When buying a 87 w124 300D/TD

[layback40]

I found this & thought it may be useful. I dont know if it has already been on here. If it has let me know & we can delete it, no point in duplication!! It is so long I need to put it over several posts.
Feel free to add extra comments.


This only applies to the 1987 300D/TD, not all 124's in general. Some
of the items may apply to other 124's, for example the A/C information.


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1- The cooling systems are notoriously problematic. If the car hasn't
had a recently replaced radiator and/or fan clutch, watch the temp
gauge closely. It should almost never exceed 100C except in extreme
conditions (blazing heat, A/C on, climbing a mountain.) A new
radiator is about $350, or you can send yours out for a rebuild at
Reseda Radiator for ~$125 + shipping. I haven't seen great results
with rebuilds... and since the old radiators don't have metal
reinforced necks, AND the plastic can get brittle, I'd lean towards
a new Behr (not Nissens) unit if possible. "Normal" operating temps
are between 80C-100C. Don't freak out if the car runs at 90-95C all
the time, that's fine, as long as you can't make it go past 100-105C
under high loads. Mercedes says temps above 110C are excessive, and I
have the TSB to prove it. Damage should not occur until temps pass
120C... but I'd have the heater on 'max' long before that point.

2- Fan clutch: This is probably about worn out after 15 years. If the
fan has metal blades, it's the old style clutch, and may need
replacement. The new style clutch requires a new plastic fan (from
the OM606 motor.) Total cost ~$250 for both parts. If the old clutch
has a good bearing, it can be re-filled with silicone fluid. If the
bearing is bad (fan blade moves front to back more than a few mm),
you must replace the clutch. Don't be too quick to condemn the clutch
as not working - it requires hot air to engage! That means the radiator
must not have a cold spot in front of the clutch, and must not be
plugged with bugs & dirt, because if it is, even a NEW clutch won't
engage. To test the clutch (after making sure the radiator is clean &
hot), get the engine to 100C or higher, and kill the engine while
watching the fan. It should stop immediately, or within 1-2
revolutions. If it spins 5-10 times, it was not engaged. Compare this
to how it acts a warm or cool (40-80C) engine. And yes, the fan does
do a lot at freeway speeds, the ram air effect at 70mph IS NOT enough
to cool the engine with a bad fan clutch.

3- Electric cooling fan: There is a 3-prong electrical switch at the
water outlet, near the upper radiator hose, on the front of cylinder
head. The 2-prong connector triggers the electric fan on high speed
when the temp exceeds 105C. To test the fan, short the two wires
together (with the engine on), and the fan should run on high. This
switch is almost always bad and should be replaced on principle. The
original switch is usually blue (and rated 105/128C), but I recommend
replacing it with the gray-colored switch (rated 105/120C) from the
W126, which costs half as much (order part number 006-545-61-24,
about $30). To test the low speed fan, find the green or red pressure
switch at the AC receiver/drier (behind the left headlight), and short
the wires together - the fan should turn on low speed. (If you ever
open up the AC system, replace the green switch with a new red one.)

4- Serpentine belt tensioner: The pivot bearing tends to fail and lean
at an angle. Look at the tensioner pulleys near the water pump, if they
don't look perfectly straight, you likely need a tensioner. ~$100 in
parts, not a bad job if you have the tools to R&R the fan clutch
quickly. Also the belt shock tends to have it's bushings fail and it
causes a rattling noise. Press on the shock body with your fingers with
the engine running to see if it quiets down. It will be obvious if it
does. Get a new OE dealer "Genuine Mercedes" shock if it's bad, the
aftermarket ones seem to have shorter life spans - the OE part is
~$60, the aftermarket shock is ~$30. The OE shock has an improved
bushing design, as noted in the factory OM603 engine manual.

5- Oil leaks: Hopefully you don't have many. Cost and location vary
widely. Depending on the severity you may choose to ignore them. The
oil level senders tend to seep oil through the body. This is ~$50
and simple to replace at oil change time. Also, the tranny cooler
hoses at the front of the engine tend to leak too, just replace them.
The injection pump often leaks oil from all over, you can pull the
pump and re-seal it for about $30 in gaskets, and 4-6 hours labor
(not a fun job but it's nice to have a dry pump!). The turbo also can
leak oil externally, but that's expensive to repair, so try to ignore
that leak if possible. A leaky front crank seal will cost ~$200 to
have a shop fix it (this requires several special tools for DIY).
Fortunately, the 603 engine does not suffer from the severe oil leak
problem at the turbo oil drain, like the older 617 engine did.

6- A/C system: This can be expensive to fix. The manifold hose (the
main hose assembly) tends to weep oil *through* the barrier, making
the outside of the rubber hose wet and oily. Yes, it's actually
refrigerant oil! This is the big hose near the ABS pump. If it's
oily, you will need to replace it eventually. I just did mine, it's
a massive unpleasant job. The hose is ~$350 new from Rusty or you
can have your old one rebuilt for about $100. Stick with R-12 if the
system hasn't already been converted - please! - and don't ever
convert an R-12 system to R-134a, or any other refrigerant that
requires PAG/POE oils. You'll be sorry in the long run. If the car
has already been converted, well, cross your fingers and hope it was
done properly. If the evaporator is leaking, it's a massive job
(15-20 hours) to replace as the dash must come out - see next
item (#7, below).

7- The center vent vacuum actuator is often broken (ruptured
diaphragm.) Put the A/C on max and see if you get a constant stream
of air out the center vents over 10-15 minutes operation. If it EVER
stops, the actuator is bad. There's a diverter flap (not the recirc
flap) that allows air out the center vents, but this flap cycles
during normal operation- hence the intermittent airflow. When the
center vent flap is open, you can look into the center vent itself
with a flashlight and just see the aluminum evaporator down below.
If you can't, the door is shut - because the flap isn't open. To
replace this $20 part, you must R&R the dash - a 5-10 hour job.
NOT fun. I've done this on both of my cars, and my sister's car,
and most early 124's need this done. If you pull the dash, replace
EVERY vacuum pod, not just the center vent pod! You will not want
to pull the dash again to replace the next failed one a year later,
believe me. To test all of them, pull the glovebox out, and use a
MityVac to apply vacuum to all 7 of the vacuum lines at the manifold.
All should hold 25" vacuum. If not - if they leak down, or don't
hold vacuum at all, that pod is bad. DO NOT bother with the rebuild
kits sold by Performance Analysis... that kit only works properly
on the center vent pod, and on one of the two chambers inside the
dual-chamber pods. Just order new pods, it's NOT worth pinching
pennies here, given the significant labor time involved in R&R.