Engine tries to rotate but stops, does not start - Help! - Mercedes-Benz Forum
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post #1 of 30 (permalink) Old 06-07-2019, 05:55 PM Thread Starter
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Engine tries to rotate but stops, does not start - Help!

2000 E320 wagon 185k miles. A few days ago turning the ignition key produced no click/no engine turning. Checked the ignition with a scanner - no errors, start signal given. The 10A fuse which controls the little fan for cooling the engine ECU/electronics is blown. I replace the fuse, the car starts and runs ok. I try to start the second time - fuse blown again. So I replace the little fan and now the fuse does not blow.

But the car still does not start, the starter is clicking, engaging but stops after the engine rotates by a few degrees. I check the starter relay just in case, try the relay from another good car, all good. I try a good battery from another car disconnecting the old one just in case it has some internal fault, nothing changes.

So I replace the starter, nothing changes.

Looks like even with a strong battery the result is the same as with an almost dead battery - lights dim, voltage drops, the starter does not have enough power to turn the engine past a few degrees. The starter's 40A fuse does not seem to go to +12V, or maybe it goes for so short that the voltmeter only registers a few volts.

Is something shorting? Where? Is that starter control board with 40A fuse busted and if so how to test it? Any suggestions? Thanks!!!
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post #2 of 30 (permalink) Old 06-07-2019, 06:58 PM
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Hello

1) There is no 40 amp fuse for the starter. If you are talking about the orange / yellow fuse by the starter relay, then this is not the starter fuse, but the secondary air pump fuse.

2) You can remove the starter relay (the black ice cube sized relay) and short the relay contacts with a jumper wire, and then insert the ignition key and briefly turn the ignition key to position 2. The starter should engage. Do not crank the engine for long, just few seconds to confirm that the starter is fine.


Note: VERY important make sure you short the contact side of the relay socket terminals, not the coil side. You need to short the terminals that the pins 30 and 87 of the relay go into.




3) I remember a case like yours and it turned out to be a bad K40 relay module (the module that the starter relay is mounted on)


Once you completed step 2, and the starter actually cranks the engine, you need to look at other areas.
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post #3 of 30 (permalink) Old 06-07-2019, 07:06 PM
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Here is your reading material.

https://www.benzworld.org/forums/w21...ight=antiseize
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1972 280SEL 4.5
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post #4 of 30 (permalink) Old 06-07-2019, 07:42 PM Thread Starter
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Thanks @mrboca and @Witek_M . (and thanks for the 40A fuse correction)

I'll check it out, but I've actually skipped the relay shorting as I've also tried another good relay plus I know the starter is engaging since the engine rotates by a few degrees. Will shorting the relay socket terminals do anything different from the actual relay closing and giving power to the starter?

It may be the K40 though if pulling it out for the fan change I accidentally twisted it or pulled on some wire etc causing a damage...
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post #5 of 30 (permalink) Old 06-07-2019, 08:02 PM
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The Starter relay is activated by the ECU (The engine Control Unit). The idea is not to test the relay itself, but to make sure that when the starter solenoid is activated (by the starter relay), the starter turns. The jumper emulates a closed contact. If the starter turns, that means the wiring from the relay, the starter solenoid and the starter motor is fine. In this case, the ECU is deciding abandoning the cranking because something else failed.

The fan fuse also powers the ECU. A voltage drop in the K40 module will cut the voltage below certain operating level. To reduce the voltage drop, maybe you can try starting the car with the module fan unplugged (the 2 pin connector by the 40 amp fuse and the starter relay).

There is another suggestion in the above link which is lubrication of the ring gear. But it has a tell-tale indicator that the tachometer goes above 2000 rpm briefly when cranking.
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post #6 of 30 (permalink) Old 06-11-2019, 04:32 PM Thread Starter
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@mrboca , excellent points which I have not been aware of, thanks! I did short the relay and the starter turns the engine just fine, so most likely I've just wasted time and money replacing the starter.

I've re-ran all the error code reading and checking etc and there are no errors. So I'm guessing K40 could be bad (I will look inside it next for obvious solder problems) or maybe something else like the brake switch? Any ideas for next step reliable testing?

thanks
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post #7 of 30 (permalink) Old 06-11-2019, 04:48 PM
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If you have a scanner that can read the brake light switch (There are two sets of contacts one for the brake light and the other is for the BAS/ABS) states, it will tell you if you have a problem with it or not. With brake pedal left alone, the brake light switch will be open and the BAS/ABS switch will be closed. When you depress the brake pedal the switch states will toggle.

In any case, I do not believe a defective brake switch will prevent you to start the car, but it may not allow you shift gears.

You can check the solder cracks, but I do not think you will be able to isolate relay contact issues.

Have you tried removing the module case blower fan connector (connector G on the starter relay side, 2 pins)? This may reduce the load on the power circuit and reduce the voltage drop during start up.
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post #8 of 30 (permalink) Old 06-11-2019, 06:32 PM Thread Starter
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@mrboca I have not tried to disconnect the fan yet, looking at the K40 (photos attached) I see a diode between the points shown with a red line looking burnt out - right below the starter relay. It looks fried and one of the solder ends is brown not shiny like all the others.

But when I check the forward bias of the diode it shows 0.540 which means it's a good diode, as it should be 0.5-0.8 (same readout as for an identical one next to it - above the red line on the photo). But since the diode is not removed from the board, it's possible that the readout is affected by other components. So I'm a bit confused, maybe the diode overheated, destroyed it's case and one solder point, but it's still good inside?

I'm going to re-solder it first and see.

But if you or anyone else is familiar with this particular component failure and what to replace it with it would be great! Thanks
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post #9 of 30 (permalink) Old 06-11-2019, 06:44 PM Thread Starter
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And can the STAR scanner read the brake light switch? When I try to get physical access to the switch, it is seems so buried behind the pedals that pulling it out does not look like fun - checking it first with a scanner would be much better.
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post #10 of 30 (permalink) Old 06-11-2019, 07:16 PM Thread Starter
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Looking with a magnifying glass through a few mm gap between the PCB and the black plastic base, where the diode is visible, diode's case is cracked open and portion of it is missing - so does not look reliable even if testing OK at room temperature and with no load. So I will try to solder in a replacement diode (looks like it's FR107 diode or something close to it) and see...
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