[dataway]

Any reason I shouldn't use this original vacuum modulator?

It works, holds vacuum ... appears to be adjustable. The new replacements are so radically different in appearance I'd rather use this one if I can.

Or ... is this style still available? NOS examples out there?

(application is a 68 TH400)

[Kenth]

They did what was supposed on many millions of vehicles from factory.
If it works, don´t fix it.

JMHO

[Schurkey]

The way I remember this deal--the huge OEM canister holds an aneroid bellows that compensates for altitude vs.manifold vacuum. Very sophisticated.

The "adjustment" screw is probably spot-welded in place. PITA to break free, but that makes it adjustable.

[dataway]

Yep, so far the adjustment doesn't want to budge. Has some kind of sealant on it that I have also seen on other originals.

[65 421 TODD]

When my ST300 was rebuilt they put a new modulator on. The car ran fine and shifted fine. Then I found an NOS correct part number Original Delco Modulator and put that on. In my opinion there was a slight noticeable difference that the car shifted just a little bit "nicer" and a little more correct.

[65 421 TODD]

"In my opinion there was a slight noticeable difference that the car shifted just a little bit "nicer" and a little more correct" WITH THE NOS DELCO MODULATOR, I Forgot to include that.

[76TA462]

I am still using the original vacuum modulator, and after a few tranny rebuilds over the years, its still doing its job just fine.

[dataway]

I guess I'll use this original then ... easy enough to test them if I think it has failed for some reason.

[242177P]

I was bored one day, had a dead one and a new one...
Didn't get any further than this, I'm easily distracted.
Ooh, something shiny! Gotta go...

[dataway]

Just tested out an aftermarket modulator compared to my OEM type. With adjustment you can get the new type close.

OEM starts movement at 7-8" of vacuum, maxes out at 15"

Aftermarket before adjustment starts at 10", maxes out at 22"
After adjustment starts at 7-8", maxes out at 19"

So basically the OEM unit is "all in" at 15" and the aftermarket at 19"

Not sure what this would mean to how the trans will shift.

[Formulajones]

It just affects your part throttle shifting. Elevation will play a roll. Adjust to your liking.

Up here at 5,000 ft I have to adjust them to spread the shifts out pretty far at part throttle so that when I drive down to sea level the shifts are more normal since they tighten up at sea level. If I adjust for something near normal up here at 5,000 feet then when I go down to sea level the darn thing is in 3rd gear before I can get out of the intersection, lol.

[Schurkey]

Originally Posted by dataway

Just tested out an aftermarket modulator compared to my OEM type. With adjustment you can get the new type close.

OEM starts movement at 7-8" of vacuum, maxes out at 15"

Aftermarket before adjustment starts at 10", maxes out at 22"
After adjustment starts at 7-8", maxes out at 19"

So basically the OEM unit is "all in" at 15" and the aftermarket at 19"

Not sure what this would mean to how the trans will shift.

No way to know unless you measure vacuum vs. valve movement. Vacuum alone isn't enough info--although we could guess that the shifts will be slightly more harsh/fast because full travel isn't achieved until additional vacuum is generated.

[vitamin c]

I was just looking into modulator adjustment in a 74 service manual and found no reference for adjustment. Are the modulators with the screw in the end "factory set" and not meant to be adjusted?

[Schurkey]

Originally Posted by vitamin c

I was just looking into modulator adjustment in a 74 service manual and found no reference for adjustment. Are the modulators with the screw in the end "factory set" and not meant to be adjusted?

Originally Posted by Schurkey

The "adjustment" screw is probably spot-welded in place. PITA to break free, but that makes it adjustable.

They're not intended to be field-adjustable. Requires some rework.

[dataway]

I was also using the tail of a caliper to watch the movement as I applied vacuum. Stroke seemed to be exactly the same, movement was quite linear on both until the last 25% of the stroke on the aftermarket unit, and maybe the last 5% on the OEM unit ... which is probably due to the much larger diaphragm on the OEM unit.

I'll put it this way ... if you had to use vacuum to control the position of the piston, the OEM unit would be way easier to use. Each inch of vacuum moved the piston about the same amount for about 95% of the stroke. Whereas the aftermarket unit in the last 25% of travel, each inch of vacuum would produce progressively less movement.