I have a 1969 220d. The other day the car would not start. It usually starts on the second try as the compression is probably low. I bought a set of new glow plugs and it still won't start. I have checked the pin style plugs on my 300cd by taking the plug out, connecting it to a battery via jumper cables. When I touch the end with the cable the tip starts to get hot, indicating that the plug is good. I tried this technique with the loop plugs and I had no reaction on the old ones or the new ones. Should this technique work with these plugs? Could all of my new plugs be bad? I would appreciate any help. thanks, andy

The loop plugs are a completely different animal than the pins.

The loop plugs usually don't open, they usually ground out in some way against the block shorting the circuit which prevents all the plugs from making heat. You need to start by measuring the voltage at each plug. The 12 volts should be equally divided along the string. You should measure 12 volts at the first plug, about 9 at the next, 6 at the next and so forth. Start by ensuring that you have 12 volts at the fuseholder. If not check the fuse. If the fuse is good go to the relay.

Also ensure that you got all the connecting wires and insulators back on correctly, if you don't this will also show up in the voltage test.

Good luck,

Thanks for the suggestion. Is there a way to test the loop style plugs with them out of the car? Maybe the old ones were just fine. I guess I have to buy some sort of voltage tester. thanks, andy

I have to go to the library to check out the manual for my 220d. I looked for a fuse but couldn't find it. I also don't know where the relay is for this car. The car turns over but just won't fire. I'd like to check the fuse and relay before I move on. They are very easy to spot on my 1980 300, but I don't know where to look on my 220d. thanks for any help. andy

Andy,

The best way to tell if the glow plugs are working is to observe the glow indicator. If it glows bright orange after about 20 seconds, the plugs are glowing about the same inside the engine since the indicator is designed to simulate the temperature of the glow plugs. If it glows too bright, one of the glow plugs is shorted. Only that plug need be replaced, not all.

The loop glow plugs are rated at 1.5 volts so don't connect them directly to the battery except for a short time or they will burn out. The zig zag wires between the glow plugs are actually resistors that drop the voltage to the plugs. Don't replace them with copper wires and be careful because they get very hot.

The glow plug indicator goes out when you go to the start position from the glow position. The glow plugs are still energized but the indicator is shunted to make up for the voltage drop in the battery caused by the high current going to the starter.

This is a very inefficient system because 1/2 of the energy is wasted in the resistors. It is hard for me to understand why MB used it for so many years. It probably goes back to the 6 volt systems.

P E H

I tested the plugs and they seem to be fine. The indash glow light also seems to be working as it should. I still haven't found a fuse for the glow plug system. Or a relay box of any type. I checked to see if fuel was flowing to the injectors, and it is. However, I don't know if the injectors themselves are clogged. The car turns over fine just as if I took out the relay fuse on my 300cd. It just won't fire up. Thats what I need to look for, a fuse or relay of some type. The previous owner had the idiocy to put a GM alternator in this car by cutting out part of the wiring harness and removing the voltage regulator. I fixed that, but it wouldn't suprise me if they also removed some other part on the car. The Chiltons book I have doesn't have much info. Any help would be appreciated. andy