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The truck will up shift fine but I think it does not down shift correctly. When stopped, it will downshift to first no problem. The issue is while moving. Slow down for a left/right turn ( so about 15-20mph) it will stay in third gear and ping bad until reaching a higher speed. Same thing when climbing a hill with a load. I thick it is not down shifting correctly. I have not tried to see if mashing the go pedal will engage the passing gear.
Would the vacuum modulator on tge side of the trans cause this issue? Thanks
I'd 1st ck the carb to trans kick down lever is still connected at both ends. Then check the vacuum modulator and the line that goes to it. Check the line for kinks or other problems, make sure it hasn't come off. If the line checks out, take the line off where it connects to the modulator. If trans fluid leaks out, its bad. If no fluid, it still could be bad. They're simple to replace, disconnect the vacuum line, remove one bolt that holds the clamp in place and the modulator comes out. It has an o-ring seal. When you pull the modulator out, be careful. There is a pin that goes in the modulator. Sometimes it comes out with the modulator, sometimes it stays in the tranny. If it comes out, don't lose it.
Modulators are identified by the color of the stripe. Pretty much the "go to" one is the black stripe. If they don't show that one in the catalog, go with the green stripe. Avoid the pink stripe. That one is a dual diaphragm version that has two vacuum line going to it. If by chance you have one of these, replace it with one of the other two and use the vacuum line that goes to direct manifold vacuum, cap the other line.
If your C6 is original to your 79 you have some decent modulator choices, first confirm you actually have a kickdown lever. Check the vacuum hose that goes from the carb/intake to your modulator, I would just replace it, its a few bucks for a new hose. Really your looking at $40 for the hose and the modulator... Go from there ......
If your C6 is original to your 79 you have some decent modulator choices, first confirm you actually have a kickdown lever. Check the vacuum hose that goes from the carb/intake to your modulator, I would just replace it, its a few bucks for a new hose. Really your looking at $40 for the hose and the modulator... Go from there ......
Good advice ! In addition:
Pull the vacuum line off of the modulator. When you look inside you should see a small slot that a screw driver will fit into. Use your flat screw driver and turn it clockwise to raise the shifting points, and counterclockwise for earlier shift points. Don't turn it all the way in! Start with 1/4-1/2 circle turns, take it for a drive and see how you like it, and just fine tune it that way. Hope this helps.
Some modulators don't have the adjustment screw. If yours doesn't have it pull the modulator and make or buy a longer pin for behind the modulator. Longer pin = later shifts shorter pin = earlier shifts.
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