Weird glow plug issue
 

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?

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!

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

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.

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

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.

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

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...

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.

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.

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.

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?

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.

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?