Speed sensor shim. (When and Why?)

[Volhv]

So, after notching my bellhousing, I developed the "Starter Thunk".
Time to install the updated bracket, and shielding sleeve for the reference sensor.
While the 86 did have the smaller starter, the 8V's did not get the shielding sleeve till 87, or 88.

I'm wondering what the deal with item 32 is. There wasn't one on my old bracket. There is no difference in sensor mounting height between the two brackets..... and if the shim were to get installed on the speed sensor, this would drop the reference sensor lower and potentially contact the flywheel setscrew.

[mikepellegrini]

I've seen those but I can't remember if any of our cars actually have them.

Looking at it, I'm thinking maybe it might make it easier to get the sensor out when it's been there for a long time.

But the bracket position sets the height.

[CHICKS]

I recently notched my bellhousing too, and every now and then I get clunks on starting. What part exactly is clunking? It sounds pretty violent, sometimes I have to restart, which it does fine.

[Volhv]

From what I've read.... The small, post 85.5 create a lot of electromagnetic interference, triggering false signals at the reference sensor.

I believe the thunk comes from the starter, as it is being stopped dead in its tracks by the engine attempting spin backwards..... due to a cylinder firing at the wrong time.

Another way to fix it, would be a starter from the early models, the big huge one.

[mikepellegrini]

Interesting.

The chunk of bell housing acts as a EMF shield?

My 951 does that and it only started since I notched the bell housing.

Hmm.

[Volhv]

It sure does, Mike.
I wish I knew that prior to hacking it up.
The bellhousing mod is great, but does have adverse effects on the 85.5, 86, and maybe 87? Can anyone confirm if it was the 87 or 88 that first got the shielding sleeve from the factory?

I'm gaping the sensor at the moment.... will post pics after I get it all together.

[mikepellegrini]

Hmm. Not having to
Pull the sensors when doing the clutch or engine is worth it.

I wonder if you could place a piece of lead sheeting down there, as a shield?

[CHICKS]

Thanks for the info. Mines an 1986 NA.

[CHICKS]

Here's the TSB:

[Volhv]

The source of the problem.... a knotched bellhousing.



A 1/32" washer is about 0.8 mm. Perfect for gaping the speed sensor.



The reference sensor sleeve is aluminum, with an ID of 15mm, and OD of 18mm.



Modified bracket, is the same as the old one, but it has been bored out to the OD of the sleeve. Naturally, the sleeve has a shoulder (to prevent it from falling and contacting the flywheel), so the bracket was also bored accordingly. While you could made your own shielding and modify the original bracket, unless you have a lathe, it's better to buy the parts for $100.



Since the part where the wires go in the sensor was crumbling, and could lead to wire breakage, and loss of contact, I filled the gap with RTV.



So it looks like Porsche eventually realized that notching the bellhousing was a bad idea, and stopped doing it on later cars. Nice Bulletin Chicks!

The spacer (#32) that I referenced earlier, was not needed.

[CHICKS]

Thanks for posting! What's the part numbers for the kit?

[Volhv]

The sleeve is 944 101 320 00
and bracket is 944 101 321 07

I also bought the spacer, but did not end up needing it.

[CHICKS]

Thank you sir, I appreciate it.

[Volhv]

UPDATE

After using the shielding sleeve and replacing one of the sensors due to a break in the wire, the "starter thunk" MOSTLY went away. It would still happen when damp outside, but much less pronounced than before. The starter would no longer get stopped dead in its tracks, but it would sometimes clunk, and continue cranking.

I've just installed an early BIG starter, and that definitely fixed the clunk.
Sourced from Plyhammer. Thank you Ply!


The early starter:
-Found on the 944 1983-1985 (and the 924 Turbo)
-1.1 KW (1.4 KW for late)
-14 pounds (8 pounds for late)
-Fits all models except the Turbo (will fit Turbo if heat shield is discarded)
-Unlike the late starter, it does not have reduction gearing.


Overview:
- Don't knotch your clutch housing.
- If you've already knotched it, get an early starter, and avoid the hassle of speed/reference sensors

[CHICKS]

Volhv I finally ordered my bracket and sleeve. $55 shipped, thanks again for the help.

[Volhv]

Awesome!

One more thing to be aware of, if replacing just the sensors, you have to gap them again. My new ones were a few thou longer than the originals, and contacted the pickup stud on the flywheel.

Not a concern since you're putting in a new bracket anyways. But, for future reference.