Did a search and could not find anything. On my phone so I apologize if u did not find some obvious thread about this issue.

1985 300D. Recently got this car in and fixing it up for the winter.

When I got the car it had no service history but it is a low mileage car. I had no glow plug light and the was hard to start.

Replace all 5 glow plugs and found one bad one that did no light when hooked up to a jumper cable. Replaced with Bosch units. Also replace strip fuse and terminals look clean.

Now the problem is that every other start the glow plug light would not come on and it would have a hard time starting. I can't say for sure if it is every other time but at least last night when I pulled it into the garage to change the transmission gasket it would not light. Then later that evening I jump in the car to roll down the windows the gp light did come on.

Any guesses? Bad gp relay?

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198? 300D ???K - Future DD
1989 300TE 285K - traded in '12
1984 300SD 186K - Sold in 08
1978 300SD 330K - Died in 05

I always have used the following link for glow plug troubleshooting issues Mercedes Diesel Glow Plug Repair It could be the relay, but then again, maybe not. Could be something as simple as a loose bulb in the instrument cluster. Try the steps outlined in the link. Good luck!

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2012 Mercedes ML350 Bluetec 91K (hers)
2005 Corvette 55K (fun car)
2002 VW Jetta TDI 231K (mine)
1998 Volvo S70 T5 Turbo 196K (kids)
1994 Ford F150 4WD 249K (firewood hauler)
1983 Mercedes 300D 376K (diesel commuter)

What about the fuse? Have you checked out Diesel Giant's glow plug fix page?

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'83 300DTurbo http://badges.fuelly.com/images/smallsig-us/318559.png

Broadband: more lies faster.

Thank you for the link. I checked them all out and looks like the gp relay is bad. Some times there will be voltage to the gp wires and other times there are not.

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198? 300D ???K - Future DD
1989 300TE 285K - traded in '12
1984 300SD 186K - Sold in 08
1978 300SD 330K - Died in 05

you can take the cover off the GP relay and check the board to see if you find any bad spots that need resoldering

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1983 300SD (daily driver)
1991 420SEL (work in progress)
1979 300SD (future replacement for 83 SD)

RIP:
1983 300SD (totalled)
1986 420SEL

Take a good look at the replaced Strip Fuse to see if it is cracked.

Also don't discount that there is an issue with the 28 year old Ignition Switch. If the Ignition Switch is not sending the Juice to the Glow Plug Relay the Glow Plugs will also not get Electricity.

Also you can see a Flat Copper Wire when you can see the Strip Fuse on the Glow Plug. Make sure the screws are tight on the Wire and the Strip fuse.

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84 300D, 82 Volvo 244Gl Diesel

Thanks for the suggestion guys. I took the relay apart and nothing jumps out as needing re soldering. The ignition seems to be reliable as in getting 12v to the relay reliably every time. However the solenoid does not always engage.

The relay does work about half the time. And when it sticks and I disconnect everything and reconnect it seems to work again. Decided I want a manual system rather than worrying abiut this relay or a used one from the junk yard. and so I ordered the manual kit from Mercedes source to try out. I always wanted a push button relay and having a pre assembled kit worked out well for my schedule.

I am glad I did pin point the cause. And luckily it is my winter car so I can afford the down time.

Thanks again for all the help

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198? 300D ???K - Future DD
1989 300TE 285K - traded in '12
1984 300SD 186K - Sold in 08
1978 300SD 330K - Died in 05

it sounds like you are getting a light when cold, but not when warm...

85 there is a temp sensor on the engine that may be telling the gprelay that glow is not needed...

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John HAUL AWAY, OR CRUSHED CARS!!! HELP ME keep the cars out of the crusher! A/C Thread
"as I ride with my a/c on... I have fond memories of sweaty oily saturdays and spewing R12 into the air. THANKS for all you do!

My drivers:
1987 190D 2.5Turbo
1987 190D 2.5Turbo
1987 190D 2.5-5SPEED!!!

1987 300TD
1987 300TD
1994GMC 2500 6.5Turbo truck... I had to put the ladder somewhere!

Hello thank you for the input. Part of me is having remorse buying that manual kit and should have figured out or try to fix the old relay. Never had gp relay on my previous 2 diesels so I was stumped on this one. I am afraid it's going to stick on and drain my battery.

Both instances yesterday were cold (I moved my car in the garage in the morning, then started it again later to get it on the ramps in the evening). Luckily I have good compression and got it starting after about 5 seconds or cranking. The morning start the light did not come on. But the evening start the light did come on and didn't have the crank.

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198? 300D ???K - Future DD
1989 300TE 285K - traded in '12
1984 300SD 186K - Sold in 08
1978 300SD 330K - Died in 05

Could be poor connector contact. Take off the connectors and if there's corrosion, spray some wd40 and burnish the corrosion away with the smooth shank of a drill bit. Take the connector apart (observe color codes so you can reassemble) and crimp the female pins for a better connection.

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85 300D turbo pristine w 157k when purchased 161K now
83 300 D turbo 297K runs great. SOLD!
83 240D 4 spd manual- parted out then junked

Actually Your Glow Plug Relay is supposed to be on during Cranking.

I have the what Members call a Ford type Starter Solenoid that I used as a Relay on My Volvo Diesel and have had zero problems with it. It was $5 as a Junk Yard. The rest was just finding a Switch and wiring.

I have no Fuse at all in that system that is wired directly to the Battery and as I said I have had no issues.

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84 300D, 82 Volvo 244Gl Diesel

Thank you for the suggestion. I did do this and did not seem to help.

I do get 12 volts on pin 1. And a start signal on pin 2. During one of my tried the relay did click and I can take the connector on and off and hear it click on and off. Then after a while it would stop working so I cycle the ignition and it would not work. . Then after a few tries it would work again. does this sound like the gp relay is sticking?

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198? 300D ???K - Future DD
1989 300TE 285K - traded in '12
1984 300SD 186K - Sold in 08
1978 300SD 330K - Died in 05

I am having the exact same problem as you in my 85. Maybe not as bad and it hasn't bugged me enough to do something about it. I cleaned the connector contacts and it hasn't helped any.

I would suggest in your next step of troubleshooting, to rig up two light bulbs at the relay connectors and observe their behaviours. 1 light bulb to ignition signal, the other to the glow plugs. You have three bulbs to observe including the one in the cluster. It will tell you what's going on.

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85 300D turbo pristine w 157k when purchased 161K now
83 300 D turbo 297K runs great. SOLD!
83 240D 4 spd manual- parted out then junked

You might want to check your grounding connection. Even if it looks ok, appearances may be deceiving; you can unscrew the nut, clean up the contacts and re-tighten.

By grounding do u mean from the battery to the chassis?

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198? 300D ???K - Future DD
1989 300TE 285K - traded in '12
1984 300SD 186K - Sold in 08
1978 300SD 330K - Died in 05