I found the #2 glow plug faulty with with ohms test. The remaining 5 GPs showed a resistance of 0.03. ohms.

The spare set of 6 GP's I had on hand show all 6 to have a resistance of 0.05 ohms. What can be known about these ohms numbers and what do they mean? Most say 1.0 is good, but what does 0.3 mean?

I disected the faulty GP to see what makes them tick and basically are a coil of wire with insulation encapsulating. I suppose the wire parted somewhere inside!

Found this post (below) on how to test GP's and thought it may be useful to post again. BB
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Hello again.

Glow plugs last until they fail. Whether to replace them all before they blow, only those that blow, etc. has been discussed at length elsewhere, so I won't go into that now...

GPs can be tested in place. You will have to locate the glow plug relay.

Disclaimer: My experience is with a different body (W126) but the W210 (yours) will have similar components. I also assume you have a bit of basic knowledge of electricity and various underhood components...

Find the GP relay

The GP relay is a black box, likely to be in the area in front of the left front fender. Mine has markings for the 6 glow plugs "G1" "G2" etc. on the right side, where the GP wires enter the relay, which serves as a connector reference.

Have a look at the GP fuse

If the GPs draw too much current, or one fails 'shorted' the GP system will draw too much current, and the fuse will open. You can have 6 good GPs and they won't be able to function. You may still be able to start the engine, but without GP assistance.

The fuse is a strip of metal between two screws, visible when you remove the relay cover. Have a close look for any gaps or cracks.

Test this with the ohmeter, as looks may be deceiving. Touch a test lead to each end, such as the two screws that hold it. There should be zero, or nearly zero, resistance. "Infinity" or blinking digits, etc. means it is open, or blown.

It's a cheap replacement part if it's gone.

NOTE: you must continue to test to find the short or the new fuse may blow again.

Unplug the GP harness

The top cover pops up and off, exposing two plugs. The smaller plug is the control, the larger 6 wire plug connects to the glow plugs.

With a bit of coaxint, you can unplug the 6-wire plug. You will see 6 prongs on the relay box and 6 socket holes in the plug.

Test each GP

Get (buy or borrow) a multimeter and set it to measure resistance (Ohms) in the smallest measurement number. Good glow plugs will measure 1 Ohm or less, but NOT ZERO.

Connect one lead (I use Black, but it does not matter) to Ground. Any bare metal on the chassis, or better yet, bare metal on the motor, should do nicely. (I clipped the Black lead to a metal fuel injector line.)

With the other lead in the air, the meter will read 'infininity.' This may be a a large number that blinks on a digital meter, or the needle is full scale on an analog meter. Use this to become familiar with how your meter displays Infinite Resistance or Open.

Now touch the other lead to bare metal. The meter now should read zero resistance. (My Radio Shack cheapie has a Continuity setting, which means that it beeps when there is little or no resistance. This was very handy!)

Now you are ready to read glow plug resistance. Touch the 'other' lead to one of the socket holes in the 6-pin plug. If it is connected to a good glow plug, you will read 1.00, or less. (My meter beeps so I didn't have to take my eyes off of the connector.)

Touch each socket in the plug in turn. If all glow plugs are good, each will read 1.00 or less. If any are burned out, you will read "Infinty" by flashing number, no beep, etc.

If you find one (or more) that test 'open' read the number next to the plug socket, and confirm it with the corresponding number printed on the relay cover. I my recent experience, #4 and #6 both tested open, all others were good.

Test the GP wire

See if you can locate the GP itself, and if you can touch one meter test probe to the nut or wire terminal on the end of the GP. If so, touch one probe to the GP and the other to the corresponding socket in the plug. A helper may be necessary. (My test leads have a slip-on alligator clip, works perfectly for this.)

A zero reading here says that the wire itself from the GP relay to the GP is OK. An open reading mean a bad wire. Take your time and be sure that you have contact on the GP end of the wire, or you will continue to read Open when you in fact haven't made the connection.

This test is a bit frustrating, but I recommend it since a 20 minute struggle to change a good GP, only to discover a bad wire, can be more frustrating!

Sorry for being long-winded, and I hope this makes sense.

Drop a line if you need more detailed help.

Best Regards,
Jim
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'86 300SDL 251k+, part of a fleet