I have a 1978 F-150 with a 400 and C6 Transmission. I rebuilt the engine about 12000 miles ago (Fall of 1999). The engine is great. I used a bunch of information from this website to build a real stump-puller. At the time, I replaced the transmission with a rebuilt unit that was supposed to be set up for towing. I use my truck to tow a boat almost all the time. In fact it is always hooked up to my work boat. I would guess that at least 9000 of those miles were towing either my boat or travel trailer. I use other vehicles to do any running around. It has an additional B&M cooler which is hooked up inline with the cooler in the oversize radiator. I have noticed the last few trips when I was hauling my boat that the transmission seems to be be slipping on its shift from 2 - 3. It seems to shift kind of slowly and allow the engine to build up some extra RPM before it makes the shift. Sometimes it helps if I let up off the gas for a moment. It generally makes this crazy long shift when I am going up a slight grade and am accelerating to about 45mph. I have not noticed any problems with it shifting into any other gears including reverse. I checked the fluid level. I disconnected the vacuum line that goes to the vacuum modulator on the transmision and sucked a vacuum on it with my vacuum pump and it holds. I am going to contact the rebuilder tommorow but was hoping to hear some more words of wisdom from the group. If you have any additional checks, please let me know.

Thanks,
MasterMarine

 

You may want to ask this question in the [link:www.ford-trucks.com/cgi-bin/dcforum/dcboard.cgi?az=list&forum=DCForumID27&conf=misc|Tr ansmission, Differential, Axle and Transfer Case] Forum.

 

I have had my 79 F-100 for 18 years. I have had the trans rebuilt 3 times. 4 years ago I rebuilt it and put the B&M transpak kit in it and have no more problems. I used the RV setting.

 

Mine was doing that and the modulator valve was the culprit. maybe not enough vacuum getting to the modulator?

 

I contacted the place that built my transmission and they had me check the kick-down lever. It is operating correctly. Next he asked me to put a vacuum gauge on the line under the truck that runs the modulator. My reading at idle was around 12 inches of Mercury. He said that was not enough and that I must have a vacuum leak. I replaced all of the vacuum lines except the one to the power brake booster. Then I used some needle nose Vise-Grip pliers to crimp off each line and help me to isolate my leak. I adjusted the idle a bit but could not get more than 14 inches. I did find that the timed vacuum port on the back of the carburetor had a cap on it that was wasted and was a serious leak (at least when the throttle plates were open). Now that I have plugged it, it seems to shift much better most of the time but it still slips a bit going from 2 - 3 when climbing a grade and accelerating with the boat in tow.

What should my vacuum be at idle? I tried to search for some kind of article about tuning with a vacuum gauge but found nothing.

Also, don't forget to take the Vise-Grips off of the brake booster line before taking a test drive. The truck is real hard to stop.

Thanks,
MasterMarine

 

21 inches used to be what we looked for in the "old days" pre emissions. It kind of depends on your engine build, but if you built a real stump puller I would say at least 20". A big cam can reduce your vacuum. Does your cam make the idle "lopey"?

I am glad somebody else does things like that:-) I was tired the other day and thought I had picked every thing up and promptly ran over my 6 month old (ex)creeper when I went for a test drive.

 

Mine has done that when it is hot. A tranny guy told me it is in the governer.