Alex Cat

I have the following symptons on my 2 speed auto ST300 gear box which I'm trying to fix and would need some help troubleshooting and finding the root of the issues.

I'm guessing the switch pitch isn't adjusted properly and plan on following the adjustment advice given here:

https://classicoldsmobile.com/forums...ustment-17232/

However, I don't fully understand the logic of the adjustment and how this gear box works and before I start I thought I would share the symptoms I have to make sure I'm on the right path:

1) the gear won't shift into second gear without me shifting into low gear and then shifting into high gear manually.

2) when the car comes to a stop it downshifts into first gear with a strong "thud" or "chug".

I'm guessing this all comes from the switch pitch adjustment but want to be sure that's the only issue. Any help would be appreciated.

joe_padavano

The switch pitch function only changes the angle of the stator vanes in the torque converter. It has nothing to do with shifting. This sounds like an internal trans problem.

JohnnyBs68S

Some thoughts (assuming you've already checked to be sure the fluid level is proper): Before I owned my '68 w/ the Jetaway/ST300, there was a time where it had the same symptom of a hard downshift to 1st. Dad discovered he had the idle speed set too high, and after proper adjustment it stopped doing that.

The manual upshift may be caused by a shorted downshift switch, keeping the trans in 1st until you manually "command" it w/ the shifter. There might be an issue w/ the vacuum modulator causing this as well. Normally at WOT it would shift to 2nd at ~60 MPH (70 MPH if I stood on it hard enough to engage the downshift switch). If I put it in 1st and floored it, I could get it to shift to 2nd early (before 60) if I moved the shifter to D even while still at WOT (minimal vacuum). I don't recall ever trying this while standing on it w/ the downshift switch engaged.

oldcutlass

When I first got my 67, it would slam into low gear when I came to a slow stop. It turned out to be the modulator was connected to ported vacuum. Check your modulator vacuum and also see if its leaking fluid.

Alex Cat

Thank you all. Indeed I do have a transmission fluid leak for which I can't seem to identify the source. From what you are saying it could be the modulator vacuum. I don't even know where it is or what it does but I will have to do some research there then.

Regarding the shifting into second it does shift up when it goes above 60mph. Is this standard then? I thought it should shift up much earlier. I just find driving it in first gear below 60 doesn't feel right because the car goes so high in the RPMs but maybe this is what it is supposed to be like?

joe_padavano

This is a classic vacuum modulator problem. Either the vacuum hose is disconnected or plugged, or the modulator is bad. The hose to the modulator must be connected directly to a manifold vacuum port.

oldcutlass

There is a metal vacuum line running down to the transmission with a short hose on the end connecting to the modulator. Looks like this.

joe_padavano

Actually, the original vacuum modulator will look like the one on the right. New replacements look like the small one on the left.

oldcutlass

Neither of my 67's had the large one, I guess it was changed over the years prior to me getting them. Either way, we agree, his description points to a modulator issue.

Alex Cat

Thank you all for all the information. Very helpful.

Just so I understand this correctly the faulty vacuum modulator is :
1) causing the car to have very high RPM when in first gear?
2) ausing the "thud" I can hear and feel when I'm coming to a stop and the car shifts back down from second to first gear?
3) most likely source of the transmission oil leak that I cannot seem to locate?

I just want to be sure that I'm diagnosing all the issues correctly before I start doing the repairs and changing the part out.

oldcutlass

Yes, those are classic symptoms of a modulator problem. It can be a leaking or disconnected vacuum line to it or a failed modulator. The leak can be the failed modulator allowing transmission fluid in through the vacuum line and then burned in the engine. A lot of times you'll see white smoke out the exhaust pipe.