'85 300SD emergency manual start

Without warning, no glow plug light in the dash and glow plug relay under hood clicks on then off immediately. (Chicago) I use the block heater overnight so morning starts are a given, but after teaching all day, afternoon starts, it seems the glow plugs sort of work still if I wait 15 seconds, but startup idle is a bit rough and a little black smoke puffs out where before there was none. The engine was factory replaced before I got it and has about 40K miles on it. QUESTION: For emergency starting until I can get the replacement parts for repair, can I run a wire from the battery terminal to the # 1 glow plug top, hold it there for 10 seconds, and then go start the engine? (maybe 12 sec. if in lower teens outside temp).?? TIA in advance for your most valued advice to everyone.
Sincerely,
Tom Norris

Forums

Diesel Engine Specific Aid

Chassis Model Number

W126

Clarification

Hi Tom, A couple quick questions before I advise. 1) Has it blown the strip fuse yet (in the relay)? 2) Do you get a blinking glow light on the dash after it starts?

FYI If you are planning to remove the plugs make sure to ream out the carbon in the glow plug holes...

'85 300SD emergency manual start

I'm not new at working on diesels (15+ years general maintenance through complete engine rebuilds) but the '85 300SD and the '84 300TD are "new" to me. I found where I think the strip fuse is on the SD - behind a plate on the firewall side of the glow plug "control box" assembly, at the bottom...but how to open that "door" w/o breaking it? On Mercedes cars, if you have to force it, I'm probably doing it the wrong way, so I haven't used any excessive force and haven't yet seen that strip fuse. The glow light emits absolutely no light at any time for any reason at the present time. The G.P.'s worked perfectly when I shut it off on a Tuesday evening, and the next morning no glow plug light and no extended click under the hood. As I stated, the silver box in the G.P. "control box" clicks on then clicks off within a quick second. Before, the 300SD started into the teens w/o block heater and ran smooth right away. Below that temperature range, I automatically used the block heater and with similar quick starting results and smooth idle immediately. I hope this helps with the diagnosis. : )
Thx!
Tom

'85 300SD emergency manual start

I hope to receive the Glow Plug book by this weekend. This morning, with only block heater on overnight and upper 20's outside, I tried an instant start...would NOT fire up. Sooooo, I let the "imaginary glow plugs" have a 10 second wait with key in "on" position, then tried to start again. Fired right up and ran smooth, so glow plugs are really working to some extent. TIA for all suggestions on this situation.
Tom

Try this now

The manual will help you troubleshoot the problem so you don't end up buying a relay you don't need. It could be a faulty temp sensor or just bad wiring contacts. Try this now. Remove the relay cover by prying up on it. It just snaps into place. Remove both plugs from the relay - clean contacts thoroughly - the coat all pins with dielectric grease and reinstall. Report back what happens... Kent

Seen this before, several times

Anecdotal stuff...my (and a couple of other cars) exhibit erratic glow plug light behavior, but the relay works. In cold temperatures (down to -20F below here...not much hope below that), I wait until I hear the "thunk" of the relay OR turn on the dome light and wait until it brightens slightly indicating the plugs are finished heating.

It's also easy to run a pair of 10ga. wires from the battery (fused, of course!) to a switch on the left side of the dash for manual emergencies. Use only a normally OPEN switch rated for 40+ Amps. (Holding two 10ga wires together for the glow plugs with your fingers will burn your fingers.)

How do I know the above? Done it before as I replaced the relay, which was actually bad in my case (but still doesn't accurately report glow plug status) and I have a starter switch (hence the current rating warning) that is intermittent...my manual switch one time welded its contacts shut leaving the starter engaged for a long time while I cut the wires to it!

The inconvenience for me is minor...there are other things I'd rather fix first.

Cleaning the contacts didn't help......

I took out all three plugs in the glow plug box and cleaned the contacts and coated them, but no difference. The book isn't here yet and it's Saturday. My home is in N.W. Indiana and I teach Math in Chicago. I have a studio apartment in the Chicago area during the week, so, though I drive it daily, I don't know how much more I can do until next weekend back in Indiana. All suggestions appreciated and followed through. TIA
Keep on happily clattering everyone...
Tom Norris

It's fixed and purring again!

Thanks everyone! I found one bad glow plug and it's too cold here in the Chicago area to be "messing around" so I replaced all the glow plugs. Everything works fine and purrs right from the beginning again. Thanks so much for the helpful hints. I tried them all!
Tom Norris