Strip fuse or pre glow time relay shenanigans? W123 300D - PeachParts Mercedes-Benz Forum

unkl300d · #1 10-27-2007, 10:09 PM

Searching the archives, this is the closests to ideas that I found...

240d eats # 4 glow plug every day

My question arises from the fact that lately my 1979 300D has been going through 80 amp strip fuses.
First incident happend in Feb 2007 at the time of a failed glow plug (all glow plugs were then replaced)
Second incident happened only 2000 miles after that in Sept 1-07, and now again after 600 miles since latest fuse replacement.

I just checked the old plugs and only one had a split loop but not a chance of loosing metal into a prechamber. At this point I rule out shorting by that reason.
I just checked all the wiring around the glow plugs and around the strip fuse holder (box). All wires are OK, not frayed, not touching metal objects which may cause shorts,arcing etc.
Ground wire is good. Battery is new and cables good.

I do not suspect the pre glow timer relay (part no. 001 545 35 32 seven pin california relay with bracket) because the car has not had problems starting.

However, my question includes whether it is possible that the relay can release more than 12 volts for some reason?

My strip fuse box (part no. 123 540 0450) had been burnt up at some point in time on the inner back side. Perhaps on Sept 3 of this year.
The one threaded nut in the fuse box that accepts the relay wire is loose and the plastic is warped. I suspect MAYBE the threaded nut has heated and maybe it is slightlt sticking out the back of the plastic fuse box. Sublime shorting onto firewall metal????

The second fuse burn I suspected being caused by a moment of quick ignition on/off prior to glow light turning off.

Recently, I had the valves adjusted and used the ignition to move the engine several times in order to dispaly the valve adjustment points.

So I am now suspecting that this use of the ignition during the glow plug relay heating (but not yet finished in its time cycle) may have softened up the strip fuse causing the final failure.
The fuse is not broken . It is darkened a bit but probably lost its properties.

I am going to order a new strip fuse box. Once I remove the old, I can observe the back side.

I know the car will start up once reset with new strip fuse and box, but I am leary of burning up another one prematurely.

Does the use of ignition as described weaken a strip fuse?

The only other symptom I have noticed is one that has occurred for decades.

The glow plug light does sometimes blink on and then off while driving.

This happened for years before the glow plug failure and continues with the new glow plugs. I assume this is the normal temperature sensor in the block momentarilt cycling the glow plug fuse.

At this point I don't want to invest in a glow plug relay.

Eduacated opinions and tips welcome.

THANKS

P.E.Haiges · #2 10-28-2007, 12:47 AM

unkl300d,

Its impossible for the GP relay to "put out" more than the input voltage. Direct current cannot be Transformed like AC current to increase the voltage except in a pulsed inductor system such as a spark ignition or a electronic system such as a voltage inverter.

U say U have the old style "loop/series" GP? I would suspect a carbon short to one of the GP. Remove the connecting resistor and ground wire and check the GP terminals to ground. If U find a low resistance reading remove the GP and clean the hole with a GP hole reamer. Then blow out the GP hole with an air gun.

GP light does not indicate the GP are energized. Its just a timer controlled by engine heat. I cannot see how switching the key switch (Dieselds do not have an ignition switch) off an on will affect the fuse U are asking about. As long as the current thru the GP relay does not exceed the rating of the fuse, the fuse will not be damaged.

Its possible U have an intermittant short somewhere in the wires.

Many GP indicator lights flash for unknown reasons. This isn't the GP relay cycling

P E H

Jeremy5848 · #3 10-28-2007, 12:56 AM

The flashing glow plug light is the glow plug relay's way of telling you that it has a problem. Regretfully, it isn't smart enough to tell you what the problem is. Probably the most common problem is one bad glow plug.

PEH is right, the fuse blows when the circuit tries to draw too much current. There has to be a short circuit somewhere, if not in the wiring or the relay, then in the glow plugs themselves. PEH's suggestion is sound...check it out. The glow plugs have very low resistance, a cheap ohmmeter may not be sufficiently sensitive. (I've never worked on the older series glow plug setup, we need to hear from someone with the right experience.)

Jeremy

P.E.Haiges · #4 10-28-2007, 01:31 AM

unkl300d,

I don't know what are good resistance readings for the series/loop GP. But since they are in series and have to draw about the same current as parallel/pencil GP, I woujld suspect the reading of each plug would be about 0.01 to 0.02 ohm cold. That's about 1/5 the resistance of a single parallel plug. That may be below the capabilities of your meter. But unlike the parallel plugs, the reading of either GP terminal to ground should be infinite if the ground wire is removed from GP #1.

The above reading would be across the 2 terminals on each series GP. Although these readings look like a short circuit, the resistance increases as the GP get hot.

P E H

unkl300d · #5 10-28-2007, 02:19 PM

Thank you for the responses !

I'll check the resistances after I install the new strip fuse box.

Eventually I will post results.

Thanks again.

unkl300d · #6 10-28-2007, 11:50 PM

Here are the pics of the strip fuse box front and back. Burnt.

I await the replacement from FASTLANE PHIL.

P.E.Haiges · #7 10-28-2007, 11:59 PM

unkl300d,

Yes that terminal box needs to be replaced. I'm surprised the fuse didn "blow" before the box got so hot.

I wonder what caused all that heat?

I had a 220D that had a GP shorted to ground. There was no fuse in the circuit. It melted some of the GP wire insulation. I had to disconnect the shorted GP and wire around it to get started. Later I replaced the shorted GP.

P E H

unkl300d · #8 10-29-2007, 12:44 AM

PEH I was running this problem thru the ol bean today.
I don't recall replacing a fuse that was totally crisped burnt in half or melted. I was presuming that at some point in time, it had a bad failure of the fuse and burnt up some of the box. But this recent one heated it enough to deform the terminals inward.
The fuse was still not burnt or broken or crisp. Just kind of black and spent and bowed outward.

Also I just discovered that the correct fuse for my California pre glow timer relay and loop plugs is the 50 AMP not the 80 AMP I was using.

Anyway, I had hoped to find evidence that the terminals or dripped fuse bits from some distant past had been toching the chassis.
I can't seem to see evidence of this although the bad shape of the box would lead one to suspect it.

bustedbenz · #9 10-29-2007, 12:51 AM

My fan blower for the climate control system burned up a plastic strip fuse box just like that glowplug system did. Seems like a fire hazard in the making to me...

unkl300d · #10 11-01-2007, 02:15 PM

Well I received the strip fuse box from Fast Lane Phil, it does not come with new machine screws to hold the fuse. Bought new ones and washers.
Installed a few minutes ago.
Car started up with no issues and 50 amp fuse looked normal afterward.

I'll see how it fares and do the resistance tests later.

unkl300d · #11 01-03-2008, 10:24 PM

Well, the relay stoped working mid December and the fuse did not blow this time.

I checked resistance of the plugs and found 3 to be unsatisfactory.
Bosch plugs about 10 months old.

Got warranty replacements from Bosch and 2 were bad. Bosch (India).

Got credit for all 5 and bought Beru plugs from Fastlane (made in Ireland).

Replaced relay and installed plugs.

Car starts up fine.

Hope for the best.
All wires look good.

Jan 23-08 No problems with the system. In fact my gas mileage has improved from an average of 19 MPG in City to 21.1 MPG in City !!