Hello,

I just recently purchased a 1980 MB 300TD, really beautiful. It's a bit chilly in the NE USA right now, and it idles rough. Decided to ohm test the glow plugs. I found that there were 7 wires/posts on the relay, not 5 as I was expecting. There seems to be an additional connection between cylinders 1&2 and 2&4 (first forward couple and second forward couple), a hex key blank between 4&5 (3rd forward couple), and then a heater line between 5&3 (rear-most couple).

When I ohm test them, it's bizarre which is which. One on the relay does correspond to glow plug for 1, but also for cyl 3?? and 2 on the relay is to the sensor(?) between 1&2, and I can't really figure out what is going on with the others. Do I have the wrong relay in this car? PN on the relay box is 001 545 78 32. What are those sensors? Quick googling seems as though it should have relay PN 001 545 98 32?

Thank you! I'll post some pictures of the car in a showcase topic sometime soon.

Vincent

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1980 MB 300TD
1975 Jeep J10
1975 DAF 66 Marathon Coupe
1965 DAF Daffodil 31

WELCOME VINCENT ! .

You have a series glow plug setup, this means that when one dies (goes open circuit) ALL of them stop working .

To properly test you'll need to search up an older post, there are plenty .

Many here replace those early typ glow plugs with parallel ones and have far less issues cold starting .

It's normal for your engine to run roughly on cold start, that's why there's a fast idle knob on the dash board .

DO NOT rev. the crap out of your cold engine ! .

__________________
-Nate
1982 240D 408,XXX miles
Ignorance is the mother of suspicion and fear is the father

I did then what I knew how to do ~ now that I know better I do better

Thank you for the reply! I just learned about the fast idle switch on the dash today. I'm hoping that clears it up. I think the thermostat is bad, also, as it hasn't gotten to temperature. For now it's in my garage because the brakes need replacing. I've bought a bunch of maintenance items; I haven't even owned it a week, yet!

Anyway, it does not appear to be in series, there is a loom running along the length of the engine and individual wires running to each one. They appear to be pencil type. When did MB switch to the pencil type?

See here, it appears to be the pencil type. What is between the plugs?


Thank you,

Vincent

__________________
1980 MB 300TD
1975 Jeep J10
1975 DAF 66 Marathon Coupe
1965 DAF Daffodil 31

Sensor for the temperature gauge.

__________________
78 W116 300SD 'Desert Rose' new as of 01/26/2014
79 W116 300SD 'Stormcloud' RIP 04/11/2022

Okay, thank you. Does that wire normally go into the glow plug relay box along with the glow plugs? Is there another temperature sensor (kind of hard to see, look in the eye of the dip stick)?

Thank you,

Vincent

__________________
1980 MB 300TD
1975 Jeep J10
1975 DAF 66 Marathon Coupe
1965 DAF Daffodil 31

Yes that wire does go into the glow plug relay box.
Since it is the wire to the rearmost temperature sensor, it provides the signal for glow plug relay timer.
Less glow time for a hot engine, and more glow time for a cold engine.

The front temperature sensor (which I didn't see in the ring of the oil dipstick)
is the one that has the wire carrying the signal to the temperature gauge in the instrument cluster.
Easy enough to test, unplug it and the temperature gauge stops working.

__________________
78 W116 300SD 'Desert Rose' new as of 01/26/2014
79 W116 300SD 'Stormcloud' RIP 04/11/2022

O.K., you're in a better place now, unplug the glow plug relay and use the $5 Harbor Freight analog meter to discern the resistance of each of the 5 glow pugs ~ if any are not close to equal they're in dire need of replacement .

DO NOT use Champion nor Autolite brand glow plugs ! .

There are prolly other low quality brands I've forgotten about .

In this colder weather, use the seat belt lamp time to decide when to begin cranking, you'll discover it really helps ~ the engine should start easily even if it runs bumpy when cold .

The valves on these cars need periodic adjusting, the job isn't difficult but it does need a pair of specially bent 14MM open end wrenches, easy enough to make if you have access to a welder, or less co$t to buy then paying a shop to do the job .

For now, go to your auto parts store and buy a bottle of "Diesel Kleen" by 'Power Service', get the white plastic bottle, (White for Winter, Silver for Summer, remember this) , pour about 1/2 of the bottle in the fuel tank then fill it up, it'll ease cold starting plus help loosen crud that's likely accumulated in the fuel system .

There's a bunch of basic things you'll need to do of have done by a competent and honest M-B repair shop, they're out there but you'll have to hunt them up in your locality .

_YOU_ can do most of the basic tune up stuff if you're handy with auto repairs and you'll usually do a better job because YOU CARE unlike most mechanics .

Be aware that German vehicles need more attention than do others, if you're willing to do this then you'll be very happy with an old Mercedes, if not clean it up and by Spring have it waxed and polished, then you'll easily sell it before it drives you crazy .

__________________
-Nate
1982 240D 408,XXX miles
Ignorance is the mother of suspicion and fear is the father

I did then what I knew how to do ~ now that I know better I do better

As these cars aged real efforts to maintain them declined. In all too many cases they were just treated as old cars. There is no dispute about this really as that is understandable. For possibly the majority of them.

In my opinion just do a total maintenance run on any example you are not sure of. As a general rule you should not believe the indicated odometer readings either. If you are very lucky perhaps some owner not long before you acquired it did this. Or there are records you possess that came with the vehicle.

If there are physical indications the accumulated milage is not too great. I think they can be a very reliable car after a real check up. Fortunately it involves more time than money. Remember it is 40 years old. So there is a lot to check.

I have a really soft spot for these cars. Also believing nothing like them will ever be manufactured again. Rust and extremely high accumulated milage are their downfall. At least for practicality. Our kids years ago would tell the wife dad is coming home in the tractor. Like the dogs they heard them before they came into sight as our house is built on the leeward side of a hill.

The car you acquired was a very high quality car when it was new. This makes a major difference now. Also welcome to the site. This site makes a real difference to owners of these cars. As mentioned checking the valves and at the same time checking for too much wear in the timing chain. The valves required checking every fifteen thousand miles. If for example this has been ignored you may have to adjust them. Drive the car for a few hundred miles and check them again. Carbon can build up on the seats that will get pounded off closing the valve clearances up again. If they have not been adjusted for decades.

Since you are going at the brakes. If the brake fluid is anything but pretty clear. Change it. If the car is drivable take it for a short drive and coast to a stop before starting the brakes. Check each wheel for temperature. You have eight caliper pistons and now is the time to check them initially for dragging. If not hit the brake pedal and check that the wheel has released. Then repeat this for each wheel. The rubber brake hoses should also be changed out if they are old. I am not for pumping an old master cylinder to bleed the brakes. There are other methods if you have to. In other words tackle the brakes in a restoration approach. Check everything carefully.

Fortunatly any parts if needed are reasonable in price still and available for this brake system. They had a really good brake system when new and it really is not that hard to get it back. Perhaps this is also a good time to clean and check and repack the front wheel bearings as well. They are pretty long lasting on these cars if not totally neglected. Even neglected they have done well in general.

I attribute this to quality. In setting them back up a dial gauge should be employed. Almost everything and the best way to tackle it is somewhere on this site.

I believe you have the non turbo engine the same as in my 1977 300d for all practical purposes. Checked over and in decent condition mine seems okay on power. To me there are three indicators of initial engine condition before any work on them.

Where is the oil pressure gauge after the engine is at operating temperature at hot idle? What does the oil cap do if loose? A little dancing is fine but it should not be blowing off. Plus how many miles per quart of base oil consumption is occurring.. Making sure there is no signifigant engine leakages present.

Piston speed on the highway is somewhat higher than the turbo engines as your car has a higher ratio rear end. The piston rings are possibly less efficient at dealing with the oil I personally believe. Especially in the four cylinder 616 engines. Where the rear end ratio is even worse. So some base oil usage is not as signifigant in comparison to the turbo engine versions. Again these are just my opinions.

I know I can do a massive amount of checking and mechanical refurbishing if needed for less cost. That being far from home with a roadside breakdown today because I did not. Where you are generally at the mercy of those that get you going again. If you can even find anyone there to work on it.

Suprising enough the majority of these engines still do pretty good in these simple tests.

Thank you guys so much! I want to make a post about this at some point, been busy lately, but I will give a little insight. The car was incredibly maintained. One owner from new to 2012, unknown history from there to 2018, some college kid bought it, then I bought it from him.

The paperwork is insane though, the guy dumped over $30,000 into it in the 1990s to... get this... fully restore it. I'm talking: detailed invoices of every thing done from a repair shop: every panel, every line, suspension, climate control, engine and transmission rebuilds, the whole wazoo. Even got it appraised. I'm utterly astounded by this. Clearly that owner loved this car.

That being said, it is in dire need of normal maintenance. The college kid did literally nothing (which he admitted), which is evident by the metal to metal on the brakes. I'm sure it needs valve adjustment, which I am no stranger to (I have other classic cars). I am so excited to own this car (especially since I got it for next to nothing). I know it's running rough because I suspect the thermostat is bad... it never since I've owned it got to temperature, even after a 30 min drive home with it.

At any rate, I'm still really confused about diagnosing the glow plugs. I got get a few to read, each were between 0.9 and 1.1 ohm, but there are 5 plugs and 2 temp sensors going in, and no clear understanding on which line is which! And with my ohmmeter in number one socket, I can read two different glow plugs, and the socket is 1-8 with #4 socket missing (on purpose, it looks like). The fuse in the relay looks good. The plugs look like they have been in there a looooong time, very crusty. Could really use some help with this, like which number should correspond to each plug. It's also possible I'm doing it right and the infinite readings on the meter are right and the plugs are just toast, but I want to make sure before I buy new ones.

At any rate, I am SO excited to own this, the car is really growing on me. I never thought I would own a Mercedes, haha. I will start a thread with a bunch of pictures and even some of the restoration invoices.

Thank you,

Vincent

__________________
1980 MB 300TD
1975 Jeep J10
1975 DAF 66 Marathon Coupe
1965 DAF Daffodil 31

One of the temp sensors drives the dash gauge. The other informs the glow plug relay on the progress of the glow operation. They could have used one sensor, but they didn't, they never do.

.9-1.1 is the normal range of the glow plugs. The light in the dash will let you know if there's a problem. When it doesn't display a glow cycle, one of the plugs would have gone bad. Since your car was probably converted from series plugs, the relay may be non-stock. Leave it alone, you're going to find your share of problems without going out of your way to look for trouble.

The early pencil type glow plug system had a temp sensor that goes to the cylinder head. I can't remember if it is a separate temp sensor from the coolant temp sensor for the gauge or not.

The later versions have the temp sensor built inside of the Glow Plug Relay and no temp sensor on the head.

Look in the service manual.


W123 Service Manual

https://www.startekinfo.com/StarTek/outside/12265/?requestedDocId=12265


CD1

http://mb.bolinko.org/15/w123-mainmenu-50/w123-dokumentacja-cz-1-mainmenu-59.html



CD 2


http://mb.bolinko.org/15/w123-mainmenu-50/w123-dokumentacja-cz-2-mainmenu-83.html

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84 300D, 82 Volvo 244Gl Diesel

Can you just probe directly to each glow plug to read the ohms. Or scan each of them for temperature after a glow cycle? If there is any confusion on the wiring.

Since they are not the series plugs it is only important to separate the common power feed to all of them first for a resistance check.. Sounds like this may not be practical. So if you do not have a thermal temperature reader perhaps borrow one?

I hesitate to suggest touching the ceramic portion of a glow plug right after a cycle with a wet finger. As I have no idea of how hot they would be. I have never used a thermal reader either after a cycle on them. If we found they are really not that hot the finger test should have evolved by now. This suggests they really may get too hot externally for this as well. The threaded post from the element to the external nut should absorb at least some heat. From the white hot element.

Sounds like a very good find. I would not be absolutely certain it is a glow plug problem. Although it usually is.

Or just read the seven connections with the ohmmeter. If you find there are five similar readings is all you are interested in. Without knowing I doubt the sensors read similar values to the glow plugs. Although with my limited knowledge they might.

Don't worry about the sensors now, just test the glow plugs ~ if you cannot figure out which hole to probe in the relay's MPC, touch each glow plug with the Ohmeter's probe, very easy and simple .

FWIW, as soon as it starts, if the outside temperature is in the 40's F, the engine should run smoothly off idle even when it's stone cold .

If not the trouble is else where .

Like everyone says : DO THE BASICS IN THE PROPER ORDER .

__________________
-Nate
1982 240D 408,XXX miles
Ignorance is the mother of suspicion and fear is the father

I did then what I knew how to do ~ now that I know better I do better