Mercedes-Benz Forum banner

1 - 13 of 13 Posts

·
Registered
1972 350SL
Joined
·
441 Posts
Discussion Starter #1
My 72 350SL went from running perfectly to not running at all over a 150 mile out and back drive to the coast. The symptoms started as a rough idle at stop lights and progressively got worse. The misfire was irregular. Soon, I needed to work the throttle to keep it running at stop lights. Early on there were no symptoms at freeway speeds, but the last 20 miles or so, I could detect an irregular misfire. When I left the freeway, it simply died at the stop and would not re-fire.

The "feel" is that the timing is off, rather than a fuel issue - though could be fuel. Will dig further in the morning.

Recent work includes new AAV, full tune-up, cap, rotor, wires, vacuum advance. Timing chain done at the dealer about 25K ago.

Ideas and suggestions welcome.
 

·
Premium Member
1975 450SL
Joined
·
2,440 Posts
Check the dwell / gap of the points. The rider may be wearing in and your dwell may be off. (If you want to cure that problem permanently, one word: Pertronix.)

The bolt on the distributor may not have been tightened correctly. (Way down at the bottom of the dizzy - adjusts the timing.)

Check the cap & rotor for cracks, arc traces, correct 'seating', etc...

Look for any loose or missing parts. Look for cracked or broken vacuum tubing / fittings. Check the small, 'stiff' plastic lines. When I went through my '75, they were so fragile that they broke at a hard look.


Just a few of the easy places to start, I expect others will chime in with more ideas.

Good luck and happy troubleshooting.
Scott
 

·
Registered
1972 350SL
Joined
·
441 Posts
Discussion Starter #3
Check the dwell / gap of the points. The rider may be wearing in and your dwell may be off. (If you want to cure that problem permanently, one word: Pertronix.)
Thanks Scott. Rather funny actually, the Pertronix was ordered last Thursday with overnight delivery from a place called Summit Racing. The overnight delivery was to have it in hand for my Mercedes-trained tech son to install over the weekend. Haven't seen it yet. I'll be calling them tomorrow.

The vacuum system has been gone through quite thoroughly, but you are right, it is a suspect none-the-less.
 

·
Registered
Joined
·
444 Posts
Replaced wires, coil, checked resistors on fender-within spec

Installed pertronix ignitor..

Crank, no start.

Found no spark at coil.

Found no 12V to coil.
Found no 12V to series resistor that powers coil.

Eyeballed fuses, they all look good, though, i dont have a fuse schematic, nor a decent wiring diagram.

Anyone know which fuse powers the coil?

Any other thoughts?

Thanks
 

·
Registered
Joined
·
444 Posts
OK, car is running now. timing has been set.

The problem were running into now, is that after the Pertonix has been installed, and the switchgear/transistor unit had been removed, and the tach on the negative side of the coil, and the tach is registering about 1500RPM (guess) higher than it should be.

Any thought or ideas about why?
 

·
Registered
1986 560 SL
Joined
·
7,355 Posts
OK, car is running now. timing has been set.

The problem were running into now, is that after the Pertonix has been installed, and the switchgear/transistor unit had been removed, and the tach on the negative side of the coil, and the tach is registering about 1500RPM (guess) higher than it should be.

Any thought or ideas about why?
I recall MBGraham talking about tach troubles with this setup. Search his posts.
 

·
Premium Member
'72 350SL, '85 300D, '98 E320, '19 Subaru Outback (sold '14 GLK250)
Joined
·
10,338 Posts
OK, car is running now. timing has been set.

The problem were running into now, is that after the Pertonix has been installed, and the switchgear/transistor unit had been removed, and the tach on the negative side of the coil, and the tach is registering about 1500RPM (guess) higher than it should be.

Any thought or ideas about why?
I haven't bypassed the switchgear for that reason. But others who have, have the same problem as you do. I believe that you can add a resistor to correct the reading, but there was also something else about there being two types of tach, with different problems/solutions. But exactly what, I can't recall.

Try asking Summit or Pertronix about getting the tach to work. I can't find anything on the net that says exactly how.
 

·
Registered
1972 350SL
Joined
·
441 Posts
Discussion Starter #9
I haven't bypassed the switchgear for that reason. But others who have, have the same problem as you do. I believe that you can add a resistor to correct the reading, but there was also something else about there being two types of tach, with different problems/solutions. But exactly what, I can't recall.

Try asking Summit or Pertronix about getting the tach to work. I can't find anything on the net that says exactly how.
Yes, we bypassed the switchgear; we concluded it had failed.
 

·
Premium Member
'72 350SL, '85 300D, '98 E320, '19 Subaru Outback (sold '14 GLK250)
Joined
·
10,338 Posts
Yes, we bypassed the switchgear; we concluded it had failed.
Did you also install a Pertronix coil? They suggest it because other coils may have incorrect resistance which results in high current through pertronix module, which eventually will burn it out.

If you find out what value of resistor should be in line to tach, please let us know!
 

·
Registered
1972 350SL
Joined
·
441 Posts
Discussion Starter #11
Did you also install a Pertronix coil? They suggest it because other coils may have incorrect resistance which results in high current through pertronix module, which eventually will burn it out.

If you find out what value of resistor should be in line to tach, please let us know!
OK. Ordered a Flame Thrower. Inexpensive, so why risk damage to the module. We WILL work through the tach issue and report. Drives me nuts.
 

·
Premium Member
'72 350SL, '85 300D, '98 E320, '19 Subaru Outback (sold '14 GLK250)
Joined
·
10,338 Posts
OK. Ordered a Flame Thrower. Inexpensive, so why risk damage to the module. We WILL work through the tach issue and report. Drives me nuts.
It's hard to imagine how a FLAME THROWER :rocketwhore:
can in any way safeguard a system ;)
 
1 - 13 of 13 Posts
Top