Need help with finding a problem. Recently rebuilt c4. Did all air test during rebuild. All check out fine. After two years in the rebuild of the car, this is one of the things I’m having a problem with. I have a strong reverse, 1 st and 2 nd gear when I manually shift to them. When I put it in D, acts like it is in neutral. I have adjusted the modulator from all in to all out with little change in the control pressures. Other have said it is in the valve body. So for I have been back into the valve body five times. Can’t find anything wrong there. Have taken the governor out and there is no problem there. Nothing sticking anywhere. Here are the control pressure test. P - 75 R - 140 D - 75 2nd - 92 1st - 92 at 17" on the modulator.
Not sure what to go next. Tough problem. Any takers
Thanks
Putting the car in D and it not moving is totally different than no 3rd gear. Sounds like your low roller—or one-way clutch—or sprag in the bottom of the case (the same part–just different names) is not holding. When looking into the case, the inner race should turn clockwise and lock when attempting to turn it counter clockwise.
If you put the car in manual 1, will it drive? If yes,then will the trans shift to 2nd when you move the shifter to 2? When you get fast enough, if you put the shifter in D, does it shift to 3rd?
If you have a power flow chart, you’d see that the only things applied when in D at zero mph are the FWD clutch and the low sprag is holding. We know your FWD clutch is working because you have forward movement when shifted into 2 or 1. When you put the shifter into 2, the intermediate band applies and the low sprag is over-run. When you put the shifter into 1, the reason you get first is because the low/reverse band applies to hold the drum, where as in D (at zero mph), the L/R band is released and only the sprag holds the drum.
Here is a theory you can try. Pull the driveshaft, take the car out of park, put some vise grips on the output shaft. Turn the output one way, then turn it the other way. If the resistance to turn isn’t a lot harder in one direction, then the sprag is not holding.
I haven’t tried this, because the diagnostic to determine a failed low sprag is “it doesn’t move in D, but moves in man 1”. Either way, sounds like you will be pulling this trans.
Thanks for the information. During the rebuild, I didn’t really do much to the one way clutch. I did not disassemble, but should have. I’ve own this car for 30 years and it slipped a little before the rebuild, but I assumed it was the band and clutches. Not too upset that I might have to pull it out. Put a new shifter seal in and it leaked. I really hate one that leaked especially after putting so much work into it. I would have pulled it for that reason alone. I’ll try your theory and will let you know how it goes. When the one way goes bad, is there any damage to the one way roller and springs and if so, where can they be bought. Thanks again for the help. It’s been a little frustrating.
The rollers and springs can be bought through any trans parts suppliers—like Transtar—they have a lot of locations. The counter guy there can generally find you the races if you need, but they would be used.
For further verification without pulling the whole unit, one thing you can do is take the pan off and drop the valve body…again. If you look up into the open casting, the drum that the rear band wraps around is the componant the sprag allows to only turn one way. Use a long flathead screw driver to try turning it both ways----make sure you are not in park.
Frustrating? Yes, imagine having a GM 4T65e that you’ve done everything to in a Grand Prix with a 5.3 V-8 that has repeat torque convertor failure. This one cost me quite a bit under my warranty to finally find the root cause. Luckily, the customer had a secondary source of transporation and was understanding and easy going. There have been a few others that really sucked to not go right. But, it just happens to be part of the job----luckily it’s not very often. It seems the times when it does happen it’s usually a unit that can be built with our eyes closed—like a C-6 or 400. Perhaps it’s because these units are no longer run of the mill and add in a bit of complacency working on something perceived as “old and simple”. This week I have two C-4’s on the bench. The last time I saw a C-4 was 7 years ago.
" I have a strong reverse, 1 st and 2 nd gear when I manually shift to them. When I put it in D, acts like it is in neutral."
I did this exact same thing on my C4 rebuild. The piston that the modulator pushes on is installed backwards. Take the modulator out, use a magnet to pull the piston out, flip it 180 and put it all back together.
Go to page 3 of my thread in the following link and see if this sums up your problems:
https://cccforum.discoursehosting.net/t/c4-rebuild-servo-kit/77/1
At first, the shifts are pretty harsh and doesn’t want to go into 3rd gear.
After running a while, the shifts soften up and it now runs through all 3 gears on its own, sweet.
I hop in the truck, run to the store, get 4 more quarts of fluid, restart the car.
Pulled the dip stick and can hear a sucking noise from the tube, so I know its low on fluid. Add 2.5 more quarts of fluid, reading right at full on the dipstick and no more sucking noise.
Drop the car in gear and the shift are back to being harsh again, so the pump and shift kit are doing their jobs.
Now, the car is not wanting to shift automatically, I can drop it in 1, run it up to 20 MPH, shift it to 2nd, shifts like a dream, put it into Drive, and no 3rd gear again…
Here an update. Hard day yesterday pulling the trans out. Lots easier the first time when I took motor and trans at the same time. Got it out with out much problem. Opened and broke it down again to find that three of the oneway clutch springs were backward. During the rebuild I did have one or two slip out but thought I had them back in the way they came out. Got them all lined up and they locked up by hand before putting back together. Typical rebuild, you get it almost back together and notice the snap ring laying in front of you.
S with that said, I had not taking the car out since tearing it down. Had to put the instrument panel halve way back in and the steering wheel back on just to test drive. 1 st and 2 nd were again fine. Could not get it to hook up in D, took it thought the gear and manually shifted unto enough rmp to shift to D. Everything shifted great all three gears. No tag, no insurance, no seat belts, no interior and parts laying everywhere and the only car I met was the highway patrol. Though his hand up and so did I.
Even though I do not have D at 0 mph, I can almost live with manual shifting unless there is a easy fix for this problem. I was concerned that on a a long run, not having 3 gear would run the motor to hard at low speeds. I have not tried adjusting the modulator. While putting it back I adjusted it about half and half, but could go some more if it would help. I was told by another trans guy that with high pressure it would overrun the D gear I don’ t know if that is true. Maybe you could explain.
Thanks
Do you actually have vacuum to the modulator? I had this problem once and it turned out the vacuum tree was plugged up and no vacuum was reaching the modulator.
So you are saying that after find the low sprag had a problem and fixing it, you installed the trans and it still had no forward engagement in D? Even though the sprag might lock when turning by hand, it might not hold the weight of the car and the torque of the engine. It’s a good possibility that the springs are fatigued. Also, if the races are not completely smooth, or if they are worn, the locking action would be weak. A cracked race might hold by hand yet slip under power. We replace low sprags on certain transmissions as part of rebuild. C-4 is one of them.
After taking the trans out and fix the sprag. Felt 
ok putting it back in because I’ ve own the car thirty years and when starting the renovation I only did the trans because it was out and not because of a failure. When it was last run, it slipped a little but in the forward gear and not take off from 0 mph. Is there a change the control pressures are over running the sprag. I know that I if I have to take it out again, I’ll change everything in the sprag or is the a chance the modulator valve is sticking and would it cause this.
Regardless I thank you for the help. I think we were heading in the right direction. I have learned a a lot about transmission with this first rebuild.
Thanks again
I changed the subject line of this post as the original title was not the correct symptom. People seem to be replying to the original title and not what was the complaint in the message body.
Line pressure has no effect on the sprag’s ability to hold. The only oil the sprag uses is for lube. This lube oil comes from the return line to the trans—the rear cooler line. A plugged cooler circuit would cause the sprag to wear and eventually cause this symptom.
If the modulator valve were in backwards causing the symptom on no forward clutch apply in D, it would also have no fwd clutch apply in manual 1 and 2. If I get a chance tomorrow I will look at an oil circuit diagram. But, I am pretty sure the modulator valve is not the problem. I’d be surprised if it were the cause. However, if you wanted to put a pan under the trans and pull the modulator. Use a pen magnet to pull the valve. The spool end goes in first----and----the stem end faces the modulator. Install the pin then the modulator.
When you get tired of manually shifting the trans. Pull it and check the sprag really well. Look at the outer race for cracks. Run your finger nail across the outer diameter of the inner race. If you feel grooving or any step between where the rollers ride and the untouched area, the race is worn. Just replace the springs and rollers.
If you have questions when this is apart, give me a call 413-663-3336. Open 8am to atleast 6pm on weekdays. I am usually not there on weekend days, but sometimes at night.
Thanks, had to leave the car alone, my daughter gets married this weekend and she wanted me to have the car ready so they could leave the wedding in it, but not enough time and to much to do. I’ let you know how the trans does and give me time to get over the last tear down. Working on the interior for a while, but once I get it finished, I’ll probably take it out again. I really want to get it right.
Thanks again for your help and the phone number. We’ll talk soon.