C6 not shifting down
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Elder User
Joined: Jun 2004
Posts: 747
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From: Pelahatchie, MS
C6 not shifting down
I have a 1986 F-150 4x4 with a 351W, a C6, and a NP208. The transmission shifts up through the gears fine when the truck is cold, cool, and slightly warm. Town is about 20 miles away. By the time I get to town, redlight to redlight, the thing stays in 3rd gear. It only shifts down when I really get on it, and sometimes is just stays in 3rd and bogs. I can shift manually, and it's really peppy. Drives great. But the second that shifter hits D, it goes to 3rd gear. The thing doesn't have but 60,000 miles on it. It sat up for a long time. I've changed the fluid and the filter. I have not changed the modulator, although I'm contemplating replacing it and the vacuum hose going to it. I'm wondering if the torque converter is causing this... One thing is, I'm not a transmission guy, and this is all foreign territory to me... Any ideas?
Senior User
Joined: Oct 2001
Posts: 258
Likes: 1
From: Central Virginia
You likely have a governor and/or shift valve sticking. That normally indicates there's trash in the fluid. Fluid dirty enough to stick valves tells me that there's stuff coming apart in there so it may be in need of a rebuild.
Cargo Master
Joined: May 2003
Posts: 2,561
Likes: 1
From: Earth
It would be cheaper to start with replacing the vacuum modulator, or even adjusting the current one. On my new rebuild, it wouldn't shift down after coming to a stop, it wanted to start off in second gear. I had to turn in the adjustment quite a bit and it finally started to run right. It took quite a bit of fine tuning and now it's fine. However, if you have quite a few miles on it, it could be what kenneth suggested. I would start with a new modulator and go from there, or try adjusting your current one first. If it works, great. If not, well, time to explore other possiblities.
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Elder User
Joined: Jun 2004
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From: Pelahatchie, MS
I'm gonna try the modulator first. It's getting it's vacuum from a little black distribution point on the firewall. I'm wondering if it would work any better to bypass that and get a new vacuum line going straight to the manifold, and use a new modulator. I'm all about trying the cheap route first.
Like I said, though, the fluid has been changed, and what was in there was old, but not in too bad of shape. The truck has 60,000 one owner original miles, no rollover....
It has no slippage, no funny sounds, no bad smell, the new fluid still looks good, and there was no debris in the pan when I changed the fluid and filter...
Like I said, though, the fluid has been changed, and what was in there was old, but not in too bad of shape. The truck has 60,000 one owner original miles, no rollover....
It has no slippage, no funny sounds, no bad smell, the new fluid still looks good, and there was no debris in the pan when I changed the fluid and filter...
Laughing Gas
Joined: Mar 2000
Posts: 1,191
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From: USA
Originally Posted by 53fatfndr
If I remember right, if you pull the vacuum line off of your modulator and there is fluid in it, then the modulator is shot.
Fluid coming out of modulator valve = junk valve
Cargo Master
Joined: Jun 2003
Posts: 2,982
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From: Somewhere Saskatchewan
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Elder User
Joined: Jun 2004
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From: Pelahatchie, MS
I've got to wait until the weekend to tinker with the modulator.
Is there no way around a rebuild if it's the governor or shift valves??
Is there no way around a rebuild if it's the governor or shift valves??
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