80L48

I have a 1980 L48 with an automatic transmission. I have had the car for 2 years. I have had allot of other challenges but am now down to one that has also been there since I acquired the car.

while driving, and letting off the gas, the transmission stays in 3rd gear until I tap the brake and then it releases.

I have the service manual and have tested the Converter lockup vacuum switch and it appears to be working correctly. However,I traced the vacuum line and it connects into the Headlight vacuum lines right after the check valve with 1 port going to the manifold and 2 ports (1 going to Air tank and the other going to the headlight switch. The smaller line going to the headlight switch has a "T" connector and this is where the Torque converter lockup switch is. This seems odd as I would think there would always be vacuum pulling on this while the engine is running (and the Air tank has vacuum for lights. )

My questions is: Should the vacuum line for the Torque Converter lock-up be connected directly to the manifold (so changes in vacuum can be detected)?

MelWff

The service manual you have doesnt cover the vacuum set up? Is this the factory shop manual?
Here's a link with the diagram. The vacuum to the switch is temperature controlled.
https://www.corvetteforum.com/forums...um-switch.html

80L48

Yes, gm service manual, only shows a vacuum line connecting to the torque converter vacuum delay and torque converter valve. Doesn't show other end

80L48

Thanks for the pic. Yes, I had that one in the manual. Unfortunately, (in this case) all the pollution control stuff has been removed by prior owner. Different carb and manifold as well. So I am just not sure where the vacuum line to torque converter lock up switch should originate.

TedH

I will check some of my diagrams from Dr Rebuild and the Shop Manual but I recall the 3-way port screwed into a port on the floor of the intake manifold (between carb and distributor) may be source for the vacuum.

Please advise if you can post pictures of the vacuum hose connections currently on your engine/vette. The challenge will be how well the PO removed the emissions and adapted the vacuum lines to compensate for the missing devices. I was very careful to study my vacuum lines/systems so that as I removed the sensors and emissions, the hoses still had the correct signals for the parts/systems that remained. But, I kept the Q-jet and purchased the Edelbrock Performer 2101 so it is very close in config to the OEM intake.

Thankfully, when I converted to 700R4, I no longer had the need for vacuum signal for the transmission as it is now driven by the TVS cable connected to the progressive linkage on the carburetor.

80L48

Thanks, I may try it and see if it makes a difference. If you get a pic, that would be great.

The headlight vacuum is now correct as dipicted in the will *** diagrams with the exception of this torque converter lockup. I have an edelbrEde performer carb, manifold, camshaft and fuel pump. There is a vacuum port as you describe with 2 ports. 1 goes down to transmission and the other goes to filter/check valve for headlights. The only other vacuum is from carb to brake booster.

Prior owner left ecm canister, but all vacuum lines are cut.

I​​​​​​I am going to test so tomorrow: disconnecting and plugging from headlight lines and try a carb port and also to manifold.

Thanks for the info

MelWff

Going back to the diagram it shows the vacuum source is from the carburetor ported connection at the thermal vacuum valve. So no vacuum at idle.

80L48

Thanks for info. Will try reconnecting to carb tomorrow.

80L48

Well, moved the vacuum line for the Torque converter lock-up directly to the ported Carb outlet (no vacuum at idle) and it works great now. I confirmed that the electrical switch was closed all the time with the engine running while in the old configuration, but now it opens when engine is at idle.

Thakns for the replies