HT4100 4.1, 4.5, 4.9 Discussion, transmission bump on downshift in Cadillac Engine Technical Discussion; I'm looking for help from any of you transmission gurus. My 92 allante 4.5 thmF7 has a bump on downshift ...
I'm looking for help from any of you transmission gurus. My 92 allante 4.5 thmF7 has a bump on downshift to first. This happens at about 18mph when coasting to a slow stop. If doesn't happen when driving more aggressively. I have replaced the vacuum modulator and the vacuum line going to it, I've also relaplaced the isc and adjusted the throttle valve cable, and applied full vacuum to the modulator instead of going thru the restriction. Nothing helped. Can anyone offer some advice? thanks
First of all, if it occurs at 18mph, it's likely a downshift to 2nd, which is actually a common complaint with the F7 and THM440T-4 transmissions (I've had 3 cars with 440T-4's so far and you could feel that downshift on ALL of them).
However, does it seem like you're power braking your car when coming to a stop? Does the engine suddenly slow down when you reach about 2mph? If so, your TPS may be misadjusted and could cause the car to downshift under a greater than normal amount of power, and thus, you'd get the thump.
Automobile(s): 1988 Allante' (sold), 1984 Eldorado, 84 Sedan DeVille
Re: transmission bump on downshift
There was a repair bulletin issued around the 91/92 era regarding the TV cable binding when the engine reached normal temperature. The fix was to replace the TV cable. The black sheath around the cable would constrict warm and make the cable within bind. The cable is available and reasonably priced. Simple to replace and adjust. Remove the one bolt holding the cable to the trans, lift the cable straight up gently and unhook the wire to it sticking up from inside the trans (make sure you replace the o ring on the cable where it goes into the trans!) Another thing to do is remove the intake duct hose to the throttle body and clean the area surrounding the throttle plate, with the throttle plate held fully open. A spray can of brake clean and an old tooth brush will work. The residue that builds up around the throttle plate can alter the air speed into the engine and contribute to idle recovery issues when coasting down. I cleaned mine once a year and it made a difference! Restarts very roughly then once revved a couple of times, smoothes right out.