Speed Control

[ODZKing]

I've posted this over at the third gen forum as well. I am having a problem with the speed control.
I have gone through the service manual as well as reading all the info I have on speed control units.
Checked the switch at the column, that checks out good. Checked the stop lamp switch, good ... needed adjustment but good.
I Checked the wiring to the servo, good. In fact the manual says that if you take the double plug and unplug it while the system is on, you will hear a click and I did. Servo setting up or some such thing. I adjusted the linkage to the carb best I could according to the instructions.
I swapped servos with another one I have, ... still doesn't work.
So I checked and rechecked switches and ran a new hose to the servo for vacuum.  Even checked to be sure there was vacuum with the car running.
Is it possible the linkage is that far out that it doesn't engage ... it is tight with the wiggle room they recommend, 1/16. The cable is fine as it was tight when I replaced the servo.
Still I click the button and nothing. What is left not to work? What am I missing here?
Is it possible I have 2 bad servo units?


PS, Not all servos are the same.  I had an issue with the first one that there wasn't much room with the support bar there. I corrected it BUT, the next servo I put on was the one from a 74 Charger/B Body.  IT TILTS DOWN to give more room for the cables ... so even though the servo's may look and work identical, they are not the same physically.
Just FYI.

[A383Wing]

what speed are you trying to engage the servo at? Some won't work below 40 MPH

[ODZKing]

what speed are you trying to engage the servo at? Some won't work below 40 MPH

55-60

[Nacho-RT74]

what speed are you trying to engage the servo at? Some won't work below 40 MPH

actually just read they don't work below 30 mph

[John_Kunkel]

Is it possible I have 2 bad servo units?

A resounding YES.

I once went to some Pick-N-Pull wrecking yards and collected a bunch of complete systems ($9.95 for a complete system) and less than half of the servos worked. Some didn't work at all and some had other issues like surging and flareup on engagement.