C4 Modulator vacuum question

[Trappey74]

My C4 transmission builder said he didn't think he could change the particular type modulator out to soften the shift from first to second but said I could unplug I think the front vacuum line and put a screw it the line and it would soften it some. Anybody know about this? I recall him saying something about a double vacuum or something with that modulator. Sorry for the stupidity and poor info but I know nothing about transmissions.

 

[Tiko433]

I would look at a few things . Behind the modulator is a pin. Stock is 1.5 in A Mod to make it the shift point hold longer is to add a longer pin .. no longer then 1 5/8 . You may have a longer pin. Then a different modulators . They have colored strips on them . Also the modulator should have a lil adjustment screw inside where the vac line connects. That can be turned to adjust shift points also. Make sure your kick down is hooked up

 

[Rustytruck]

Does your 1974 bronco have its vacuum can mounted on the firewall behind the throttle lever mounted on the firewall?. It may soften and delay the shift a little.

 

[DJs74]

My '74 had the double vacuum fitting modulator on it when I bought it and I don't think it had the screw in the port for adjusting (I'm not 100% sure though) but when I rebuilt everything, I replaced it with the green striped adjustable style modulator valve.
I had to modify the mounting bracket some or you can purchase the correct bracket for the single port / adjustable type - the brackets mount to the transfer case bolt in the same location but the bracket itself was a little bit different for the double versus single vacuum port modulator.
If you happen to have the screw in style modulator as opposed to push in / bracket style, I would imagine they offer an adjustable version but I'm not sure
If you install an adjustable modulator, it might come with a new pin as mentioned above but regardless you will have the benefit of being able to adjust the shift points to your liking.
If you decide to install the adjustable type, the adjustment is pretty sensitive, 1/4 to 1/2 turn adjustments at a time result in noticeable differences but you may have to make a few attempts to dial it in.


DJs74

 

[jckkys]

Your '74 may have 2 vacuum nipples, explaining the "double vacuum" reference. Some '73 and '74 C4s used this in conjunction with the EGR. Ford figured out this wasn't one of their better ideas, and dropped it, in late '74. I would just get a yellow stripe single nipple modulator and use one of the vacuum lines to connect it directly to the vacuum tree in the intake. These modulators are adjustable by turning the slotted screw in the nipple. Also ask your transmission builder if he installed a shift kit. That would also explain the hard shifts. Some of them put shift kits in every job. The Bronco C4 never needed a shift kit. I had the same problem and had to scour the junk yards for an unmolested C4 valve body, to fix it.

 

[Trappey74]

He said he didn't put a shift kit in it. I'm just curious as to why he wouldn't have changed modulators to and adjustable one. He did say that whatever C4 I had was the one ford made that had the trouble with ahifting points before they moved to the newer modulator. I don't hve the modulator color handy but can get it soon today

 

[Trappey74]

My modulator hs a red ring around it and two black hoses connected to it.

 

[jckkys]

The green and yellow stripe modulators are listed as correct for the '75-'77 Bronco C4s. I've used both and can't tell the difference in the way they shift. Their price is close too.

 

[Trappey74]

Upon looking closer I noticed that the two vacuum lines that come from the modulator and run to something on the back of the intake by the firewall with multiple nipples for hoses to slip over. One of the black hoses is connected to it and the other hose it cut off and not connected to anything. Could this be part of the problem

 

[jckkys]

My guess is, the PO or your transmission guy disconnected one of the lines. Most likely, the larger nipple near the top is the best one to keep the vacuum to. The vacuum tree in the back of the intake should be hooked up to it. Any unused nipples need to be capped or plugged. When you get a chance, get a green or yellow striped single nipple modulator. The transmission shop should have them.

 

[Trappey74]

I'm assuming either color will work fine and work with myC4. I use this to ride around town sometimes and will hunt out of it very little. Just enough to enjoy it for its original purpose and make me feel like a kid again

 

[DJs74]

I'm assuming either color will work fine and work with myC4. I use this to ride around town sometimes and will hunt out of it very little. Just enough to enjoy it for its original purpose and make me feel like a kid again

Just a reminder and something to keep in mind while you are shopping... you might need a new retaining bracket for the adjustable modulator valve or you might need to make a modification to your current one.

My '74 Bronco also had the double fitting modulator valve and both fittings exited the valve on the side at a 90° angle, the retainer bracket presses on the end of the modulator holding it in the transmission and the bracket on mine was solid where it fit against the valve.
The new adjustable modulator will have the hose fitting on the end where the bracket sits so the bracket will have to have an open slot for the hose to pass through and attach to the vacuum port.
I cut the slot in mine using a drill bit and dremmel tool.


DJs74

 

[jckkys]

The tranny shop may have one of those too. One of the EB venders had them but not now.

 

[Ol'Blue]

I tried both yellow and green modulators in my '77 C-4. The green had better shift points by far. When I tried the yellow, I used the adjustments screw and didn't seem to have any effect. Installed green and no adjustments needed. Worked great right off the bat.

Neither changed to firmness of the shifting.

 

[BwoncoHowie]

Sometimes it helps to search the archives for modulator help...

http://classicbroncos.com/forums/showthread.php?t=69111