Hi Guys, another glow plug question, but with some unique qualities...

Just recently I've noticed that my pre-glow indicator light on my 85' 300D N-T has been acting strange. When I turn the ignition sw. to run position on a cold day, the PreGlow light will not come on till about 6-7 seconds after I turned the key. Then it would go out 1 or 2 seconds after that. On warm days the light would not come on visibly at all. In either case, the car starts normally, but immediately idles slightly rougher that it did before this problem (at least for the first few seconds).

What I am wondering, is this some type of indication that a Glow plug may be damaged/gone-bad or is this problem just a slow relay or a bad connection? Does a separate relay power the illuminator and if so, where is it?

I pulled the GP relay out and opened it up, cleaned contacts and all connections to it and around it. The relay does click and energise the GPs whether or not the light comes on. Also, if I immediately turn the key to stop when running, and then back to run, the light works normally and will do this for about the next 5min or so, then works as stated above.

I've just returned from an 800mi trip with my MB, so I'm thinking that maybe the extended 75-80mph driving broke a plug or rattled something loose??? Otherwise the car handled the extended stress fantastically and even runs better now than before (less smoke too).

Thanks

ATLD

Any ideas???

Forgot to mention, the trip I recently took brought me into areas where the outside temperatures were far below 0 deg F. My problems started after I left this area. My car does not have a block heater...

ATLD

Sincerely,

ATLD

Hello,
You may have a plug with an intemittant crack in it.
Take the 5-pin plug off the glow relay and with a DVOM
(Fluke Meter) set to ohms and check between pins 1 thru 5 and
ground. Do this with the engine cold and all your glow plugs
should have a resitance of around .7 ohms or so. if you get one
that has .0 ohms or is higher than 2.0, it is probably bad.
usually 4 will be identical and 1 will stand out with a whacky
reading. If they all test ok, run the engine and re-check.
The resistances will be slightly higher, but all the same.

If still all ok, the glow relay could be your problem.
It controls the indicator lamp and is a stand alone unit.
when the glow lamp fails to illuminate, that is the warning
signal from the relay that there is a fault.

Good luck.
DR. D

Thanks, you Gods and Goddesses of the w123:

I'll test the plugs tomorrow when I get a chance. I know that the battery is good; it is less than 6mo old and reads 13.2 V at 32 deg with no help from the engine. V drop is minimal at loading and test-loading revealed a very low spec internal resistance indicating the battery is good. Thanks for the idea, anyway. Nice to get the simple things squared away first.

Is there anyway that I can check the relay's internal operation: a schematic; anyone???

So then, the GP relay directly controls the GP indicator and there is no external relay or something (though I don't see how it would need one with a single lamp, but it's something else I could check)? What's confusing is that the lamp is not just out, but it comes on after a delay, and works fine the second time. I haven't read anything about this before.

ATLD

Nope, all the fuses and goodies are on the relay.
if you have good power to the big cable on the relay and your strip fuse is good, check fuse 12.
check pin 1 on the other connector. that should have power when
your circut 15 or ign. is on. check pin 2, that is your crank signal
from the ign switch. should have power when craking.
the temp sensor is internal and there is no way to test.

it's probably a glow plug however.
good luck

DR.D

A very simple way to check the glow plugs is to start with a stone cold engine and then run the plugs through the cycle with out starting the car. After two or three times you can feel the warmth of the plug on the block if they are working. If one is cold replace it. This won't solve a more serious problem but I have used this method to solve my glow plug problems with minimal effort and tools.

Hello again,

Recent investigation has revealed that my number 4 cylinder glow plug is shot. Reads about 70 ohms cold and hot, versus the .7-.8 ohms for the other plugs. I guess that this must be the problem so now it's time to replace the whole set.

Anyone know which plug is better from Fastlane (I have to replenish some other Standard Maintenance parts form there anyway):



Product Brand List Price Core Ships

F1100-117853 Glow Plug Bosch $13.50 $10.70

F1100-11683 Glow Plug Beru $13.50 $11.47


Thanks,

ATLD

Personaly, I have had a whole bunch more problems with failures and intenal shorts with the Beru's.
The Bosch have always treated me well.
Either will work for you, so go with the cheaper or the one with the better warranty.

DR.D

Thanks, Bosch it is!!!

ATLD

Well, thanks to everyone for their help again.

Had to pull out the bad plug to check its style (because Philip at fastlane was not sure about a Euro 617-912 engine in 85') before I could order one, so, I figured that whilst I have it out, instead of putting it back or leaving my car dead, I'll stop by my dealer and pick up a new one there (in another car) so I don't have to remove it again. Cost $14 and fixed the problems altogether.

I'm still going to order 5 more from Fastlane, 4 to replace olds, and 1 as a spare, as well as some other wear parts (like brake liners and filters etc) but at least I'm in no rush now, or have to pay for air-shipping.

ATLD