We're trying to diagnose a 1990 300TE4Matic, 3.0 straight-6 SOHC 4-valve
that started running like crap. It's gutless, and can barely get out of its own way.
Started by checking the usual suspects... main and "differential" fuel pressures
are within specs, ignition parts are good, timing is okay (crank-triggered),
cat doesn't seem to be plugged, etc, etc.

While checking all that other stuff, we noticed some clattering coming from
behind the distributor cap that sounded like a loose timing chain, so we
popped off the distributor and valve cover to see if maybe the chain had
jumped a tooth. However, the only sources we have to verify the timing
marks are being very unhelpful, and basically giving the same useless info.

They all say to set the crankshaft at #1 TDC, which we did using the metal
pointer that is part of the same bracket that holds the crank sensor pick-up.

Then they say "Ensure timing marks on camshaft and bearing cap are aligned",
and show a drawing of the backside of the cam gear bearing flange...
http://www3.telus.net/neatcrap/300TE/topview.gif

The bearing cap has the appropriate "mark" (big tab in the casting), but the
back of the cam's flange doesn't have a nice big mark as shown. What it
does have is two BIG bosses located 180degrees apart (which look more
like cast-in handles for manufacturing), a smaller boss located between the
two big bosses (looks like a much thinner version of the big bosses, that's
been filed down to almost nothing, possibly a casting line), and a tiny dot
on one of the cam's "ribs". The big bosses do not line up with the caps's
mark (6 teeth before it), the smaller boss doesn't either (3 teeth after it),
but the tiny dot DOES line up.
(Note: pic is NOT at TDC (about 180 out), but I've shown where our marks are).
http://www3.telus.net/neatcrap/300TE/back-of-cam-flange.jpg

Our sources also show drawings of the front of the timing components, with a
"timing mark" on the left side, apparently level with the top of the head...
http://www3.telus.net/neatcrap/300TE/frontview.gif

Our gear also has a mark in the same place when at TDC (pic out about 180),
but since the gear can be flipped front-to-back if necessary to alter the timing by
3 degrees, the arrow may just point to the oval hole for the oval pin. Didn't look
at the back to see if there's another arrow...
http://www3.telus.net/neatcrap/300TE/front-of-cam-gear.jpg

So the problem is, we have no cam tab, a dot that's not mentioned anywhere, and
none of the sources explicitly say if the arrow should be level with the head at TDC,
so we can't be sure if it's still in time or not.

(Cross-posted to alt.autos.mercedes and rec.autos.tech)