I believe that later relays have the thermistor in the relay itself, not in in the block. Don't know when the change was made, 83 could be either way.

The W126 and W124 diesels don't have a block temp sensor.

Peter

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1972 220D ?? miles
1988 300E 200,012
1987 300D Turbo killed 9/25/07, 275,000 miles
1985 Volvo 740 GLE Turobodiesel 218,000
1972 280 SE 4.5 165, 000 - It runs!

Just changed 4 of my 5 glowplugs today. That 5th plug will be a bear, I think those new ratcheting wrenches are the way to go, not enough room for a deep well socket. Mine wouldn't start cold unless plugged in. I purposely left it unplugged last night, then after installing the plugs it started right up! I checked the resistance on each new plug and all were .4 to .5 ohms. Then after I started the car and it was warmed up (not hot) they were 1.5 to 1.6. I guess if someone were to check theirs it should be done cold. When I first bought my '82 TD about a week ago, the glow light stayed on about 5 seconds, the last few days it wouldn't come on at all, now it stays on about 10 seconds. Tomorrow morning should be the test (32 degs.)

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'93 300D 200k (since Aug 04)
'82 300TD 205k (since March 04)
'02 Ford Escape 192k (bought new)
'90 Ford Festiva 310k (since '92 with under 10k)
'01 Jeep TJ Wrangler 278k (since 07 with 235k)
'76 VW Westfalia van 150k ? (since '90 with 100k)
'72 Chevrolet Corvette conv. (base not a LT1) 175k (since '84 with 100k)
71' VW Squareback (since '72 with under 9k, odo turned over, then stopped then replaced. maybe 250k)
'87 Yamaha Riva 125 scooter 7k

I agree with on your comment about checking the glow plugs cold. I think it is just assumed by everyone that you check them cold. The manual states that the glow plugs are in order if you measure 1 ohm or less at 20°C/68°F.
I did notice a difference on the resistance reading depending on when I measured the glow plugs.

I also agree that the fifth glow plug will be more difficult than the other 4. I used a boxed end wrench to remove the four. You may have a good idea about using wratchiting wrench to remove the glow plugs.

After changing 4 of 5 glow plugs and installing used relay, I still had trouble starting the 82 300SD last Saturday. It was 3°F at the house. I did not think I would need to use the block heater becuase of the parts I had just installed. Bottom line was the car would not start. I wore down the battery after first glow cycle and cranking episode. Used the block heater and the battery charger for about 45 minutes. Then she started. I took the car to friends shop and we tested the battery. It indicated it was good. It is about 4 years old. It still may be the problem. I looked at the prefilter and noticed it was blackish in color. My problem may now be bad fuel/algae. I put some fuel conditioner in and will see what happens.

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'82 300SD 206K miles Anthracite Grey received 250,000 Km badge
'93 GMC Turbo Diesel 1-Ton Dually 113K miles Stolen 17 Jul 05
2005 GMC 2500HD Crew Cab D-max 22K miles Love it!
'68 Corvair Monza 110 Coupe 26K Sold
'66 Corvair Monza 110 Convertible 123K
'52 Ford 8N Sold
'66 Ford 3000 Diesel (204 hours) For Sale
'86 White 2-65 MFWD Iseki Diesel

OK, I am now more intrigued after reading these posts. I have a 1980 240D, which has the sensor you described between the #3 & 4 plugs, but in addition also has one between the #1 & 2 plugs, also in the same harness and both go to the module. The frontmost one is stamped "NTC" in the brass nut along with the part #'s and goes to pin #8 marked "NTC" on the module, the rearmost of the two goes to pin #6 and is marked "B" on the module, any idea what each one is for?
-Chris

It's not clear to me, but the wiring diagrams appear to show a separate sensor to control the timing of the glow plug light on the older engines. The wiring diagram shows a "Temperature Sensor, Preglow Indicator" going to the "NTC" contact on the module. The newer engines appear to have a "NTC" circuit internal to the relay. I assume the newer engines perform both functions with a single sensor. This is mostly a guess.