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  #1  
Old 12-13-2010, 10:44 PM
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Location: Princeton, NJ
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vcv trouble 1985 300D

In reviewing the basics about possible 2-3 trans flair causes, I found 18-20"hg at idle in the vacuum amplifier to trans modulator line. Also found this line going to the modulator won't hold vac, and instantly bleeds down though I don't have overly harsh shift issues so not my biggest worry right now. The 18-20"hg in this line with the motor running is unchanging regardless of trottle position. So, I gathered the cause is a bad VCV. To test, I removed the supply vac connection to the VCV and connected my hand vac, pulled 20"hg and actuated the throttle and observed no change in the vac on the hand vac gauge.....it should bleed off the vac when you actuate the throttle correct? Double checked that the vent line running to the firewall was free and unblocked and all ok there so I feel ready to blaim the VCV.

So, what do I do? In my searches I find the VCV is NLA. What actually fails with these things? Can they be repaired? I notice the adjusting nut and the rod the nut threads to feel rather wiggly and loose (not that the nut is actually loose, just that the entire rod seems like maybe the bearings it turns on are shot). Anyone have success finding replacement VCV's or rebuilding them?

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1985 300D 231K anthracite grey w/black int.
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  #2  
Old 12-13-2010, 11:24 PM
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Does sound like there's a problem with the VCV. The vacuum should bleed down as the rack is opened. Have you hooked up the MityVac and run it out onto the cowl and taken readings as you drive around? I'm pretty sure I remember a thread on here where someone opened one up and tried to fix it.
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1977 300d 70k--sold 08
1985 300TD 185k+
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1985 409d 65k--sold 06
1984 300SD 315k--daughter's car
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  #3  
Old 12-14-2010, 07:36 AM
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You can still find some in the junk yards. The only time i've seen the VCV not releasing vacuum is when its overly adjusted. You can try blowing thru it, maybe the piston just got stuck.
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Old 12-14-2010, 10:04 AM
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Location: Princeton, NJ
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sounds like I'll have to pull the vcv and see if I can figure out what makes it tick.

About connecting a vac gauge to the bleed off line that runs to the the firewall and then driving around: Won't connecting a gauge here block the system's ability to bleed off vacuum? And so, won't you then read an unchanging vac? With the gauge in place aren't you essentially blocking off bleed line?
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  #5  
Old 12-15-2010, 12:19 PM
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Location: Princeton, NJ
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pulled the vcv last night. Ended up being an easy fix, that I could have just done with the valve in place on the IP, though pulling it took all of 2 minutes (1min. 30sec. of which were trying to figure out how to unclip the pushrod from the vcv).

Anyway, cool little valve that vcv. When adjust right it is a very precision vac regulator. Mine was not dropping the vac signal at all except just at full throttle, and even only down to about 11"hg. By removing the plastic cap on the side and exposing the nut with two flats on it, I found you just turn the nut to change the range of motion required to activate the vac regulation. Turning CCW makes the valve dump vac sooner into the rotation of the lever (i.e. it will start dropping vac level sent to the trans with less and less throttle movement as you turn the nut CCW). Setting mine to the vac just drops to zero when the lever hits the stop resulted in vac at idle of about 11"hg and vac a 1/2 throttle of about 5"hg which seems to be about where you want it based upon other posts I've seen.

With these settings I find my 2-3 flare is now gone. It was intermittent before and mostly occurred during warm up so I'll drive a few more days before saying it's fixed for good......but so far all indications look positive. The other shifts also feel firmer with a shorter shift duration especially noticeable as heavy throttle (make sense I guess as at heavy throttle I'm now near 0"hg whereas I had previously been at 18"hg since the VCV wasn't adjusted)

Amazing to see first hand how big an impact vacuum has on these trannies

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