the real glow plug light question - PeachParts Mercedes-Benz Forum

alan911 · #1 03-22-2008, 11:51 AM

OK, just yesterday, turned the key on my SDL and no glow plug light.

I know, I know,.....check the plugs, check the relay, check the fuse. Well.... all the plugs were changed approx 6500 miles ago. Found two bad and changed them all for the heck of it. Just so ya know.

The car starts perfectly fine as it has for the last 22 years. No ruff idle, no hesitation, no spitting, no coughing ..... no nothing. Just Perfection starting and running.

Question: And I've searched. Has anyone ever-ever tested JUST THE BULB ???

How do I test just the bulb and absolutely nothing else. Oh,.... without taking the cluster out.

Brian Carlton · #2 03-22-2008, 11:59 AM

Quote:

Originally Posted by alan911

How do I test just the bulb and absolutely nothing else. Oh,.... without taking the cluster out.

On the glow plug relay there is a four pole plug. In this plug is socket #3.........a black wire. Check for ground on this socket. If you have a ground, the bulb is probably OK. If the circuit is open, the bulb is NG.

alan911 · #3 03-22-2008, 12:06 PM

I knew you'd come through Brian. Thanx.

alan911 · #4 03-22-2008, 12:58 PM

OK...slight glitch...

Relay box has 2 plugs along with an 80A strip fuse.:

One with 6 polls G-1 through G6 for the glow plugs

One with 4 polls labeled 51...31...etc Looks like a small relay plug.

Nothing with a #1 #2 #3 #4 on it. This is an 86 SDL

Brian Carlton · #5 03-22-2008, 01:10 PM

Quote:

Originally Posted by alan911

OK...slight glitch...

Relay box has 2 plugs along with an 80A strip fuse.:

One with 6 polls G-1 through G6 for the glow plugs

One with 4 polls labeled 51...31...etc Looks like a small relay plug.

Nothing with a #1 #2 #3 #4 on it. This is an 86 SDL

It's the four pole plug. On the relay itself, there will be four designations: "15", "50", "31" and "LA".

The wire that you want plugs into "LA". It should be the only black wire of the group. For reference, the brown wire plugs into "31".

alan911 · #6 03-22-2008, 01:19 PM

OK will give it a shot

alan911 · #7 03-22-2008, 02:08 PM

OK...

The LA socket checks out OK, so the bulb is good.

While in there checked out the GP's. Looks like # 1 is open. GP-2 to 6 all measured a slight resistance. (digital meter)

So.... Looks like it's a plug.

I thought they went in pairs when they go to effect the glow light ! I guesss not. Must be a myth.

And, I guess with a six cyclinder, 5 out of 6 working shows no discernable starting or warmup problems. Ah.......here in Florida.

Brian...you da man.

markg612 · #8 03-22-2008, 03:38 PM

Quote:

Originally Posted by alan911

Ah.......here in Florida..

You warm weather guys have all the fun--5 out of 6 at 75° is a tough catch.

alan911 · #9 03-22-2008, 03:52 PM

Glow Plugs ?

What Glow Plugs ?

We don't need no steen-kin' Glow Plugs.

They're just an added accessary item for you northe'n yankee boys.

markg612 · #10 03-22-2008, 04:22 PM

Yeah, no kidding.

Brian Carlton · #11 03-23-2008, 09:07 AM

Quote:

Originally Posted by alan911

OK...

The LA socket checks out OK, so the bulb is good.

While in there checked out the GP's. Looks like # 1 is open. GP-2 to 6 all measured a slight resistance. (digital meter)

So.... Looks like it's a plug.

I thought they went in pairs when they go to effect the glow light ! I guesss not. Must be a myth.

And, I guess with a six cyclinder, 5 out of 6 working shows no discernable starting or warmup problems. Ah.......here in Florida.

Brian...you da man.

From my recollections, the relay compares the current draw of #1 to the average of the remaining five. So, if #1 is out, it immediately provides an indication to the driver by failing to illuminate the light. If any one of the remaining plugs are out, the driver will not get an indication because the differential is not great enough. If any two of the remaining five are out, the differential with #1 is now great enough for the relay to respond by failing to illuminate the light.

markg612 · #12 03-23-2008, 10:18 AM

Quote:

Originally Posted by Brian Carlton

From my recollections, the relay compares the current draw of #1 to the average of the remaining five. So, if #1 is out, it immediately provides an indication to the driver by failing to illuminate the light. If any one of the remaining plugs are out, the driver will not get an indication because the differential is not great enough. If any two of the remaining five are out, the differential with #1 is now great enough for the relay to respond by failing to illuminate the light.

I was in the 602/603 book for something else and flipped over to the schematics for the glow controller, any of the non-afterglow systems is exactly as you mentioned--#1 is independent of the group 2-5/6.--The quick delineator is the presents of the 80A fuse under the glow-controller cover. The after glow models don't have the strap fuse or the averaging circuitry.

alan911 · #13 03-25-2008, 12:34 PM

OK....Since 2-6 tested ok,( slight resistance) I guess it would be safe to assume that the issue is with the #1 GP and I think I can get to it without all the hassle of removing a bunch of stuff.

Of some concern is, that the plug died after only approx. 6500 miles. I replaced them all with new Bosche ones back then. weird...... the originals had about 100,000 mi on them.

go figure.

Brian Carlton · #14 03-25-2008, 12:37 PM

Quote:

Originally Posted by alan911

Glow Plugs ?

What Glow Plugs ?

We don't need no steen-kin' Glow Plugs.

They're just an added accessary item for you northe'n yankee boys.

Can you start the 603 without glow plugs in FL?

I'd bet against it.

alan911 · #15 03-25-2008, 12:48 PM

Yup.......I do it by accident once in a while, especially after driving the Porsche for several days. I tend forget what I'm driving........ and other stuff as well.

Oh.... and I didn't need them in CA when I lived there either.

Here in Florida, I think it gets an extra boost on compression with all the water in the air. Seems to run a little better when it's humid. Pls. no flames....haven't thought through the logic yet.