'77 512 Hot Start issue...suggestions?

Do you mean:

1. Nothing at all happens, or

2. the solenoid actually moves (engaging the pinion gear to the ring gear), but the starter motor doesn't seem to engage, or

3. The solenoid moves (engaging the pinion gear to the ring gear), and the starter motor does try to spin the engine but can't?

Are you measuring this on the large main +12V battery cable or on the start command wire connected to terminal 50 of the solenoid? And under what condition -- 1, 2, or 3?

 

Yes, that's normal (due to the internal resistance of the battery when huge current is flowing). If the starter motor was actually turning (and developing some back EMF), the voltage wouldn't droop quite so much, but getting down to 9~10V when the starter motor is actually cranking is typical/normal.

Have to agree -- either weak starter or weak battery. If the current is high (like >150~200A) during the starter motor stalling period = that would be a sign that the starter is weak. You can usually get the battery "load tested" at an Auto Part chain store (if you can't test the actual current in-situ). If the battery is OK, that would be more evidence to try a starter-ectomy or rebuild.

If the ignition timing was too advanced, I think it would initially crank, but you'd get more burping/farting/backfiring up the carb stacks -- are you getting any of that?

 

only 8.5 V is low
I once had the same or nearly teh same problem with a mercedes. there the battery was not good anymore. so if you have the possibilty to jumpstart then try this. when the starter will turn then the crankshaft you need a new battery. when you have a battery tester you may check if there are still the complete amp available. my battery only had about 30 % of the max amp. for starting cold it was ok, but not when the engine has been warm.

you may check the battery voltage directly at the battery when starting or when you try to start and see how many V are at the battery. if also only 8.5 V I think the battery has seen better days.

easiest way first is to try to jumpstart. good luck

 

Only other possibility is that there might be a poor connection in the battery primary wiring/grounding. The later BB512 OM doesn't show a battery cut-off switch (so I'd assume your earlier car doesn't have one either), and, since you've had the battery out and in to test, the battery post connections should be fresh/clean, but that does leave where the negative battery cable connects to the chassis.

You should be able to get the starter rebuilt/tested by an auto electric rebuilder shop (at the non-F price ).

Do please let us know what gets it working.

 

the battery in the mercedes also has been good in static condition: 12,8 V
so please try to jump start and see what will happen

 

Had this problem once on a late '70s Porsche. I installed a GM ignition solenoid...problem over. I later changed the starter.

 

Not sure this is really it (and the picture is not the best for showing where it's really located) , but item 26 here:

Ferrari 512 BB Parts : Table 105 - INSULATING MATERIAL AND BULKHEADS

looks (and translates) like a fairly serious ground (earth) strap -- maybe have a poke around the RH rear corner area?