Suddenly it does not shift?

So the gearbox on my 1970 Buick Electra has always worked very well and shifted really smoothly. The other day I decided I wanted to dry the L2 and L1 gears for the first time out of curiosity as I have never found use to them. Standing still, putting in L2 and driving a bit, no problem. Then when standing still again, putting in L1 and the gearbox makes some weird noise Not a very loud one, but it scared the crap out of me. Thankfully I could both drive forward and reverse still, park and neutral working as they should.

But, it seems it stays in the first gear and does not shift up when in drive. I think. So I understand I sound a bit clueless now but the the gearbox was so smooth that gear shifts often went by unnoticed before. But I could feel them from time to time, and now they do not seem to be there at all. I drove it up to around 50 mph a few times in drive and I cannot remember it revving that much before at that speed.

I have the troubleshooting chart in the Chassis Service Manual but I think the events that lead here is a better starting point. Did the gearbox maybe get stuck in whatever mechanism that only permits shift up to L1, disabling higher shifts?

 

I also drive a 1970 TH400, but I had it completely apart for
a soft part refresh a dozen years ago. If yours is original, it
is way, WAY overdue for that work. Original rubber that
directs the oil pressure can become quite stiff and hard by
now. good luck, Bruce Roe

 

Both of these checks are the first things to do . Larry is correct again .

 

Thanks guys! For some reason shifting seemed to work again again next time I drove it. Weird. I guess I have to wait and see if it pops up again.

There is a small vacuum leak in the vacuum modulator. If I hand pump it down to 20in-hg it leaks back to atmospheric pressure in 10-20 seconds. Is this acceptable?

 

It should hold 20 inches for several minutes.
You can get the "red line" adjustable type at most autoparts stores for about $20.

 

Check the electric kickdown switch, which on a 70 should be on the gas pedal..if that switch is stuck in an engaged position, it thinks your at full throttle and wont shift until you hit the governor limit, like 4500-5000 rpm. Ask me how I know....

 

I see! It does look that it should be possible to replace with everything in place, just crawling under the car?

Interesting! I have been looking through my 1970 chassis service manual for this but I cannot find any such switch. Then in Section 64-3 I read

From what I read in the below forum post it sounds like what the manual calls "switch-the-pitch" is what you are referring too? https://www.jalopyjournal.com/forum/threads/th400-switch-pitch.674586/

 

No. Switch Pitch is not included on your transmission. The last year GM included SP feature was '67
The reference is to the electric switch mounted to your gas pedal in the cabin. You may need to spritz a little electric lube onto that switch. Vacuum leak for the moderator not good.

 

Switch Pitch was a system that changed the angle of the torque converter vanes under certain conditions, and subsequently changed the stall speed of the converter. Neat system, but it was gone by 1968 so it doesnt apply here.

The electrical stitch I'm talking about SHOULD be mounted somewhere on the accelerator linkage. In 1970 is was often mounted on the actual pedal assembly and activated by the pedal hanger rod above the pivot, but in older versions it was mounted on the intake manifold, and had a contracting rod and bellows actuated by the carburetor primary linkage. Either way, this activates the "downshift" circuit in the transmission, which overrrides the vacuum modulator and will not allow the transmission to upshift by sending electrical current to a connector on the drivers side of the transmission case. If you cannot find the switch, another quick check will be to disconnect the electrical connector at the transmission case and road test. If the transmission shifts normally, you've found your problem. If not, you could have a stuck valve in the valve body, or any number of issues. Good luck.

 

Attached is a ‘71-76 pedal assembly. See the rectagular switch with the yellow wire at the top? You should have a very similar switch with a piston actuated by the pedal rod. It can stick in depressed mode, especially on a car that doesn’t see lots of pedal travel.
Patrick
https://www.ebay.com/itm/293708225010

 

There was indeed a switch below the throttle pedal and it had quiet a lot of binding in it. I treated the plastic rod on it with silicone spray and it seemed to free up pretty well. The roads are icy and actually the car would not even start today so I could not road test it. But now I have another place to check should there be issues with the gearbox later on.

Thanks for all your awesome help so far, this has all been very helpful!

 

Yes. There is a small bolt that holds the retainer against the modulator.
Remove than and the modulator pulls out of the bore.
There is a small O Ring, make sure that is on the back of the old modulator tube that goes into the bore or remove it from the bore before putting the new modulator back in (lube with petroleum jelly or ATF before installing).
There is a small spool that is in the bore, and may be a spring, so make sure it does not fall out.
There will be a few drops of fluid, but nothing to be concerned about, so you won't get a bath.
If you can, spray some carb cleaner and a small brush and compressed air to clean before you remove the old parts, to minimize getting any grit when you put the new modulator back in .

(My apologies about the "most auto part store" comment, I failed to read you were in Sweden.)

 

if you jack car up on passenger side only, it tilts fluid away from the modulator, little to no mess to change it out.

 

Awesome, thanks! Will use all these tips and tricks when I get around to change this.

 

So my hand vacuum pump is leaking, this was stupid. It is almost new so I am going to return it. Meaning I cannot tell if the vacuum modulator is leak free, but it looks rather fresh on the outside at least. Will have to make it a routine to always check vacuum pumps before usage, thought this one was so well made I shouldn't have to.

Under the car I found some other stuff though. The atf refill tube had popped out a bit and settled maybe 3/8"-1/2" over its nominal bottom position. As this biases the level measurement I probably unknowingly have overfilled the gearbox slightly. Also some of the oil pan screws were a bit loose. So hopefully I will not have the slightly leak of transmission fluid anymore now

 

Before we had hand pumps, we used our lungs. Put a clean hose on it, suck on it and cover the hole with your tongue. It will be very obvious if there is a leak!

 

My Vacuum pump looks like this.
If i push the handle full, it will leak.
If i push the handle half it will work.
Was like this from new. Goran