DonaldSmith Posted May 27, 2015 Report Share Posted May 27, 2015 The other day, I made a diagnostic trip around the neighborhood, to see why my 47 DeSoto wouldn't downshift after its tiptoe upshift. Underhood diagnostics didn't show anything wrong. It turned out that I had to limp home, with no idle speed. The throttle crank would not stay on the shaft, and by the way, the fast idle cam would hang up. I removed the carburetor and worked with it on the bench. So much better to see and reach everything. I filed away the mating surfaces of the throttle crank, so that the screw could pinch it tight on its shaft. But the fast idle cam would keep hanging up. It's been maybe fifteen years that I had this car. I had added the automatic choke, and I must have fabricated a fast idle cam from heavy sheet metal. A strip of the metal wrapped the shaft at increasing distances, making the cam surface. The space between the strip and the shaft was filled with a crescent of sheet metal. I think the crescent was glued in place, and the glue gave out. The crescent would shift, jamming on the shaft. I got rid of the the crescent shim,and partially built up the space along the shaft with applications of of arc-weld gobs, and filed out the hole for the shaft. Then I JB Welded the space to a form the diameter of the shaft. It ain't pretty, but it stays true and pivots on its own weight. I'll road-trip the car tomorrow. I'll see if this makes a difference with the downshift. 1 Quote Link to comment Share on other sites More sharing options...
DonaldSmith Posted June 1, 2015 Author Report Share Posted June 1, 2015 Some comments lost with the site down, but did nothing new before today. Floor the accelerator and it doesn't downshift? It's supposed to. It upshifts when it gets to speed, and when I stop, it has downshifted properly. I went through the diagnostic checks in the Shop Manual, with a light bulb and test leads, but went through a period of confusion. For a while it seemed that everything was malfunctioning. Read the manual carefully, and follow it exactly, using a good bulb with stout leads. Connected between the kickdown circuit and ground, the bulb is supposed to be lit with the engine idling, and go out when engine speed is increased. OK, but for some reason only after the engine warms up. Pull the linkage to speed up the engine, and the light should go out. OK. Pull the accelerator linkage back (toward the right fender) till the light goes out, and push in the kickdown switch on the carburetor- the light goes on. OK so far. Pull the linkage all the way, and it should push the button and light the test light. NO? Shortened the throttle accelerator linkage; pull it all the way back and the light comes on! Test drive: Get the car up to speed to upshift, and depress the accelerator pedal all the way down. It downshifts! Wait, it doesn't!! Wait! I have to press the pedal to resistance, and then some, and the car downshifts. Good enough. Case closed. . Quote Link to comment Share on other sites More sharing options...
Niel Hoback Posted June 1, 2015 Report Share Posted June 1, 2015 You just need to shorten the linkage a touch more. Quote Link to comment Share on other sites More sharing options...
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