So if this '79 300TD I'm looking at comes with a "needs new vacuum
hoses" message, what all is involved in "updating the system" with
all-new hoses? I understand that vacuum controls everything, but how
accessible is it all? I suppose I need to talk to someone other than
who I did about the car (talked to the driver, her husband did all the
maintenance on it though), but where is vacuum routed, and how
accessible is it throughout the car?
I've only ever owned GMs and one Honda, and the prospect of what might
be involved in this is a bit frightening, but perhaps the vacuum system
either doesn't degrade too bad outside of the engine bay (which I'm
guessing is relatively straight-forward to just whack some new hoses
into?) or it's all fairly accessible unlike the other cars I've had?
thanks for any tips, although maybe I should skip this car (there's a
nifty Peugeot 505 turbo wagon available for sale down here for half the
money, but that frightens me in other ways <g>).
I'd be skeptical about the car "needing all new vacuum hoses" - unless
its been in a fire!
(IMHO, that sounds like something told "the spouse" to justify replacing
But it may well need repairs and these could range from:
a rebuilt vacuum pump
a new brake booster
repair to the vacuum powered door locks
The brake booster is the $$, the door locks just time consuming.
Vacuum aside, you should know that the climate control system on the '77
- '80 MY cars is quite expensive to repair - so much so that there's now
a digital retrofit kit to replace most of this OEM system. (I installed
one on my '80.)
Considering these points, and that this is a non-turbo engine, I'd look
for a TD (if that's what you want) in the '82 to '85 MY as these have an
improved climate control system and a turbodiesel.
Of course if this car is very cheap and you like it then.....you know.
Thanks for the warning, not heeded soon enough but hey the car was
cheap. ;) The digital system looks like a good investment, although
I'm going to tear apart other bits first just to make sure all is well
and good before spending "real" money.
Would it be correct to assume that despite these being the same engines,
pulling a turbo off a later MY wouldn't be something I'd want to
install? I'm asking 'cause I assume that compression ratios in turbo
engines are probably much lower, so I'd need to get shorter rods to make
it not blow up, in which case I might as well replace the whole engine?
As it is, the engine is strong enough for me. It's not the 3.8L
supercharged V6 I'm replacing, but it gets me where I'm going fast
The Pug is a good car. Very soft ride. A buddy of mine had one, it's impressive.
The vacuum hoses are a stupid simple fix. You need two things 1) a supply
of the hose and a few of eevry connector, end and t them make. 2) A car
like yours that's plumbed correctly.
You just put on the vaccum bits till it matches exactly the other
You'll need new little white fingers for the things in the black box
on top of the valve cover where the vacuum goes into.
Rumor has it the hose is no longer available in clear or colored and
that it only comes in black so your car will look logically, not
physically the same as the other car.
But that's all there is to it. Vacuum bits are cheap at the dealer
and it shouldn't take you more than a half hour to put them all on.
Get a vacuum handpump/gauge. You'll need it anyway if you hve one of these cars.
DO NOT cheap out and get the plastic one, they break, get a metal one.
Need Mercedes parts? http://parts.mbz.org
Richard Sexton | Mercedes stuff: http://mbz.org
I wouldn't characterize a major replacement of most, if not all, vacuum
hoses as a simple fix. Vacuum is used in far more applications in
these cars than any other car I have ever seen, including, for example,
door locks, fuel fill door, trunk lock, and shutting off the engine.
The reason I would say it's far from simple is that many of the
vacuum hoses are in difficult to access locations, like under the dash.
You can track them down and get to just about all of them, but it
can be quite a task. And there are still a few, like the lines that
control the center dash vents, that are very inaccessible. For those
particular lines and the actuator, the only access is if you remove the
Also keep in mind that for every application there is a small actuator
somewhere that produces the desired movement, whether moving an air
flap or locking a door. These have a rubber bellows that doesn't get
better with age. Mine are all still original and OK, but I keep the
car garaged here in NJ which isn't the worst environment. If the car
has spent 25 years in say Phoenix, the results could be much different.
And each of those acuators could be $100+.
On the other hand, with my exper with a well maintained 80 300SD, only
a couple of the lines have actually failed. And both of them were at
the same spot, where they passed through the firewall grommet. If
that's all you have to deal with, then it is relatively simple, but
still time consuming to track down and find the problem.
Also, I agree with the warning about the climate control system. Not
sure if that model has the auto climate control, but if it does, these
are particularly problematic and involve the vacuum system. MB used a
strange contraption, called the climate control servo, that combines an
electric drive motor, a coolant control valve, and gear driven switches
and vacuum valves that serve as the master control for the heat/ac.
There must be a dozen electrical connections and about that many vacuum
ones going into the thing, together with coolant hoses. These fail,
either by cracking and leaking coolant, or by just quitting. If this
car has auto climate, I'd inspect servo for leaks and see if it works.
If it doesn't, which you may not even be able to tell since the vacuum
system is kaput, then you may be in for quite a repair bill, even if
you do the work yourself. Last I checked, a good rebuilt one, using
an aluminum housing, was going for about $600. As TG pointed out, you
can get a conversion kit that replaces the whole works for good, but
that is even more expensive.
It turns out that it's not nearly that bad, so I bought the car today.
There are a few issues, but I hope they're relatively straight-forward
to fix with my knowledge + some sort of useful shop manual + the local
resident experts on alt.auto.mercedes.
The only vacuum problem I've noticed so far is that the passenger-side
doors, fuel door, and hatch don't lock when I turn the key in the driver
door. I also can't manually operate the lock on the front (passenger)
door, so I'll pop the panel off and see what's binding, and hopefully
the automatic actuation problem will be obvious.
Beyond that, this car does have the auto climate control. Wow, that's
some system you've described. It blows a little bit cold, but it
doesn't seem that I have manual control over the fan speeds. Hopefully
I can just recharge the system and it'll be all dandy.
The hatch doesn't stay open by itself, but this one I'm not too worried
about just yet. I also have to get a lens for one of the reverse
lights. Oh yeah and the hatch lock doesn't seem to work from outside,
but it operates from the button inside, manually, ok (and since it's a
wagon, I /can/ climb back there even if I prefer not to, heh).
It's pretty clean, though, and even the floor mats are in nice condition
with the 300TD badging on them. ...and this car really cruises on the
freeway; 80mph for a half hour and it seemed really happy to be
cruising. 154k miles ain't too bad for a 28 year old car.
Baby blue, and it's even got the old blue California license plates
which look real nice, oddly enough. (California cars get transferred
with plates, unless you specifically request new ones.)
So since I'll sell my old car for more than I paid for this one, I get
out ahead even if I do have to drop some bucks into it. I just gotta
get used to not locking my car. That'll be some fun. ;)
Yes, if the manual lock button doesn't work, then that's the place to
start. Here;s the free tip of the day:
To remove the interior door panel, besides the obvious 2 screws by the
latch, there is a screw near the middle of the armrest, accessible from
the underside on an angle, and another screw behind the door release
handle. To get at that one, you have to use a small screwdriver or
similar to pry out the thin black plastic trim cover, which is somewhat
flexible. The screw is behind that. You also need to remove the
thin trim piece on the door that runs from the door panel in the latch
area, up to the top corner of the door. It;'s held in place by one
very small screw near the top corner. After all that the door panel is
held in from behind with clips, so to remove it you need to lift
upwards about 1/2 inch and then out.
There is no manual control, the servo controls the fan speeds. Did
you check that you can get heat out of it? If you start with a room
temp or below cabin and set it to max heat 85, with car running and a
warm engine, the blower should go to max heat speed. I say max heat
speed, because in heating mode, the max blower speed is 4, while in max
cooling it's faster at speed 5. Heat should be coming out of the floor
vents, the two side vents by the windows, and a little bleed air out of
the center vents. If you then get the cabin nice and warm and rotate
the temp setting to max cooling, 65, the blower should slowly step down
through the heating speeds, then slowly step up again to max cooling
blower speed. Air should now be coming out of the side vents and
center vents, with the floor vents closed off. All this should work
even if the compressor system is kaput. If it does all this, I'd say
95% chance the servo is good, provided of course it's not leaking
Another tip: You should periodically do this, like once a month.
Forcing it to go from max heat to max cooling moves the servo gear
mechanism through it's full range of motion. That helps keep it
lubricated, the electric contacts from getting dirty/bad spots, etc.
I hope you pulled them out and looked underneath. One of the main
failures in this car is rusting and this is one prime location. Part
of the problem is that at this age these cars can develop rain leaks
and that's where the water accumulates. Left unfixed it's a disaster.
I recently pulled some parts off a similar car, where the front floor
area was completely shot. In fact, I suspect that may be what sent it
to the scrap yard. My suspicion is many of these may rust out from the
process above, rather than from salt/etc on the outside. Once those
mats get wet, with a rain leak, they can stay that way forever.
If it was in CA all those years, that is real good news!
Good luck and hope it works out well for you!
Great, thanks for the tips. Chilton's didn't tell me what I actually
had to do with the "switch plate," so this was helpful. Pretty wild
setup there, but I guess it keeps it all nice and tidy.
FWIW, my '79 back doors at least have two screws as the armrest meets in
two places with a gap in the middle. ...the manual lock 'peg' wasn't
attached, so I took the panel off to reconnect that and now at least I
can lock up the car. ;)
Oh, and the front passenger side door lock can be manually operated for
"a while" after the car's been on. Ugh, and the key doesn't work from
the outside on the passenger door; guess they had a new ignition lock
put in and didn't bother re-keying everything (discovered that the key
doesn't lock the glove compartment either).
I'm still feeling ok though 'cause this car was 1/4 the cost of many of
the other 300TDs I've seen around, so this sort of stuff is not
It was 80 in LA yesterday and I didn't think to check. Alas, it does
not now appear to control the temperature at all. If I can find useful
information anywhere I'll see about taking that apart, but it may be
that the drop-in replacement option may be desirable as well.
The fan does crank up, it just doesn't warm up in the compartment. Maybe
the coolant lines are clogged or something? When running, the engine
heats up to 175 and goes no further, so I guess it's got a 170 'stat in
it, which appears to be functioning at least.
No problem, living in southern California gets one into a state where if
it drops much below 70 you crank up the heat in your car and wear
I looked at it today, and now I'm thinking this'll be a major PITA to
fix, but it's a good excuse to look for 300TDs at the wrecker and
practice taking things apart. ;)
I have looked under the floormats. The floors are in good shape.
BEFORE you remove the passenger door panel:
Your central locking system is single door only. By this I mean that ONLY the
drivers door locks and
unlocks the whole car. The passenger will only be able to be unlocked IF the
door is locked from the
drivers door. You cannot lock the pass door if the drivers door is unlocked.
Sorry if I'm being a pest. :) I really appreciate all y'all's help.
The mechanical bits are more up my alley, this vacuum stuff is all
German to me.
Thanks, I actually discovered that today while playing around with it.
If I lock the driver's door, I can lock the passenger side doors
manually; the front goes down easily and the back feels like it is
getting /some/ vacuum assist. If I unlock the driver's door, the
passenger side doors unlock as they should. ...which means that in
order to lock my car, I need to turn the key in the driver's door lock,
then go around the car to lock the other two doors! (Driver's side back
door operates properly, and I'm not 100% positive but I think the hatch
auto-locks properly. Fuel door will not lock automatically, and
manually operating the mechanism doesn't 'stick' in the locked position
like the doors do, i.e. it just releases as soon as I let go.) Is the
"can't lock passenger door if driver door is not locked" mechanism
vacuum-driven as well? I can't imagine a mechanical solution to this
that wouldn't be really ugly, but if it is a fact that as long as the
driver door is unlocked the passenger door is unlockable, then I must
have some good vacuum functionality. I know there's a leak somewhere,
though, since if I leave the car locked overnight, turning the driver
door lock doesn't unlock the other doors, but they'll pop open soon
after starting the car.
I haven't taken the door panels off yet (except to reconnect the back
door plunger knob thinger, after which I forgot to reconnect the window
switch ugh), and I have discovered that Chilton's is amazingly useless
on vacuum issues, are there two lines going to the lock, e.g. a 'lock
now' line and an 'unlock now' line, or is it one line with positive and
negative pressure that causes the lock to operate? Would the behavior
I'm seeing be automatically indicative of bad servos, or do I really
need to get in there and test the vacuum with the door apart? Might
there be another 'tap' to test the vacuum operation, say in the engine
bay? I have a decent vacuum pump, and certainly the more I can diagnose
before taking it apart, the more likely it is I can get the right parts
before doing so.
Oh- when I open the doors, there's that plastic sheeting in there.
Clearly I need to remove it to do any meaningful work; what should I use
to reattach it to the door when reassembling?
Oh again, it typically takes a second or two for the engine to shut off,
which is not the behavior of the old lady's '85 300D, so I figure that's
also a vacuum issue that's hopefully pretty easy to rectify. Today the
car failed to shut off entirely, but I discovered if I operate the
windows it will shut right off (and I do know about the STOP lever under
the hood). Fun times, I actually don't mind all of these issues 'cause
I used to spend a lot of time working on cars when I had 'simpler' ones,
and this seems like all stuff that one can figure out without needing to
attach the vehicle to a computer. (...says the software developer!)
Maybe I'll go dig out the vacuum pump and try to diagnose the shut-off
issue in the morning.
Is it relatively straight-forward to test my reservoir? Is there
somewhere I can hook my vacuum pump, give it a few pumps, and see if
it's ok? When I first saw it, I had no idea what it was, but then
browsing here and there something had a picture and it clicked. I don't
remember seeing any easily-accessible ports on it for testing, though.
Yes, only vacuum driven. One vacuum line moves the piston one way, and
another vacuum line moves it the other way. What MB calls the vacuum
switch, which is really a valve, is in the drivers door and linked to
the lock. When the driver's door is locked/unlocked this valve
connects the incoming vacuum vac line to either the lock or unlock
line that goes to the other doors.
I can't imagine a mechanical solution to this
I think someone was selling DVD's or books on how to fix MB vac on
are there two lines going to the lock, e.g. a 'lock
My guess is it's probably not related to the servo. First place I'd
look is on the driver's side firewall, near the brake booster, where a
bunch of the vacuum lines go through a rubber grommet. Try pulling
those lines back and forth a bit, work them around. The only two
leaks I had were right there and the lines just snapped when I tried to
move them to find the problem.
The vacuum originates from a pump on the front of the engine. You
should see a hose, about 3/8 coming from it and then getting split off.
Keep in mind that there are check valves along the way to keep one
segment seperate from another. You'll see those in the lines at
The line for the driver doors goes from the engine compartment, through
that grommet at the firewall, under the dash, and on to the door via
the hinge area. The hinge area is another areea to check.
I have a decent vacuum pump, and certainly the more I can diagnose
Clear package sealing tape works well. If the sheet is shot, you can
just use a clear poly sheet of the stuff these use to cover things when
That could be due to either a vac leak or bellows problem with the shut
off circuit, or there could be other leaks on whatever is shared with
that line. I'd work on the obvious vac problems, like the door first,
as you may cure this in the process.
The shutoff works via a vac line that runs to the ignition switch,
where a valve turns with the key. From there a line goes back to the
rear of the injection pump, where it works a bellows.
Well that's true, I was thinking about the underdhood ones really
they're the only ones exposed to the heat anf vibration of the engine
and seem to go first. The long runs in the door seem to last
forever. The diaphragms go on the door and heater flap
actuatros but they're not too bad to replace. The underhood
ones can be done in a leisurly afternoon though.
Need Mercedes parts? http://parts.mbz.org
Richard Sexton | Mercedes stuff: http://mbz.org
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