'75 Eldorado Transmission Vacuum Modulator

[benji808]

Rebuilt my carburetor this weekend, and when I got it back in the car, she started up and ran fine (ok, there was a fuel line issue I had to creatively resolve) but would not shift out of first gear. I did some research and experimented further, and discovered that she would shift, but I had to pretty much floor the accelerator. More research and sounds like the transmission vacuum modulator.

Since I had the carb out earlier, I first looked at that connection. It was loose, so I hammered the nipple in, and checked the steel line going down to the modulator. The steel line holds vacuum, and I have double-checked that the connection at the carburetor seems good.

Still not shifting right.

What are next steps? Below is what I am considering:
1) Triple-check the above things
2) MAYBE put some gasket maker around the nipple going into the carb...really don't want to do that, but would help narrow possibilities
3) Replace actual vacuum modulator at trans.

Any other ideas or things I can do to troubleshoot? I have noticed zero transmission issues prior to today.

Thank you!

[Dave Shepherd]

Put a vacuum gage on the hose down at the modulator, you should have manifold  vacuum there.

[TJ Hopland]

Since you were messing with the carb make sure you didn't do something that is keeping the electric kickdown activated all the time.  The kickdown is the 2 wire switch that sort of looks like a throttle position sensor that interacts with the throttle shaft on the cable/linkage side.  Its only supposed to be activated at WOT but they can be easy to damage especially at this age.  Circuit is activated when the switch sends +12v down to the trans.  Easy to test just unplug that 2 wire connector.  No connection = no activation so it should shift normally.

On the vacuum circuit no vacuum getting to the trans makes it think you are also WOT so it will firm up and delay the shifting so checking that was also a good idea.

[cadillacmike68]

I'm having a similar problem in my 1968. It won't shift into 2nd until I get to about 30mph and then into 3rd at about 60mph, and it's a harsh shift. It recently had a lot of climate control work, so there might be some misplaced vacuum line(s).

[Dave Shepherd]

You need full manifold vacuum to the trans modulator.

[35-709]

Also check that you do not have transmission fluid in the vacuum line at the transmission, if you do, the vacuum modulator is no good.

[benji808]

Ok, follow-up question and test results:

1) Under what conditions should I be getting manifold vacuum out of that port on the rear of the carburetor? Because I can't get my gauge to read anything at idle, warm idle, or in gear even at the carb. Shouldn't this be "always on"? I checked another port on the carb (the one that goes to the choke) and it was reading around 18. I'm a vacuum newbie, so any help here is appreciated...possible I'm doing something wrong by my connections seemed solid.

2) I used a pump to test pressure at the modulator. Seems to hold pressure fine. There was some liquid junk down there but not an appreciable amount, so I'm guessing it's just residue from various sources (may check on that again to confirm).

3) TJ - I tried unplugging the kickdown...no change.

[benji808]

Well...the issue seems to be that the carb is not pulling vacuum through that port. I plugged the modulator line into a manifold vacuum source and it's shifting fine...guess I screwed something up in that rebuild...maybe put a gasket on backwards?? LOL...will have to pull it and take a look later this week. I learned a lot through this thread - thank you all!

[TJ Hopland]

Yep you are supposed to have manifold vac there which is all the time.   Yep could be the wrong gasket or maybe flipped the wrong way.  Its been a while since I did a Q jet so I don't remember what the options for going wrong were but I do remember they include a few different gaskets that cover different models so its possible you got the wrong one that just had some slight difference but still seemed to fit alright. 

[klinebau]

You said that you "hammered" the vacuum modulator nipple into the carb.  This really only needs to be lightly tapped in.  Perhaps you damaged that nipple a bit and it is no longer pulling vacuum from that port.

[TJ Hopland]

Ya Kurt makes a good point.  You may have pushed that nipple in so far its basically sealed against the opposite wall.

[benji808]

Turns out the carb rebuild guy sent me the wrong gasket - one that did not have the hole to route vacuum to the modulator. Fairly simple to pull the carb, remove the baseplate, drill the hole out and reinstall. Shifting great now!
Thank you all for the comments - I learned a lot, even if it was an inane issue.

[TJ Hopland]

I forgot we were talking about a 75.  Does yours have the 6 jets? Rather than the usual 4?  One of the extra is the bellows one and the other is a single rod kinda like the main rods?   If so I can see how that mistake was made.  That 6 jet was a rare carb and historically they included its unique gaskets in all the kits even thought 99% of the people never needed em.   Maybe they changed that recently?   I may still have a dozen of them in a junk drawer.  I never knew what they fit till I did the one on my 75. 

Side note my 75 with that wacky carb would get me almost 20 mpg under optimum conditions.  My 73 and 78 would never get close to that unless it was all downhill.   Had to be that carb.