250S ignition wiring - PeachParts Mercedes-Benz Forum

djd · #1 10-25-2008, 04:48 PM

1967 250S W108 automatic, standard point ignition.
Looking for a little advice before I waste more time and some money.... (and let me apologize up front for what I think will be a long post.)

Once upon a time I was working on the power windows and accidentally left the ignition switch in the RUN position for about an hour. Fixed the power window, but now the car won't start!

My guess is when leaving the key on, the points must have been closed and current was flowing thru the coil circuit. Must have overheated something somewhere. But where??

I've tracked it to low voltage at the coil. If I jumper around the ballast resistor, it starts right up. However, if I hot wire from the battery positive directly to the ballast resistor "input" side it starts right up then also. In either case I can then remove the jumper and it will continue to run.

The resistor measures about 0.9 Ohms which my info says is correct.

I'm inclined to throw a resistor at it anyway, but the resistance measures OK and it does work if I give the resistor a direct battery connection. Maybe the problem is high resistance in the wiring to the resistor? The path is from the battery positive -> starter -> light switch terminal -> fuse 1 -> ignition switch -> fuse 2 -> ballast resistor. Some of those terminal are hard to get at to check and clean.

Anyone have a suggestion?

Thanks,
Dennis

Arthur Dalton · #2 10-25-2008, 04:52 PM

Check for 12v at ballast input side.

Anthony Cerami · #3 10-26-2008, 06:30 PM

I converted my 250S to a pertronix system. Digital Ignition...
Best thing I did! I converted all my other classics
Best price was at Summit racing.

Arthur Dalton · #4 10-26-2008, 06:32 PM

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Digital ???????????????

Anthony Cerami · #5 10-26-2008, 07:19 PM

Digital electronic ignition module.....solid-state electronic ignition
may be its....High fidelity ignition??

Arthur Dalton · #6 10-26-2008, 07:37 PM

Solid State....but not Digital.

It is Hall Effect Electronic switching, using a transistor to switch the coils primary circuit..... vs mechanical point switching.
But I like your Hi -Fi idea........

Anthony Cerami · #7 10-26-2008, 07:44 PM

it does have a micro controler ...is that not a digital device?
Or whats the differance between a microprocessor a micro controler ??

In any event the old girl runs like a Mercedes Benz....(swiss watch)

Arthur Dalton · #8 10-26-2008, 07:53 PM

Pert 1 module is nothing more than a pick-up with an amp circuit to trigger a switching transistor...which replaces the mechanical points. Which is why it is a Point Conversion. It converts the Ignition Coil Switching from Mechanical To Electronic ...that's all.
And it is a great Tech advance for switching the circuit. Very few devices use mechanical switching today , and if your computer were to still use that type switching, it would take up 6 city blocks.
Probably the best performance modification for the $$$ that one can make to a Vintage Benz ..or any car w/points..........there is no comparison to the two systems....Points went out with Horse Shoes.

Anthony Cerami · #9 10-26-2008, 08:05 PM

yes ...like a hall effect sensor. However there seems to be additional circuitry it also adjusts the dwell / spark timing through the RPM range ...With circuit protection as well. A very cool device.

Arthur Dalton · #10 10-26-2008, 08:08 PM

That would be the Pert 2 system..
You do not want that with a Benz ignition b/c they run Solid Core plug wires.

Anthony Cerami · #11 10-28-2008, 03:04 PM

well .......Just tryin to help ...do what you want....
bottom line
three of my old cars have a pertronix system installed...
1967 250S
1965 220 sunroof coupe
1972 4.5
all run great!

djd · #12 10-28-2008, 07:42 PM

Ok...Ok....back on track here gentlemen....
I've heard mostly good things about "Hi-Fi" ignition systems (Get it? Oh never mind.) and someday maybe I'll convert over, but right now I just want to get the point system working again. Before I fixed the power window, it started and ran just fine.

Yes there is nominal 12 volts at the resistor. The input voltage is slightly less (~0.3V) than battery post voltage but this is probably normal considering the number of old connection points and length of wire. Is it better to check this "under load"? (That is with the points closed and current flowing thru the coil.)

What should the voltage be at the coil during cranking? I'm getting something like +8.5V.
Dennis

Arthur Dalton · #13 10-28-2008, 07:59 PM

Suspect is poor connection at or to the ign sw...fuses mentioned are not in the ign circuit. Ign circuit is non-fused.
Take coil primary and hold it near a ground a crank engine..do you have spark??
If NO then try again with bat pos jumpered to ballast . if yes on one side but not on the other , then ballast is open ..

A common poor connection on those is at the light switch for ign feed.

djd · #14 10-28-2008, 10:17 PM

It starts right up if I jumper from the battery post to the resistor input side. I was afraid of this... probably best to clean ALL the connections. They're just so damn hard to get to, I was hoping to reduce the pain.

Light switch ay. That's that thing that's way up under there and hard to reach while laying on your back with your head on the pedals..... ugh.

How do you get the knob off? (Flat black rubbery thing with chrome bezel around the switch.)

And to get to the ignition switch, you have to pull the instrument cluster out - right?

The fuses aren't in the circuit per say, but according to my diagram, there are connections at the fuse panel, no?

Thank you for the suggestions
Dennis

Arthur Dalton · #15 10-29-2008, 12:13 AM

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The fuses are AFTER the ign circuit, so it is a mute point. The fuse panel gets fed from the ign sw, but the IGN circuit is BEFORE the fuse panel and is NON-FUSED. They do not want a fuse on an Ign circuit.

If you look at the schematic, you will see the battery voltage simply goes to the light switch as a connection terminal and then to the ign sw. The fuse feeds are not for ignition.

So, the suspect is the terminal where the battery 12v connects to the feed to the ign sw. atn the light sw. They get loose and drop the circuit under load right there.
[ circuit # 30]. You may also want to check the B+ terminal on the alternator, as that is the Bat feed to light sw #30.
As you have the schematic, juat follow the circuit from the battery + to the light sw and on to the ign sw ..it is a simple series circuit.