Silver C

I forgot to mention that once in a while back then it shows up a visit workshop, check coolant, display faulty and brake lining wear msg, but when checked my brake pads and brake pad sensors are just fine..

Moviela

The fat cable to the battery must deliver enough current to the motor to turn over the engine. The voltage will read 12V even if the wire is the thickness of a human hair. A wire that size would be incapable of carrying the current and burn up.

I can't imagine how a failure of the HT wire could cause a short unless the insulation was peeled away. It will cause a situation where the current carrying capacity is reduced if corrosion has reduced the copper to cuprous oxide.

In the present situation a simple voltmeter and a high current ammeter are needed to identify the bad actor. It could be the lack of ability of the battery to source enough current also. Here is where the high current ammeter is most helpful.

I still consider the possibility that the solenoid plunger is not moving the full stroke causing the fuse to blow. Any solenoid requires a very high current to start the movement of the plunger. The motion is complete within milliseconds and the current then drops to about 20% of the initial starting current. This 20% value is what the fuse is protecting. It is when the plunger is restricted from finishing its motion that the current remains higher than the fuse can withstand. If either of the fat cables from the battery or the one from the engine to chassis is corroded, or if the battery is weak in the knees, the solenoid cannot complete its stroke and will blow the fuse.

Glyn M Ruck

Movie is as usual being absolutely correct in theory. However the constant draw while cranking is what heats the fuse & finally blows it with a 15 amp in place. Not so with a 20 amp. These are not quick blow fuses.

That said ~ the solenoid could well be the problem.

Silver C

The 20a fuse still blows as well, so if not the HT fat battery cable so what?

Silver C

Then I guess my mechanic meant that the power supplied from the fat HT cable us not enough to power the selenoid, thus the selenoid cannot perform its full action, thus trying to draw more power and heat up and burn the fuse. Does that make sense?

Silver C

Now they have rethreaded and checked my battery HT cable and said its all fine. They are conviced its the bad selenoid now. They didnt manage to install the s-class one because there are some differences in the gears.

Moviela

No, that does not make sense. The solenoid draws less than 20A once the plunger is seated. This would be a much smaller wire, on the order of 1.31(mm2.) This wire runs from the the relay near the fuse to the solenoid terminal of the starter. This wire could have a problem, or the relay could be pitted. I would disconnect the wire at the starter solenoid terminal. Then connect a jumper from the starter motor terminal to the solenoid terminal. If the motor then cranks the engine, the solenoid is OK. You will need to find the problem with the wire from the relay through the fuse to the solenoid terminal. You can also measure the amps flowing into the solenoid while cranking. It should be less than 20A. If more, the solenoid needs to be serviced.

Silver C

What is the normal amp that runs through the cable to the solenoid? Thanks.

Moviela

I have never measured it before so I grabed a Bosch starter from a 2,7l Porsche engine in my garage. This will be a bit long winded, so grab a bowl of basko and follow along. I peeled the solenoid off and discovered that there are actually the start of two windings in the coil. One wire is larger than the other and the finish of that winding is conneceted to the (+) brush on the starter motor. The other smaller wire element e is connected to chassis.

When 12v is applied to the solenoid terminal current flows through both portions of the coil. As the plunger pulls back it slams a washer into the terminals that connect the fat wire from the battery to the (+) brush on the starter motor. Since that coil would now have both ends connected to 12v, the current in the winding drops to zero. The other coil having the bottom connected to ground continues to carry current until the starter relay is released by the front SAM.

The two coils operate together to more quickly pull in the plunger. The starter I have is a Bosch, and I can't imagine it is much different than the Mercedes unit. The plunger pulls in in 50 milliseconds. During that time the the current is 35A. This is more than the 20A fuse rating, but because of the short duration, the fuse element laughs it off. At the end of 50ms the current is flowing only through one winding and drops to about 10A. The fuse will handle that without problem.

Possible failure modes.

1. If the contacts that get hit with the washer on the plunger are not a dead short, current can continue to flow through the larger wire winding of the solenoid causing the fuse to blow.

2. A build up of carbon dust from the brushes on the (+) terminal of the motor could allow a sneak path to ground for the solenoid winding, blowing the fuse.

3. An insulation defect in the solenoid coil allowing a short to ground, or between coils. This could blow the fuse.

I noticed several problems in this starter. The pinion gear slides on a shaft and appears to be lubricated with MSO2 paste (Glyn might know better what lubricant is used) that looked like black stiff butter. I cleaned the interior of the starter, ground and polished the armature contacts, and relubed the shaft. This starter had a spade lug to attach the wire from the starter relay. The lug was corroded, and was soldered to the solenoid windings. I polished the lug and resoldered the connection. I sanded and polished the washer and terminals that connect the motor (+) to the fat wire from the battery.

To recap:

The solenoid current should be quite high for 50ms until the plunger is fully seated, then drops to a low holding current until the starter relay de-energizes.

35A for 50ms.
10A while starter motor is turning.

Glyn M Ruck

Most Mercs have Bosch Starter Motors, Some Denso & some Valeo.

The basic operation is the same for all. Motor power varies with application.

Pinion is lubricated with a 5% MOS2 high temp paste.

Silver C

It turns out all 3 were damaged:

1. The fat HT cable that runs down from the battery to the starter and alternator.

2. The small blue green wire that runs down from the relay.

3. The motor starter itself.

Hopefully when those 3 are replaced and problem will be solved. We managed to test and trial with external wires and managed to start the car when we use additional wires for those.

Silver C

Cant wait for tomorrow to get my car started again.. Please pray for me guys..

Glyn M Ruck

Good luck. You have to be the most unlucky person. The chances of trouble with all three is close to nil.

Silver C

Amazing isnt it? And now they told me the main fat HT battery cable is out of stock everywhere in town and they have to order it in or look for a replacement..

Glyn M Ruck

It's not a common replacement item. Benz should be able to get it from Frankfort to Jakarta within 24 hours. They pretty much guarantee that.

Silver C

Now they are telling me that somehow theres no power at the 52 fuse, my God I dont know whats wrong!!!

Glyn M Ruck

Tell them to check that the fusible link has not blown.