Turbo 350 no kickdown after changing carb

[l.neidlinger]

Hello all,

I have an 82 GMC c1500 with a 305/TH350 in it. After the third "professionally" rebuild quadrajet sucked a gasket I got a smoking good deal on a brand new Holley 4175. Probably a bit of overkill for a 305, but it runs very good and my pinging problems are gone now. However, now I have a problem with the trans.

The trans doesn't want to kickdown anymore, and it is short shifting. I checked the throttle movement, and it goes full throttle. I also checked the trans kickdown, at full throttle, I can't pull the cable out any further.

The only thing I can think of is that the vacuum modulator is receiving a different vacuum signal now causing the early shifts and no kickdown. Could an adjustable modulator fix this?

[l.neidlinger]

Thought I would add, the short shifting happens at WOT and hard acceleration only, otherwise, shift points are normal.

[LTZ C20]

Is the modulator hooked up to manifold or ported vacuum?

[l.neidlinger]

Is the modulator hooked up to manifold or ported vacuum?

It's tapped directly into the manifold.

[mcintyrederek]

I would start my double checking the vacuum line.  If it's rubber/vinyl towards the modulator end, I've seen them "suck shut" and cause it do to strange things.

[l.neidlinger]

Turns out there wasn't enough adjustment left and it wasn't pulling the kickdown cable far enough. I spaced it with a ziptie (heh) and it kicks down and winds out just as far as it use to.

[philo_beddoe]

Isn't there a range where the kickdown or "passing gear" as we used to call it, will work? In other words, if your going to fast it will not kick down? Mine kicks down up to about 30 mph i guess, wont kickdown at 40 mph.

[l.neidlinger]

Isn't there a range where the kickdown or "passing gear" as we used to call it, will work? In other words, if your going to fast it will not kick down? Mine kicks down up to about 30 mph i guess, wont kickdown at 40 mph.

Yes. This is where I noticed the problem. With the old carb, it would do a 3-1 kickdown up to about 35 MPH, but I couldn't even get it to kickdown to first at all after I swapped carbs. I guess the clip for the kickdown on the Holley is a bit further back than the Quadrajet, so I had to space the cable with a zip tie for now until I can properly space it.

[philo_beddoe]

Vile mentioned something about that on my truck, but i cant remember what he said. I'm not really sure how a kickdown works.  Where is this clip/cable?

[VileZambonie]

You have a detent solenoid which is activated by a switch on the accelerator. The TH350 uses a detent cable for that purpose.

[philo_beddoe]

Ok, thats that switch then just above the gas pedal.
Got it. Thanks.

[VileZambonie]

Yes the switch is adjustable

[philo_beddoe]

Ok,,,so what does adjusting it do? Do i need to adjust mine?

[VileZambonie]

Only if you want to change the downshift points

[philo_beddoe]

I do, it needs to downshift at higher speeds.