1979 F150 4 x 4 with C6 transmission------Had the transmission shop rebuild the transmission. Now when I am driving it if I leave from a dead stop real slow it is in 3rd gear right from the get go. If I give it some throttle it will usually drop down to first and then shift normally. This does not happen all the time but probably does it about 25% of the time. I can set at a stoplight for a minute or so and then take off and it is in third gear right from the start. They rebuilt the tranny with an RV shift kit and a new torque converter. I do not have a kickdown rod on it yet but from my knowledge the kickdown rod only comes into effect when you are driving at speed and give it the gas, the rod will make the tranny kickdown to a lower gear. I am going to take it back to the rebuilder but I want to have an idea whats going on before I talk to them. Is the valve body causing the problem or ?????? Thanks for the help.

 

Check to make sure your vacuum line is connected well and not leaking. Possible that it's not seeing vacuum at idle and therefore not down shifting. Kickdown linkage is as you stated, so not the problem.

 

Sure sounds like a sticking valve in the VB. Increase in pressure when throttling it pushes the 2-3 shift valve back.
i would be pulling the VB if I had built that unit.

 

This is common in c6 trannys what it is from is usually the modulator valve (thingy that the vacuum line hooks to) is too long, too tight, or incorrectly configured in a way which boosts line pressure enough that the governor Is stuck over or open, thus making the truck think that when stopped you are traveling at 25 mph under high vacuum and we need high gear. the governor spring is not strong enough to overcome heavy line pressure. The tranny shop might have test drove it and maybe didn't start off easily enough to notice. The kickdown could be adjusted to help but I would start with modulator, back screw out and/or remove (park down hill to avoid spilling fluid) the modulator in order to measure rod, and DO NOT DISCARD ROD, use a round stock, like a nail, or a long bolt with no threads on shank, or a wire coat hanger, or a bail from a rat trap, and build a shorter rod, like 0.020-0.050" shorter, and see how it responds. A sticky valve body would be after the modulator and after the governor, Another easy test is unhook the vacuum line altogether and go drive it, you should run 1st gear all the way out even under part throttle, and get a pretty harsh shift. Cap or put a screw in the line to not leak vacuum. Good luck.

 

I would get as smart as you can on possible problems, but DO NOT touch, adjust or work on it at all. Doing anything other than returning it could and probably will void ANY warranty you got from the shop.

I hope you got a warranty and if you did you have not went over the X amount of days or mileage. I would of thought that the shop would of put a kick down rod on it.

Fluid, filter, vacuum line, modulator. I am sure it was not cheap to get that done, so it should be right out the door and right down the road.

A fellow FTE member says...Check the vacuum modulator and the line that goes to it.
Check the line for kinks or other problems, make sure it hasn't come off.
If the line checks out, take the line off where it connects to the modulator. If trans fluid leaks out, its bad. If no fluid, it still could be bad. They're simple to replace, disconnect the vacuum line, remove one bolt that holds the clamp in place and the modulator comes out. It has an o-ring seal.

Modulators are identified by the color of the stripe. Pretty much the "go to" one is the black stripe. If they don't show that one in the catalog, go with the green stripe. Avoid the pink stripe. That one is a dual diaphragm version that has two vacuum line going to it. If by chance you have one of these, replace it with one of the other two and use the vacuum line that goes to direct manifold vacuum, cap the other line. When you pull the modulator out, be careful. There is a pin that goes in the modulator. Sometimes it comes out with the modulator, sometimes it stays in the tranny. If it comes out, don't lose it.

Thanks mikeo0o0o0

 

I have fixed a handful of these that do that, always was lighten modulator either back screw out, shorten rod etc. The shop will tell you unless they are just up for wasting more of their time to obtain and install a kickdown linkage, this won't help but it might appease them into taking another look. I would say that if it works in the lower 2 ranges manually selecting the thing is healthy and not necessary to come apart, just needs more fine tuning.

 

My .02... Most likely (and easiest to fix), the modulator valve needs turned inwards to firm up shifts - but I agree with 77&79F250 that I'd take it back to the shop and have them adjust it since they did the work. It takes about 2 minutes to adjust modulator valve and then you have to drive it and see how things go. Very much a guess and check.

Modulator valve can make a lot of difference. Assuming your valve is adjustable, you can adjust that bad boy in until it won't even shift out of first gear. The way I understand it - The modulator wants to prevent shifting, the governor wants to create shifting. When the governor overcomes the modulator (by pressure), the transmission shifts. Therefore, adjusting the modulator can make the transmission hold gear longer.

I was having a very similar issue with a C4 where it would sometimes remain in 3rd after stopping (typically after it was nice and warm). My theory was that the modulator was not adjusted properly to get back to first gear. When the engine warmed up vacuum supply to modulator dropped slightly and must have been just enough that it couldn't pull the tranny back to 1st. Kind of weird - someone feel free to contradict me, but adjusting the modulator solved the problem for me...

 

Thanks everyone. I did check the vacuum line to the modulator. I found a kink in the line up at the engine. I got it straightened out like it should be and now it seems to be shifting correctly now----although I have only driven it to work and back two times which is about 4 miles thru town. Going to try to take it on a longer drive tomorrow to get it to complete operating temperature and see how it does. Will let you know how it does.

I didn't worry too much about buying a kickdown rod yet because I am planning on installing FI Tech fuel injection on it in the next 2-3 months. Will get a kickdown rod of the correct length at that time. I see there is a guy selling custom made ones on EBay.

Does anyone have any experience with the FI Tech fuel injection?? Does it work as good as they say it does?? Planning on installing it with an Edelbrock Performer intake. Any input would be appreciated!!

 

Local shop I trust implicitly recommended the FiTech setup without reservations. But at $1600 or so, 'way higher than I would up paying for a DP manifold and rebuilt double pumper.

 

Right to a point. The modulator an develop so much pressure that the little spring behind the governor weight can't overcome the oil and the weight is forced over to the open position, by oil, with greater force than the spring is capable of, thus causing for the valve body to want an immediate upshift since the governor is fooled into thinking that ground speed is throwing the weight over, when really it is oil. Turning the modulator inward, *clockwise* actually boosts pressure, asking the governor weight to throw even harder against the spring, out *counterclockwise* actually drops pressures, so long as the pin/rod are correctly sized. In this situation, 2 possibilities can happen, the kinked line might let vacuum in and not out, holding modulator in low pressure position, or no vacuum signal at all. No signal often wants to cause late shifts, like holding in 1st for too long.

 

glad you found it and got it fixed, pleasures to you and your drive. Maybe you can see my dad while he is cruising my dent, since his 2016 f150 went in the body shop.