Porsche put a "sleeve" in the reference sensor bracket to minimize the high torque starter electronic interference with the reference sensors.
Part 28

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Porsche also put out a TSB technical service bulletin about the hole in the bell housing for the sensors. The bell housing sensor holes protect the sensors from the interference as well. Should not be removed.
Conclusion: If you have predetonation while starting----it could be caused by cutting out the sensor area in the bell housing.
Do not cut out the sensor area when you do a clutch job or you may get predetonation.
YMMV

Attachment 566205

John

This phenomenon is a common problem. There are two types of starters for these cars (early and late) and when Porsche switched to the later style starter (smaller body) the electro-magnetic field created by this new style starter confused the reference sensor that monitors the flywheel postion. The "fix" was a magnetically shielded bracket for the reference sensor. My '87 has this updated bracket, but it still had the problem occasionally. The only thing that completely eliminated the problem for me was to install a rebuilt old-style starter. (This larger-body starter won't fit on 951s, only NAs.)

(Looks like John_AZ just beat me to the explanation!)

looks like im not the only one with this problem then. it really doesnt bother me as it only happens once out of every 7-8 times that i start the car. i just want to make sure its not damaging anything. im still going to check the starter though to make sure its all snug in there and looks OK.

I get this also ounce in a while, I've always thought it was an issue with my starter where the bendix does not disengage in time and you get this loud thunk. Not a pleasant sound, every time it happens and someone is with me they think my engine just died.

Just listened to the video and my problem seems different, yours keeps on turning after the thunk, mine stops dead. Yours really looks like detonation of some sort.

Anytime my battery was even the slightest bit low it would stall after the clunk and I'd have to turn the key back and try again. That was with a freshly charged battery so it kinda powered through (probably not good but I just wanted it running).

Yea I need to check the year on my battery too..

My car does the same. Happens once every few months maybe. Just waiting for it to leave me stranded (knock on wood it's probably been 5 years!!!)

Did you ever find out what causes the noise?

My question was aimed at Will.

I don't get it, the OP is not the same guy that busted his transmission???

Well this is embarrassing... :icon501:

Nope.....still rolling the dice. Though.....I haven't driven this car more than a couple hundred miles in the last few years.....

I'm reading all this info and thinking this is the solution to a clunk sound on my brothers 944S when starting. What exactly is the solution though? A new starter or just the shield on the reference sensor bracket and where do you get one of those?

He has had this problem and it telegraphs feedback thumping through clutch pedal on startup. The previous owner thought it was a flywheel problem. My brother Alan thinks it's something to do with the classic timing belt/slack etc. Says there is a metal sound upon starting, but after car is started it runs fine. Thanks for any help.

super old thread but the question is valid, our '87 924s with fatty starter and ref. sensor shield STILL does this regularly, the starter needs to be replaced anyway but we replaced it once before and the kick back actuallly exploded the internal planetary gears, just blew them up, then we actually put the old starter back in. An after market gear reduction starter perhaps? $$$ It would seem like one could add another shield or something to address this