I just went out and did the same test myself. I could not get the VCV to hold a vacuum either. I even went and isolated the VCV from the rest of the vacuum system, still wouldn't hold vacuum. Is it supposed to hold, or bleed off?

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'81 MB 300SD, '82 MB 300D Turbo (sold/RIP), '04 Lincoln Town Car Ultimate

Sooner or later every car falls apart, ours does it later!
-German Narrator in a MB Promotion Film about the then brand new W123.

im going to do the following...

1.) trace and replace rubber cap, re test.

if that doesn't work...

2.) buy and replace modulator

Is that the course i should follow, or is there an intermediate step before buying a new modulator?

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'85 300D, 'Lance',250k, ... winter beater (100k on franken-Frybrid 3 Valve Kit)
'82 300D, 'Tex', 228k body / 170k engine ... summer car
'83 300TD Cali Wagon 210k, wife's car

I agree with Brian... the most likely culprit, and IF you are lucky, will be either the cap or o-ring [IF it has one] on the modulator... but I would also suggest you consider these related test "intermediate steps":
[1] Put a golf tee/cap on one end of the hard vacuum line that runs to the tranny modulator and use your new pump on the other end to test this hard line by itself. It's not unheard of that these can be abraded or cracked somewhere and thus leaking.
[2] It's also a given that you should replace any associated straight-line rubber connectors at either/both ends. Buy yourself a yd/meter or two of the size [ 4mm ID?] suited for the most common [small] hard vacuum line running around the engine compartment. Also consider using small electrical ties to sinch(sp?) down and tighten any questionable soft rubber connector-to-hard vacuum line connections [see: W123 Vacuum InterLock Diagram which will [if the link is working] lead you to: http://ericandkat.com/mercedes/W123VacDoorLockDiag.JPG for examples of how smaller electrical ties can me useful in solving nuisance vacuum leaks. I have found these a little tricky to install but invaluable in my efforts to make for tight vacuum lines.
Regards,

thanks for the tips sam...once i get this sorted out i will replace most of the rubber components. its amazing more of them arent leaking...

i took a look and the black rubber cap on top of the green modulator was cracked for sure. i took it off, and am patching it up with superglue/silicone to see if that fixes it. a rubber cap at that harsh enviro spot is quite the weak point...

sam: how would i check the o-ring?

im guessing to do anymore work on that area other than replacing this cap i am going to need to pull the transmission pan?...it's tight up there. i can barely inspect anything, i only can whats going on with a mirror.

thanks again,
dd

__________________
-------------------------------
'85 300D, 'Lance',250k, ... winter beater (100k on franken-Frybrid 3 Valve Kit)
'82 300D, 'Tex', 228k body / 170k engine ... summer car
'83 300TD Cali Wagon 210k, wife's car

Well I really didn't test it, but what I did was to take the old o-ring down to the plumbing section of my local ACE Hardware and found one that had a little smaller ID and "fatter" in girth, and this made a big difference in how the tranny was shifting. This was before I picked up a Hand Vacuum Pump!
Regards,

I tested the hard tranny vac line and the rubber connector to the modulator - they held vacuum so it's defintely the modulator/rubber cap/o ring.

I'm going to order one of the little rubber caps and hope. It's the little cap thats about a half inch wide?

Sam - I'm confused as to where this o ring is located. do you need to take the modulator off the tranny to get at it? Hopefully it doesn't come to that...but it will based on how things are going thus far.

-dd

__________________
-------------------------------
'85 300D, 'Lance',250k, ... winter beater (100k on franken-Frybrid 3 Valve Kit)
'82 300D, 'Tex', 228k body / 170k engine ... summer car
'83 300TD Cali Wagon 210k, wife's car

... so it's entirely possible that your tranny's modulator does not have a separate O-ring that seals the cap! Sorry if I gave bad advice.
Regards,

I've been doing my own troubleshooting regarding this vacuum system.

I applied vacuum to the system and it didn't hold. I tested the source line going to the brake booster and it was giving out good vacuum. I tested downstream of the orifice and it was reading good vacuum levels. I tested the line down to the modulator and it held vacuum for a good, long while. I then tested the VCV by itself and it would not hold vacuum. So, I guess I'll have to order a new VCV sometime in the future.

__________________
'81 MB 300SD, '82 MB 300D Turbo (sold/RIP), '04 Lincoln Town Car Ultimate

Sooner or later every car falls apart, ours does it later!
-German Narrator in a MB Promotion Film about the then brand new W123.

I dont know which vacuum system you have but i have whats known as "California 85" the most complex one. Boy, you get one vacuum leak and everything goes astray, the doors wont lock and sometimes the car wont shut off.

Hope you can go to a Salvage yard and find a vacuum control valve. i would send you one free if I had one...or post a message asking for one. Boy the dealer is expensive!!!

I've been shopping around, pricing new VCVs, the lowest I saw still wasn't low enough for my wallet. I may have to go used or check eBay.

__________________
'81 MB 300SD, '82 MB 300D Turbo (sold/RIP), '04 Lincoln Town Car Ultimate

Sooner or later every car falls apart, ours does it later!
-German Narrator in a MB Promotion Film about the then brand new W123.

H-townbenzoboy,
DO NOT go out and buy any kind of replacement VCV just yet.
Since I started this THREAD I usually monitor it and try to help when I can but I just returned from a 3-week vacation to Italy.
(1) Q - Would you please clarify which MBz YR/Model you have with this problem. I want to make sure you are working from the correct vacuum diagram and that your various vacuum hoses/lines are hooked up properly.
(2) Yes it is entirely likely that your VCV will NOT hold a vacuum and yet be perfectly OK. I say this because it's function is to act as a variable bleed of vacuum from the tranny vacuum control system... thus simulating and signaling to the tranny via the modulator what your throttle demand is. So don't chuck this expensive piece of plastic just yet.
Regards,

I was hoping you'd day that.

(1) I have a 1982 300D, Turbodiesel. It does not have the EGR valve connections anymore if you need to know that.

__________________
'81 MB 300SD, '82 MB 300D Turbo (sold/RIP), '04 Lincoln Town Car Ultimate

Sooner or later every car falls apart, ours does it later!
-German Narrator in a MB Promotion Film about the then brand new W123.

This means that originally:
(a) your chassis was a W123.133,
(b) your engine was a 617.952 I5, and
(c) and your model was as stated, a 1982 300D, Turbodiesel

This means that regardless of whether it is a Federal or California car, I believe your vacuum diagram can be found at: http://www.peterschmid.com/vacuum/1977_1985/617_95/1981_1984.jpg
Take a look at this and check around the engine compartment to see that the vacuum plumbing is according the diagram and in fact you have the same components that are shown. Yes, count your blessings you do NOT have a 1985 Calif. car!!! I note that you have somehow disabled the EGR... probably by disconnecting and plugging/capping off the lines. Unless your State has an active inspection program, I would leave the EGR disabled.

I see that you already have the mandatory hand vacuum pump.

So this is how I recommend you do a basic/easy check of your VCV.
(1) With the engine shut down and the throttle closed, hook the vacuum directly to the top of the VCV by disconnecting the green dashpot/surge damper [shown as #72 in diagram].
(2) Pump the hand vacuum pump to see if the VCV hold any vacuume at all. Report back to this THREAD.
Regards,

I did exactly as you said, I cannot get vacuum to hold at the VCV at all. What's the next step?

__________________
'81 MB 300SD, '82 MB 300D Turbo (sold/RIP), '04 Lincoln Town Car Ultimate

Sooner or later every car falls apart, ours does it later!
-German Narrator in a MB Promotion Film about the then brand new W123.

... relocate my Vacuum pump and check something before I advise further tomorrow.

Anyone else can feel free to jump in here IF you have any ideas!!
Regards,