M120 V12 variable valve timing solenoids question - Mercedes-Benz Forum
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post #1 of 16 (permalink) Old 06-09-2019, 11:35 AM Thread Starter
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M120 V12 variable valve timing solenoids question

I've scanned my '94 S600 coupe a few times with a Snap On MT2500, and noticed the cams are not advancing at 2100 rpm like they should. it's staying at base valve timing throughout rpm range.
what actuates, grounds, an/or switches on the intake camshaft solenoids to advance the cams at higher rpm ?
I'd like to just put those on a switch manually and switch is on/off at will.
obviously the design was used primarily as a compression release by delaying exhaust valve closing,
bleeds off compression so it cranks on starter easily.
the amount of cam advance is drastic with the solenoids, 20 crank degrees by what I read,
i.e. 10 cam degrees.
is this correct ? it moves the cams a lot ahead, greatly aiding power.
especially if you have a higher mileage motor that has some stretch in the chain already
any help appreciated.
is this circuit fused ?
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post #2 of 16 (permalink) Old 06-09-2019, 03:50 PM Thread Starter
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ps- I've checked for voltage and found there's 12 volts at the solenoids at idle, or with engine off and key on.
if this is the correct operation, I suspect the following:
the solenoids are energized from time the key is turned on, and engine starts
the intake valve cams then advance automatically using engine oil pressure when the rpm rises past 2000 rpm,
thereby supplying enough oil pressure to advance the cams.
above 5200 rpm, the solenoids are de-energized to take away cam timing advance,
for high rpm power.
is this how they work ? I suspect so, any info welcome, thanks.
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post #3 of 16 (permalink) Old 06-09-2019, 05:26 PM
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Trouble shooting for the solenoids is here:

http://manual.startekinfo.com/manual...1/m23.jsp#29.0
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post #4 of 16 (permalink) Old 06-09-2019, 09:02 PM Thread Starter
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Quote:
Originally Posted by jal1224 View Post
Trouble shooting for the solenoids is here:

STAR TekInfo

could you simplify that in your own words, as to how it's checked, the startekinfo is inconclusive at best, it doesn't mention the M120 V8 engine specifically.
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post #5 of 16 (permalink) Old 06-10-2019, 01:48 AM
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Chain stretch is from something wrong happening, not from mileage.

Cam on/off is elec. only.

Previous: MY97 CL600, MY92 600SEL, MY98 S420, MY96 S280 x2
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post #6 of 16 (permalink) Old 06-10-2019, 03:46 AM
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Quote:
Originally Posted by 1994v12 View Post
could you simplify that in your own words, as to how it's checked, the startekinfo is inconclusive at best, it doesn't mention the M120 V8 engine specifically.
I accidently gave you the M119 reference. Here is the whole M120 section.
As Joe-V12 said, the system is electro- mechanical only for cam timing. Hydraulics aren't involved.
Run thru the various tests listed. That will tell you if your system is working right or not
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post #7 of 16 (permalink) Old 06-10-2019, 04:15 AM Thread Starter
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Quote:
Originally Posted by Joe-V12 View Post
Chain stretch is from something wrong happening, not from mileage.

Cam on/off is elec. only.
the cam solenoids on this engine, are energized with 12 volts with the key on.
so I'm pretty sure cam is off/on then back off again at higher rpm.
it depends on oil pressure to advance it.
a 100,000 mile chain will have natural stretch in it just from wear and tear.
that goes for any engine. from a 4 cylinder right up to a V12 or V16.
anything with a timing chain, suffers from timing chain stretch.
the chain gives up a degree or so right off the reel the moment you fire up a new engine.
a spring loaded or pressurized tensioner will take up the stretch,
but the chain is still being stretched,
eventually it stretches to the limit of the tensioner,
or wears down the tensioner, a groove in it.
ask me how I know, I changed 100's of them.
they get so stretched that when you teardown the engine, the chain is literally falling off the timing gears.
eventually it will get so stretched that the engine will not run or start. it goes out of time.
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post #8 of 16 (permalink) Old 06-10-2019, 04:27 AM Thread Starter
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I found it in another thread somewhere, here's how it works
it's an electrical-mechanical-hydraulic actuator
OFF below 2100 rpm is for smooth idling,
ON from 2100 to 4700 rpm for increased torque
OFF at 4700 and above for volumetric efficiency

basic mechanics is this, advancing the cam timing of the intake valve, into the direction of rotation gives more low-mid range torque and horsepower, but sacrifices high end horsepower.
retarding the cam backwards counterclockwise against engine rotation, acts as a compression release, making it easier to start, and smoother idle, but sacrifices low rpm torque.
being a 10:1 compression engine it could use that feature, to save strain on battery/starter on hot restarts.
retarding the cam back to base timing at high rpm gives more high rpm horsepower.

it doesn't switch on automatically at 2100 rpm using 12 volts. the 12v is already there at idle, check the voltage at the solenoids yourself and see, there's 12 volts there with key on, engine off. it advances at 2100 rpm when engine rpm rises, and oil pressure rises and actuates it.
all the solenoid does is uncover the oil port to allow it to advance with increasing rpm and increasing oil pressure.
oil pressure increases with rpm, that's what actually makes it work.
at 4700 rpm the voltage is cut, closing the oil pressure port, and the cams go back to base timing.
the only way to turn it on at will, would be to install a high pressure relief spring valve in the oil pump, so there was enough oil pressure to actuate it at will at all times, and put the 12 volts on a toggle switch.
without the oil pressure, the voltage can do nothing but open/close the oil port. not advance the cam.
the muscle that moves the cams ahead, is the oil pressure, not the 12 volts.
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post #9 of 16 (permalink) Old 06-10-2019, 04:50 AM
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My bad. I'd looked, but never seen an explanation like that. Thanks.
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post #10 of 16 (permalink) Old 06-10-2019, 05:39 AM Thread Starter
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no worry, we're all learning here as we go.
I thought cam advance switched on and off with 12 volts
until I checked it with voltmeter, it went on with key on, was on at idle.
and 12 volts stayed there from idle up to 4000 rpm
made me think huh ?? how's this switching.
watch oil pressure gauge. its on 1 at idle, then goes to 3 at high rpm
made me think ah-ha, the varying rising oil pressure is what actually engages it.
the oil pressure engages it as it rises, only way it could work
if 12 volts is there at all times.
I didn't want to free rev it past 4700 rpm to see if it shut off the voltage at high rpm,
to retard the cam timing.
but that's probably how it works. when it gets to 4700 rpm,
the voltage is cut, and it goes back to base timing at high rpm.

my logic, if it cut in/out with the 12 volts at 2100 rpm, the voltage should be zero at idle and with key on, and 12 volts should come in at 2100 rpm. that doesn't happen on my car.

basically the rising oil pressure turns it on at 2100 rpm, and the removal of 12 volts turns it off at 4700 rpm.
the port is always open and 12 volts present from 0 to 4700 rpm.
at least that's what my car does. unless maybe mine is malfunctioning and the 12 volts should not be there at idle. my gut tells me that's how it works though.
i.e. there's not enough oil pressure from idle to 2100 rpm, to advance the cams, by design,
even though the port is open...
I think....
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