Twin Stick OD Problem

Thanks. I'm pretty sure it is something in the kickdown circuit. The only thing I have no idea how to test is the vacuum switch. i have a suspicion the switch is sticking when it kicks into overdrive but looking at the diagram, I don't see how that could be the problem.

I don't think you have to have the vacuum switch. It appears to be a safety switch of some sort so that the coil won't be grounded with the engine running. So to test your system wire around it -- connect the two leads together. BUT FIRST make sure the switch is not normally closed -- closed with the engine off. From what I can see it should be open with engine off, closed with engine running.

There is no clear indication of what the vacuum switch is for in the TSM. I think it's to prevent dropping out of overdrive during acceleration, as that would be bad! During acceleration there is low vacuum, so the switch would open. Let off the gas pedal and there is high vacuum, so the switch is closed. This isn't mentioned in the TSM -- the vacuum switch isn't mentioned at all! You can test the vacuum switch by checking continuity with engine off (should be open) and with engine idling (should be high vacuum and contacts closed).  At least that's how I THINK it should be working...

You have to understand how shifting is conducted with the Twin-Stick. Start off in first gear, use clutch and shift to second. Let off gas pedal so OD activates, shifting to "third" (2+OD). HIT THE KICK-DOWN BUTTON ON TOP OF THE GEAR SHIFT AS YOU USE THE CLUTCH TO SHIFT TO THIRD (fourth forward gear). You do those two simultaneously, and that is when torque on the trans could damage the OD unit. To prevent this there is the vacuum switch -- just in case you don't use the clutch or have the gas pedal down a bit when OD is trying to drop out. Once in (actual) third let off the gas to automatically shift into "fifth" gear.  This only works because there is a big gap between actual 2nd and 3rd in a Twin-Stick trans. On a normal three speed with OD there is little difference between 2+OD and 3rd.

Down-shifting can't be done through all five forward gears. You have to hit the kick-down button to drop from "5th" to "4th". Then you use the clutch and drop all the way to 2nd (actual 2nd), skipping "third" (2+OD). These are older non synchro first/reverse trannys -- you should NEVER down-shift into first unless at (or very near) a complete stop.