Here is some valuable the-devil-is-in-the-details info on what I found while doing air pressure tests on the hydraulic passages of my C6 transmission.
As a brief recap of where we last left off in my personal Odyssey, I had rebuilt my engine, passed its dyno test, installed the engine along with a C6 transmission I paid “an expert” to rebuild for me, and successfully started the engine. While it appeared the truck would go forward, it clearly would not go in reverse. D’OH!
As such, I took it upon myself to attempt to learn how to rebuild a C6 transmission using the education materials described in an earlier post, “Ford C6 Transmission Testing, Diagnosis, and Repair Resources for DIY Mechanics.” During the learning process, I hope to diagnose why the transmission does not work when shifted in "Reverse.”
What I Have Done Since Then
Because I already had the valve body removed from my transmission last Spring when I was making sure that the shifter, linkage connection, manual lever, and valve were all aligned correctly and that the correct detents were engaged with each shift, I decided to perform air pressure tests of the transmission by blowing air into the hydraulic passages shown in the image below, recommended by the manual.
Per the "Automatic Transmission Diagnosis C3-C4-C6-FMX" manual, applying 25 psi blasts of air into the following hydraulic passages should produce the following results:
1. Forward Clutch Test
· A dull thud sound, if it is okay―I got a dull thud.
· A hissing sound if it is leaking―No hissing.
2. Reverse-High Clutch Test
· A dull thud sound, if it is okay―I got a light thud and possibly a little hiss, but not 100% sure.
· A hissing sound if it is leaking―Not sure.
3. Low-Reverse Clutch Test
· A dull thud sound, if it is okay―I got a dull thud.
· A hissing sound if it is leaking―No hissing.
4. Governor Test
· A click, whistle, or buzz sound means it is okay. Another manual describes it as likened to a toy plane motor type of sound―I got a hiss or wide open blow of air sound.
· A hiss sound if it leaks―Not a hiss so much as a wide open hole sound.
5. Intermediate Servo Apply Test
· The clutch band tightens, which means it is okay―I saw no band movement.
· It does not tighten, or hisses means it is leaking―Air moved right through and there was no band movement.
6. Intermediate Servo Release Test
· The band releases it is okay―Again, no movement of the band.
· The band does not release, and there is a hiss, which means it is leaking―No band movement was observed, and the air blew freely, but it was not a hiss.
Notes About Air Pressure Tests 1-6
After trying the tests several times and playing around with how I was doing it, I came to the following conclusions:
1. You have to ensure a very good seal between the air hose gun nozzle and the passage holes. While playing around with repeated tests, I found that making an improved gun nozzle, as shown in the image below, improved the test results in all tests. The air gun nozzle improvement involved an added long hose and a caulk tube nozzle that fit the hydraulic passages better than the smaller snub-nosed air gun pictured below it that I used initially.
2. The manuals differed in which passage hole the governor air pressure is used. In one manual, it indicates blowing into the Governor "input passage" to do the test; however, another manual indicates blowing into the Governor pressure "output passage". When I used my improved air gun and did both holes separately, neither produced the desired sound, indicating it was okay. However, by accident, while fumbling around with the two Governor passages, I discovered that by blowing into the pressure output passage and covering the input passage slightly with a fingertip, I did get a strong vibrating sound, much like a toy airplane motor running. Go figure.
3. Encouraged by the improved air gun and the new Governor test results, I repeated tests 1-6 three times with strongly positive results in everything but tests 5 and 6, where the loose band now moved but only slightly. Testing it a little further to determine where the air was flowing, I found that to air pressure test the Intermediate Servo in tests 5 and 6, you have to cover all of the passages immediately surrounding the Intermediate Servo "apply" and "release" passage holes to get the blown air to actuate the band. Only after using a very strong duct tape to cover the surrounding passages, did the band began to move more pronounced. Furthermore, by readjusting the band tension as described in one manual by tightening the band adjustment screw to 10 ft-pd torque and then backing off 1 and ½ turns before tightening the locking nut, the servo band now was not as loose as before and now had between a ¼ to 1/8-inch gap between the band surface and the drum. Blowing air into the intermediate servo apply passage showed marked improvement in the band tightening and releasing.
4. For a better idea of what those air pressure tests look and sound like, here are some videos to clarify the sounds I observed that appear to be positive.
Forward Clutch and Reverse-High Clutch Test Sounds
Low-Reverse Clutch Test Sounds
Governor Test Sounds
Intermediate Servo Apply Test Sound and Band Movement
Lessons Learned Thus Far
· Always consult multiple manuals to confirm whether a procedure is correct or not.
· Play around with the tests to see why or how a negative result occurs. If I had not inadvertently covered one passage while applying air to a nearby passage, I would have missed what would otherwise have been positive results.
· Never rely on only one round of testing. The position of the air gun nozzle and improving its mating to the passage holes made a big difference between the first round of tests and the ones that followed.
Where to Go from Here
My transmission's problem of not engaging in reverse most likely has to do with the Reverse-High Clutch not engaging with the drum band because the band was not properly adjusted by the person who rebuilt the transmission. From the appearance of how loose the band was, it is likely it was just slapped together, and no adjustment had been made before the transmission was returned to me.
My hope is that now that the band is tightened, once I have re-installed the valve body and poured another $50 or so of transmission fluid into the transmission, my truck will go both forward and reverse. I will be cautiously optimistic here.
Should the problem remain (or a new one arise), I will then proceed to attach my vintage Ford transmission tester and do the control pressure tests of vacuum and transmission fluid pressures under varying gear shifts to see what the pressure readings indicate toward a diagnosis.
In other words, if readjusting the drum band does not solve my problem, there is more I can do to narrow down the diagnosis and find the cause(s) of why my "professionally rebuilt" transmission is not working as it should.
Until then,
Adios Amigos
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