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Early shift to third?


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Hello everybody, I recently started having an issue where my 70 Monte started shifting into 3rd very prematurely. It’s a 402, th400 car, and I was driving it this past weekend both making some 1/4 mile WOT pulls and just regularly driving it. I noticed my shift from 1-2 occurs regularly, around 4200 rpm. Then when accelerating in second it’ll shift into third at roughly 3,000 rpm, the shift doesnt feel very crisp either. At part throttle cruising it will kick down with more throttle but shifts back up quickly.
 

Fluid level is full and doesn’t smell burned. If I take and manually shift the car the shifts are fine. My first thought was the vacuum modulator and I have ordered one, it’s going to be a bit of a hassle with my headers…. In the meantime has anyone had any comparable situations? If so what was the fix. Pic for attention. 

E391C9F9-1BFC-4F13-8747-60CAA7CAE184.jpeg

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Vac Modulator would be my first guess, then double check the timing. Both are complete guess on my part. Hopefully someone has some more pertinent suggestions 

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 If I am correct there is an adjustment screw in the center of the vacuum module that has an effect on shifting rpms. I trial and erred this on mine and got it set properly... mine was shifting too early... problem is, you can only get to the screw with the vac line off the module.. just a thought... 🤔 I only turned the screw 1/4 turn a try...

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Aftermarket vacuum modulators have an adjustment screw to help dial in shifting. Governor weights, gears, tire diameter all have an affect on the trans shifting. 

Have you changed anything lately? 

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The only thing I changed between driving it with normal shifting and since it started shifting early was changing out a set of spark plugs. The car has a valve guide issue on cylinder 7 that causes the spark plug to foul out every 8-10,000 miles. So I put new plugs in, same part number, same gap, same wire routing and I double checked they are routed to each cylinder correctly.
 

I did check underneath the car while it was on the hoist and the modulator (and a few other trivial trans parts) are leaking fluid, mine still has the original modulator that’s double the size of the aftermarket adjustable ones. The new one should be here Friday and if I get far enough on the wife’s patio project I’ll try to get the header loosened up and the new modulator in this weekend. A day prior to the shifting change I did swap from 275/60/15 MT ET radial pros to the 275/60/15 MT ET street SS’ and I believe the SS is a little taller but it drove 25 miles home just fine with them on it, and 1-2 hasn’t changed, just 2-3.

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The tires are out of the equation now. One down.  Lol. How did the trans shift before the possible problem? Has the trans been rebuilt to your knowledge? Possibly a spring on the governor broke.🤔

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I’ve owned the car for about 20,000 miles and it hasn’t skipped a beat transmission wise, until now. This is mostly single lane roads, 50-65 mph. I rarely do highway being a big block with 3.73s and a 28 inch tire, but this also includes 200-250 1/8-1/4 mile WOT pulls. I would guess that the trans hasn’t been out. Still had what was left of the original mount last year when I replaced it because it was torn apart. 
 

Prior to this it shifted nice and crisply, both shifts were good and quick, and very repeatable. Tonight or tomorrow I’m going to visually inspect the transmission lockdown switch on the throttle pedal just to make sure there’s not something glaringly obvious. 

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Hopefully you will find something obvious. I feel it may be governor related more than modulator. Do you have a "transmission guy" or someone does you know maybe. It may be time for a freshioning up. 

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Yeah I work at a Chevy dealership and we have a good transmission guy who has a lot of experience with these but he has been out with some family things so when he gets back I was gonna have him drive it. I got him on the phone the other day and sent him a picture of the modulator and he said start there. 
 

I just wish mine didnt have the factory modulator that hits the header before it can come out. Lol

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Just throwing this out there, remembered a day or two after the post. When I went to check out my Monte there was issue with shifting, don't remember if it was 1st to 2nd or 2nd to 3rd. PO took it to his mechanic and it was vacuum related. I only mentioned timing in my original reply because even tho I wasn't seeing an issue but when I had my mechanic friend work on mine, he redid the timing and it shifts much nicer now. Good Luck 

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