C-4 Question

I was wondering if someone could give me some tips on my c-4 transmission. I am not getting the performance that I would like out of my '76 because the transmission changes gears too quickly. I can accelerate from a dead stop and be in third gear before I reach 100 yards. The transmission is not allowing the motor to wind out at all. What do I need to do? I know there are shift kits that can be put in, but I was wondering if I could just have someone tinker with the kick-down linkage to get it adjusted to what it needs to be. Thanks in advance.

Would also like to ask if anyone has used James Duff's headers on their rides and what they thought of them?

Check your vacuum modulator, located on the passenger side of your trans by the pan. This will cause exactly what you described. You must have a vacuum line hooked to it. You can adjust it by turning the screw inside the vacuum port in or out. My advice is to change it then adjust it. They are about 15.00 from any auto parts store.

The usual suspect when dealing with a C4 that isn't responding to engine load is go to the vacuum modulator, passenger side of transmission case, just behind the transmission pan.

Change it, don't lose the pin that goes in there and expect transmission fluid to come running out when you remove it. No pin = problems.

Install a new one, most new one are also adjustable. Instruction in the box. Check all the vacuum lines at the same time.

If that doesn't fix it, then it is a hung valve on the valve body. That doesn't mean a transmission rebuild unless the transmission shop has a boat payment to make. Just need to take the valve body out, clean it thourly (dissassamble to clean inside) to remove the dirt/trash that is hanging the valve.

If you go for a valve body kit while you are in there, set it for "tow".

I also think it is the vacuum modulator valve. It should be adjustable, if not I would replace it with one that is adjustable. Also check your vacuum line. If any of the vacuum hose looks cracked replace it. I ran a C-4 without kickdown linkage for many years and never had any problems. Is this problem something that just appeared or is this a new install?

I had a new carb put on a while back and while that was going on we noticed that the kick down linkage was put on backwards, therefor I had no passing gear. We turned them around, and I now have a passing gear, but everything is now changing to quick. Thanks for the help!! I'll be willing to try anything to get this horse up and kicking like it should. My previous two Broncos were 3 on the tree and I never had a problem winding them out a little.;D Just miss the spunk of the standard shift.

Thanks!

STICK

A number of things or combinations of them could cause the symptoms you are having. This really sounds like a vacuum modulator adjustment could be a good place to start. It is a result of not having enough modulator pressure. The pressure is increased by a drop in the intake manifold vacuum signal coming from the engine. This signal actually increases or decreases the spring tension on the modulator valve and by doing this boosts a certain circuit of hydraulic pressure inside the transmission. If your engine is running good and well tuned, it may be delivering too strong of a vacuum signal to make the transmission behave satisfactorily. One of the ways you can compensate for this surplus of good vacuum is to either change the modulator to a different one or re-calibrate the existing one. In order to help you accurately, I need to know which version of modulator you have, how high above or below MSL (altitude) you are operating the vehicle when the problem occurs, and how many inches of mercury, intake manifold vacuum (in.Hg) your engine is putting out. This measurement should be taken with a vacuum gauge @ idle, fully warmed up, with the choke completely off. I have seen 4 basic types of modulators available for C-4s built between 1972 and 1981. Each is used in specific applications.

1) Red-stripe with two vacuum inputs (large & small) Usually found on earlier (73-74) vehicles where EGR was first introduced. This can in most cases be replaced with the green stripe version and get better results for the shift characteristics.

2) Yellow stripe, with one single hose connection for High Vac lower altitude driving. Usually found on larger engines with strong vacuum signals, and trucks with earlier governor shift point calibration.

3) Blue stripe, with one single hose connection for engines that generate slightly higher than average vacuum signals. These fit a small variety of applications of the C-4. This version seems to very uncommon but finds its performance levels between that of the yellow and green paint stripe codes.

4) Green stripe, with one single hose connection for average settings that fit the widest variety of applications of the C-4 and C-5. This version seems to be the most popular and widely used of them all.

Adjustment screw location:
All 3 versions may or may not have an adjustment screw, which is usually located inside the vacuum hose connection. You will find it by unplugging the hose connection to the modulator at the trans. The red stripe dual input version, the screw is not located inside the hose connection but in the middle of the rear portion.

It would be better to find out what version modulator you have and if it is adjustable first before going too much further. Also, Is your vehicle presently equipped with a functioning EGR system?

Not knowing what you have, I can only guess and tell you what you might try. This procedure is reversible. If your modulator has an adjustment screw, try turning the screw in 2 complete turns. This procedure will cause shifts to be firmer and stretch out the shift points to happen at higher road speeds. If this doesn't help your situation, It may be an issue of needing a modulator with the correct calibration for your engine. Fine tuning the actuator pin length is a more difficult service that should perhaps be done by an experienced technician who is very familiar with the C-4.

Sorry for the essay, hope this helps,
~BH

are there any ways to adjust the module without removing it? i cant get a screwdriver in there because i only have about 1 inch clearence between the module and the crossmember

Thanks for all the help. I have already spoken with my buddy who does most of my work and he said that he had already thought of that and has me a new vac mod on order. Will try and get it put on next week and see what happens! Thanks again guys!!!

Stick

76 explorer said:

are there any ways to adjust the module without removing it? i cant get a screwdriver in there because i only have about 1 inch clearence between the module and the crossmember

Click to expand...

There is not a lot of tension on the adjustment screw so you dont really need tool steel to turn it. I made my own modulator adjuster tool from a piece of welding rod. You flatten one end with a hammer, then grind it square. Then cut it to the length you need. If you shape the tip of the blade like one of those giveaway type pocket screwdrivers, it will fit the adjustment screw just fine. then bend a loop in the handle end so you can turn it with your fingers while counting the number of turns.

Good luck