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Discussion Starter #1
My C-4 tranny isn't shifting properly. Originally, it shifted firmly from 1st to 2nd, and took a little bit til going into 3rd. Now, I don't feel the 1-2 shift, and the 2-3 shift takes forever, and I have to lift a bit. Could it be modulator? What checks can I easily do?

Thanks,
Perry
 

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First thing to do is check the rubber ends on the vacuum line, make sure they are not dry and cracked. If they are fine and are connected correctly then it may be you modulator. The diaphragm in the modulator could of ruptured which means the modulator needs to be replaced. It is about $25 for the modulator and 5-10 minutes of your time (very easy fix, little to no mess).
I take it your fluid is at the correct level and doesn't smell burnt.
By the way if you do change the modulator you will need a 3/4" open end wrench, the thinest one you can get.
 

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What happens when you shift manually, compared to leaving the shifter in D? Are you saying, at WOT, the transmission won't shift either way?

Vacuum modulator and attendant vacuum lines are the first thing to check, as mentioned. Test the line at the modulator with a vacuum gauge. If you're seeing 12-16" of vacuum or better, replace the modulator with one that is adjustable (most are now).

Next, adjust your shifter linkage according to the service manual. Improperly adjusted shifter linkage can cause shifting issues.

Lastly, verify condition of engine and transmission mounts. Damaged/broken mounts can cause torque-over, changing shifter linkage position and causing shifting problems.

The above presumes the ATF looks as normal and is filled to the proper level.

Good luck!

Pat
 

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I have a similiar problem with a c4,, when you drop it in drive,, it feels like it is taking off in 2nd gear,, but if dropping it in low and starting off,, it feels like it has more power,, do you think it is a linkage adjustment problem??? thanks...
 

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stang73 said:
I have a similiar problem with a c4,, when you drop it in drive,, it feels like it is taking off in 2nd gear,, but if dropping it in low and starting off,, it feels like it has more power,, do you think it is a linkage adjustment problem??? thanks...
I think that is normal for the dot Trans. It was for a reason. The dot was for 2nd to 3rd and 2nd was...awe heck I can't remember

Your trans must still have an orriginal build inside. I wish mine was the Dot trans on the inside.
 

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Discussion Starter #6
Thanks for the tips. I can shift it manually. Once in a gear, it doesn't slip. I was thinking of putting in some Lucas additive. Haven't changed the fluid, it's up to level OK. It had been stored since last Sept, and puked a bunch of fluid out. Someone said they do that if overfilled. I have a vacuum gauge so I'll check the vacuum.

Thanks,
Perry
 

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Discussion Starter #7
I checked the fluid and it does smell a bit burned and the tranny slips a bit in second. I got a filter, and am getting a modulator. I think I may have used a wrong fluid, a Honda fluid. I have 4 quarts of type F Castrol, and a quart of Lucas. How many quarts will it need if I drop the filter and change it? I need to check the vacuum line and the modulator. Any other advise?

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Perry
 

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As mentioned above, if you used the wrong fluid then you need to drain the convertor, it is not to good to mix fluids. (best to get it all out and start fresh)
If you just do the pan drop then you will need 5 quarts.
If you do the convertor and pan then you will need 9 quarts.
When I first got my car many years ago I drained the t.c. and the pan, didn't want to mix old fluid with new fluid. It is a very straight forward job just take your time.
1. Start by dropping the pan, loosen the rear bolts and let it pour out some, then slowly loosen all the bolts unit most of the fluid has flowed over the edge, then remove the pan and dumped the remaining fluid that is in the pan(always support the pan with your hand).
2. Remove the filter
3. If you are going to drain the t.c. remove the inspection plate (two bolts), then look for a bolt head not a nut, the bolt head is your drain plug. Loosen the bolt and the fluid will flow. Once there is no more drips re-install the bolt and tighten to spec's.
4. Return to your pan and check to see if it is flat (sometimes the bolt leave nipples if they were over tighten(you must have a flat pan).
5. Install the filter.
6. Place the gasket on the pan and install (tighten to spec's don't over tighten).
7. Pour 5 quarts of type 'F' into the filler tube (check for leaks).
8. Start the car, slowly shift the shifter all the way down and up a few times (pause at every gear). You will not feel it going into gear, keep the car running and check for leaks, add some more fluid.
9. Try shifting again this time it will go into gear return the shifter to park and top up the transmission to full on the dip stick (don't over fill) remember to check for leaks. If no leaks take it for a test spin around the block and park it and shut it off and once again check for leak. If no leak re-install the inspection plate.
When you buy the gasket they usually come in a small box and the gasket is twisted everywhere which makes it hard to install. A good tip is to flatten it out the night before, put it between two big books and put a weight on top.
This is a fun job but a dirty job
PS. the fluid amount mention here is for a complete drained system (pan+t.c.) less fluid is required for just the pan
Torque Spec's
Pan bolts 12-16 pound
Convertor drain plug 20-30 pounds
 

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It had been stored since last Sept, and puked a bunch of fluid out. Someone said they do that if overfilled
That issue, while not affecting your operational problems, can be caused by a sticking converter check valve in the front pump, causing the fluid to drain out of the converter and ultimately overflowing the case vent line. The C4 in the race car developed this issue and, rather than pulling the trans during the season, I merely ran a line from the vent tube to my puke tank with a one way valve and periodically checked the level. Fixed the issue during operational maintenance.

C4's are pretty reliable as long as they're maintained. Now you know to use the proper fluid. The wrong fluid has a different co-efficient of friction and can cause slippage. Good luck!

Pat
 

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Discussion Starter #10
Well I dropped the pan, changed the filter, straightened out the pan. It had some silicone on it. Put 4 quarts of Castrol Type F in it, and 1 quart of Lucas. Went through the procedure, with the engine running, shift through all the gears. At first, ti seemed to want to move. Now it doesn't move. Do I not have enough fluid in it? I didn't change anything else. The fluid was pretty toasted smelling, but before I changed it , it at least went into gear. Any help?

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Perry
 

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what does the dipstick read, did you go through the gears with your foot on the brake? You must do this so the fluid gets to move around before you take it for a drive.
When you sit in the car and put the car in reverse or drive do you feel it engage? (rpms drop) or does it feel like it is still in park/neutral?
p.s. I just read some reviews about that lucas transmission fix, and they say the stuff is very thick, which could be sticking the valves or blocking the filter.
 

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Discussion Starter #12
At first it seemed to go into gear, rpm drop and it moved a bit. Yes, the Lucas stuff is thick. I thought it may thin out with the other fluid. I am hoping I'm just not filled up yet. I'll re-try it today later.

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Perry
 

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Discussion Starter #13
Got everything buttoned up, added 2 more quarts, ran through gears and it works! It still shifts at what seems a high speed 2-3, at @ 28 mph. I don't have the tach hooked up, so don't know rpm's. What's normal? It may get better as I drive it, or I may need to adjust the modulator. i bought one and need to get a 3/4 wrench ground down to fit in to get it apart.

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Perry
 

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2-3 @ 28mph is normal, my car shifts at the same 26 to 28mph cruising at normal speed.
If you go to your local hardware store and pickup a cheap 3/4" wrench they tend to be on the thin side (so you won't have to grind it down), just take the modulator with you.
 
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