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Old 09-04-2004, 07:26 PM
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Join Date: Jul 2004
Location: Palm Springs, CA
Posts: 166
A/T modulator vacuum pressure and rpm's

I measured my vacuum at the hose just before the line to the A/T modulator(just after the surge damper and orifice). At idle the pressure reads 14"hg. As I increase the rmp's just slightly the vacuum decreases to 3." So, at only 1200rpm I have around 3" vacuum. As rpm's increase from 1200 to 4000 the vacuum remains at 3."

Does anyone know the proper linear change in vacuum in relation to change in rpm's. I understand that at full throttle there should be nearly zero vacuum, but more specifically how should the vacuum change all the way through the rpm range?

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Old 09-04-2004, 07:57 PM
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Zen And The Art Of Diesel
Join Date: Dec 2003
Location: Oklahoma City
Posts: 2,050
There is a spec to set it at a given throttle position, but other than that, I have never seen a curve graph of what it should be. I imagine there's so much variation in the system, vacuum and transmission, that it'd be about pointless. I'm also a member of the "I hate MB auto trans" club BTW...

One more Radar Lover gone...
1982 VW Caddy diesel 406K 1.9L AAZ
1994 E320 195K
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Old 09-05-2004, 01:09 AM
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Location: Palm Springs, CA
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I have read 10" to 20" at idle from the line to the modulator, and I have read zero or nearly zero and full throttle. What about everywhere else between idle and full throttle. I am getting clunky shifts and feel that I don't have enough vacuum with light to medium throttle accelleration. I have a feeling that my vacuum control valve is not working as it should, and that it should be decreasing vacuum to the modulator in a more gradual way as the throttle position increases.
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Old 09-05-2004, 03:43 AM
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Join Date: Apr 2004
Posts: 270
Very interesting! I was just about to embark on the same topic until I saw this thread.

Until a couple of days ago, my tranny shifts have been perfect, no harsh bangs, and no flair. Now all shifts are flairing except on a light gas pedal.

A couple of days ago, I repaired my Alda which returned my benefit to recieve turbo boost, and everything is fine except this flairing. I looked at all my vac hoses and all are fine.

Based on my knowledge about knowing zero vacuum will cause the tranny to shift with a bang, and flairing as a result of too much vacuum to the modulator.

So I'm inclined to find out if there is a setting on the modulator to adjust the levels of vacuum + or -.

I'll connect my vac guage from idol and run up the rpm's and see what I get. Seems odd though that if the vac drops at higher rpm's that it should flair, and with higher vac at lower or idol it should shift without banging?
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Old 09-05-2004, 06:33 PM
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Join Date: Apr 2004
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Follow-up on my earlier post here. I found the vacuum hose connecting from switchover to Alda disconnected this morning, hiding from view! Hooked it back up and all is back to normal.
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Old 09-05-2004, 11:48 PM
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Join Date: Oct 2003
Location: Denver Co.
Posts: 39
The curve should be pretty much a straight line with about 15 at idle down to about 0 at full throttle. full vac at idle is a function of one of the vacuum switches on the valve cover ( the lower one I believe) as you come off idle that switch closes and vacuum is controled by the vac control modulator on the back of the Injector pump. it sounds like your vacuum switch is working to give you full vac at idle. your vac. cont. mod. doesn't seem to be working correctly though. The vac. cont. mod. is adjustable if you remove the cap just forward of the lower vac line you can adjust it. only small adjments. you might also consider removing the vac. cont. mod. and taking it apart and cleaning it. having said all this quite alot of the vac. cont. mod.s are just plain bad. it's plastic with a spring in it and it's over 20 years old. there are a few articles available on adjusting the vac. cont. mod. has a good one and steve Brotherton did a really good artivale on adjusting it. do a search on it, there a few threads with links. Good Luck

82 300D
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