Seems like the other post is locked so I'll follow-up here. When I downshift manually to first the transmission responds well and normal, but as I understand it there are different components being used in manual 1st both in the transmission and the valve body. I think the 2-1 shift IS happening because when I INITIALLY take off it seems bogged down but if I press the gas further it seems to finally lock up and get going. My first thought was the FWD clutches slipping, but then I think it would ALWAYS slip not just under slight throttle conditions. I'll let you know what happens. Tommorow I will add a bit more fluid and see if the problem gets fixed, you never know... I don't see how it could be anything else.


BTW, Tomorrow it's going to be put under a test of fire, I'll let you know why later, I don't want to ruin the surprise.

Well this weekend the Missus and I took a trip down the backroads of South Texas. A 2 hour drive to the beach. The transmission responded well all thoughout. On the way back I noticed something strange though, the car seems to love 2nd gear. It holds it until 40mph (2.78 rear end). Once in third gear if the car even goes to 39mph for a second the transmission downshifts to 2nd... if I press the gas slightly it will upshift to 3rd. It will downshift and upshift a couple of times in a short time period as long as the car is around 39 to 42 mph. I think this has something to do with the vacuum modulator.

Any tips?

Thanks.



Greetings from SOUTH TEXAS!
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Make sure you do have a good vacuum supply to the mod. If not you can get as you describe with a too early downshift with little load.. If that is good the SK-C5 kit corrects what they call 2-3 shuffle of too easy pass gear.. See below describes i think what you may have...
Nice looking 68 you have there ...


http://cgi.ebay.com/ebaymotors/TRANSGO-SKC5-FORD-C-4-C-5-TRANSMISSION-SHIFT-KIT-C4-C5_W0QQcmdZViewItemQQ_trksidZp1742Q2em153Q2el1262QQcategoryZ34208QQihZ007QQitemZ4582299929QQtcZphoto

Thanks and thanks. The strange part is I don't even have the passing gear cable attached! I am still looking for the bracket and the cable on ebay and such. It JUST started doing this, it worked fine before and for half of the trip. I will check the vacuum mod. line. It might have been pinched off or caught on something.

I just checked the line and it does not leak. I also didn't find fluid in the line so I am assuming the valve is not torn internally.

So, if the vacuum line checks out OK and no fluid spilled from the modulator line, what else can go worng with these modulators? Should I replace it anyway or would I be wasting money? Perhaps my problem is elsewhere? I did plug a manifold vaccum leak at the carb just now, but I don't think that was the problem... I'm going for a test drive right now but wanted to ask what else can go wrong with the valve or what else the problem can be just in case the car is still acting up after the test drive. Thanks

Update: car is still acting up... could it still be the modulator or should I check something else?

Measure the amount of vacuum at the hose at the modulator.

It must be the same as engine vacuum off the intake manifold.

If there is any difference that must be corrected.
Even a small insignificant leak will throw a monkey wrench into the works.
By you description it sounds like a vacuum problem.

I will check vacuum and if it tests out ok I will try adjusting the valve tonight and see how it goes tomorrow. If it still acts up I will replace the valve tomorrow. What else would cause this abrupt shift delay in the 1-2 and 2-3 shift? Stuart I really hope you are correct and it is only a vacuum or valve problem.

Other than a ruptured diaphragm (which I believe would be apparent if there was fluid in the vacuum line) how else can the modulator fail? I ask because I have no fluid in the line and the line itself is good. So Im puzzled...

BTW originally these cars had a metal line for vacuum but mine has all hose (about 3 feet). I made sure it wasn't cracked, but is it bad practice to have all hose? It is a thick hose and it does not collapse under vacuum but Im not sure if this is correct.

If the hose has an inside diameter larger than the steel tube it was supplied with there could be a variation in vacuum values.

Get a length of steel brake line and bend your own.
The kind with a steel coil on the exterior works best.
Get one that fits into the "L" shaped hose at the modulator.
The flare on both ends will make sure it doesn't fall off.

If the screw in the modulator is in too far it can cause a similar problem.

Back it out then turn it in until the end touches the diaphragm.

Then in 2 turns.

You are taking vacuum from the manifold?

Yes, the vacuum is from the manifold. I'll try to adjust the modulator like you described. I'll look into getting some line tomorrow. Right now I don't have any "L" shaped piece of hose or a hard line for that matter, only a 3 or 4 foot piece of hose from the modulator to the manifold.

Usually when I order parts for my C5 such as the gasket or the filter I use "1982 ford mustang with 302 and C5" I tried that today at the parts house and they don't have a vacuum modulator for my transmission!

They have two options for me though. One is a 1974 mustang 302ci with C4 (yellow stripe) and the other is a 1982 mustang 255ci with C4 (green stripe). They are a bit physically different from each other but both would fit my transmission, so which do I get? What is the difference in either one?

Thanks.

I would be sure to get a metal line first , with only a few inches of rubber hose at each end. I have seen long hoses collapse and suck together shortly after beig subjected to operaring temp. A soft spot mid way up the transmission tunnel will be almost impossible to find. You should be able to pick up some brake line at most any auto parts store. If that doesn't help check into replacing the actual valve.

OE is green but any will work.

I'd be more inclined to look at the vacuum first.

I'll make sure and buy some line tomorrow. I went ahead and replaced the modulator with the green striped one and I got a false alarm... The first 5 minutes of test driving the car behaved well. It shifted to second around 10mph and into 3rd around 25mph under light throttle. After those first 5 minutes the transmission went back to its old routine of delayed shifts and holding onto second gear past 40mph. Will post the results tomorrow after I add the line. Thanks for responses so far.

The green is listed here as a heavy duty. I use them if i can get them but in saying that not noticed a huge difference. I think the green gives a slightly higher shift point.

If you look at the SK-C5 shift kit it lists the too easy pass gear and 2-3 3-2 shuffle. I think thats what you have and is more of a valvebody calibration issue that is not easily corrected with basic fixes .The shift kit will sort it if thats what it is designed for...

Good point, and you've never steered me wrong, but I gotta ask. What would be better... to buy the SKC5 or get a C4 valvebody? What year C4 VB fits the C5? I think I can get either item cheap. The SK you showed me last time was under 20 and I don't think the VB will be much either. I have a 68 VB but it will not work.

You need a vb with a sliding manual valve.

What do you mean?

Oh and could something have broken or gone bad in the valvebody that causes these symptoms? Thanks.

Really any 70+ valvebody will work. The SK-C5 kit will fit either. I would go to C4 valvebody since IMO it is simpler and more common.
I don't think it's the case that something is broken, more poor calibration from original .

A sliding manual valve means the detent is in the case vs valve body.

Yu should have a flat spring with a roller as the detent.

I thought that's what you meant as I compared the 68 VB to mine but wasn't sure on the vocabulary. I'll post today what happens with the vac line.

Before you replace the valve body, fix the vacuum line (if you haven't already). A hose in place of the metal line will cause the exact symptoms that you are describing.