Hi Biggs.
There are several negative factors to watch out for concerning jump starts. The two most dangerous are ripples and high impedance shocks. These two factors alone are enough to make you want to avoid jumpstarts at all costs.
If you have an old 240D, it doesn't really matter. It will take severe punishments, and still likely work fine. I can testify to some real horrible stories that ended well. On a modern MB with ECU's, CPU's, and CAN buses, it's vital to only use professional startboosters or taking out the battery for reloading. If the owners of these cars were fully aware of the consequenses, they would never ask anyone for a jump start. Here's why.
Let's say you have a 2004 E220 CDI W 211. An old car might or might not produce ripples. If you use another 2004 W211, you will not have any problems with ripples. It produces the same clean current your car needs. Once you have seen the monster water cooled generator in a W 211, you'll know it's a sofisticated thing. But the question of impedance still remains. The shorter the cables, the better. Long cables produce high impedance values, if not corrected by an "impedance swallower" (Help me out here, YAL)
Imagine you have two gas tanks. Tank A is pressurized and tank B not. Each tank has a + and - valve connection. Your task is to connect these following certain rules. When you connect the red hose, you may open the + valve at tank A, but not B. When you connect the black hose, you may open - valve at tank A, but not B. Then you will have pressure in the hoses and tank A, but not tank B. Imagine then that you open the - valve at tank B. Then these tanks will want to equalize the pressure. The result will be gas rushing out of tank A through the black hose. The stream will be high in the beginning, and lower in the end.
This is not a model of how electricity or your batteries work, but it's an attempt to visualize for you what impedance is. The stream of gas can be compared to electrical current which will act similarily.
In the case of the W 211 a collegue can probably help out with short cables and correct procedure, but is it worth the risk?
That's why you should call the MB Mobility Go, or whatever it's called in your country. When I rush out to rescue a stranded MB customer, I use a real high tech start booster, which we modify a lot to satisfy the needs of digitalized modern cars. If that's not possible, take out the battery and take a taxi to a gas station or whatever to reload the battery. In an extreme emergency, I would rather buy a new one. It might pay off very well in the end. Try to inquire about prices on some ECU's and other electronic devices in your car. You'll see what I mean.
Another thing to consider is that the malfunction might appear many months afterwards. Get a Star Diagnosis scan as soon as possible after a jumpstart.
A W 124 is in the border line in my opinion. They do have an OVP. I treat it just like I treat a W 211. Just to be safe and sound. Even though my experience tells me it can take a lot of punishment and still work fine.
Regards
Geir
] She can use my cell phone though[
