Today’s Technology
Ken Adams
 
Head of the Class
AMG engine refinements
 
Last issue we took an in-depth look at the beefy lower end of the 6.3-liter M156 engine built by AMG at Affalterbach.  For this installment we will take a look at the top end and some of the refinements of the cams and valve timing components, with particular attention to the very modern technology of variable cam timing.

The M156 uses a dual overhead cam, four-valves-per-cylinder hemispherical configuration. Nothing radically new here. The major changes are how the cam timing is controlled. The timing of the intake and exhaust valves actuated by the four cams is electrohydraulically controlled via solenoids. All four camshafts are continually adjusted for optimum performance and efficiency. Cams are continually adjusted up to 42 crankshaft degrees. At low engine speeds and light throttle, higher valve overlaps can be used. By doing this, less fresh air is drawn into the engine, and combustion temperature drops, thus nitrous oxide is reduced. During higher rpm and load situations, the cam timing is either advanced or retarded according to a predetermined map programmed into the ME engine control module. These specs seem to be a highly guarded secret by the folks at AMG.

The control of the cam timing is accomplished with the aid of oil pressure controlled by electronic solenoids. On the front of each cylinder head is a cover that houses an electrically controlled solenoid, an oil control valve and a position sensor in front of each camshaft. Depending on the pulse width modulation (PWM) signal, oil pressure is either applied into or released from the camshaft adjusters as needed. By applying full pressure, a camshaft moves in the direction of advance up to 42 degrees. When the oil pressure is released, the adjuster returns to its base position. The positions of all four camshafts are constantly monitored by individual position sensors.

Traditionally, a camshaft is positively connected to the cam sprocket or adjuster by means of a dowel pin or woodruff key. While this technique is reliable and strong, compensation for timing chain wear and stretch is difficult. On the M156, a completely new method is used to keep the timing adjuster correctly positioned on the camshafts (shown above).

Actually, there are no keyways, dowel pins or splines used to unite the two components at all. In the case of the M156, the camshafts are mated to the camshaft adjusters by means of a single bolt in the center of the adjuster into the center of the camshaft (photo 1). So what keeps it from slipping, you’re wondering? A very thin washer impregnated with diamond particles is placed between the camshaft adjuster and the camshaft (white washer in center of picture). The diamond particles provide enough friction to properly secure the two components together without any slippage.

Timing chain configuration has been completely redesigned. The timing chain drives two idler gears, one placed in each head, that drive the two camshaft adjusters in each head via straight-cut gears. Each camshaft adjuster has an anti-backlash gear to reduce noise usually associated with straight-cut gears (photos 2 & 3). Special procedures and tools are used to correctly preload the anti-backlash gear/adjuster assembly.

Cam Adjuster showing anti-backlash gear

Backside of Adjuster

Some significant changes have been made in how the cam timing must be set up after repairs. Several special tools are absolutely essential to set up cam timing following repairs or assembly (photo 4).

Special Mercedes-Benz tools used to adjust cam positions

Setting up the cam timing after a repair at first seems like a difficult task. Not so; in fact, it’s fairly straightforward, but it is essential that the technician has the correct tools. Here’s a quick overview of how this procedure is carried out.

The engine is rotated until the 40-degree ATDC (at top dead center) mark lines up with the casting mark on the timing cover. A fixture mounts to the front of the cylinder head and secures the camshafts in their basic position (photo 5).

Pulse wheel index fixture

On the back of each camshaft is an 8mm slot cut in at an offset to allow the use of a special straightedge tool to be slipped into place. This further positions the camshafts into the base position but with more accuracy. At this point, the securing bolts on the front of each camshaft adjusters can be removed and reinstalled while the camshafts remain secured in their respective base positions (Photo 6).

Cam lock installed with adjusters removed

During re-assembly, the anti-backlash gears are preloaded, then  a new diamond coated washer is always fitted in between the camshaft and the adjuster, using a new stretch bolt. These bolts do not get tightened yet, however. The next step is to align the pulse wheels using the locating plate. Once the locating plate fits completely on the cylinder head dowel pins, the bolt can be torqued to spec. Camshaft adjusters look identical, but beware: There are different adjusters, marked “EIN” for intake and “AUS” for exhaust.

Once fully assembled, the final step is to connect the vehicle to a Star Diagnostic System (SDS) (Photo 7). Once connection is made, the engine is fired up and engine speed revved to approximately 2,500 rpm. The camshaft positions are learned electronically in just a few seconds (Photo 8). Keep in mind: This is only a short overview of the procedure. If you are so inclined to tackle this project, be sure to follow the Mercedes-Benz Workshop Information System (WIS) procedures. Failure to do so could be catastrophic to your beloved power plant.

Screen shot of computer in process of running learning process

Star Diagnostic System (SDS) screen during cam position teach-in procedure

A few side notes. Yes, some early production head bolts were prone to breakage. Early production head bolts had a female head torx configuration. The current and revised bolts have a male torx head. The good news is the early bolts have now been replaced on most cars and I see very few failures due to head bolts. Automotive Racing Products offers a stud kit for this engine, which is a better way to secure a cylinder head. The downside is the engine has to be pulled out to remove the heads. Unless you’re a hardcore racer, the results are probably not worth the investment.

Some M156 engines are prone to tappet noises when starting cold due to bleed down, the condition where oil leaks out of the tappet lifters too quickly, allowing them to lose contact with the tappets before being pushed back into contact. Bleed down is normal to a degree. If you find the tapping sound to be excessive, there is a cure. Tappets from a M159 (SLS) fit perfectly. These tappets bleed down much less and as a bonus, have an anti-friction coating on the friction surfaces. Don’t run down to your local dealer and request these be installed on your M156 under warranty. Dealerships are required to install only the correct parts listed for the particular vehicle being repaired. If you choose to update the tappets, it’s on your dime.