67 c6 servo adjustment

I’ve been checking servo adjustment advice but there seems to be conflicting suggestions. I’ve carefully set it as per shop manual, but the 1-2 upshift is very strange, to say the least.

It may be time to disassemble the transmission to see why. You have to drain the fluid and then remove the valve body to see the band and the servo and the apply arm. I took mine apart and found the apply arm was broken in half. My transmission (1968 C6 with the R Servo) was working but the 1-2 shift was sloppy. It could be the wrong apply arm is installed. Every servo takes a specific apply arm; it could be yours was replaced with the incorrect one. Or the band material could be worn away. Or the servo could be leaking. All sorts of things can be wrong - you have to disassemble it to find out what.

For the 1-2 shift you are only adding the servo to stop the sun gear. I would perform an air pressure check with 25-40 lbs of regulated air with a rubber tipped gun. The pressure should hold for several seconds after releasing the gun lever. The rubber on the servo has probably delaminated. Servos can be replaced in chassis with only valve body removal. Ive attached a photo of a delaminated servo.

Can you define “very strange” ? What adjustment are you doing to the servo? Or, are you moving the stud and nut on the driver’s side for the band adjustment?

Hi guys, thanks for the replies. The problem suddenly appeared some time ago. In “D” the 1-2 upshift would not work. I found I could manually select second, but it would stay in second unless I went back to D. First thing I checked was the modulator and vacuum supply - both OK. I decided to check the servo setting which appeared to be fine. Pulled the valve body out and dismantled it. No debris at all, and the valves were not scored. Decided to put a Transgo spring kit in, had a couple of minor assembly problems so contacted their support, and they were really a pleasure to deal with. Got it all assembled, with a new filter. Road test, still no 1-2 upshift – Oh bother (or words to that effect) I did find that if I drove it at high revs, then released the throttle it would then do both the 1-2 and 2-3 upshifts.
Sorry about rambling on, but possibly some may have some (Polite) suggestions.

It had no 1-2. Did it do a 1-3?----as in 3rd gear coming in on time and only missing 2nd?
Can you apply the servo using air? Regulated to 30-40 psi.
If you can get manual 2nd, it sounds like the servo is working. It’s a control problem, but not necessarily in the VB. There could be ring wear in the governor support. The modulator valve could be stuck inwards.
How did you test vacuum to the trans? Restricted vacuum or vacuum leaks will cause a disruption of signal to the trans.
The problem came on suddenly…was the car sitting or stored for a while then the next time you drove it there was no 2nd? Or did it drive fine one minute and no 2nd the next?
Mostly, valves don’t all of a sudden hang up. They do hang with debris contamination. When I rebuild a C-6, I end polishing every valve. I put the valve in the drill press chuck and spin it while applying 800 grit sand paper to the valve spools.
Do you know when the last time this trans was rebuilt? If it wasn’t too long, you should keep diagnosing. If it’s been a couple decades or never. You ought to put a rebuild kit in it. If you can do the valve body, you should be able to make your way through the whole trans with success.

Air check the servo or pull the servo cover. Seal is delaminated

If the trans has the original servo, it should be aluminum with o-rings. The bonded rubber servos came out later. Either way, the O-rings can fail too. He said he gets manual 2nd which should prove the servo as functional.

Again, many thanks for the advice. It does not shift into 3rd. I put a vacuum gauge to the line right at the modulator and got app. 14-16 inches. The car had only been sitting for 3 - 4 weeks. There was evidence of the transmission having been looked at previously, no idea when, but feel it was not a long time ago. Pleased to hear the servo is probably functioning. The modulator valve is not stuck the pin is installed and free. I shall continue trying to diagnose, as the thought of overhauling the transmission sounds a bit daunting to me with my very limited knowledge of automatics.
(Ask me about Lucas electrics then I’m in my element) Cheers, Dale.

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Dale, based on your last post it sounds like the transmission will automatically shift if you get the RPM’s/ vehicle speed high enough correct?

If that is the case the issue most likely is either in the governor or the modulator.

Does the transmission automatically downshift to first gear at a stop or do you have to do so manually? If you are having to manually shift back to first you may have worn or damaged governor seals or a sticking governor valve.

If the downshifts are occurring properly consider removing the vacuum modulator and bench testing it with a vacuum pump to ensure it is working. Verify that the pin between the modulator and the valve is in place as this sometimes is left out on a repair or overhaul. If the pin and modulator are good check the valve for proper installation, if memory serves me correctly the valve can be installed backwards.
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