Does your MB have the automatic glow plug controller or are they the manually operated glow plugs? Are these the 2 types you are talking about. The manual glow plugs (old style) are 1.5 volts and must be connected in series along with other resistors so they can be operated on 12 volts. The auto controlled glow plugs (new style)are 12 volt and are connected in parallel. Therefore the 12 volt plugs have a much higher resistance and if one or more are in series with with the old glow plugs, they probably will not get hot enough to start the engine. If an old style glow plug is connected to 12 volts (parellel connection), it will burn out like a 12 volt bulb connected to 120 volts.
You can tell which are which by looking at them. The old style (series) have a loop on the end and the new style (parallel) have a single shaft on the business end. Also the old style have a double connection and the new style have a single connection. On the new style the return current goes thru the cylinder head (ground). On the old style, the current goes from one glow plug to the next with only the last plug grounded to the head.
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