Trans Issue- late shift

[Subzero]

I drove the F100 to a local car show today and went I pulled from the neighborhood onto the 2 lane (35 mph) as I sped up to get from 0-35 the trans refused to shift to a higher gear out when it hit 30 and wouldnt speed up. I backed off the gas to roughly 25 and accelerated again and it shifted when it hit 30 mph like usual. This stoped after it warmed up. The shifting may have been slightly more hard for the rest of the drive but not very noticeable.I heard that this happened one time before ( about 1 week ago- also the last time its been driven) but I wasn't in the car. The truck when shifting into reverse somestimes makes a "clunk" noise but all drive gears have been fine previously. I checked fluid and it was full. What could be causing this? It seems my truck is breaking down more now recently

PS: truck has around 142,000 miles on it

[colnago]

I'm having some engine issues, which I think is creating problems with the C6 tranny. In a nutshell, it idles rough (can't seem to completely smooth it out). Not sure what's causing the rough idle, but I think it's also affecting my vacuum. It bounces around 12" at idle. The tranny shifts hard and late if the engine is too cold or too hot, but after five minutes of driving, the tranny is fine. You might warm the engine up then check your vacuum.

By the way, my odometer has 110K on it, although I don't think I have the original engine (so I really have no idea how many miles are on the engine). The tranny is definitely not original to the truck (originally came with a Ford-o-Matic (or Cruise-o-Matic, whichever was the later version)).

Joseph

[Subzero]

Thanks Joseph, I think that could also be my issue. I have yet to change out the points and the engine does idle roughly. I'll try and change them out this weekend and see if the trans gets back to normal.

[Donnie]

Sounds like a vacuum problem............when you let up on the throttle your modulator got a strong vacuum signal & allowed the governor to signal a shift........check your modulator & ALL vacuum lines...How much vacuum do you have at idle?.............Donnie

[Subzero]

I'm not sure as I don't have a way to really tell the.
vacuum pressure. But I do know the truck has never done this till now so could a leak just develope like this fast? I do remember a slightly harder shift into that gear before but never anything like this. I probably won't get around to the points this weekend but I'll sure do them this week. I haven't been driving the 68' much since my master cyl has a leak. Thanks for the tip

[colnago]

You should be able to borrow a vacuum gauge from AutoZone or O'Reilly's. If you have to buy one, they're only around 20 bucks. If this isn't an option, then check all of your vacuum lines. PCV, brake booster, distributor, auto trans, anything else. Take a can of starter fluid or carb cleaner (I prefer the starter fluid), and spray the vacuum lines. Also spray around the carb base gasket. Listen for the engine to rev or stumble. If it does, you found a leak. If you can, cap off the port going to the brake booster, because you might also have a tear in the booster diaphragm. These tests won't tell you what your vacuum level is, but it might find where a vacuum leak is. I'm thinking that you might have an old vacuum line that cracked, and now you're leaking.

Keep us posted,

Joseph

[Subzero]

Issue hasn't returned after those two incidents. I'll post up if it happenes again.