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Join Date: Aug 2008
Location: Bulverde Tx
Posts: 118
Garage
86 trans issue, low vacuum

Hello Folks,

It's been a while since I posted anything. I have 1986 928S, I think there is a picture in the Gallery. She has about 111k miles and I have had her for about 5 years. My problem is she has decided not to shift out of 1st, but if you run it up to about 6000 RPM, SHE WILL SHIFT INTO 2ND. No way to run a Rail Road, gets mighty scary fast. I had this problem before and it was a bad vacuum line from the Manifold to the trans modulator. I checked this line and it is good, using my mini vac I determined there is no leak in this line. Using my vacuum gauge, I only get about 5# of vacuum at the modulator at idle, virtually no change in vacuum with increased idle. I have tested the modulator with my mini vac, she begins to open at 7# and is fully open at 12#. My problem is not enough vacuum? I checked the vacuum at the intake manifold, the port that feeds the power brake booster. 18# at idle and 22# @ about 3000 RPM. I have checked all the lines at the Spider Junction, all are attached. Every time I reattach the MAF, air breather I was pulling vacuum lines off. The lines under the manifolds are impossible to get to, so I might have a leak there. I might have to remove the intake manifold, looks nasty. Any idea would greatly be appreciated. Do the limits on the Vacuum Modulator sound correct. 7# BEGGING TO OPEN, 12# FULLY OPEN?

Old 09-15-2020, 09:07 AM
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Join Date: Aug 2008
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I forgot to mention in ,my post. By using a long piece of vacuum house, with thew car on floor jacks. I can use the mini vac to activate the module and the car will shift. its shifting soft, but that I can be adjusted with the adjusting lever on the modual
Old 09-15-2020, 12:28 PM
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Join Date: Mar 2010
Location: Texas
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p930man,
With that same Mityvac, and since the transmission vacuum is connected via the 3/4/5/6 way vacuum connector back by the brake booster, you can check the other vacuum lines to see which line is bleeding off your necessary vacuum.

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Grand Prix White 1994 GTS AT (The GTS)
Black 1989 GT (The GT) and Cobalt Blue 1989 S4 AT (The Blue Car)
Indishrot 1984 Euro S AT (The Stepson) and Black Metallic 1984 Euro S 5 speed (The Schwartz)
Old 09-16-2020, 08:08 AM
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