OK, where's the fuse in my gp relay- grrrr - Page 2 - PeachParts Mercedes-Benz Forum

benmack1 · #16 06-22-2009, 08:54 PM

I understand what you are saying about the wiring but one thing still confuses me. I can measure good conductivity with the ohmeter from pin hole 1 to gp/nut 1 and etc for the other lines. I would expect the opposite if I am not measuring voltage down those lines when hooked and energized since the pins are hot. Also, all 5 lines seem to have failed simultaneously. It may be I am just too stupid to get it. However, I will ask at risk of the aliens getting mad, is there any situation that the relay kicks out and fails when it is under load to the gp's and would measure fine with only voltmeter 'load' and seem fine. These have seemed to fail overnight. I believe all 5 were fine as of last Friday. I had one bad one this past spring and it was very evident even during warm days.

Brian Carlton · #17 06-22-2009, 09:03 PM

Quote:

Originally Posted by benmack1

..........is there any situation that the relay kicks out and fails when it is under load to the gp's and would measure fine with only voltmeter 'load' and seem fine.

Yes, there is a situation.

The situation is a dead short in one of the glow plug wires.

The relay works fine and operates perfectly. When connected to the glow plugs...........the moment the key is turned, the relay sees the dead short and trips the circuit breaker. When you pull the plug to the relay, it resets and it provides proper voltage to the pins.

So, now the game begins............you need an ohmmeter to test each wire to each glow plug..........one of them is shorted. To make this test, the wire must be disconnected from the glow plug. You are looking for infinite resistance to ground on each of the wires. One of them will have a dead short to ground...........when disconnected from the glow plug.

One additional possibility..........one of the glow plugs has a dead short to ground..........although this is less likely.

benmack1 · #18 06-22-2009, 09:24 PM

Now things make sense in my mind. Thanks. I dread pulling the intake. I can't get to those wires without pulling it I don't believe.

Bio300TDTdriver · #19 06-22-2009, 09:27 PM

Quote:

Originally Posted by whunter

It should have a strip fuse in the relay.
Enter this part number in FastLane, click the part picture to enlarge it.

MB# 007 545 99 32
Diesel Glow Plug Relay (Preglow Time Relay)
Chassis W124.128
Model 300D 2.5
Years 1990-1993

Have a great day.

Have a great day.

Fastlane might be fast, but they are definitely wrong on that one. There is no strip fuse in that year of 124!

compress ignite · #20 06-22-2009, 10:11 PM

Brian Carlton,

For Interpolating !

whunter · #21 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.html#post1019018

Brian Carlton · #22 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 · #23 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 · #24 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.

benmack1 · #25 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 · #26 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.

barry123400 · #27 06-23-2009, 01:28 PM

Quote:

Originally Posted by benmack1

I understand what you are saying about the wiring but one thing still confuses me. I can measure good conductivity with the ohmeter from pin hole 1 to gp/nut 1 and etc for the other lines. I would expect the opposite if I am not measuring voltage down those lines when hooked and energized since the pins are hot. Also, all 5 lines seem to have failed simultaneously. It may be I am just too stupid to get it. However, I will ask at risk of the aliens getting mad, is there any situation that the relay kicks out and fails when it is under load to the gp's and would measure fine with only voltmeter 'load' and seem fine. These have seemed to fail overnight. I believe all 5 were fine as of last Friday. I had one bad one this past spring and it was very evident even during warm days.

You are not too stupid to get it. Simple things with too many viewpoints can get complicated for all of us and hard to deal with. Everyone means well.
Take your meter and read each wire on the glow plug harness . If all readings are the same from the relay plug to ground the wires and glow plugs are fine. You are looking for one glow wire harness terminal that shows no resistance at all but instead a dead short. Or a ridiculously low ohmage value. You can calibrate your meter just by touching the probes together to see what it's zero ohms is. Remember you are reading very low ohmage readings so be observant.

An open line for example will not trigger a circuit breaker. So too much continuity is what you are looking for. Not a lack of it.

If this test shows all five wires to read about the same. You need a test relay. There are many reasons that relay can be open or shorted internally once current is demanded. If internally shorted it certainly is capable of getting warm. The internal circuit breaker still could be opening to deal with an internal relay short. I do not have a schematic but if the relay breaker is opening at least the key activation circuit is intact.

If the relay circuit breaker were to open with the glow harness off you would have proof the relay has a heavy short. Conversly if it only opens with the glow harness plug hooked to it you have a shorted wire or glow plug. It is not just a tired circuit breaker I believe as the relay gets warm.

whunter · #28 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 · #29 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 · #30 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!