documented on dinoplex.org : http://dinoplex.org/instruments/vegliaspeedosensor/index.html

 

Thank you Steve. If replacing the sender unit is needed, do I have to drain the transmission?

 

One of the 328 DIYers or F Shop people would be better suited to answer that, but my (foggy) recollection is people report that a couple of quarts of gearbox oil might drain out if 116987 is removed from a full gearbox -- so you'd need to: 1) collect that and reuse the best of it to refill plus any needed new to top up, 2) refill with a couple quarts of new, or 3) drain it all ahead of time and reuse the best of it and top up with some new, or refill with all new.

 

If you are prepared and work fast, not much will come out.

 

Thanks Tim. No gearbox oil came out because I raised the car on the right side only and the left side was still on the ground.

Update: Speedometer/odometer is operational again. After the static 12V test passed, I decided to forgo additional debug and replaced the sender since I already had a new one.

This is the tool used:
Sunex 32mm 1/2in Drive Extra Deep Socket
https://www.amazon.com/dp/B006L1K4CQ?psc=1&ref=ppx_yo2_dt_b_product_details