I recently put a new LS7 motor in my '71 Chevelle. The TH400 that is in the car worked fine with the old low performance motor. It kicked down fine when I stomped on the gas. I realize that the low vacuum condition that is typical for a motor with a lumpy cam causes things like the brakes to be hard to operate at low speed conditions. Is there a way to adjust the vacuum feed into the tranny so that it will kick down when appropriate? Thanks, Carl

Are you sure it's not that the kickdown wire has come disconnected somehow?

Maybe? Where does the kickdown wire physically come from on the TH400 tranny and where does it hook up to?

There should be a slider switch above your gas pedal in the passenger compartment that activates when you floor it. A wire coming off of the switch goes thru the firewall (by itself) and connects to the driver side of the tranny to a spade lug which is in the general area of your shifter linkage (above and slightly forward, I think).

There are a couple of things that may be unique to your car, and maybe someone else can clarify the issues if they apply:
1) If you have the cowl induction flap, I believe it uses the same gas pedal switch
2) If you tranny was/is equipped with the TCS system, you will have a two spade lug connector on the tranny, one spade lug for the kickdown, and the other for the TCS. I'm not sure which is which.

OK, car sat for 5 years while I dealt with life. Got ready to move it and TH400 slips MASSIVELY. Pushed it to a shop in Feb 2007, had to rebuild tranny. fast forward: Aug 2007, picked up the car from the shop and it won't upshift unless I give it 3/4 throttle . . . not too popular a thing to do in residential streets. Took car back, shop said that in Feb 2007, it was raining so much that he wasn't able to test it. hmmmmmm.

He calls back in a day and says the most vacuum he can get off of the intake is five pounds (I think he meant inches). He said the cam was too lumpy I had to get either a belt-driven or electric vacuum pump. I didn't want a non-stock belt so he suggested the electric Summit P/N# SSB28146 Stainless Steel Brakes Electric Vacuum Pump. Here: http://store.summitracing.com/partdetail.asp?autofilter=1&part=SSB%2D28146&N=700+115&autoview=sku

What do you guys think?

You need a little vacuum for the modulator valve but not downshifting. That kickdown switch wire comes off the drivers side of the tranny and that is what downshifts the trans. Even if you don't have vacuum you should be able to shift manually by pulling it down into 1st gear and manually upshifting. I have had the modulator valve hose come off before and it would feel like it was slipping, then I would just manually shift it and it worked fine.

I'm sorry, my latest reply had a misleading title. It would not UPshift after tranny was rebuilt.

I'm sorry, my latest reply had a misleading title. It would not UPshift after tranny was rebuilt.

How high did you rev the engine to see if it would upshift? If power is always applied to kickdown solenoid, the trans will upshift, but at a very high RPM. Disconnect the kickdown wires from trans first. Does it upshift then? If not, could be the governer is sticking. There is a small cover on the side of the trans, remove it to get to governer. Governer has weights and springs (something like a mech adv for a dizzy) see if they move correctly.

I'm sorry, my latest reply had a misleading title. It would not UPshift after tranny was rebuilt.

A little misleading? Upshifting versus downshifting is more than a little misleading.

I think you should be looking at what kind of work can be expected from a shop that has the transmission from Feb to August and can't test their own work because it is raining.

How high did you rev the engine to see if it would upshift? If power is always applied to kickdown solenoid, the trans will upshift, but at a very high RPM. Disconnect the kickdown wires from trans first. Does it upshift then? If not, could be the governer is sticking. There is a small cover on the side of the trans, remove it to get to governer. Governer has weights and springs (something like a mech adv for a dizzy) see if they move correctly.


:beers: X2

I recently put a new LS7 motor in my '71 Chevelle.
Is this an LS7 new-style not-Chevrolet-design titanium-rod 427 engine that should be developing plenty of vacuum; or is this an LS-7 454 crate engine with the radical cam?