2003 glow plug module dissected

surlyoldbill

Well-known member
{NOTE: Subsequent to this excellent hack, Mercedes upgraded the glow plug module to include circuit breaker type links that are resetable. It is available for around $110. }
Annotation 2020-05-22 101157.jpg


Upon learning that the modules fail because of a blown internal fuse, I decided to open my old one up to see if I could hack the design and create leads to external (replaceable) fuses. The donor was from a 2003 Sprinter.

Upon inspection, it appeared to be a one-piece unit. I chose a japanese style saw to cut away the back and top to see inside. I dabble in japanese carpentry, and have better control with one of these saws than a power tool such as a dremel with a saw blade attachment, my second choice for the procedure.


With the back and top cut off, I found out how the module was made.


The module is made by slipping the electronic parts plate in from the front, where it is caught by tabs on the top bottom and sides, then it is sealed with what appears to be black silicone, making it look like it is a one-piece mold, which it is not. The module can be taken apart by cutting the silicone with a razor, and prying open the tabs, allowing the module to be slid forward out of the enclosure.

The next step will be determining the proper size fuses, and soldering in jumper wires to a 5-bank fuse holder, one for each glow plug. The design is a little funny, because though one side of the metal fuse is blown, the other remains intact and would thus allow current to flow (that is a one-piece metal tab with a slot in the middle). I am not familiar with this style of "partial" fuse.
 

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220629

Well-known member
surlyoldbill,
Doktor A recommends a 20 amp fuse for his glow plug testing rig if that helps. In looking around it appears that this fuse block might do the trick. Maximum load per fuse is 30 amps. Max load capacity for all circuits combined is 65 amps
73820.jpg

http://www.wiringproducts.com/contents/en-us/p348.html
That said, the fuses probably come off a common buss and may not be able to be separated easily if that's necessary. Still worth considering if nothing better comes up.

Could you see the contacts on that mechanical relay to determine whether the contacts had more miles left on them? There have been a few reports of that module/relay causing battery drain down problems. It's probably because the relay either hangs up mechanically or the contacts fuse together as it ages. Those are guesses. Thanks for sharing this info. vic
 

Hoppingmad

Member
Fuse size
I'm pretty good at this (so far)
I'm fond of guessing the fuse thickness(thickness and width- can you look at an unblown 1?), and those circuit board fuses are thin, study one of those little plastic ones ( in line adapters are available at napa) I'd start with 2 amp or less(remember circuit board fuses are small)
Theres probably a larger fuse( 20 amp?) for the glow plugs elsewhere on the van, tapped into one leg of the relay.
I'm certain that little fuse just protects the insides, and if a 1/2 amp blows right away you can always go bigger, there cheap.

I haven't a wiring diagram handy, but I don't believe even german engineers put a fuse on each glow plug. So I don't understand why you'd want a bank of 5 glow plugs>

But then again I've been wrong before, fortunately alimony ended 2 years ago.

Feel free to straighten me out as I may have the problem show up here sometime.
 

surlyoldbill

Well-known member
The relay unit has internal non-replacable fuses for each glow plug as part of the circuit board. I would assume that they are 15-20 amp, because that's what is used to test the plug for fault to ground. These are a little different than I'm used to, there are two "legs" that are connected at each end, and the smaller of the two is the one that is blown; meaning that theoretically a current can still pass through the internal fuse? That doesn't make electrical sense to me.
 

abittenbinder

Doktor A (864-623-9110)
The design is a little funny, because though one side of the metal fuse is blown, the other remains intact and would thus allow current to flow (that is a one-piece metal tab with a slot in the middle). I am not familiar with this style of "partial" fuse.
It is not quite a "partial fuse". The more substantial leg of the fuse link is the current flow path of a voltage comparator circuit.

When activated, the control windings of the glow plugs experience a gradual, small voltage drop due to their PTC (Positive Thermal Coefficient of resistance) properties. As temp of glow plug increases, the resistance of the windings increases, thus limiting current.

The glow plugs typically draw from between 8-30 Amps (the actual amount is dependent on temp of the plug) . The fuse links are rated MUCH higher than 30A to allow for protection from actual direct shorts only.

When the fuse link is blown, the comparator circuit detects no voltage and DTCs are then set in the glow plug module. Doktor A
 
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surlyoldbill

Well-known member
Upon closer inspection, the internal fuse is connected at two seperate places on the circuit board end, and is joined at the GP end, as Dr. A described. ALL fuses are joined by a copper plate at the circuit board end, at least the leg that blows, the other leg goes to an individual circuit, probably for individual monitoring and control.
 

maxextz

Rollin Rollin Rollin.....
good post and pics:clapping: its good to open stuff up even if it turns out to be a can of worms:thumbup:

max........
 

ppm.ross.lee

Owner, EliteService&Fleet
Beautiful modification idea. Works perfectly though I chose to use 30 amp fuses. I even numbered them on the fuse block so I know immediately which glow plug to inspect. This is going to save my fleet a lot of money.
 

220629

Well-known member
Beautiful modification idea. Works perfectly though I chose to use 30 amp fuses. I even numbered them on the fuse block so I know immediately which glow plug to inspect. This is going to save my fleet a lot of money.
Welcome to the forum. If it's not too much trouble I'd be interested in some pictures of your design/installation. Thanks in advance. AP/vic
 

Ciprian

Spark Plugs not allowed!
Just a thought here. Andy said that the glow plugs draw between 8 and 30 amps and the fuse links inside the module are rated at much higher amps than this. Would a 30 amps fuse in-line with each glow plug protect the glow plug module in case a glow plug shorts to the ground? As an added bonus you can instantly know which glow plug failed (if it shorted to the ground, it will not show anything if it failed open). The 30 amps fuse might be too small, but the idea is to have a fuse smaller than the link in the module, but big enough to not blow under normal operation. What do you think? It would not be expensive to do.
 
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220629

Well-known member
Ciprian,
Nice idea. It seems the individual glow plug fuse blowing is what wrecks the module. One fly in the ointment is that fuses can have different peak current tolerance design characteristics so your added fuse may not be the first to go even if it is a lower amperage. As you say, it isn't an expensive modification so it's probably worth a try. Given the OEM design it does seem a shame to need to replace an entire module for what comes down to a blown fuse. (Thanks again to Bill for cutting his module apart and posting pictures.)

So far my MIL maintenance indicator light triggers related to glow plugs are very intermittent and jump around to blaming different glow plugs. I would presume if one of the module fuses were blown the MIL would return quite quickly and the glow plug number fault would not change. That said, it is a guess as I don't have any of the glow plug monitoring circuit design details. AP/vic
 

Ciprian

Spark Plugs not allowed!
What is the root cause of the module's internal fuse failure?
The root cause of it failing is the glow plug failure. There are 2 kinds of glow plug failures.

1. the glow plug opens, so no current will flow through it. In this case the glow plug module is not affected. You just replace the glow plug and you are good to go.
2. the glow plug shorts. This will put a big current through the circuit, so something must burn out. And in this case, the fuse in the module burns out and you need to replace the glow plug and the module.
 

220629

Well-known member
I suppose that a wiring harness shorting to ground should be added to the fuse failure cause list. That isn't to say I've seen many comments about that as a problem. AP/vic

Edit: The root causes mentioned so far are external. There is also a set of contacts, relay coil and other internal circuits that can cause a general module failure.
 
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sikwan

06 Tin Can
This idea certainly has peaked my interest as a blown fuse would certainly pinpoint the glow plug and verify the P-code generated!

According to the 2006 schematics, the external fuses should be placed near the connector C2.
gp_module_sch.JPG
I can see the comparator circuit as mentioned by Dr. A.

The source fuse (I haven't verified my own fuse) is 125A, which would put the value at 25A if all 5 glow plugs were to short. I suppose a 30A fuse per line would be a safe rating; 1 of the 5 (30A) fuses would blow or the big 125A would blow which would protect the module. The module could still fail if the comparator circuit was the fail. No way to protect that.

Note: A blown up PDF file of the above schematic is included.
 

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220629

Well-known member
Sikwan,
Thanks for the circuit. It's interesting that there is the one unused comparator circuit. I wonder if open circuit voltages on that unloaded and floating comparator could cause any erroneous fault codes? I know that may be a bit of a stretch. AP/vic
 

Scott_Mc

Sprintering Since Aug/02
......:thumbup:thanks gents

This thread makes me want to change all of my original glow plugs just as a precaution:hmmm:
 

Ciprian

Spark Plugs not allowed!
......:thumbup:thanks gents

This thread makes me want to change all of my original glow plugs just as a precaution:hmmm:
I thought the same thing, but the thought of breaking a glow plug in the head and having to replace the head made me change my mind. Now, if the glow plug is bad, I will risk it.
 

ppm.ross.lee

Owner, EliteService&Fleet
I love this forum. This genius deserves :thumbup::bow:
This great idea works! I snipped out all 5 fuses in the relay leaving only the unused one. Then I soldered a wire just after the solenoid for power for my fuse block that was glued on top. Then I soldered wires on the bottom to where the internal fuses were. I drilled holes in the lower right corner of the housing near the other connectors keeping everything in order to mirror the connector to the left of the wires. On the fuse block I numbered the fuse locations and placed the wires in the corresponding fuse location. I have used it on the vehicle, it's worked for the last month flawlessly, and now I am going to modify my entire fleet of 30 as they go bad. I usually get 5 to 8 a winter to go bad. BEST PART IS..... I will be able save time by knowing which glow plug to replace in the future in seconds!!!!!!!!
 

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