My '82 is the first automatic transmission I've owned. It was a pain at first but I'm starting to like it.

I changed the fluid and filter a few months ago because it was shifting waaay too quickly. It would go through all 3 gears by 15mph and was likewise very low-RPM sluggish. In addition, it always felt like it was locked in gear, pushing against the brake and coasting like a manual still in gear.

Just after changing the fluid, it ran like a dream, allowing the engine to get up a bit in the rev band before a crisp shift into 2nd, revs build and another well-executed shift into 3rd. By the first hour or two after the change, it degraded to it's old habit.

I realized that I overfilled the transmission. In addition, the vacuum line into the modulator on the pass side of the tranny looked like it was going to break off; old and crusty. I pulled out the modulator and drained a good amount of still-red fluid out, turned in the small screw in the modulator to delay the shifts, and reassembled.

Now, fluid level (when warm) is at the full mark. The trans shifts much better, but only because the "still-in-gear" drag is gone. I can get up to a good speed and let off the gas pedal, and it will return to idle but coast very well. However, the shifts are still pretty close together (even under full throttle) and it just acts like a CVT...revving up to a higher stall speed before pushing off, and then two relatively quick shifts to 2nd and 3rd. It's better but the quick shifting is still there.

Should I turn in the modulator screw some more? Is the transmission slipping more and will that be a problem? It feels better and "happier" around town, but it wasn't as good as when I first changed the fluid.

been a while since i had a 400 apart but i think your issues are in the valve body. probably a sticking valve. let someone else chime in. if you ask me thirty years ago i could have told you off the top of my head what to look for. being they are to close of shifts i don't think the vacuum modulator adjustment will help . you may want to pull the gov and check springs and weights but i think its in the main valve body.

I agree with farmer0 1, but I don't know too much bout the older trannies. Maybe Mike L. will chime in.

Riley

maybe a tv cable...but most likely the valve body

the 700r4 had a tv cable i belive all the 400 ran vacuum modulator with vacuum from the pump on the engine.

The governor controls the max speed in each gear before the shift. The modulator "modulates" this speed, based on engine load (throttle position). The vacuum system is rather difficult to adjust but it can be done. The vacuum regulator valve on the side of the injector pump is supposed to act as a controlled vacuum leak varying the signal to the modulator, based on throttle position. I start by checking vacuum signal to the modulator with a vacuum gauge while applying vacuum to the inlet side of the VRV with a vacuum pump. I use my Robinaire A/C evacuator pump to supply the vacuum. If the VRV will not hold 11" at closed throttle, I try another VRV. Once the VRV works, I check engine vacuum pump signal to the VRV, if low, look for a leak, check the vacuum pump, etc. Most tight systems will pull 26 - 28" of vacuum.

Once vacuum signal is established, I install an after-market, adjustable modulator and begin road testing, stopping and adjusting the screw inside the vacuum hose fitting of the modulator until I arrive at shift points that approach normal. It's usually a little below a good gas engine shift point.

Yes, it's time consuming but most early 6.2 trucks w/THM 400's had problems with shift points being too low. I've seen some trucks that had the vacuum modulator disconnected and governor weights/springs changed to "tailor" the shift to work without the VRV. I prefer to use the VRV and make it work.

Fred, you sound like the guy I need to talk to! :)

If there is a higher "stall" speed from adjusting the modulator "in", am I burning the clutches in the transmission more because of the slippage? Should I back the modulator screw back out, or am I not doing any damage?

replace the modulater. when you have oil on the vacum side it is toast. if you still have stacked shifts you need to check the vacum supply. you have to realize that this is a artificial vacum system. not as sensitive as a gas pot. first check the pump. if you have 20" your doing good. next go to the ip. the regulator valve is on the right side. plug into the output side and it should still read 20" maybe a little less thats ok. no less than 15" though. crack the throttle and watch the gauge. it should drop to zero and bounce back up again. check all vacum lines for swelling and mushyness[is that a word?] atf destroys rubber lines. remember that this was a very poor system at best and may take some fudging to make it sort of work. no vacum no shift, constant vacum early shift.