I have a C-4 that shifts as it should when cold but when it reaches operational temperature the shift points are a little delayed,when cold it will be in third by 25 when driving through residential streets but after it's warm it wants to go into third between 35 and 40. I always thought it could be a vacuum issue but on my trip yesterday going down a long hill with no foot on the gas it still would not go into third until I reached over 35 so it's not vacuum so where should I look. It's done this since rebuilt about 4000 miles ago and should of taken it back but the warranty is long gone.
On the C-4 the modulator that controls the shift from first to second is vacuum actuated. Its possible a simple adjustment of the modulator is what's needed. The modulator is located on the right rear corner of the transmission. To adjust the modulator remove the vacuum line off the nipple going into its center. Inside this nipple you will find a slotted screw head. Turning this screw will alter when the transmission will shift from first to second. You have to experiment with which direction will allow the shift point. You can over adjust it to the point where it will not upshift if you go to far. So overall by using this adjustment you should see a difference. Also be sure the location of this vacuum line is seeing full vacuum all the time so the modulator works correctly no matter what the engine temperature is. Ronnieroadster
Depending on what modulator is in it, it may or may not have a screw. if it has a screw back at off about two turns. If there is no screw in it, you will have to take the modulator out and very slightly shorten the pin behind it. Maybe take .030 off it. Also make sure if it has a kickdown, that it’s not binding and being pushed prematurely. Could also be a sticky valve, sticky, governor, wrong servo seals, and a host of other things.
If all I done was highway driving it would be no problem but had been thinking it was vacumn related and have it hooked to where it gets vacumn from all eight cylinders,when it would not go into third going down hill with no foot on the gas it has to be something else.
The vacuum hoses rot out / dry rot down by the modulator. Just because the engine is making vacuum doesn’t mean the modulator sees it.
Shift points are a balance between modulator pressure and governor pressure. Governor pressure is always trying to force an upshift while modulator pressure is trying to delay it. I would T in a vacuum gauge very close to the modulator and see what kind of vacuum the modulator is seeing. On an lift foot upshift it shoule see 18" or over. If that's the case, I would check trans operating pressure. With high vacuum, above 18", the operating pressure in any forward gear should be 55 - 65 psi. At the Lincoln Mercury dealer we had a petcock in a vacuum line that allowed us to create a vaccum leak in the modulator line. By adjusting the petcock you could lower the vacuum and monitor the increase in pressure. At low vacuum in a forward gear you should get up to about 150 psi. If your operating pressure is correct, in that it is between 55 - 65 psi at idle, your shift points sould be more in the normal range. If they are above 65 psi, you will start seeing delayed shifts. If the pressure is correct, then the problem is most likely in the governor. Ford uses a two valve governor. The large valve prevents any governor pressure below about 5 mph. This give you a nice coast downshift. Once is moves outward the small valve is the one that controls governor pressure. I suppose it could be sticking in the bore causing your problem as it warms up. That's the only thing I can think of that would allow it to shift normally when cold, but have delayed shifts when warm. If it had the wrong spring, shift points would be delayed when cold. That's also assuming the operating pressure is correct and doesn't raise when the trans warms up.
The Jalopy Journal
