Hi,
I've been lurking here for a while and figure it's time to ask questions. My wife and I just bought a '79 240d and it has some quirks. The first is that the glow plug light is kinda sporadic, it goes off but never in less than a minute and sometimes it goes long enough that we just turn the car on (it starts up ok, we live in LA so it's never too cold here). Is this an electrical problem or does this indicate an issue with a glow plug?
Also, at night only some of the dash lights come on (which makes me think again about the electrical system) the ac dial lights up, so does the reserve tank light, but none of the gauges or the heating dials. Any thoughts?
I'd appreciate any wisdom, this is my first foray into mercedes maintenance.

Josh

Check out diesel giantsGP teting DIY

RE lights, if some of them comeone its probably just burnt bulbs. 15-20 minutes to replace them. Also listed in DIY section. GL.

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http://superturbodiesel.com/images/sig.04.10.jpg
1995 E420 Schwarz
1995 E300 Weiss
#1987 300D Sturmmachine
#1991 300D Nearly Perfect
#1994 E320 Cabriolet
#1995 E320 Touring
#1985 300D Sedan
OBK #42

Your car has the old style "Loop" glow plugs, which take longer to operate than the newer style. You can upgrade them as described on diesel giant's site if you would like a shorter glow time.

Have you checked out the bulbs in the instrument panel? My guess is that either they or the ground wire is to blame.

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1979 240D- 316K miles - VGT Turbo, Intercooler, Stick Shift, Many Other Mods - Daily Driver

1982 300SD - 232K miles - Wife's Daily Driver

1986 560SL - Wife's red speed machine

You might want to borrow a known good or even just another glow plug relay of the loop plug type. That one was in all 123s up till 80 or 81 unless they were converted to the parellel plugs.
That will separate if you have a flakey relay or indicator bulb circuit problem. The glow plug indicator light is controlled basically by the relay. Maybe you have a pick and pull parts yard locally. Even with your erratic light the plugs might still be cycling properly. To find that out you have to measure the voltage at the plugs and how long it stays on.
If even one instrument cluster light is coming on and some not. I agree it is likely burnt out bulbs. If none are coming on first try to set the dash dimmer control at the full clockwise position. The small rehostat that controls the intensity of the cluster lighting is known to go bad often enough. You can bridge it with a wire both for a test and if that is what it is just solder the wire across. Maximum brightness of the cluster is still below what most people want. All bulbs burnt out is also a possibility as well.
Seldom is it a conductor problem for the dash cluster. I am aware the rehostat is a variable conductor yet is excluded from the general statement. I guess I should have said seldom is it a fixed conductor problem like wiring.
I have a beige 1979 240d as well. You also live so far south you should be just fine staying with the loop series glow plugs. Do carry a spare plug and wrench plus a test light in your trunk.. There is a chance if you ever burn one out the car may not start. When one is out they all are.

Thanks for the suggestions! I have tomorrow off so I'll take a look at the lights and see what I can do about testing the glow plugs

Josh

Hi
On My 79 300D the length of time your glow plug stays on is related to the water temp of the engine,the hotter the engine the shorter the time they are on, they also go out after engine start (79 300D). There are two temp sensors on the block, 1 for the gauge and the second for the GP relay.
I would think the 240D 79 is the same but cannot be sure. Bosch make a kit for upgrading to a better system http://www.***************/search/product.aspx?sid=32p1ikqqq2zn2r45txyfx045&makeid=800016@Mercedes&modelid=1193699@300D&year=1979&cid=glow%20plugs@glow%20plugs&gid=7164@Glow%20Plug%20Kit
Some people just replace the plugs with pencil type, add jumpers and remove the grounding strap, but the plugs will still be controlled by water temp and go out as soon as the car starts. The after market kit comes on for 10secs and remains on for 2 Minutes after the car starts, this did wonders for my cold idle. Also the kit is designed for the 240D so no extra relay that is required for the 300D. anyway just FYI either way works.

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1979 300D.

Check your 80AMP srip fuse.

Maybe it is weak. cost is 75 cents.

I just had my glow plugs replaced. 1979 300D one plug failed after 160K miles. original plugs.
I think the loop-series plugs may last longer than pencil type.

anyway, get the multimeter out and search the archives for help and diagrams !!


Interesting anecdote about the water temp sensor and relay timer !!!

Great Forum, great members!

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1979 300D 220 K miles
1995 C280 109 K miles
1992 Cadillac Eldorado Touring Coupe 57K miles SOLD
********************
1979 240D 140Kmiles (bought for parents) *SOLD.
SAN FRANCISCO/(*San Diego)
1989 300SE 148 K miles *SOLD

Hi
Thats how it works on a 79 300D, spent months figuring it out(due to my limited intellect) pull up the wiring manual.maybe my relay was bad but it gets a input from the water temp.

Check MB manual 15-505 Operation of pre-glow relay of 1979 300D 617 pg 15

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1979 300D.