Shifting issue
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2nd Gear
Joined: Nov 2025
Posts: 2
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Shifting issue
I have a 1991 f-250 that I just bought and plan on building and I’m having issues with shifting, when I’m driving sometimes 1st gear to 2nd is harsh but that’s not much of a concern that only happens about 30 percent of the time, I’m concerned about when it goes from 2nd to 3rd, it’s super hard every time and at 30 to 50 mph it seem to not be able to pick between 2nd gear or 3rd and it just shifts up and down the whole way, does anyone have an idea about what might be the cause
Mountain Pass
Joined: Feb 2025
Posts: 120
Likes: 22
From: Marietta, GA
I have a 1991 f-250 that I just bought and plan on building and I’m having issues with shifting, when I’m driving sometimes 1st gear to 2nd is harsh but that’s not much of a concern that only happens about 30 percent of the time, I’m concerned about when it goes from 2nd to 3rd, it’s super hard every time and at 30 to 50 mph it seem to not be able to pick between 2nd gear or 3rd and it just shifts up and down the whole way, does anyone have an idea about what might be the cause
Is it a 3 speed auto? If so, it’s a C6 and they’re pretty durable and simple, and it’s all hydraulic/mechanical.
Cheapest and easiest first step is a service. Drop the pan, replace the fluid, filter, and pan gasket. If you haven’t done that before, get a big plastic tub from the concrete aisle at Home Depot to minimize the mess. There are 2 diameters of bolts holding the filter on, and one diameter has two different lengths so pay attention. The filter should also come with a new gasket.
Also change the vacuum modulator valve. It’s on the back passenger side of the trans. About $10 for the valve, one bolt, and 5 minutes. Check that vacuum line.
This will resolve a lot of issues with a C6. If it doesn’t fix your problems, one of these experts can advise on next steps.
Mountain Pass
Joined: Apr 2020
Posts: 241
Likes: 30
From: NC Mountains
I have a 1991 f-250 that I just bought and plan on building and I’m having issues with shifting, when I’m driving sometimes 1st gear to 2nd is harsh but that’s not much of a concern that only happens about 30 percent of the time, I’m concerned about when it goes from 2nd to 3rd, it’s super hard every time and at 30 to 50 mph it seem to not be able to pick between 2nd gear or 3rd and it just shifts up and down the whole way, does anyone have an idea about what might be the cause
Mountain Pass
Joined: Feb 2025
Posts: 120
Likes: 22
From: Marietta, GA
Bummer for 2 reasons. First is that it could be one of several electrical/electronic issues, but those you can check without dropping the trans. Second, if it’s the line pressure modulator, that’s internal. Cheap, but you have to drop the trans.
Check for codes first. I’d also still do fluid and filter if you don’t know for sure it’s had a service. If it ends up being internal you can save the fluid and reuse it.
Check for codes first. I’d also still do fluid and filter if you don’t know for sure it’s had a service. If it ends up being internal you can save the fluid and reuse it.
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