If the VCV won't hold 9" vacuum, with the modulator disconnected and the rack closed, there is no further testing or investigation to be done. It's got to be replaced.

... I have been distracted by several familymember's car problems that had accumulated while I was away in Italy and also have not found my MityVac unit in my over-stuffed garage. Time to do some winter-time house cleaning!

Thanks Brain for coming to the rescue. I WILL go my Independent MBz's garage here today and see IF I can refresh my memory about how we might further test these gold-plated plastic VCV(s).

I destinctly rembember bench testing several different used ones including one that was properly working and it seems like I remember being able to get a good one to hold vacuum as Brian indicates so long as the throttle linkage is in the closed position and then depending upon how much you open the throttle linkage, the vacuum would drop slowly IF you opened the linkage just a little or more quickly IF you opened the linkage wider. I did NOT find it necessary to get into the internal adjustment of these, however I do know that some VCV(s) are NOT adjustible and some are.

So by all means test yours as Brian has indicated which I thinks is with the engine running at idle. And I'll do my best to refresh my memory here today at my Indy friend's shop by also testing my 1980 240D's VCV using a vacuum pump and the engine off and without taking the VCV off the IP.

Regards,

The test is done with the Mityvac without the engine running. The Mityvac provides the vacuum to the VCV.

... with the engine shut down and the throttle minimum [not cranked up at the dash either] I found that I could pump it up to the pump's usual 15 to 25 but it would quickly drop and eventually go to zero. So in order to get a quantitative measure I timed the drop from 10-in to 5-in and this time was 25 seconds.

This is entirely consistent with what my understanding of the function of the VCV and that is to trigger/regulate a variable vacuum signal to the tranny modulator that simulates throttle position [ demand/load ]. The time it takes for the VCV to bleed down my static vacuum from the pump or to act as a bleed to the dynamic vacuum that is fed by the engine's vacuum control system [through multiple restricted orifices] is no doubt adjustible, at least on those VCV(s) that are designed to be adjustible.

My engineering sense of the above is why I initially chose to start this THREAD and also why I selected the title as I did, for I saw that the VCV might very well NOT be adequately adjustible IF it were fed too strong a vacuum... thus the need to restict the vacuum using the various sizes of small multiple plastic orifices and in the 1981 and later models, I'm now convinced that the inlets and outlets of the small dashpot chambers very likely also serve as restrictions. I am more than ever convinced that the balance of ALL the restricted orifices is IMPORTANT if not CRITICAL to being able to properly set up [" balance "] these vintage engine/tranny vacuum control systems AND certainly CRITICAL to maintaining a reserve of vacuum strong enough to operate the brakes IF there is a rupture somewhere in these less-than-critical vacuum using/consuming systems.
Regards,

... to get around to performing the test I promised and that I POSTed just prior to this.

It might also be interesting to remove the VCV from the IP and with the throttle linkage shut off fully... then to see what the vacuum bleed performance might be. Again on adjustible VCV(s), I think we should expect differing results. IF you have an unadjustible VCV, I suggest that balancing the vacuum being fed to the VCV by using the proper size restricted orifice is even more CRITICAL just because you canNOT fine-tune the system as you can IF the VCV is adjustible.
Regards,

My suspicion is that this performance is not per design. The VCV will typically bleed off vacuum to a setpoint that is variable depending on rack position. Once this point is reached, the vacuum will remain at this level indefinitely if there are no leaks in the system.

With the results you have observed, I'd guess that the VCV is not completely sealed internally and is bleeding vacuum to the vent. While the level of the leak is not significant in this case, I don't believe that it's by design.

... then both of our 1980 MBZ(s) [240D & 300D] VCV(s) have the same kind of defect despite the fact that these cars' transmissions are function quite well. As a mater of fact the 300D just came back from the tranny rebuilders where a leak was repaired under warranty AND while they had the car they made it a point that they also tweaked the associated engine/tranny vacuum control system adjustments for they find the installing field mechanics often do not know how to do this very well. They made no mention of a defective VCV.

If while working with my independent MBz mechanic here in town I find that these vacuum bleed characteristics I have reported earlier are not normal for a VCV, I'll be the first one to announce this to those who read this THREAD.
Regards,

Since my experience is with the later 617 engines and my comments refer to the performance of the SD, it's entirely possible that the VCV on the earlier vehicles was of a different design and construction and permitted a very low level continuous bleed. If both vehicles are performing in an identical manner, the credibility of this theory is reinforced.

We need more data from others that have the older 617 and 616 engines for final confirmation.

I went through this week and replaced the rubber cap and the rubber hose connecting the vacuum line to the VCV. The modulator now holds vacuum over a very long period of time (went down 1mmHg after 10 minutes).


Now connecting everything back up the system doesn't hold vacuum (rack closed). I am using the MityVac testing right at the point where the vacuum that goes to the transmission comes off the main line. Plugging the output from the dashpot to the VCV with a golf tee holds vacuum fairly well going down 1mmHg every 30 seconds. So the VCV is leaking.

I read up top that with the rack closed the VCV should hold vacuum or be replaced. Are there any adjustments, gaskets etc. to check before I invest in a new one of these (or go junkyard roaming)? Ill bring in and verify its operation on a bench as well before calling it toast.

this is now problem #4 in my tranny vac system debug extravganza up till now theyve been cheap, i may not be so lucky here...

thanks -
dd

__________________
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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

With the Mityvac connected to the supply vacuum on the T (opposite end of the T goes to the transmission and the bull goes to the VCV), how fast does the vacuum drop?

Can you pump it up and will it hold any vacuum?

Does this VCV had a round plastic cover facing you that can be removed...........with an adjustment screw beneath?

DieselDan44,
Unless I have missed it, you have NOT removed the VCV from the engine/IP and bench tested it all by itself and this is a relatively easy task. If you do this and you see leak-down results that indicate it is leaking vacuum much faster than I reported my 240D's VCV is leaking, then I would agree to the notion that you might have to replace the VCV... but not until then. AND IF you do have to replace your VCV, at ~$200 new, I would recommend that you try and find a used salvage yard VCV that holds vacuum reasonably well and is adjustable. I think that $30-$50 for such would be possible.
IF you decide to remove the VCV and bench test it, let me know and I'll pass on my suggestions.
Yes, these engine/tranny vacuum problems can be frustrating but they are NOT insurmountable... and yes for that plastic VCV to cost $200 might seem a bit of a rip-off, but price it at a Dealer's parts counter, I suspect it's noticeably higher there!
Note - That misadjustment of the idle on your IP could cause the VCV to bleed excessive vacuum... thus my current recommendation to remove the VCV from the car!
Regards,

A Mityvac, connected to the VCV with the engine not running, is just about as good a bench test as you'll get. The only difference is that the VCV remains mounted on the vehicle.

... you can examine it more closely and see if it is the adjustable type... OR you could just throw $200 at your problem and hope it works!
Regards,

Sure......but, you can also verify this on the vehicle........that plastic cap will be looking right at you.......

It falls almost instantly, i can pump the MV to 10 mmHg and it falls to 0 within a second.

It does have a round plastic cover that looks as it *may* be removable, but I can't see how this could be done. It does have two screws which holds the side on. I have an 82 300DT, which I would think would be the same VCV as an 84 SD. The part number on it is 1230700046.

Just did this. Took me a minute to figure out how to remove the throttle linkage. Now it's staring at me on the table here.

I'm assuming that testing it is as easy as closing the input from the rack all the way, and punping it up from where the dashpot would usually go. No connection required to the other port on the VCV , since it's just for bleeding vacuum. Initial test results indicate I have a bad VCV.

Sam, I'd be interested in your bench testing notes to verify.

all made possible by you guys, thanks again. Im having a lot of fun diagnosing this one. It's relaxing only because Im not depending on the car...

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