I'm new to this whole vacuum modulator concept, but I assume it functions something like the throttle valve cable setup in a GM 700R4. Mine also has a rod on the throttle linkage, which I'm going to assume works like the detent cable in a TH350. Am I close? Okay, so here's what's going on. Got a 302 with a C4, and I think my rearend is geared pretty high. 3.0:1 maybe? I can cruise at 70+, and I can cruise at 50-, but at 60 it wants to downshift with just a little bit of throttle. Feels like it's too easy to kick it down. I don't think it's slipping... So should I adjust that kickdown rod? It would appear that the rod doesn't engage until well over 3/4 throttle, so unless it moves while I'm driving, I don't think that's it. How about the modulator? Is this stuff common to a C4 and just a matter of adjustment, or is this a sign of impending trouble? Thanks for your thoughts...
I recommend contact PaulS with any c4 questions...in my opinion he would be the board expert on these. http://mmb.maverick.to/member.php?u=1477
Well it depends. The modulator works off of vacuum. The more you open the throttle, the less vacuum you have. So if you have a vacuum leak, low vacuum due to a lot of overlap, etc, then yes, it will kick down early. I would disconnect the hose that goes to it and look for signs of trans fluid. If there's any in there, time to replace the modulator. If no fluid, disconnect the kickdown rod from the linkage and go for a test drive. If it solves the problem, you need to adjust the kickdown linkage. If not, you can adjust the modulator with a screwdriver. Chances are good that it's most likely in the kickdown linkage though. Just an educated guess...maybe the expert PaulS will chime in here.
Alright, thanks. That makes sense. I'm almost convinced it's not the kickdown, but that's easy to check and I'll do it on the way home from work today. As for the modulator, I'll check that out too. Do I pull the hose from under the hood and check for fluid, or from the modulator, which I'm assuming is down on the side of the trans? Guess I'll also make sure there are no vacuum leaks. And just to be thorough, where should that hose be connected? To the manifold as I assume, or the carb? Engine does have an upgraded cam and a 600CFM carb, so maybe it just produces less vacuum than stock, you think? Got power brakes too, if that makes a difference...
You can check the adjustment of the "kick-down" linkage by performing the following: 1. shut the engine off - remove the keys 2. have someone hold the throttle pedal to the floor while you hold the kick-down rod fully depressed (like it is being kicked into passing gear) 3. there should be .060" gap between the adjusting screw and the carb linkage. If there is no gap then back the screw out until you have .060" gap between them. Check to make sure the vacuum line has manifold vacuum at an idle. If you have more than one line to the modulator change it to a "green stripe" modulator and connect only the larger of the vacuum lines to it. Plug the small line where it connects at the EGR manifold. Throw the small line away. If the hose is loose at the modulator it may cause the C4 to shift late and very hard and downshift at higher than normal speeds. If it shifts from 1 to 2 and 2 to 3 normally then I doubt the modulator is at fault. If you remove the hose at the modulator and ATF leaks out then it has a ruptured diaphragm and should be replaced. Check the kick-down linkage and the line at the modulator. let us know what you find.
Thanks, Paul. Sounds like you really do know these transmissions... Well, I got a chance to check the rod. There was a gap, but it was more like 3/8" instead of .060". I adjusted it and it's at least in the ballpark now, will get it right later... I haven't had a chance yet to go for a test ride though, 'cause I figured last night I would "go ahead and replace my wiper linkage real quick." HA! I ended up going to bed some time past midnight when I realized I was gonna need some hardware I don't have. I'm in my wife's car today and mine is at home with half the dash laying in the passenger seat, steering column halfway to the floor and crap hanging out everywhere. Just as well though, it's raining like all hell today...
It still sounds like you don't have manifold vacuum to the modulator. You should have to go to full throttle to downshift at 60 mph. Check to make sure that the vacuum line goes from the manifold to the modulator and connects to the rear of the modulator.
Do you have a holley or edelbrock carb ? If you have an eldelbrock carb you will need to mess with the secondary linkage a little to get it open all the way up with the engine off.
New development: As of this morning it shifts late and hard. And I haven't gone fast enough to really be sure, but I don't think it will go into third at all. Not at typical highway speeds anyway. I know vacuum is connected to the modulator. There's a rubber hose going from the manifold to an OE hard line, then from that another hose to the back of the modulator. Unfortunatley while I was under there I forgot to pull off the hose and check for fluid, but I will... Given what you've said, Paul, I think this almost confirms a modulator problem, right? Either a problem with the vacuum line or a bad moulator? You mention I should be running a "green stripe." What application should I ask the parts guy to look up if I end up buying a modulator? Thanks again for all your help.
Ugh. This is starting to bother me. Just got back from my lunch break, which I spent at the parts store. Got a speedometer cable, a press-in modulator with the green stripe, and enough vacuum hose that I can run it straight from the modulator to the manifold. I haven't checked out the old one for fluid yet, but as cheap as it is I just went on and bought it. Seems to be getting worse now. At least I got on the interstate and confirmed now that it DOES shift into third. I just have to be really hauling ass to get there. The 1-2 shift is taking a ridiculously long time. The trans fluid is not dark or anything, doesn't smell burned and it doesn't seem to be leaking, but this is starting to make me worry. I'll change out the modulator tonight and see what happens...
That seems to be a fairly common problem on these transmissions. I've had the same problem a few times over the years doing that same exact way and it always turned out to be the modulator or the line so I think as long as you are getting the proper vacuum you will fix your problem here. Let us know.
Don't run a rubber line from the manifold to the modulator! the hose will collapse and you will have intermittent problems forever. If the metal line is crimped or rubbed through you can use a long piece of 3/16" brake line to replace the metal line. Keep the rubber parts as short as you can. It does sound like a modulator problem - are you sure the valve is in the case properly? The long stub end should point to the modulator - it has a hole for the rod to slide into. If it is in backwards it will act the same as a bad modulator. No matter - the green stripe is better for performance anyway.
Sounds to me like the seals inside got dried out and now the fluid is bypassing circuits. This is common in C-4's that sit for long periods of time and don't get driven much. You'll have to rebuild it to fix the problem as it'll only get worse.
The only C4s that I have seen dry out are new un-run trannies. The friction surfaces dry out and will glaze if they are run in that condition. The seals are made of neoprene and don't dry out. They can develop a "flat spot" when the weight of the parts settle on the seals and they take a set. They need to be heated up a few times before they can be expected to work properly. Sometimes, if it has been a very long time, it will never recover without replacing the seals. They will look and feel like new but when placed on the bench they will roll up at on end.
Thanks to everybody for the help and the suggestions - I got the problem solved. It's one of those situations where you're not sure which thing you did fixed it, ya know? I got under there and replaced the modulator. (Paul - the thought of the long rubber hose collapsing did occur to me while I was under it. Used the hard line.) And I KNEW already that it was going to dump ATF from that hole until I got the new modulator in, but somehow at just the right moment I completely forgot that fact and made a righteous mess. So I got some cardboard to put down so I could finish the job, I looked up, and I see that the column shifter linkage has pressed the kickdown rod all the way down! How? 'Cause of that @#$% wiper motor I replaced! I had to drop the column and when I did it caused the linkage to bind against the rod. I SHOULD have noticed that when I checked the rod earlier in this thread, but everything works fine when it's shifted into park! In drive, it will press that rod all the way down, but I would never have noticed that if I hadn't been replacing the modulator. I ever-so-gently bent the rod out of the way, re-adjusted its relationship to the carb, and now everything works great. As a bonus, I now have an adjustable modulator to play with and a good bit more Ford-lore to file away in my head for next time I need it. I always appreciate a learning experience. I really should start a thread about all the trouble that wiper motor has caused. Hell of a story...
