bw1339

Hello,
I rebuilt the TH425 in my '66 Toronado. It now starts in second gear and shifts normally to third. I can shift it manually and force it to start in first. When I put it in Drive, the first time I take off, it does 1-2-3. If I stop afterwards without taking it out of Drive, it only does 2-3.

I have cleaned the governor with lacquer thinner and has made no difference. Any suggestions?

Thanks.

matt69olds

Make sure the modulator valve is free to move. Did you replace the vacuum modulator? Any possibility that you bent the modulator reinstalling the trans? If all that checks ok, it’s time to get back into the valve body. For whatever reason, it sounds like the 1-2 shift valve doesn’t want to return to the downshift position. Make sure the valve is free to move. If you move the valve against the spring, it should snap back to the relaxed position.

bw1339

Hello,
Vac modulator is new and as far as I can see, it's undamaged. My next step was to shift the adjustment a bit and see if that made any difference. I do have another modulator I can try.

The governor in TH425s is encased in a sheetmetal can, so I can't actuate it or access the springs. I will pull it again and have a closer look. I think I can remove the spool inside if I pull the gear.

On the 1-2 shift valve - From the reading I've done, most people who have trouble with this are stuck in 1st gear. Being spring loaded, I'm assuming the resting position of the valve is in the 1st gear position.

What you say, does it involve removing the valve body, or just pushing the "plug" past the captive pin?

When I rebuilt this transmission, since there were no O-rings or seals to replace inside, I didn't remove the valves inside the valve body.

Much appreciated.

matt69olds

The shift valves are spring loaded in the downshift position. It’s a very light spring, so any crud can cause it to hang up. Since you say you have first gear whenever going from reverse to drive, and it only has 2nd gear starts after it makes a 1-2 shift, that suggests to me the valve doesn’t want to return to its downshift position.

Make sure the governor weights are free to move. It may help loosening the screw on the modulator, but I doubt it does anything. Don’t turn it more than 2-3 turns at most. If that doesn’t work, you might try loosening and then retorquing the valve body bolts, starting from the center and working your way around in a circle. I suppose it’s possible that the valve body could be distorted if it’s not torqued evenly.


If all this accomplished nothing, time to clean out the valve body. If you have a manual with detailed illustrations, that’s a big help. If you don’t have a manual, either get one of find some detailed illustrations. Make sure you have a large clean well lite work area. Work slowly, don’t use excessive force removing the valves. Remove the roll pin, remove the valves making sure you note what the order and orientation of each. If a valve won’t come out, spray some brake clean into the bore, and use a pick to work it free. Once the bore is empty, throughly clean the bore, valves, and sleeves. Carefully reinstall the part in the correct order. Make sure each valve moves freely, and snaps back against the spring tension.

bw1339

So I took apart the governor and cleaned it. The spool moved but had a small amount of drag. There must have been some varnish left from my prior attempts to clean it.

I am pleased to report that it now starts in 1st gear. As I decelerate, I feel it downshifting into 1st too. I can't describe how relieved I am, since pulling the transmission requires pulling the engine too. Thank you for all your advice.