Expert with starter or solenoid. - PeachParts Mercedes-Benz Forum

ah-kay · #1 05-30-2018, 11:20 PM

I have problem with a starter. It is the solenoid which is not pulling in the pinion. I took it out and checked it on the bench. Electrically it seems to be OK as it pulls but just not pulling hard enough to retract the level ( overcome the spring ). Can a solenoid lose magnetism over time? The starter itself is OK as it spins with I apply 12v directly to the motor.

Diseasel300 · #2 05-30-2018, 11:31 PM

The solenoid is not magnetic, the magnetism is created by the current flowing through the coil of wire. If it is not pulling in the plunger, the mechanism is either binding up, or the solenoid is failing and needs replacing.

Pull the solenoid apart and see if it's packed full of grease. I had that problem in my SDL. The solenoid should be dry. It's ok to wipe a layer of oil on it to prevent rust, but not grease.

ah-kay · #3 05-30-2018, 11:48 PM

The solenoid is apart already. I know the magnetism is created by the current. It seems the current is not creating enough magnetism to pull the plunger fully in. This is the part baffles me. I cannot explain it unless the coil degrades over time. The plunger is clean.

funola · #4 05-31-2018, 12:20 AM

Measure the resistance. I recall it draws around 7 amps. Do the math E=IR

gastropodus · #5 05-31-2018, 01:10 AM

My W123 solenoid gave me fits with the same problem: failure to pull in and make contact. And this was after I had the whole starter "rebuilt" by a shop in town; they apparently used some crap no-name brand solenoid. I eventually had to go and buy a genuine Bosch solenoid, I think it was from the dealer. And funola is right: the whole design is a bit marginal, and sometimes the total resistance from battery through switch to the solenoid is high enough to drop the current below the minimum necessary to actuate the solenoid. In my case I was so annoyed with the problem (which spanned months, and involved carrying a hammer to bang on the starter before each start

) that I installed a relay under the hood. The starter switch trips the relay, and the relay slams the solenoid with 12V straight off the battery. Bam! End of problem.

ah-kay · #6 05-31-2018, 02:16 AM

I connect 12v directly to the solenoid and it is still not pulling. So it is not voltage related. I did further test. I removed the spring behind the plunger inside the solenoid and it pulls the pinion lever ok. So all I can conclude is

1) the spring stiffened over time. Possible? I am not sure.
2) the coil is not creating enough magnetism to pull the plunger. The plunger is connected to the pinion lever. Can the coil degrade over time? Resistance of coil increased?

I probably need a new solenoid but I don't understand the failure reasons. I am an engineer and would like to understand why it happened.

Diseasel300 · #7 05-31-2018, 09:06 AM

The spring did not stiffen. The plunger did not "lose" magnetism. The solenoid doesn't "wear out".

Some solenoids have a pull-in and a "holding" coil, not all do. Not sure about the Bosch starters. If you lose the pull-in coil, the solenoid will still create a magnetic field, but a weak one that is not strong enough to overcome the spring.

Replace the solenoid.

tyl604 · #8 05-31-2018, 09:36 AM

Ah kay - I am glad to know that the spirit of "what makes it work" is alive and well in this throw-away/replace world. We are of like mind.

sgnimj96 · #9 05-31-2018, 10:10 AM

Starter motors are tough (especially these). In general, unless someone gets them incredibly hot, they are easily fixed.

Mine quit working (click,click...) and it was just a gunk-stuck solenoid plunger. Cleaned it up and it is good to go.

Take that thing apart and clean it up. Open the whole thing, there is a diy somewhere here, and plenty of internet starter fix videos

Mine actually had the brushes slightly off and wearing funny because the riveted brush holder had slipped out of position, so THAT was wierd. Straightened it out gave it a touch of red loctitte. Anyway, cleaned the armature, etc, while I had it open.

Had to drill out the solenoid screws and replace those w/ some from ace hardware so the actual cost for fixing it was $3 and some time.

funola · #10 05-31-2018, 11:05 AM

Quote:

Originally Posted by Diseasel300

The spring did not stiffen. The plunger did not "lose" magnetism. The solenoid doesn't "wear out".

Some solenoids have a pull-in and a "holding" coil, not all do. Not sure about the Bosch starters. If you lose the pull-in coil, the solenoid will still create a magnetic field, but a weak one that is not strong enough to overcome the spring.

Replace the solenoid.

I am pretty sure these starter solenoids has a pull in and holding coil. Some rare solenoid failure modes are: Solder joints on the magnet wire can fail, insulation on the magnet wires can fail resulting in shorted turns thus weaker magnetic field.

barry12345 · #11 05-31-2018, 11:23 AM

High failure rate component on these starters. One problem is over time the windings in the solenoid could increase resistance. Creating a weaker magnetic field. As an engineer you know that reduced current would occur.

I will not accept a rebuilt German car starter unless it has a new original aftermarket solenoid. Why they did not make a more robust wiring circuit that provides the trigger voltage to the solenoid has always bothered me as well. Common on German cars. The voltage drop is too substantial.

Before changing the solenoid. Check the condition of the armature for excess wear from the brushes. Plus the remaining brush lengths in the voltage regulator.

It may not be worth a new solenoid as is. You might get the armature trued up if the wear is not excessive though. Bearings and brushes are also cheap if you detect too much wear.

I will not buy a remanufactured in Mexico starter or alternator. Odds are you are better off just getting a used unit from an auto wrecker.

When the manufacture of really poor quality aftermarket repair parts started. They introduced very cheap and generally unreliable components to the rebuilding industry.

This is the reason I suspect that Mexican rebuilts can work. Still new failures with them happen in very premature time frames. Far too often. I suspect they just hope they last to over the rebuilders warranty. Far too many also do not work even out of the box.

I have no issues with companies making a reasonable profit. I do have issues with them knowingly selling really inferior components for the sake of excess profits.

ah-kay · #12 05-31-2018, 01:14 PM

Well, thanks for all the info, I think I know the answer. There are 2 coils inside the solenoid, a pull coil and a hold coil. They are both energized at the same time. There is only one outside terminal so the switching is done internally when the 2 big terminals makes contact by the plunger. It then cuts current to the pull coil. I suspect my solenoid has a broken pull coil inside. You can't measure it externally as the 2 coils are kind of in parallel when not activated. I will open it up and take a peek.

97 SL320 · #13 05-31-2018, 04:40 PM

It is easy to test resistance.

For the pull in coil: Measure from small trigger terminal to the large motor terminal on the solenoid. ( For the pull in coil to work, it seeks ground through the starter motor windings. It also allows the starter motor to rotate slowly improving engagement. This is why strong trigger circuit wiring is critical )

For the hold in coil: Measure from small trigger terminal to the ground . ( Body of solenoid )

vwnate1 · #14 05-31-2018, 06:31 PM

Any older German car Mechanic will tell you : ALWAYS replace the solenoid if you're planning to clean and re install a Bosch starter .

The only two brands I've had good luck with are Bosch and Echlin .

All others either suck out of the box or have very short service lives .

90 + % of the time you can disassemble, clean, re lubricate a Bosch starter and it'll work as new again .

Obviously replace the brushes if they show any appreciable wear, it's a simple thing even though they're usually soldered in place .

ah-kay · #15 05-31-2018, 07:52 PM

Quote:

Originally Posted by 97 SL320

It is easy to test resistance.

For the pull in coil: Measure from small trigger terminal to the large motor terminal on the solenoid. ( For the pull in coil to work, it seeks ground through the starter motor windings. It also allows the starter motor to rotate slowly improving engagement. This is why strong trigger circuit wiring is critical )

For the hold in coil: Measure from small trigger terminal to the ground . ( Body of solenoid )

This is the best info and right on. I did the test this way with the starter on the bench.

1) Pull in coil - connect the trigger terminal to +12, the starter stud with the thick cable to ground. It pulls in OK. Basically the starter motor coil ( very low resistance ) is used to ground the coil when unit is installed in the car.

2) Hold in coil - connect the trigger terminal to +12, casing to ground.

I found the problem is in the starter motor internal. The carbon brushes is faulty and the pull in coil cannot seek ground through the windings+brushes.

New brushes are in order. Thanks 97SL320.