I have a 74 C4 tranny mated to a 302 in my 30 model A pickup. Recently I am having some shifting issues, and am wondering if you guys have encountered this also. When in drive, and leave from a stop in a normal fashion, the transmission wants to stay in first, like when it should shift to 2nd, I can watch my tach and the rpm's go up and it doesn't shift, but it will eventually, sort of like a slow shift then the engine rev's, then it goes in. This doesnt happen all the time, and doesnt seem to happen if I give it more gas from the stop. The fluid is full, light pink and doesnt smell. I took off the vacuum hose on the modulator and it isnt leaking fluid(it is a green stripe modulator). I bought the truck a year ago, so not sure when the last time the fluid was changed, or whether it has a shift kit in it. Does this sound like a modulator or band issue, or something else?
I don't have a vacuum gauge, but the truck has been fine for over a year since I bought it, and just recently this issue began. I haven't made any engine modifications during that time period. The modulator hose seems in good condition also. Are you thinking that if the vacuum is too low when I take off from a stop relatively slowly, that the modulator wont assist in the shifting, but if I give it more gas, the vacuum will be enough? This truck has a healthy cam in it, and has no kickdown linkage, but this is how the truck was a year ago with no problems.
Ok, I will pull the vacuum hose off, but the end at the modulator is pushed on so hard that I suspect I will not be able to get the same seal once I put it back on. The rubber hose is about 6-8" long while the rest is metal tubing up to the intake manifold that it is bolted into. The end of the modulator has the metal nipple that the hose fits on and from pictures of it, it looks to extend a bit past the end of the modulator, but not mine, it seems fairly indented in the cavity of the modulator. I'm a skinny guy, but this truck is low to the ground and the exhaust pipe will be against my chest, so I will need to borrow my buddies metal ramps to drive the truck up on. Will not be able to do this until later in the week, but will update on my findings. Is it possible that the modulator screw is unwinding, and affecting the shifting, but not sure if the green stripe one has this design or not. It has a clamp that is bolted to the tranny that holds the modulator in place that then rests against the top of the pan.
Something else I thought about, the modulator hose end is hard to get off because the previous owner used hose that was the right size for the intake manifold line which looks like brake line tubing size, and the nipple of the modulator is bigger, so that is why it is on there way too tight. Shouldn't the same size tubing be used that is the nipple end of the modulator, as maybe if the brake line tubing is the wrong size(too small), it could more easily get clogged?
Update. Bought a vacuum gauge, figured it would come in handy anyways. Hooked it up and at idle 800rpm, reading was 13, increased idle to 1500 rpm and reading was 18. Is this within a reasonable range? My metal tubing from the manifold is 3/16" diameter. The thing is, today I drove it for about 45 minutes and it was shifting nicely as it has been for the last year up until a couple of weeks ago, and even when it doesn't shift at the appropriate time when I take off from a stop, it is not doing this all the time. Should I be worried and continue with some other tests? I was going to put an auxiliary trans cooler on now that I am reading more and more about tranny's, as heat is a killer. I tried to feel each cooling line to determine which one is the hot one, but cannot really feel the difference between either one. If I had to guess I would say the hot one is the one near the oil pan, and the cold one is at the back, sound right?
Still have been driving and now the issue is more repeatable. When truck is driven cold for first 5 minutes of driving, it has a prolonged shift into 2nd. After 5 minutes the shifting is good. I have a Lokar tranny dipstick and I have checked it several times as has someone else, and it appears to be at the full level. Could it be that this dipstick isn't really reading the level correctly? With engine oil and you are down a quart, no big deal for the short term, what about tranny fluid, how much not at full causes concern like 8 ounces, because maybe the initial 5 minutes of delayed shift is until the fluid builds up because it is actually low. Would it hurt to put in say 8 ounces and test drive again, or how much is too much also?