Need help with 1993 Mercedes 400SEL

[KurskKnyaz]

I'm trying to help my friend fix his car. In short, this is what happened: He was driving while it was raining and water leaked into the trunk where the battery is. While this was happening all the lights came on (check engine,...etc) and now the car won't start. This is what I tested so far:

1. No fuses are burned out and the relay switch that kicks in when you attempt to start the car works fine.

2. When I first removed the battery, I used a multimeter to test for continuity between the POSITIVE terminal (from the car, not battery) and the auto body (ground) and there was continuity.

3. We tried starting the car with a booster battery pack, using jumpers from another car, and tried another car battery.

4. I tested for continuity again and there was no continuity between the POSITIVE terminal and the auto body. However, the multimeter displays a number that keeps going up (or down, don't remember). I think this means there is a capacitor within the circuit.

5. When attempting to start the car, the belt on the engine rotates just a bit and then stops but the car keeps making the same noise as if it is about to start.

This is my theory of what happened: Water shorts the battery's POSITIVE terminal with the ground. This is not enough of a short to fry any fuses but enough to damage some sort of capacitor that holds charge to power the starter.

I looked up the started for this particular model and it is a 1.7 KW starter which means it draws about 140 amps! I can't imagine such a motor running without a start/run capacitor. What I think is happening is that the run capacitor (if there is one; I would think that all powerful electrical motors have one) is not holding enough charge to give the starter the torque needed to start the engine.

Does anyone have any experience with this type of problem or have any ideas about what might be wrong? Does anyone know if the starter has a run capacitor and where it would be located (and how to test it)? Thanks for any help!

Once again this is a Mercedes-Benz 1993 400SEL.

 

[drnickriviera]

No capacitor, just a big ass wire straight from the battery to the starter.

One thing i'd look up is to see if that model had MB's super special enviro friendly wire harness insulation. The insulation gets hard and falls off the wires. Our 95' E300 had it.

 

[PottedMeat]

AC motors can have motor start capacitors - the motor that turns the engine crank is a high torque high current 12V DC motor. All the current is provided by the battery ( rated by cranking amps ).

Sounds like the battery was discharged ( probably by a short from pos to ground from water + something else), but there's enough juice left to move the crank a little bit.

 

[KurskKnyaz]

I tried another battery and a booster battery with the same results. I got my hands on some wiring diagrams of older MB cars and saw that some of the starters DO have a capacitor in line. This is a V8 and the starter is rated at 1.7KW. That is about 140 amps.

I'm going to tell my friend to try leaving the booster battery connected for 5 minuted. If there is in fact a capacitor that will give it enough time to charge. We tried to start the car as soon as we connected the battery, maybe that's why it wouldn't start.

 

[KurskKnyaz]

Originally posted by: drnickriviera
No capacitor, just a big ass wire straight from the battery to the starter.

One thing i'd look up is to see if that model had MB's super special enviro friendly wire harness insulation. The insulation gets hard and falls off the wires. Our 95' E300 had it.

I did notice insulation coming off the wires. I tried using a multimeter to locate a short but can't because there seems to be a capacitor inline with the positive terminal.

 

[brblx]

insulation coming off of what wires?

is there some kind of fuse box of distro block in the trunk? i would open anything like that up and make sure it's completely dry.

are you saying you hear the engine rotating and making compression, then you see the accessories stop moving but the engine keeps cranking?

 

[KurskKnyaz]

Originally posted by: brblx
insulation coming off of what wires?

is there some kind of fuse box of distro block in the trunk? i would open anything like that up and make sure it's completely dry.

are you saying you hear the engine rotating and making compression, then you see the accessories stop moving but the engine keeps cranking?

There is another fuse box in the trunk ; everything is dry.

I see the belt on the engine budge just a bit (the starter doesn't even go around once), then the car just keeps making the nose as if its trying to start but nothing on the engine is moving. What I'm hearing is probably the sound of the starter trying to turn without enough juice, but nevertheless there is a sound and nothing is moving.

 

[KurskKnyaz]

Originally posted by: PottedMeat
AC motors can have motor start capacitors - the motor that turns the engine crank is a high torque high current 12V DC motor. All the current is provided by the battery ( rated by cranking amps ).

Sounds like the battery was discharged ( probably by a short from pos to ground from water + something else), but there's enough juice left to move the crank a little bit.

I think the starter has capacitor because:

1. There is no way that starter relay can handle 140 amps. This is a 1.7 KW
starter for a 4.2 L, V8 monster
2. I looked at some electrical diagrams of older MB cars (124 and 126) body types and some of the starters have capacitors (I believe they are right on the starter)
3. When I test for continuity between the positive terminal and the auto body the multimeter shows a number that keeps going up or don't (don't remember). I think this is the resistance that is fluctuating as it would if there was a capacitor within the circuit.

 

[Zenmervolt]

Originally posted by: KurskKnyaz
I tried another battery and a booster battery with the same results. I got my hands on some wiring diagrams of older MB cars and saw that some of the starters DO have a capacitor in line. This is a V8 and the starter is rated at 1.7KW. That is about 140 amps.

I'm going to tell my friend to try leaving the booster battery connected for 5 minuted. If there is in fact a capacitor that will give it enough time to charge. We tried to start the car as soon as we connected the battery, maybe that's why it wouldn't start.

140 amp draw is actually rather small for a starter unless it's running free with no load. Under load (i.e. when engaged and starting the engine) they can draw 250 amps or more for small V8 engines like the one in that Merc.

This is why even the tiny batteries for compact cars will be rated at 400 cold cranking amps or more. The battery in my old Lincoln (4.6 litre V8) was rated at almost 1,000 cold cranking amps. There is absolutely zero need for a capacitor between the battery and the starter motor. I can guarantee you that there is no capacitor for the very simple DC motor used in the starter on that Mercedes. There's simply no need for it.

If you are hearing a clicking noise, that is the starter solenoid and it indicates that there is insufficient amperage delivered to the starter/solenoid combination. This can be due to excess resistance in the wiring, a weak battery and booster, a shorted battery with a reverse cell, a loose ground, or any number of other issues.

ZV

 

[Zenmervolt]

Originally posted by: KurskKnyaz

Originally posted by: PottedMeat
AC motors can have motor start capacitors - the motor that turns the engine crank is a high torque high current 12V DC motor. All the current is provided by the battery ( rated by cranking amps ).

Sounds like the battery was discharged ( probably by a short from pos to ground from water + something else), but there's enough juice left to move the crank a little bit.

I think the starter has capacitor because:

1. There is no way that starter relay can handle 140 amps. This is a 1.7 KW
starter for a 4.2 L, V8 monster
2. I looked at some electrical diagrams of older MB cars (124 and 126) body types and some of the starters have capacitors (I believe they are right on the starter)
3. When I test for continuity between the positive terminal and the auto body the multimeter shows a number that keeps going up or don't (don't remember). I think this is the resistance that is fluctuating as it would if there was a capacitor within the circuit.

1) As I said, 140 amps is a SMALL draw for a starter motor. Typical load for a small V8 like the one in that Merc is going to be around 250 amps during cranking. This is well within the ability of automotive batteries to supply. It is under no circumstances a "monster". Stop thinking, you're not helping yourself.

2) Those are not capacitors. The little piece attached to the starter is the relay itself (also sometimes called the "bendix").

ZV

 

[KurskKnyaz]

UPDATE:

Tried leaving the battery booster connected for about 5 minutes (should be enough time to charge any capacitor) and then starting the car. Same problem. Anyone have any other ideas?

 

[KurskKnyaz]

I never actually got to look at the starter, but in the trunk there is a relay who's purpose seems to be cutting off the rest of the cars electronics from the battery while the car is starting. I'm still trying to find an electrical diagram for the 140 body type without luck. Thanks for the advice.

 

[Belldo]

Check fuse 16 in the fuse and relay box under the hood it says it is for the cigar lighter but it also powers the starter lock-out relay. The relay is in the back of the same box. Its K38 or i believe it uses the letter F

 

[KurskKnyaz]

Thanks. I'll try that tomorrow. Anyone think its unusual for such a short not to fry any fuses?

 

[LTC8K6]

Water won't cause that type of short at 12V DC, IIRC.

It will cause things not to work, but it won't blow fuses.

Not a great conductor at 12V DC.

 

[KurskKnyaz]

The car is fixed; after being taken to the mechanic.

When a new battery was connected properly the car started. Before we were using jumps and I insisted that there is not enough contact to start the car. A properly mounted battery did it.

However, the car would still shut off. The mechanic told my friend that he needs a new alternator because the one he currently has is damaged and puts out more voltage than it should, and according to the mechanic, this is what fried the battery.

I think this is bullshit.

According to the electrical diagram for the 1993 Mercedes 400SEL starter. It is the starter that charges the battery (unless I read that wrong). I can understand that if there would be a short between the alternator and the body, the battery could fry.

I'm not a mechanic but I know that an alternator powers the electrical components of a car and acts as a charger for the battery in some cars. AFAIK, an alternator converts mechanical energy to electrical energy. Unless the physical properties of the alternator somehow magically changes, I can't see how it can put out a higher voltage.

Correct me if I'm wrong, please.

 

[drnickriviera]

Voltage regulator

 

[PottedMeat]

Originally posted by: KurskKnyaz
The car is fixed; after being taken to the mechanic.

When a new battery was connected properly the car started. Before we were using jumps and I insisted that there is not enough contact to start the car. A properly mounted battery did it.

However, the car would still shut off. The mechanic told my friend that he needs a new alternator because the one he currently has is damaged and puts out more voltage than it should, and according to the mechanic, this is what fried the battery.

I think this is bullshit.

According to the electrical diagram for the 1993 Mercedes 400SEL starter. It is the starter that charges the battery (unless I read that wrong). I can understand that if there would be a short between the alternator and the body, the battery could fry.

I'm not a mechanic but I know that an alternator powers the electrical components of a car and acts as a charger for the battery in some cars. AFAIK, an alternator converts mechanical energy to electrical energy. Unless the physical properties of the alternator somehow magically changes, I can't see how it can put out a higher voltage.

Correct me if I'm wrong, please.

The starter has nothing to do with charging the battery, it turns the crankshaft long enough to get the engine started. The alternator and battery supply power for electrical systems, the battery powers the starter, the alternator charges the battery.

The voltage regulator on the alternator adjusts the strength of the rotor field so that it outputs a constant voltage as the load varies. If the regulator is fried, the output could be anything, higher/lower than whats needed.

 

[KurskKnyaz]

Got it.

However, the electrical diagram for that model says that the starter acts as the charger once the car is started.

 

[brblx]

how exactly does the electrical diagram say that?

not to sound like a dick, but i'm not sure you know how to read wiring diagrams. the starter is typically between the alternator and the battery (circuit-wise), but it's not doing anything when the engine is running. it's just a place for the two lengths of battery cable to be held together. current isn't actually delivered to anything at the starter unless the solenoid is energized, moving the pinion gear and completing the high amperage side of the circuit.

i'd post on a mercedes board to see if the voltage regulator is replaceable on its own. you could also just call the dealer and see if it's sold seperately. sometimes the whole alternator has to go.

 

[KurskKnyaz]

Originally posted by: brblx
how exactly does the electrical diagram say that?

It says "Starter/Charger" in plain English.