OK, where's the fuse in my gp relay- grrrr - PeachParts Mercedes-Benz Forum

compress ignite · #1 06-22-2009, 10:11 PM

Brian Carlton,

For Interpolating !

whunter · #2 06-23-2009, 12:20 PM

A bad/damaged glow plug harness is rare..

The far more common cause of no glow plug light is bad glow plugs.
For more data on this simple issue read the following thread..

Glow plugs link thread
http://www.peachparts.com/shopforum/diesel-discussion/137732-glow-plugs-link-thread-post1019018.html

Brian Carlton · #3 06-23-2009, 12:34 PM

Quote:

Originally Posted by whunter

A bad/damaged glow plug harness is rare..

The far more common cause of no glow plug light is bad glow plugs.
For more data on this simple issue read the following thread..

Glow plugs link thread
http://www.peachparts.com/shopforum/showthread.php?p=1019018

If you bothered to read the thread, you might realize that the issue has nothing to do with the lack of a glow plug light. The problem is the internal circuit breaker that is tripping due to overcurrent.

You'll also note that this relay does not have a fuse.

whunter · #4 06-23-2009, 12:58 PM

Quote:

Originally Posted by Brian Carlton

If you bothered to read the thread, you might realize that the issue has nothing to do with the lack of a glow plug light. The problem is the internal circuit breaker that is tripping due to over current.

You'll also note that this relay does not have a fuse.

Read post #1 again.

-------------------------------------------------------

I have a 93 300D 2.5 turbo.
The car won't start.
No light on the dash.

-------------------------------------------------------

The issue is misdiagnosis of failure to start.
Bad/dead shorted glow plugs are more likely than a bad harness.

Correct function of the glow plug relay confirmed = it is totally irrelevant whether it has a fuse or not.

Brian Carlton · #5 06-23-2009, 01:09 PM

Quote:

Originally Posted by whunter

Bad/dead shorted glow plugs are more likely than a bad harness.

Dead glow plugs won't trip the internal circuit breaker.

It's certainly possible that one of them has a short, as I mentioned above.

A wire in the harness may be abraded and contacting a ground.

whunter · #6 06-23-2009, 01:19 PM

Quote:

Originally Posted by Brian Carlton

Dead glow plugs won't trip the internal circuit breaker.

It's certainly possible that one of them has a short, as I mentioned above.

A wire in the harness may be abraded and contacting a ground.

A SHORTED glow plug will trip the internal breaker.

An abraded wire is LESS likely than a glow plug shorted.

Brian Carlton · #7 06-23-2009, 06:36 PM

Quote:

Originally Posted by whunter

A SHORTED glow plug will trip the internal breaker.

An abraded wire is LESS likely than a glow plug shorted.

Then the glow plugs should all be tested for resistance prior to, or in conjunction with, testing of the wiring harness.

The only problem is the need for a decent meter. The difference between 1 ohm and 0 ohms might not be readily apparent on a cheap meter.

benmack1 · #8 06-23-2009, 01:17 PM

Question on the next step of diagnosis.

I may need to pull the intake and replace either the gp's or harness but I am wanting to think about one more possibility first. (I have large hands and have a hell of a time getting under the intake and fuel lines etc to get the wires off of the gp's.)

At risk of pissing off the aliens again here goes.....

1) Can I rig up a banana plug or some sort of jumper from the relay pin to the pin hole and check the lines one by one rather than pulling off the other ends and checking. I may be not fully understanding the way this works but I am hoping I will find a bad line this way to further confirm. The reason I am not just tearing in is I am still concerned (maybe ignorantly, but..) that it would be a weak circuit breaker in the relay that under load is just failing. I don't see that anything we have tested has ruled that out. That is a simple fix if I can diagnose it back to the relay. I am thinking if all 5 gp's work under single line tests each, then the above thought has legs.

2) I am also going to return to my resistance comments from earlier about one gp to another. If I have a short I am thinking the resistance for that plug will be lowered. I measured 1.6 ohms consistently from gp pinhole 1 to gp pinhole 2 and also from 1 to 3. I haven't done all 5. I am wondering if I have a short should I expect 0.8 ohms or something less than 1.6 suggesting that I have bypassed a resistor in the gp due to a short? If not then maybe I'm back to the relay breaker strength. Maybe this is stupid but seems logical to me.

whunter · #9 06-23-2009, 01:37 PM

Quote:

Originally Posted by benmack1

Question on the next step of diagnosis.

I may need to pull the intake and replace either the gp's or harness but I am wanting to think about one more possibility first. (I have large hands and have a hell of a time getting under the intake and fuel lines etc to get the wires off of the gp's.)

At risk of pissing off the aliens again here goes.....

1) Can I rig up a banana plug or some sort of jumper from the relay pin to the pin hole and check the lines one by one rather than pulling off the other ends and checking. I may be not fully understanding the way this works but I am hoping I will find a bad line this way to further confirm. The reason I am not just tearing in is I am still concerned (maybe ignorantly, but..) that it would be a weak circuit breaker in the relay that under load is just failing. I don't see that anything we have tested has ruled that out. That is a simple fix if I can diagnose it back to the relay. I am thinking if all 5 gp's work under single line tests each, then the above thought has legs.

2) I am also going to return to my resistance comments from earlier about one gp to another. If I have a short I am thinking the resistance for that plug will be lowered. I measured 1.6 ohms consistently from gp pinhole 1 to gp pinhole 2 and also from 1 to 3. I haven't done all 5. I am wondering if I have a short should I expect 0.8 ohms or something less than 1.6 suggesting that I have bypassed a resistor in the gp due to a short? If not then maybe I'm back to the relay breaker strength. Maybe this is stupid but seems logical to me.

benmack1 · #10 06-23-2009, 02:08 PM

Thanks Barry. Your comments are in line with my thoughts. I may not be the relay but I want to check before I tear into something I don't need to. I have taken the manifold off of this thing and it really is a pain in the can. Certainly doable but not something I care to do if I don't have to.

compress ignite · #11 06-23-2009, 02:33 PM

#1 is a Wash
You measure "From Pinhole" (In the relay end of the wiring harness) to GROUND
(Not from "Pinhole to Pinhole" ...That would be measuring the resistance of Both
wire leads involved ,in addition to Both Glow Plugs involved as well as the engine block and assorted grounding straps.)[And Inaccurate]

You'll have to have BOTH ENDS of the suspect Wire DISCONNECTED and place
your DMMs probes, One at each end of the wire.(One probe at the Relay end
of the wire and one probe at the Glow Plug end of the wire.)

You're gonna have to disconnect the Glow Plug ends of each wire from the
glow plugs to test the Glow Plugs (One At A Time) for "Dead Short".

IF your Wire Leads from the Relay Pins to the Glow Plugs were a Known Good
Quantity (Proof Positive) [BUT they are not!!!]you could just probe the "Pinholes" [One at a time]with one DMM
lead and put the other DMM lead to Ground to test the Resistance of your
Glow Plugs.

Your situation is:
1.You May Have at least one (or maybe more) BAD wire in the Relay to GP harness.
2.You may have at least one (or maybe more) "Dead Short" Glow Plug.

Or you could have a Combination of the two!

Once you've been through this and understand it ...It Ain't no big Deal.

(You're Avoiding taking off the Manifold so much you're willing to "create"
a 99.99% most likely improbable Fault with your Glow Plug Relay.The Odds
are not in your favor on that issue.)

Now,Let me throw some Magnesium on the Fire...
I know the 1990 124.128s (300D) were not Afflicted with the "Super Quick
Self Degrading Wiring Harness Nightmare" option from Stuttgart.
Is the 1993 124.128 (300D) close enough to the Time Range of "Accursed"
W124 Chassis,so that IT may figure in as a Major Contributor to this
Particular 1993 300D's present difficulties ?????

Because,Half Naked Copper in the Glow Plug Wiring Harness would explain
a lot!