I have a question on a first start on day 40 degrees F or above. Should a mechanical injected 80 model diesel start right up smooth, without any bucking & cross-firing type of sounds before it smooths out running? Mine seems very rough on that first crank up for a while.

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Rebe

It should be smooth. How are your glow plugs?

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14 E250 Bluetec "Sinclair", Palladium Silver on Black, 153k miles
06 E320 CDI "Rutherford", Black on Tan, 171k mi, Stage 1 tune, tuned TCU
91 300D "Otis", Smoke Silver, 142k mi, wastegate conversion

19 Honda CR-V EX 61k mi
Fourteen other MB's owned and sold
1961 Very Tolerant Wife

I don't know how the glow plugs are actually. I was looking at some threads a while back. I wasn't sure the best way to test them. Is pulling them out and ohm testing the fail safe way to be sure they are good?

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Rebe

yep should be pretty smooth

if you unplug the harness you can ohm them out at the harness or from each plug to ground, again with the harness unplugged

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"NEW" 1984 300D TD needs some love

You can test them with the Ohm Meter by disconnecting the Electrical Connector that goes from the Glow Plug Relay to the Glow Plugs.

This is not a recomendation to buy parts there but the Disel Giant Website has a good pictorial to check the Glow Plugs under Glow Plug Repair. When the Page open up you need to scroll down the page to see the pictures.

It will take longer to read and look at the pictorial than it will to actually do the test.

Harbor Freight has inexpensive Digital Volt/Ohm Meters (Multimeter).

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84 300D, 82 Volvo 244Gl Diesel

A multimeter will confirm a bad plug, but not a good one. The best test is removal from the engine to make sure they glow in the correct spot. Failing that, a current draw test will confirm that they're at least doing something.

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'83 300D, 126K miles.

You can reject a pin type glow plug by ohm test results, but you can not necessarily accept them as good by such a test. I have seen many pin type plugs that ohmed fine, but had holes in the shells.

Twenty years ago, they were so cheap, if they were even a little in doubt, I just replaced them. They have gotten so ridiculously expen$ive now, they are worth removing and scrutinizing.

Hope this helps,

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2001 SLK 320 six speed manual
2014 Porsche Cayenne six speed manual

Annoy a Liberal, Read the Constitution

One can make a somewhat cheap ammeter using a cheap gauge like this:

Amazon.com: SMAKN 85L1-A AC 0-30A Analog Panel Meter Ammeter Gauge: Automotive

and a couple of probes. Run power to each glow plug via the appropriate pin in the connector, and watch the current flow. Should jump to around 15 amps on contact, then drop off as the plug heats up and plug resistance increases.

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Respectfully,
/s/
M. Dillon
'87 124.193 (300TD) "White Whale", ~392k miles, 3.5l IP fitted
'95 124.131 (E300) "Sapphire", 380k miles
'73 Balboa 20 "Sanctification"
Charleston SC

Consider how the glow plug is constructed.

Under the metal sheath is a ceramic core wrapped with heating wire. Just like the element in the bottom of your electric oven.

Unlike your oven the glow plug only has one end. So at the tip of the element the wire leading off the end of the coil has to double back up the glow plug element to get to ground.

It is possible for the glow plug winding to short to this return wire somewhere along the path. [Edit: or it can break and the broken end contacts the metal sheathing.] If this happens, the plug will not get hot past this point because the current will skip the remainder of the winding and go directly to ground along the return wire.

Your average ohm meter is not going to detect this. When you are trying to measure 1 ohm and you have half of the element shorted out your meter is probably going to be showing a few tenths of an ohm different than a good plug. Well within the range of error introduced by contact resistance of your probes to the connector and ground.

And if you try to measure amps, you will have the same problem. You are going to see a large quantity of current that tapers off. If the element is shorted, you will actually see more current than a healthy element.

There really is no way to determine glow plug health in situ with instruments available to the average person.

I have access to a micro-ohm meter that could probably determine this level of resistance change, but it would be difficult to get a good reading on a glow plug. It injects several tens of amps into the device under test, and that would heat up the element and invalidate the test. We use this to verify that bus bar joints are secure under load.

I think you have 2 choices:
- pre-emptive pull and replace
- chuck the glow plug in a vice and hook a battery to it with jumper cables - but that is hazardous. Danger of burns and shorting the battery trying to hook onto the little stud without touching the body or the vice.

On a 83 240D the glow plugs are a piece of cake. Took me 30 minutes with a couple open end wrenches. The trick is to pull #1 and #4 first, then pull #2 and #3, then install #2 and #3, then do #4 and #1. The tail of #1 and #4 hinder your access to #2 and #3.

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The OM 642/722.9 powered family
Still going strong
2014 ML350 Bluetec (wife's DD)
2013 E350 Bluetec (my DD)

both my kids cars went to junkyard in 2023
2008 ML320 CDI (Older son’s DD) fatal transmission failure, water soaked/fried rear SAM, numerous other issues, just too far gone to save (165k miles)
2008 E320 Bluetec (Younger son's DD) injector failed open and diluted oil with diesel, spun main bearings (240k miles)

1998 E300DT sold to TimFreeh
1987 300TD sold to vstech

I connected 1 Glow Plug to a Dashboard type Amp Meter with the highest reading of 60 amps on it. When I applied the Electricity the Gauge Needle Pegged at 60 amps and stayed there for a bout 2 seconds until the Glow Plug got hot and the amperage dropped to about 16 amps and remained steady.
Since the Fuse on the Relay is 80 amps I am guessing you need a Gauge that goes at least that high.

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84 300D, 82 Volvo 244Gl Diesel

This is a big help on these glow plugs. So, far I have only located the connection on the left fender, and removed the lid. I haven't had time to start testing from it to the glow plugs yet. I have been short on time to address it, as yet. However, on my former '83 240d I never knew that junction box was there. I owned it for years, and over 100K racked up on it, and I never replaced any glow plugs. It had about 220K on it when I traded it in. It only let me down once on a sub-zero night, and actually was my fault, knowing the battery was over due to be replaced. On setting up the ammeter in the circuit to test I am not sure how you do it? Is there any kind of schematic on doing this? I found the Diesel Giant's info on testing the glow plugs. Also, I have the database on the car with all the recorded info., that was ever done on it. It shows the #1 glow plug replaced 40K ago, the #2 replaced later, the #4 replaced at the #2 time, and then the #1 replaced again about 20K ago. It doesn't show the #3 glow plug ever being replaced? I don't know if it didn't possibly get noted or not??

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Rebe

Post #2 in this thread has the "official" MB diagnosis method for the OP car:

How To Repair & Maintain your Mercedes-Benz | Mercedes-Benz Club of America

__________________
Respectfully,
/s/
M. Dillon
'87 124.193 (300TD) "White Whale", ~392k miles, 3.5l IP fitted
'95 124.131 (E300) "Sapphire", 380k miles
'73 Balboa 20 "Sanctification"
Charleston SC

I got to doing a bit of testing on the glow plugs. I ohm'ed them from the relay to the glow plugs. I got 1.2 ohms on the first one, which is the very newest one. The others were like 1.4 ohms on #2, 1.3 ohms on #3, and 1.4 ohms on #4. They are pretty close on the readings amongst them, but according to the spec.'s they are out of range for the .6 ohms. I haven't removed them, as yet. I don't have one of those reamers for cleaning out the threads. I looked at the pdf file 2, and it is really kind of an easy to follow method of testing the glow plug system. Just comparing my start-up to the information in it...would point to my having the one glow plug defective??

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Rebe

240D on 1 bad glow plug is almost a non-starter (depends on how cold it is)

2 bad plugs = definite non-starter

The bad thing about the way the relay diagnostics are set up, is that you can be 1 plug down and still have the light to come on like it is normal. Not so bad on a 617 with 5 plugs but on the 616 being 1 plug down is a bad thing.

__________________
The OM 642/722.9 powered family
Still going strong
2014 ML350 Bluetec (wife's DD)
2013 E350 Bluetec (my DD)

both my kids cars went to junkyard in 2023
2008 ML320 CDI (Older son’s DD) fatal transmission failure, water soaked/fried rear SAM, numerous other issues, just too far gone to save (165k miles)
2008 E320 Bluetec (Younger son's DD) injector failed open and diluted oil with diesel, spun main bearings (240k miles)

1998 E300DT sold to TimFreeh
1987 300TD sold to vstech

The total current draw of all the plugs has to be less than 80 amps, otherwise the fuse will blow.

4 glow plugs, 80 amps, so no more than 20 amps per glow plug.

Am I missing something?

Perhaps the one plug that you tested which drew 60 amps was a bad plug?

__________________
Respectfully,
/s/
M. Dillon
'87 124.193 (300TD) "White Whale", ~392k miles, 3.5l IP fitted
'95 124.131 (E300) "Sapphire", 380k miles
'73 Balboa 20 "Sanctification"
Charleston SC