After facing charging problems and replacing alternator and battery and cleaning all my contacts and grounds , I realized ( too late ) that the problem seems to be from the glow plug relay .It comes on , the car start fine , but i cannot hear it " click off " and the voltage indicating a low value , below 12V tells me that the plugs are still on , even after 3 or 4 minutes . Is my relay bad ? i would hate to fork out another $250 , only to find out that there was another way !
It's a 1979 300SD ( W116)

Today is your lucky day!

That brilliant genius, jbach36, just posted an article today, on the DIY forum. Check there for some brilliant info.

http://www.peachparts.com/Wikka/OM602GlowPlug

jeff

check your plugs with a multimeter for voltage to check and see if your relay is sticking on. Personally, I wouldn't fork out $250 for a new relay. I'd either find one at a pick and pull for $5, on Ebay for $35 or just install a manually activated relay like ForcedInduction did before spending $250. I'd also take apart the existing relay to see if it could be fixed before condemning it.

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1977 300d 70k--sold 08
1985 300TD 185k+
1984 307d 126k--sold 8/03
1985 409d 65k--sold 06
1984 300SD 315k--daughter's car
1979 300SD 122k--sold 2/11
1999 Fuso FG Expedition Camper
1993 GMC Sierra 6.5 TD 4x4
1982 Bluebird Wanderlodge CAT 3208--Sold 2/13

Thanks Jeff, that is so timely for me. I just acquired a 190DT that has the exact problem you mentioned in the write-up: "My problem started when I would turn the key, no glow plug light, but after the car started, the glow plug light came on."

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1995 E300 200k
1981 300GD unknown km

Well, you could do what I used to do with mine before I replaced it. After you get the car started, pop the hood and unplug the connector to the glow plugs. Pain in the rear for sure but it will get you by. I did that with mine for a few months until I found a good used one.

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Joe

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1980 300SD - 495k miles - 'The Ambassador'
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Former Family Members
95 C280
73 280SEL
90 300D
87 300SDL (X2)
86 560SEL
84 300D
80 300SD

He , Kerri : where is the thread from ForceInduction you mention , I cannot find it . Coud you point me to it ?
Thanks in advance

I don't remember where he talked about it, but I'm pretty sure its a simple starter relay rated for continuous duty at 80 amps (or whatever your plugs pull) and a momentary push button to trigger the relay. The relay then powers the plugs directly for as long as you are pushing the button. Try searching for 'manual glow plug relay' or some variation of that for more specifics.

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1985 300TD-euro 352,000 mi
1974 240D (1?)52,000 mi - has a new home now

Thanks. I also just found out that the relay timing is controlled by a temperator sensor on the block. Do you know how to check the proper operation of this sensor ?

mmmmmm....I think that sensor just controls the light on the dashboard. The relays on the w123's stay on for about 30 seconds, regardless of the light. Not sure about the w116's though.

Don't know how to test the sensor. If its a temperature switch, you could use a volt meter to see if it either delivers or cuts out 12v when glowing. But it might be a thermocouple or something similar that the relay uses information from to decide when to turn off the glow light. All that to say, I don't really know how it works

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1985 300TD-euro 352,000 mi
1974 240D (1?)52,000 mi - has a new home now

On W116's the relay stays engaged for 35 seconds or so. The dash light does not coincide with the relay duration.

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Joe

~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
1980 300SD - 495k miles - 'The Ambassador'
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Former Family Members
95 C280
73 280SEL
90 300D
87 300SDL (X2)
86 560SEL
84 300D
80 300SD

Check this thread.

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1995 E300 200k
1981 300GD unknown km

I posted it in one of the other threads a good while ago, but I was having a similar problem.
It looked like I was having charging system issues, until I hooked up a voltage meter that I could watch, and found that the glow plug relay was staying on for 3 minutes every time I cranked the car up.

After I fiddled with the relay that was on the car, as well as another used one, I found that the one I had was sticking due to arc-transfer on the points of the actual relay contacts inside.
A little quality time with a fine file smoothed the pitting and the build up off of the points, and it's served well since.

The other controller had a relay which I couldn't take the points apart to file, so it would require a distributor points file to do the job on, but I haven't needed to mess with it since the first works now.


Summary of this whole bit being, it might be worth carefully disconnecting the controller and seeing if it's something easily repairable before forking out the dough.
As with many other parts of these cars, it's amazing what is able to be repaired, rather than replaced.

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-Josh
Testing the cheap Mercedes axiom, one bolt at a time...

Thanks a lot guys...it's a fantastic idea !!!