whats the cute little valve thingy on this bad boy?

[AuroraGirl]

I dont know what trans it is either(th350???) but the little vac. canister thing valve. Ive never noticed one before.

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[legopnuematic]

vacuum modulator, looks to be a th350

 

[AuroraGirl]

vacuum modulator, looks to be a th350

what does it modulate? and dont say vacuum ;(

 

[HotRodPC]

It is the vac modulator as Spencer said. It helps determine shifts points along with the governor and detent cable.

 

[legopnuematic]

From the interwebs, "The transmission vacuum modulator valve determines how much load is on the engine so the transmission can shift properly."

From experience the 1-2 shift is fine when it isn't hooked up, doesn't 2-3 without dropping the shifter to l2 and back to D

 

[Matt69olds]

The vacuum modulator “modulates” engine load and throttle to determine shift points in relation to vehicle speed. In other words, low engine loads=high engine vacuum, lower shift points and line pressure for smooth shifts. Stand on the throttle, low engine vacuum, that raises line pressure and takes more govenor pressure to force the shift valves to the upshift position.


With the vacuum modulator unhooked, as far as the valvebody in concerned the engine is at full throttle all the time. Unless other modifications are made to the valvebody it won’t shift until whatever vehicle speed it is calibrated to make a WOT shift.

 

[AuroraGirl]

The vacuum modulator “modulates” engine load and throttle to determine shift points in relation to vehicle speed. In other words, low engine loads=high engine vacuum, lower shift points and line pressure for smooth shifts. Stand on the throttle, low engine vacuum, that raises line pressure and takes more govenor pressure to force the shift valves to the upshift position.


With the vacuum modulator unhooked, as far as the valvebody in concerned the engine is at full throttle all the time. Unless other modifications are made to the valvebody it won’t shift until whatever vehicle speed it is calibrated to make a WOT shift.

My dads 75. th350. He had an issue where it seemed to not upshift for the life of it, we chocked it up to a poorly running carb and thus it kinda got parked when the front drivestrain got REAL bad and he had to unbolt the driveshaft in the front to get it home(np203, from what i read). I was gonna source a new manifold and a good carb for him to replace the current one(so no downtime with engine opened up) once it was warmer, im leaning toward holley, and set the distributor back to base timing. assuming that isnt the sole fix for his problems, do you think that regulator could have issues or goes bad? A meth head screwed with the entire thing when we were away, so I know there are a laundry list of things that could be affected. I know the guy said something about the transmission. The truck is stock except for a dump bed, and it came from High altitude montana and its primarily pulled heavy items or towed or moved snow in its life, id bet you it wasnt adjusted after coming to sea level either. But the WOT shifting thing really sounds like what I remember when it started getting bad. Its a SBC with a 400, if that meant anything different.

 

[HotRodPC]

I'd certainly check the vac modulator in that case. It may have even just come unplugged at the transmission end. That can happen as the rubber get's old and swells if it's got fluids on it and it gets spongy and rots, or even gets brittle and cracks and then leaks air sucking air instead of letting the modulator feell the full vacuum. So yes, by all means, check the vac hose at both ends. If those are good, then swap another known good modulator on it. Keep in mind, some look different than that biggo canister type. Most people now have the little brass bell looking type and they work just as well and maybe even better since they're adustable with a tiny screw inside the vac hole. You can turn it in a tighten up the spring that the vac needs to overcome to move the modulator valve and delay the shift points, or back it out to make them earlier shift points.

 

[Matt69olds]

The modulator can change shift points, but only 2-4 mph either way. If you need more of a change then the govenor needs to be modified.

 

[HotRodPC]

The modulator can change shift points, but only 2-4 mph either way. If you need more of a change then the govenor needs to be modified.

True, and even 2-4 can make a difference in shift quality. A tad bit firmer and a tad later can make a noticable difference. Tightening the detent cable to pull a bit earlier can also change the shift points and firm the shift too but you can't tell some that. They claim it's for downshift only.

 

[Bextreme04]

The location of that modulator is for a TH350. The TH400 modulator is on the passenger side near the front of the trans.

 

[FireTruck1984]

You gents are very knowledgeable! I learn a lot every day

 

[SquareRoot]

You gents are very knowledgeable! I learn a lot every day

Who you calling "gents"? I'm a meth head!