Transmission Vacuum Nightmare
 

Another issue with my cursed low-mileage, rust-free 300d.

Driving yesterday, all is fine. Shut off. Get back in 15 min later, can’t get out of second gear. Shut off, unplug vacuum from transmission, drive home totally fine, all gears good.

Get in this morning, can’t get out of second. Driving 20 mph. After 15 min, I have 3rd and fourth again.


This car is a mess where the vacuum in concerned. I currently have to shut off the engine by pressing the two ignition vacuum lines together (which are otherwise loose and disconnected under the dash.) the valve in the ignition assembly is bad so this is the temporary fix. see attached image

I need to make a 2 hour drive tonight. Am I asking for trouble? I really really need to make this drive.

I wouldn’t drive with an open vacuum line. May affect vac pump duty cycle and wear.

Was under the impression that vac at the transmission affected shift hardness not the shift itself. You sure the Bowden cable is OK?

Bowden is fine, I’ll reconnect and see what I get when I’m out of work. I haven’t lost a gear before on this car, never a slip or an excessively hard shift even. I know it’s vacuum but so much to troubleshoot when I really need to get on the road once I get out of work.

The vacuum SHOULD just control the quality/timing, but it was an immediate fix yesterday. I know I’m going to spend awhile chasing this down. Going to disconnect the kickdown for good measure.

Any other suggestions for a quick fix to get me through the evening?

Has it been unwise to drive it with my....improvised shutoff method (ignition lines open) I teed them off a couple times but I swear it made for a rougher idle.

Vacuum has nothing to do with gear shifting or timing other than softening shifts when at light throttle. Unhooking the vacuum line was most likely a coincidence.

Vacuum has absolutely NOTHING to do with idle or fuelling of the engine. This is not a gas engine, vacuum leaks do nothing to affect fuel mixture since the suction is created by an external pump, not the intake manifold.

You haven't mentioned where the fluid level is on the dipstick. I'd be checking that before anything else. Sticking in gears or losing gears is typically due to low fluid pressure in the valve body, valve body wear, or internal problems with the transmission (broken spring assemblies in the valve body, worn or broken bands, etc).

Making a 2 hour drive is your decision. You may be walking home or calling an Uber. Poor shifting is one thing, losing gears or hanging up in the shift sequence is a different animal.

chiming in to second other's opinions - the only issues I've ever noticed when totally disconnecting the vac line from my 82 300d trans modulator have been very harsh/clunky downshifts at 2-1. I don't think it would impact upshifts, or generally shifting through the gears at all.

Maybe. There have been threads with theories regarding vacuum pumps and “parking” pistons when a certain level is reached. In this circumstance the leak will cause maximum flow and thus maximum work. Pulling a hard vacuum is energy intensive, but these pumps don’t do that. But moving max air mass/flow is guaranteed to push you to the right:

https://production-ems.s3-ap-southea...e/Fig-2-EN.jpg

Not exactly your pump but used to illustrate.

IF our pumps operate on such a principle, and more flow = more work and energy, then it could cause accelerated wear. More load = shorter life.

So unless you’ve fully capped your vac system so the pump sees a “leak free” situation, it may be seeing higher mass flow and thus more work. That may result in shorter life.

The long and short of it is that your vac is not causing the tranny to miss shifts; you probably have something inside the tranny going bad.

Are you sure the ignition switch is bad? It could just be the rubber connectors on the plastic lines. From what I hear on the forum those switches just almost never go bad. I pulled my lines about a year ago and the rubber connectors were still perfect after 38 years (failure to cut off was a bad shutoff valve instead). You can get new rubber connectors from Pelican.