1988 TH 400 Kickdown Problem

[johnnyfever]

I bought a 1988 Chevy 30 RV with a TH 400 transmission and a 350 engine, and I'm having a problem on the highway. The trans fluid is at the proper level and appears healthy. It shifts great and seems to be working fine until I run it on the highway for a bit. At about 65 miles per hour it will do a slow downshift into second gear with no change to the accelerator pedal. If throttle is further applied, it eventually will shift back into 3rd gear. The downshift doesn't happen at WOT, just a steady speed on a flat road. The person I bought the RV from put a new carburetor on it, and I'm trying to figure out the kickdown control. I'm trying to sell the RV and would like to repair this problem, not pass it on to the new owner. Any input or ideas will be greatly appreciated!

 

[Erik the Awful]

You should have a vacuum modulator and a kickdown switch. You can run without the kickdown switch if you have enough torque versus weight, but with a 350 in an RV, you're never not going to need it. It's simply a microswitch that the throttle arm hits at full throttle.

Your issue might be your vacuum modulator leaking vacuum. As the vacuum bleeds off the transmission "thinks" you're pushing harder on the throttle, as if you're driving uphill, so it shifts down. Then again, I don't have a chart in front of me, so I'm kinda spit-balling.

 

[L31MaxExpress]

You should have a vacuum modulator and a kickdown switch. You can run without the kickdown switch if you have enough torque versus weight, but with a 350 in an RV, you're never not going to need it. It's simply a microswitch that the throttle arm hits at full throttle.

Your issue might be your vacuum modulator leaking vacuum. As the vacuum bleeds off the transmission "thinks" you're pushing harder on the throttle, as if you're driving uphill, so it shifts down. Then again, I don't have a chart in front of me, so I'm kinda spit-balling.

If it is a 1988, should be TBI and the ECM controls the kickdown through a relay on the firewall via TPS and MPH signal.

Agreed on the modulator, those short rubber hoses between the intake manifold and hard line and modulator and hard line both like to split and leak as well.

 

[johnnyfever]

Thanks for the input, guys, I'm going to put it up on ramps today and start looking around, I'll let you know what I find, Thanks again!

 

[Schurkey]

The person I bought the RV from put a new carburetor on it

If it is a 1988, should be TBI and the ECM controls the kickdown through a relay on the firewall via TPS and MPH signal.

"Carburetor" is a very scary word in this situation.

Is this REALLY a carb, or do you mean TBI unit? Is there a throttle position sensor, and is there a vacuum advance on the distributor?