W108:12 volts for cranking, 6 volts for run?

[MunichTaxi]

I have seen a written comment in these forums about the ballast resistors, and the role they perform.
It was written that when cranking the W108, one ballast resistor is bypassed to provide 12 volts to the coil. Then when the engine is running, the bypassed ballast resistor is "allowed" to perform, therby providing only 6 volts to keep the engine running.

So....., why not have 12 volts all the time for the coils use?
Seem like a hotter spark would result in better burning of fuel, and better fuel economy.

Somewhere in this questionable logic is a downside, or maybe even a KABOOM! But I am anxious to hear everyone's comments.

[Tomguy]

The starter takes a lot of power to crank the engine, so they assumed the voltage on cranking with the ballast would be too low to provide acceptable spark, so it is bypassed during cranking. Without the load of the starter (and with the alternator running) the 14 volts of the system is enough to keep it going well. Also, it helps the points last longer by having no real current pass through them (only a tiny bit to "switch")
Bypassing the resistor(s) with the stock coil may lead to premature ignition box failure, too.

However, it MIGHT give better economy. I can't be too sure, though, without testing myself.

[MunichTaxi]

So, by excessively cranking a 'dufficult start" engine, one could fry their Switchbox. That makes sense for the W108.

There are 2 ballast resistors at 0.6 ohm and 0.4 ohm, and the shop manual states the 0.4 ohm is bypassed for cranking. So, the engine 'runs' from the 0.6 + 0.4 ohm ballasts.

Experiment #1: Wonder what the result is of replacing the 0.6 ohm with a 0.4 ohm? Total resistance would be 0.4 + 0.4 + 0.4 (coil)= 1.2 ohms

Manufacturer resistance is 0.4 + 0.6 + 0.4 (coil)= 1.4 ohms.

[Jim H]

Hello all.

Less series resistance = more current for the same voltage. Current is what makes the spark "hotter" but it will also make the windings hotter, too!

When starting, the 12v system may be down as low as 9-10 volts, so one series resistor is bypassed, returning the current to proper level. 14 volts would draw 14/10 = 140% of rated current, which in turn would produce 1.4x1.4 = 256% of rated power loss, in the form of heat.

Overheated coils are not a good thing, since they fail when you're away from home, out of money, in the most miserable weather you can imagine, at the worst time and place...

Best regards,
Jim

[MunichTaxi]

The Petronix Ignitor II has a spec of 0.6 ohms(and 45,000 volts).
Since the Mercedes coil has a spec of 0.4 ohms, maybe the way to gain spark energy is to use the Petronix coil?

[autozen]

Munich,

You are thinking and worrying too much. The system works just fine so leave it alone. I was weined on these cars as a young mechanic. If you start screwing around with different components, I guarantee you are in for misery. If you want to get rid of the system and install an aftermarket breakerless, that is another story. If you are looking for better fuel economy, you will need to get rid of the 108.016 and the 114.011 and get a 124 chassis 300E. Those cars were at the beginning of the emission era and got terrible mileage. I'll bet you don't get over 13 to 14 mpg on the 250. The 300E was designed for fuel efficiency. It says so in the MB manuals.

Peter

[psfred]

The ballast resistors control the current going to the coil to prevent overheating the coil. The voltage does not change, only the current flow.

The rational is that a hotter spark from higher current is needed to fire the engine reliably when voltage is pulled down by the starter (10V is uncommon), and when running you have at least 13.5, else the battery won't charge..... 14V is more common at speed.

The Pertronix COIL has more internal resistance than the MB coil, and needs less external resistance.

If you want to toast a switchbox, use a standard non-transistor coil!

If it works, leave it alone.

Peter