I have 1985 300D
Over 5 montns ago, I realized the Glow Light on the instrument
panel will not light up when I start the engine. But somehow I have no problem start up the engine even on a cold day(San
Francisco Bay Area).
So I don't know if this problem is from the light bulb itself, the
glow plug(s), the glow plug relay or the relay fuse.
Please share your opinions.

David
1985 300D

dc88168,

From the information you provided it is less than a SWAG for someone to give you an answer. If you are letting the average time the light was lit to pass before you turn the key to energize the starter, kind of pretending there was a glow plug light, and the car starts like it always did, it is more likely that the bulb in the instrument pod is out. If you just by pass the glow plug energizing sequence by immediately cranking the engine and it starts then it is more likely the glow plugs, the fuse or the relay are not working correctly, probably in that order. Let us know what you are doing, exactly, when you start the car. You can do an experiment by trying each method and let us know which starting procedure works best. If the "ersatz" procedure works best, the glow plugs, fuse and relay are probably ok. Then your task will be to get the instrument pod out and change the bulb. If there is no difference, I would believe the fuse or the glow plugs have gone bad. Check the fuse first (inside a black 3" X 3" X 1" or so plastic protective cover/box on the driver's side front fender well under the hood). The fuse looks like it was stamped out of a piece of sheet metal. If it is droopy or broken in the middle it is shot. They are cheap.

The glow plugs ususally fail in a sequence, and the first sign is the glow plug light fails to light the first time you turn the key to the glow position. If you turn it back to the "0" position and right back to the glow position and it lights, it is an indication that one plug is gone. If you do it twice and the light lights, it is an indication that more than one plug has failed. You may want to try this process to see if the light works the second time. If so, you can explain why the car starts pretty good, as 4 of 5 cylinders fire as expected, and the weather in your neck of the woods is not dropping to the teens right now, is it?

Hope this helps and good luck, Jim

__________________
Own:
1986 Euro 190E 2.3-16 (291,000 miles),
1998 E300D TurboDiesel, 231,000 miles -purchased with 45,000,
1988 300E 5-speed 252,000 miles,
1983 240D 4-speed, purchased w/136,000, now with 222,000 miles.
2009 ML320CDI Bluetec, 89,000 miles

Owned:
1971 220D (250,000 miles plus, sold to father-in-law),
1975 240D (245,000 miles - died of body rot),
1991 350SD (176,560 miles, weakest Benz I have owned),
1999 C230 Sport (45,400 miles),
1982 240D (321,000 miles, put to sleep)

If the number one plug is bad you get no light. Check the fuse, listen for the "click" from the relay, and try changing out the first plug. My 83 300d lost number one this fall and the light was gone. Also had a couple others bad as well and only had real poor starting when 3 of the 5 were bad. When it got in the 30's north of here I almost didn't get it started one day. It had missed and sputtered all summer when starting cold and just figured it was "the nature of the beast" 5 new plugs, and a valve adjustment now it starts like a gas engine, no smoke, no miss, just quick easy starts.

Thank you, Jim Smith and rebootit for your valuable suggestions.
Here are the things I have done:
(1) Saturday @ 7AM, Temp about 50 F, I turn on the key and pretending the glow light was on and wait 30 seconds , then start the eninge. Engine started just fine.

(2) This morning Sunday @ 7AM, Temp about 50 F.
I start the engine without waiting, the engine would not start.

(3) I checked the relay box, the fuse was fine.

May be I have a burnt out glow light bulb, or just one bad glow plug. I would probably use a ohmmeter to check the glow plugs.

Where is #1 glow plug located?

Thank you again

David
1985 300D

#1 plug is the one in the front of the engine. Located right below the first injector. One of the "easy" :p ones to get to. Change that one out with a known good one and try the light. If it comes back on problem solved. Then buy some Bosh plugs from Fastlane and change the rest whenever. All of mine were factory or at least had a star on them so I guess factory. After 230k miles it was time.

Based on your problem description, you have either a bad light bulb or one (or two) bad glow plug. To check the light bulb, pull 4-point coupler from the preglow time relay; turn key into position "2"; bridge jack 1 (+12V from fuse box terminal 15) and 3 of coupler. If preglow indicator lamp is not lighting up, either the bulb is bad or the black wire from coupler jack 3 of preglow itme relay to preglow indicator lamp is bad. It is likely that you have a bad plug, not the wiring or bulb.

David

I had that happen in my 83. Even when relatively warm (~50) Ive found the glow plugs to help, otherwise Ill need to crank the engine for a few seconds instead of hardly at all. what I did was while in the glow position, I tapped the light, and the glow plug light and the one next to it both turned on. Was just a bad connection, or a bad bulb that needed a little 'coaxing'. Do check the glow plugs, I used an ohmmeter, and all were fine so I know what it was in my car, hopefully itll be as minor as possible in yours too...

JMH

To: JimSmith, rebootit, be459, JHZR2

Thank you for all the suggestions.

I used an ohmmeter to measure all the plugs (reference to Ground) and each one measured about 0.8 ohm.
However, I did not take the wires out before measurement.
So may be those readings were not accurate.

Question to: be459
In the relay box, I see 3 items:
(1) Fuse
(2) Connector (in the middle) with 3 wires
(3) Connector (on the side) with 5 wires
the " 4-point coupler" mentioned in your post, is this Item (2) of the above?
However, the terminal numbers are: 15, 31, and 50.
Will you please verify this.

Thank you

David
1985 300D

It is the connector in the middle. It should be a 4-point connector. Points are labeled 15, 50, LA, and 31 on the relay box. Here is the mating diagram:

Point 15 <-> Jack #1 (to fuse box terminal 15)
point 50 <-> Jack #2 (to starer lockout and backup lamp switch terminal 50)
point La <-> Jack #3 (to Indicator Lamp)
point 31 <-> Jack #4 (to ground)

The above info is from the service manual and is good for 617 engine starting 1981.

I do not know why you have only three wires. Are you sure about that?

Keep in mind that the plugs are wired together inside the relay box. If you did not unplug the relay box, you was just measuring the same circuit five times.

David

Hi David (be459)
Thank you for your recent post.
I just went back to check again and following are my findings:

(1) Yes, the middle connector has 4 wires, marked 15, 31, 50 and LA.

(2) I unpluged the big connector as you suggested.
I used an ohmmeter to measure each wire from the connector
(G1, G2, G3, G4, G5) with reference to Ground.
I found G1(Glow Plug#1) and G5 (Glow Plug#5) are bad
(infinite reading, open).

So thats why my glow indicator light on the instrument panel went off.

The reason I can still start the engine every time with 3 glow plugs is because I live in warm California.

But I think I better change those 2 glow plugs. Plug #1 is easy to change, there is lot of room. I don't know about Plug # 5,
I can hardly put my fingers in around that area.

Thank you everybody for your help


David
1985 300D

As I said in the past, put a towel next to the engine below the plugs. In case you drop a nut, washer, etc, it will be on the towel, easy to find. I suggest you change all the plugs that will give you a chance to clean the chambers. A bad plug can still have .9 ohms resistance.

David

The glow plugs I ordered came by UPS today.
I changed the Plug# 1.
I turned on the key, the glow light on the instrument panel is on now. Engine start right away.

As for Plug #5, there is not any room around that area. So I did not change it yet. There is the oil filter housing and a rubber hose there. There is no room for the mid-deep socket and the 3/8" drive. May be I leave this plug for the mechanic next time.


Thank you to everybody on this board!




David

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1985 300D