Glowplugs... light and circuit fail sometimes - PeachParts Mercedes-Benz Forum

landev · #1 10-12-2007, 08:34 PM

I have had a couple of occasions in the past week where I get no glow plug light and not much in way of starting in my 84 300D-T. Only half reading posts, I assumed that the problem was that I needed new plugs (which I ordered), but now, on reflection, I am wondering if my relay is the problem. I was planning on installing the plugs tomorrow (which will probably not be a bad idea at any rate) but has anyone heard of a relay intermittent... works sometimes and not others? The fuse looks fine, so I'm assuming that's not the problem. Just being lazy here... looking for some advice before I dig into it.

Thanks in advance!

-l-

#2 10-12-2007, 08:41 PM

You can do a couple of easy things. Check the resistance of each GP by unplugging the connector at the relay and checking each connection (they are numbered on the female connector). A good GP should be about 0.6-1.0 ohms when cold, a bad one usually reads infinity (open circuit). You can also verify that the connection at each GP is getting at least 11V while they are glowing (they glow about 40 seconds, regardless of the light). That shouls help you determine if it's the GPs or the relay.

Brian Carlton · #3 10-12-2007, 09:08 PM

There is an 80 amp fuse under the cover of the relay..........a copper strip.........unprotected with any insulation...........

Replace this fuse before you do anything else.

landev · #4 10-13-2007, 12:36 PM

I ordered a couple of fuses yesterday... (can't find any Merc parts in northern Idaho to save one's life!) so will try this once I have the part. In the meantime, will check resistance on the plugs. Something tells me that I have a plug out, as the car runs very rough at a cold start. Is it dumb just to go ahead and replace the plugs with the new set of Berus?

#5 10-13-2007, 12:44 PM

Quote:

Originally Posted by landev

Is it dumb just to go ahead and replace the plugs with the new set of Berus?

It's not dumb, them you will know how old they are. Personally, I treat them like light bulbs and replace them as they fail. Either way will work. Rough starting sounds like one (or more) bad GPs.

landev · #6 10-13-2007, 08:41 PM

O.K....
I measured the resistance on the plugs and they are 4 X .5 and 1 X .6. I took out the fuse and closely examined it... sure seems to be fine. I took the car out for a spin after changing fuel filters and all... it started just fine... glow plug circuit seemed to work fine. I then let it sit a bit while I worked on the '96 300... I then went to start the '84 and the 'lil GP light didn't come on (again.) I then got out and rapped on the relay with my knuckles, got back in and the light came on. Doesn't this sound like a relay problem?

-l-

bustedbenz · #7 10-13-2007, 08:51 PM

That's exactly what it is. Mine hasn't been worth replacing to me yet, I just rap it with the Mag-Lite whenever it sticks.

landev · #8 10-22-2007, 02:03 PM

Welp,

Replaced the relay... man, what a difference! The car starts much smoother! I also noticed that the glowplug light stays on much longer than it did with the old relay.

Way mo' beddah!

Thanks!

-l-