TH400 Modulator removal

Tranny still only shifting at high revs. I'm considering takeing the modulator off and adjusting the valve in it. had a new one installed when i first had gear changeing issues, gave it to the autoshop and had them install it, but don't remember any screws on it. Is it a simple pop out with a flathead screwdriver item or is there screw mouted on it ? TH400 Modulator removal

 

You do not need to remove the modulator to adjust it. The adjustment screw is inside the vacuum port and can be adjusted with a small screwdriver.

I do not think this is causing your high RPM shift problem. it sounds like the downshift soleniod is stuck open simulating how the transmission is designed to shift (or downshift) at WOT, at other operating speeds.

This could be caused by a stuck downshift switch. Also, if the transmission pan has been dented, sometimes the cover on the downshift soleniod also gets dented causing it stick open.

If the downshift swtich checks out to be OK, you will have to pull the pan to check this out. The downshift soleniod can be checked with a 12 volt power source.

Thanks,


Ken

 

"BAM" !!! Sitting at my desk at work, provisioning a modem, and "BAM" !! Transmission solution pops into my head.. "Like a light just came on, out of no where" !! Here are my thoughts and tell me what you all think ? the original Vacuum modulator on my 74/TH400, was a large canistor style module (allowing more vaccum,also meaning more pressure) where the new one, "is a small tube type module", probably not even allowing half of what was given before. guess i need to go get the older big canistor again.

 

The additional volume of the larger canister will not "allow more vacuum". As manifold vacuum varies with load and throttle position, so will the pressure inside the modulator. The calibration of each modulator is done with the diaphragm's area in mind so the transmission won't know the difference.

Devon

 

So your saying small or large wouldn't be any difference ? the larger module wouldn't hold any additional volume of pressure in it then the small one. I'm doing a comparison to the two, like an vaccum tank does for the A.C. unit on some cars. tranny changes only under heavy load in the beginning of driveing. If i stop at the the light rev the engine a few times, and then go to drive, it goes down the road as it should,"no problems after that"

 

Nope, something else is up, possibly with the governor or governor circuit which by default comtrols WOT upshifts when the modulator sees zero vacuum. The modulator does not hold a vacuum like your power brake booster does. The modulator must see manifold vacuum in real time in order to give the modulator circuit the correct input.

Devon

 

spell check is a good option

 

So is capitalization and punctuation, Mr. Wizard.

Welcome aboard. Great way to make an entrance, by the way.

Devon

 

make sure you have manifold vacuum down to the modulator. without it it will shift extremely late.

 

There is an option on modulators. 6 cylinder engines have a little less vacuum, and a larger area diaphragm (and larger outer dia) vac modulator. I don't know if one of these will help or hurt, but its easy to try. The vac mod has basically a ramp and a set point (set where the ramp starts). You adjust the set point, but the ramp is fixed.

Same modulator works on THM350 or 400.

I think I have a big container of modulators, never knew what to do with 6 cylinder types. good luck, Bruce Roe

 

Great end to the modulator drama.. Yesterday install new shocks on the front of my 68, and "Wow" what a difference, between a new set of shocks and a old 40+ year set is ! (Lol). ok ok ok, well while under the car i decided to rerun a new vaccum line to the modulator and also adjust the valve some on it. Two turns of a very small flat head screwdriver to the modulator, new vaccum line, and "Bam" a smooth, but powerful running 68 skylark with a 455/400 tranny under the hood !! Changes gears so smoothly, as a passenger you wouldn't even notice it. Wow what a difference