1987 gd300 glow plug relay

4 replies [Last post]
mikemcdonnell15's picture
Joined: 27.07.2017
GWOA Groups: Members

Where could I source a replacement relay. Cold starts are poor and the glow plugs have been changed.


fraj's picture
Joined: 15.06.2009
Location: Somerset
GWOA Groups: Members
Re: 1987 gd300 glow plug relay

Hi Mike,

              Some of the saloon car relays are the same.  I got mine from a breakers for peanuts compared to the MB item. Suggest you confirm with Raj or Gavin.   Unfortunately all G parts now come under Mercedes car spares and are not available from Mercedes Truck centers so Gavin can only advise.  Two years ago he could have supplied the relay.

              The only problem with parts from breakers is questionable durability.  The one I acquired is now giving problems after six years so will soon need a new one.



Arnie's picture
Joined: 12.04.2004
Location: London
GWOA Groups: Committee, Members
Re: 1987 gd300 glow plug relay

First look here for the parts you need.

(click on ENGINE tab)


Look for the part number and then look here:


I suggest Bosch or Beru


mgrays's picture
Joined: 07.11.2005
Location: Aberdeen Scotland
GWOA Groups: Members
Re: 1987 gd300 glow plug relay

Are you sure there is something wrong with the relay box?

One of the glow plugs is used within the timing circuit on a OM617/300GD so if one is broken it stops the glow relay working. This has happened a couple of times to me so I ended up tracing out the circuit to find out how it works. When it was like this you would see the glow light but if you listened to the relay it clicked on for only about 1 second and then flipped off as the load path from the broken glow plug was not there. Normally it should click on and stay on for 5-10 seconds and then click off - you will need to be right by the relay box to hear it click.

See attached for the basic circuit .. pretty basic circuit with the relay being that squiggly bit at the top, the single glow plug is the top terminal and the dash bulb is the bottom right driven by the transistor with a few resistors.

If you open it the relay is the bit with the contact to the side of the circuit board.

g wagon g
g wagon g's picture
Joined: 21.04.2014
Location: uk
GWOA Groups: Members
Re: 1987 gd300 glow plug relay


As said get someone to turn ignition on while you are by relay. They are usually pretty good

You could try a voltmeter on the glow plugs to make sure they are getting power for the pre heat

They might not be getting full amps but its a starting place

I might have my old heater plug relay from when i changed to om 606. Let me know and i will have a look if you need one. It was perfect if i can find it