Parts Catalog Accessories Catalog How To Articles Tech Forums
Call Pelican Parts at 888-280-7799
Shopping Cart Cart | Project List | Order Status | Help

Go Back   PeachParts Mercedes-Benz Forum > Mercedes-Benz Tech Information and Support > Diesel Discussion

LinkBack Thread Tools Display Modes
Old 12-15-2002, 02:25 PM
waybomb's Avatar
Registered User
Join Date: Nov 2002
Location: St. Joe, MI; Manitowoc, WI
Posts: 1,553
Question Need Glow Plug Relay Help

I just bought a 1982 300SD. The glow plugs are not working. Found the procedure for checking everything out. This is my first diesel, so bear with me.

I have main power. The fuse has power both ends. With the relay apart, there is power at the solenoid.

The relay looks almost new on the inside. The one contact has minor arc mark. The secondary contact is new.

With the 4 pin plug off, I have voltage at pin 1, the red wire, with the key on. I do not get a glow plug light on the dash. If I jump 1 and 3, I get the dash bulb, so it works.

I took the 4 pin plug apart, to tighten / crimp the connectors a bit. with the key on, I plug/unplug the 4 pin pin plug, the relay pulls in, but imediately releases, so I figured I had a bad connection.

The connectors are very clean.

I plugged them in without the plug body on to check voltages and grounds. Everything checks out. I then checked for volatge and grounds on the inside of the relay. Everything is there. But again, if I pull off / put on the red wire, the relay pulls in but immediately releases.

I get this reaction whether the 5 pin glow-plug plug is attached or not.

I've only had the car for a few weeks, but this relay looks brand spankin new on the inside. I read somewhere about charging an internal capacitor by energizing the coil a number of times, I've done this, but still the same results - the relay drops out as soon as it pulls in.

Any simple fixes - or do I buy a brand new relay? I gotta tell you, this one looks brand new on the inside. Outside ain't too bad looking either.

Hoping it is something simple

*****More questions - I bench tested it. Ran negative 12vdc to pin 31. Positive on big lug 30. Jumpered from 30 to 15. The solenoid pulls in, but immediately drops out. Pin "LA" feeds the dash lamp. Anybody know what pin 50 is for? The sheet I have says it goes to the back-up lamp? Huh?

BTW - with no glow plugs, she started in 6 degree weather here in Wisconsin week ago!
It was a Florida car before a few weeks ago.

Last edited by waybomb; 12-15-2002 at 04:18 PM.
Reply With Quote
Old 12-15-2002, 05:28 PM
Aaron's Avatar
Join Date: Aug 1999
Location: Cincinnati, Ohio
Posts: 1,935
If your light is not coming on in the instrument cluster, pull the 6 point plug from the relay and jump terminals 1 and 2 with a wire then put the key on to see if the light illuminates. If it does, I would check each of your glow plugs individually with an ohm meter to make sure they are all functioning. If you have a couple faulty ones, then replace as needed (or put a whole set in) and that should take care of the problem. If the plugs all check out ok, your relay would most definitely be at fault and should be replaced.
Reply With Quote
Old 12-15-2002, 09:36 PM
Registered User
Join Date: Aug 2001
Location: Evansville, Indiana
Posts: 8,150
Sounds like the relay is bad, and not staying on with temp like it is supposed to.

Much better than the failure on the Volvo -- glow plugs stayed on all the time, ran the battery flat on the highway if the headlights were on.....

1972 220D ?? miles
1988 300E 200,012
1987 300D Turbo killed 9/25/07, 275,000 miles
1985 Volvo 740 GLE Turobodiesel 218,000
1972 280 SE 4.5 165, 000 - It runs!
Reply With Quote
Old 11-26-2009, 02:50 PM
Registered User
Join Date: Nov 2009
Posts: 16
pre glow plug relay

We checked the voltage with a meter. The voltage was fine across the fuse inside the relay box and to the relay itself but no voltage went to the glow plugs.
We then took our relay apart, this was a little tuff. There are snaps about one inch up on the inside of the relay box that you have to really work a screwdriver to pop open.
Inside of ours we had water (must have been from washing the engine). Cleaned all corrosion off circuit board and dryed all components and contacts with hair drier. The board had a small break in a trace. Solder this trace and then tested it on the car.
Worked perfect.
Reply With Quote
Old 11-27-2009, 02:01 AM
Diesel911's Avatar
Registered User
Join Date: Sep 2007
Location: Long Beach,CA
Posts: 26,003
If you have not already seen it The Diesel Giant site has a pictorial on checking the Glow Plugs with a meter. When the page opens up it appears that there is not much there; you have to scroll down to see the pics.

If you remove the Glow Plugs the carbon should be reamed or otherwise cleand out of the Glow Plug Holes.
Totally Avoid Autolite Glow Plugs and Monarks. Use Bosch or Beru Glow Plugs. Bosch are good, cost less and easier to find.
84 300D, 82 Volvo 244Gl Diesel
Reply With Quote
Old 11-27-2009, 10:44 PM
wahip's Avatar
Registered User
Join Date: May 2008
Location: Chicago
Posts: 10
Originally Posted by Diesel911 View Post
Totally Avoid Autolite Glow Plugs and Monarks. Use Bosch or Beru Glow Plugs. Bosch are good, cost less and easier to find.
Agreed that the Diesel Giant tutorial depicts the proper glow plug check sequence and is pretty straightforward.
I had good luck with the Monark injectors, but what is the issue with their glow plugs? I thought they would be a reasonable German-made replacement.
Reply With Quote
Old 11-29-2009, 01:37 AM
Custom User Title
Join Date: Jun 2008
Location: Simi Valley, CA (SoCal)
Posts: 454
50 is the start circuit. Power is applied to this circuit when the ignition is in the Start position (Position 3).

15 is the run circuit. Power is applied to this circuit when the ignition is in the Run (Position 2) or Start positions.

The Starter Lockout and Backup Switch (part of circuit 50) is just a switch on the shifter to allow the starter to energize, and the same switch also senses reverse to engage the backup lights, hence its name. This is simply where the connection is made, since you must be in Park or Neutral to start the car, and you continue to glow if you're starting the car. When you release the key from start, this kills the glow. So this provides a signal to the glow plug relay circuitry (transition from 12V to floating/open) that tells the glow plug relay to shut off.

FYI, just because the glow plug light doesn't light up doesn't mean that your glow plugs aren't being energized, it just means that something is not up to snuff. You're system could be working at 90% and you may not get a light.

Question: When you apply power to pin 1 of the 4-pin connector of the relay, are you applying ground to pin 4 at the same time? If not, this may be why the relay drops out immediately. If circuits 15 and ground check out okay to the 4-pin connector, then I would leave the plug connected to the relay and move on. Circuit 50 tells it to turn off, so even if it doesn't check out it shouldn't enter into the equation for this situation.

Even though the relay pads look good, drag a two-sided nail file or sandpaper back and forth between the contacts anyway. It's for good luck. And to make sure the smoke can get out easier. And to remove the relay pads as an area of doubt.

Also, pull the 80amp fuse and make sure it is intact. These fuses can develop a microfracture in the center and look good and may pass voltage checks but won't pass any significant current. If you pull it and it's bad, you will be able to tell immediately.

Assuming that your connections to the four pin are okay, your relay connections are fresh and your fuse is good, pull the other plug to the relay. Turn the key on and see if you have voltage at the pins on the relay that connect to the glow plugs (you probably have 30-40 seconds to perform this check before the relay times out). If not (i.e., the relay is still dropping out immediately), then it is most likely that the electronics in the relay are bad and you'll have to replace it. If you have voltage at one pin, then you should have voltage at all pins. If you have voltage at all five pins your problem is in the glow plugs or in the wires to the glow plugs.

Leave the connector off and check the resistance of the glow plugs by connecting one end of the ohmmeter to ground and the other end to the connector pins. Resistance should be on the order of .6 to 1 ohm IIRC. What you may find is that glow plug 1 has a different resistance (probably higher?) than the other four. This is a fault that will cause your glow plug light to remain unlit. I suspect this is your problem as you state that your car starts even at cold temps. I live in SoCal, and I had a relay fuse break late afternoon as I was getting ready to leave. It was a warm day and it took a while for the engine to start. The engine only has 109KMiles, so it's in good shape and it still took a while to start. I can't imagine trying the same thing at 6 degrees. You guys in Wisconsin still use Fahrenheit even though you're up there next to Canada, right? If you find you have high resistance in one of the glow plug lines, check the resistance with your ohmmeter between the tip of the plug and its body (not the engine block - we're trying to rule out a bad connection between the plug and the block).

Let us know how these checks turn out, and if you find anything. These cars are amazing when they are well maintained, and the folks at this site can help with just about any problem you encounter. Good luck!

'84 300SD 119KMi (Liesl der Diesel)
'84 300D 326KMi when the oil left (former parts car)
'82 300SD 253KMi (new parts car)
Reply With Quote


Thread Tools
Display Modes

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is On
Trackbacks are On
Pingbacks are On
Refbacks are On

All times are GMT -4. The time now is 10:23 PM.

Powered by vBulletin® Version 3.8.7
Copyright ©2000 - 2019, vBulletin Solutions, Inc.
Search Engine Optimization by vBSEO 3.6.0
Copyright 2018 Pelican Parts, LLC - Posts may be archived for display on the Peach Parts or Pelican Parts Website -    DMCA Registered Agent Contact Page