Is there a way to test the resistance of a glow plug without taking off the intake manifold and what not? What is the simplest way to achieve this process if I just want to test them to see if it needs to be done.

The reason I ask is that I am having a starting issue whether it is hot or cold and I am wondering if it is the glow plugs or not.

Edit: The car has 62000 on the clock and is a 99. So, another question is how long do those plugs last? TIA

I don't know about Mercedes but Ford GPs can be tested in place. Pull the lead going to the GP and put an Ohm meter from from the top of the GP to ground (head). 1 Ohm or thereabouts is good, open cct is bad.

When the engine is hot the GPs are not really necessary so it sounds like something else. Do you have any fuel leaks, places where air could be getting into the system when it is at rest.

IF one of the glow plugs are broken, the glow plug light will remain on while the engine is running, along with the check engine light. There is an easy way to see if the glow plugs are drawing any current but it's too risky explaining it. Gotta be shown rather than written.

Since you stated previously that the engines stumbles while starting I would asume that one of the glow plugs arent' working. Recently had the same issue with our '99 stumbling and then the glow plug light stayed on so it was obvious it was a glow issue. After replacing the glow plugs she started right up, although the check engine did last a few days or so and went away on it's own...

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#dieselFLEET
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'97 E300
'99 E300

But if it is the glow plug or plugs, why is it still problematic when it is hot? Although I must say it is easer to start hot than cold.

Even though my car even when hot and I try to start without letting the glow plug light go off completely it is a little harder to start than if I let it go off and then try. I've tried that theory before :-)

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#dieselFLEET
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'97 E300
'99 E300

So how long do GPs last on this car?

The general consensus is that they last approx. 40K

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#dieselFLEET
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'97 E300
'99 E300

Maybe it would be time to do it then since we have 62 on the clock and I would need to check the service records anyways.

It won't hurt. Not hard to do either. We bought the glow plug reamer tool from FastLane. This tool cleans out the inside of the prechamber from carbon deposits. Our's wasn't too dirty at all, but then again it was a piece of mind, as some have reported problems with carbon buildup causing big problems, etc...

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#dieselFLEET
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'97 E300
'99 E300

Can't seem to find the reamer. Is it an MB tool

I believe that it takes 2 Glow-plugs to fail before the glow-plug light indicates trouble. A single glow-plug failure is not indicated.

Phil

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'95 E300 Diesel, 264,000 Miles. [Sold it]

True.... The systems read glow plugs in pairs as I recall.

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#dieselFLEET
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'97 E300
'99 E300

Call Phil here at FastLane. Very friendly and knowledgeable

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#dieselFLEET
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'97 E300
'99 E300

Hard starting on a 99 diesel either hot or cold eh? I guarantee you that the plastic formed fuel lines around the injection pump are leaking air into the fuel system allowing the injection pump to lose prime. This will cause a hard start hot or cold. They will get worse and eventually cause a no-start. The lines should be replaced with factory updated parts which are not as prone to this condition along with both the main and pre-filters.
FWIW any Mercedes diesel engine should start when hot without even glowing the plugs.

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