1970 C10 Trans Issue after distributor upgrade

[rmawyer]

375 turbo hydromatic transmission acting strange after doing some engine work (upgraded to DUI HEI distributor and replaced spark plug wires). Transmission prior had smooth shifts at appropriate speeds but now shifts from 1st to 2nd at 45 mph / approx. 2500 tach and from 2nd to 3rd at 60 mph / approx. 3200 tach and shifts hard. Assuming something was missed or not put back properly during engine work. Any ideas on what to look for? Thanks for any help.

[dmjlambert]

Did you disturb or break loose the vacuum line that goes to the transmission? It's back there by the distributor. If you have a metal vacuum line check each end which probably has rubber hoses.

[rmawyer]

Originally Posted by dmjlambert

Did you disturb or break loose the vacuum line that goes to the transmission? It's back there by the distributor. If you have a metal vacuum line check each end which probably has rubber hoses.

I'll check when I get back to the truck Friday. What is the transmission vacuum line supposed to be connected to (distributor, carb, other)? Thanks.

[dmjlambert]

You probably have a vacuum "tree" fitting on the intake manifold behind the carburetor. It connects there. It uses a short piece of rubber hose to connect to a metal line, and that metal line goes down to the side the transmission, where another short rubber hose connects to the transmission. I'm going from memory here, and I'm sure others will chime in if I have a detail wrong.

[rmawyer]

Thanks for sharing your knowledge. I assume something was buggered up during the upgrades.

[Steeveedee]

Originally Posted by dmjlambert

You probably have a vacuum "tree" fitting on the intake manifold behind the carburetor. It connects there. It uses a short piece of rubber hose to connect to a metal line, and that metal line goes down to the side the transmission, where another short rubber hose connects to the transmission. I'm going from memory here, and I'm sure others will chime in if I have a detail wrong.

I'm betting that the line got bumped and is off the modulator, assuming that the guys doing the work would have caught it if it was off at the manifold. It could just coincidentally have a failed modulator, but given the timing, I'm betting that you are correct, and the line is disconnected.

[rmawyer]

You were right. The transmission to intake vacuum line was inadvertently removed during the distributor upgrade removal / install. The hose end was laying on top of the intake stud, so it looked like it was connected but was found to be loose when I checked the fit. After reconnecting the line everything is resolved. Again thanks for the pointers!

[BigBird05]

Most modern mechanics wouldn't even know that that transmission had a vacuum modulator. But why didn't they catch the shift problem? Did they replace the wire that powers up the HEI distributor? Should bed a 14 gauge that should run from the bulk head connector to the distributor and the yellow wire that runs from the starter to the coil should have been removed ( by a good shop ) or at least taped up so it doesn't ground during engine cranking. It the little details that make the difference between replacement shop and a good shop.

[rmawyer]

Work done by a buddy with decades of experience. We finished late in the day and I did a short test drive around his neighborhood before heading home but did not get up to speed until I was on higher speed roads later when driving home. I called my friend and explained the issue, and to his credit he said it sounded like the transmission modulator or line. Since I could not get the truck back to my friend for a couple of days due to a funeral I used that time to contact my friends on the forum to get their opinions and of course they corroborated my mechanic's diagnosis. Just covering bases - thanks.