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· Master Story Teller & Fabricator
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Discussion Starter · #21 ·
There is no need for you to get one of these adjustable modulators, they are only used for special circumstances. The only conceivable need for one of these would be if you installed a Combo cam on an automatic Opel. The Combo cams lower the manifold pressure and can cause the auto trannies to shift prematurely. I only know about the Combo cam thing, I don't know what the situation is with "racier" cams. Generally, guys with the "racier" cams have stick shifts, so the premature or, in my case, late shifting isn't an issue.
 

· Master Story Teller & Fabricator
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Discussion Starter · #22 ·
I'm adding the post below from another thread because it applies to this thread:

From Knorm:

<<< Also here’s something to help you figure out the shifting issue with much more validity than my cursory knowledge of auto transmissions. >>>

https://shop.ukrtrans.biz/wp-content/uploads/catalogs/TH180.pdf

Font Line Parallel Document Letter


Font Line Parallel Document Letter


More from Knorm:

<<< So now about your vacuum modulator. You probably need 15inHg to operate your modulator acceptably. So if you are seeing 14inHG when you test 3000rpm NO LOAD on your car you have an issue. If you are seeing 15-20 like me you have no issue. The modulator works by reading the vacuum signal. When it see a large load (low vacuum) it will shift to a lower gear. When it sees a low load (high vacuum) it will shift to a higher gear. There are different modulators available for the GM180 that have different spring pressure and shift differently. There’s also an adjustable one out there.

Now to the quoted comment:

What that is referring to is the intake valve closing AFTER the optimum point of rotation.

You can try check your lash while the engine is running with the vacuum gauge plugged in. I generally adjust my valves while running and looking at a vacuum gauge so I can experiment and obtain the best vacuum at idle. You may have your intake valves too tight, but I kind of doubt it. The hydraulic lifters are pretty forgiving.

I’d check your cam timing first since we know your engine has been milled. kwilford and Mike Meier have both written stuff on that procedure I think. You may have bought firstopels timing system back in the day too. If so visit that thread and put it to use.

I would highly recommend eliminating the cam timing variable before spending time getting messy and trying anything else. >>>
 

· Can Opeler
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4,147 Posts
Don’t forget to read about the governor too. It connects to the modulator so it probably has some control over it too. I don’t know anything about how that system works though. I just know it keeps track of speeds for safe shifting.
 
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· Master Story Teller & Fabricator
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Discussion Starter · #24 ·
I did some research and it seems that a company called Rostra is the primary manufacturer of the various types of these. This is their website link to the page that lists their TH180 adjustable modulators:


Rostra lists 3 types as being available, although several more may be out there. The 3 I ordered have arrived:

Metal Nickel Household hardware Gas Auto part


That's great, but I still don't know what the vacuum range on any of these are and which would be most appropriate to try out first. It's yucky to remove them and screw in a new one because tranny fluid comes pouring out and I'd really like to only have to do this once with the most likely candidate. So I found a PDF on the Rostra site that lists their modulators, here is the page that lists the TH180 ones:

Font Parallel Pattern Number Rectangle


If you look in the Technical Data column you'll see FC and ADJ, I figure that the FC ones are Fixed and the ADJ ones are Adjustable. Notice that the first one is Blue and is spec'd for 69-70 Opels. After the FC's and ADJ's you see numbers, which I guess is the vacuum range. Now, I don't need the Blue Fixed one for stock Opels, I need one that can deal with whatever my problem is, which I figure is caused by low vacuum. Since the Blue Fixed one says "15-16" and the Blue Adj. one says "15-17", I figure that there isn't much difference between them. The Dual Green has the lowest numbers at "9.5-11.5" and the Dual Purple is somewhere in the middle at "12-14". Dual meaning: 2 stripes. So, I have the 3 types.

Which do I try first?

FYI: I think my reluctant tranny shifting is something caused by my throttle bodies, or valve adjustment, or valve timing, or spark timing/misfire, or something other than a problem with the transmission and that the problem is temporary until I find the culprit. Keep in mind that my tranny shifted better than any auto tranny GT I have ever driven over 40 years and it's a rebuilt auto tranny with new belts and stuff inside and I've never abused it, I very much doubt that there's anything wrong with the tranny, it's something external causing the shift problem. I bought these modulators in case I DON'T find the problem and I have to treat the symptom and not the cause.
 

· Master Story Teller & Fabricator
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Discussion Starter · #25 · (Edited)
I found some more info about adjusting these gizmos that puts it in simple terms:

Question:
"I did a search and found that to make my TH400 shift later I need to tighten the flathead screw in the vacuum modulator clockwise. Is this a micro-adustment or does it take full turns to get a significant difference? For example, the secondary valve spring in a Quardajet is a micro-adjustment -- every 1/8th to 1/16th of a turn makes a huge difference. Is the vacuum modulator that sensitive, or does it take 1/4, 1/2, full turns before a change is noticed? I know trial-and-error would give me the answer, but since I don't have a space to work on it (street parking and scissor jack) I'd like to have a baseline to save time before I jack the car up-and-down a dozen times. "

Answers:
"From the Summit Adjustable Vacuum Modulator instructions...."To adjust the shift points of the transmission, disconnect the vacuum hose. With a flat blade screw driver turn the screw on the inside of the modulator end clockwise to raise the shift points and firm up the shifts. To lower the shift points turn the screw counter clockwise, this will also soften the shifts. Only turn the screw two full turns at a time, reconnect the vacuum line to the modulator, and then test drive the vehicle. Re-adjust if necessary." "

"Modulator adjustment doesn't make it shift harder. You have to modify the valve body for that. Adjusting the modulator just changes the shift points or rpm at which the transmission shifts. (ie: holds in a particular gear for either a longer or shorter period of time.)"

"The Modulator will make the shift happen by engine VACUUM... The way to set the modulator is by driving slowly without using much acceleration and allowing the VACUUM to do the shift and set the screw in the modulator for that transition point I set mine about 25MPH.... Once you romp on the gas pedal then the "detent" or "kick-down" cable over-rides the vacuum modulator for the 1-2 shift... Then it is in the governor and valve body for the 1-2 shift........Programming this requires dropping the valve body more times times till you get it where you want...but this will change with temperature and RPM......that is why many use a floor shifter in manual mode so you can shift at your ideal spots where torque is peaking.. "


So, what I'm gleaning from this handful of comments and the quote from the Summit site:
1) To make the tranny shift later, turn the screw clockwise.
2) To make the tranny shift earlier, turn the screw counter-clockwise.
3) Only turn the screw 2 times on your first adjustment attempt, drive, and readjust as necessary.
4) Do your driving tests at low speed/low throttle of about 25mph. Too much throttle will make the tranny shift later than "normal".

In regards to how many screw turns to adjust them and how many turns you can do before you can't adjust it anymore or it falls apart, below is a pic of the 3 modulators I received and the screw positions when I received them. There appears to be at least 4 turns of adjustment possible and possibly several more. Read my comments in the pic:

Camera lens Cameras & optics Lens Camera accessory Circle
 

· Master Story Teller & Fabricator
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Discussion Starter · #26 ·
One thing that I am going to look into to solve my shifting problem is the possibility of inserting the venturis from my 45DCOE carb into the throttle barrels. The car shifted GREAT with a 45 DCOE on it, but now it shifts like crapp with 45mm throttle bodies. So what is the difference between the two? The DCOE has venturis/restrictors in the barrels, the TB's do not. The TB's are 45mm all the way through, the 45 DCOE had 38mm venturis in it, effectively giving it a 38mm passageway. If I slip the venturis into the TB barrels and it fixes the problem.....well......that's what the problem is: My throttle bodies are two big to let my tranny shift normally. I really should install, say, 40mm throttle bodies to mimic the 38mm venturis. That would be the simple answer, but there might be other factors involved, if this works. Venturis increase the velocity of the air through the carb after the venturis. Maybe the increase in velocity will improve the engine tranny performance with the TB's.

I'll find out.....
 

· Opeler
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69 Posts
It's been a long time since I have owned an AT Opel but my Kadett wagon shifted smoothly and I remember it had a bowden detent cable requiring an adjustment procedure. It connected a cable from a lever on the AT case to the throttle linkage. It played a role in vacuum modulator pressure. I was always happy that my car had excellent part throttle and full throttle kick down response.
 

· Master Story Teller & Fabricator
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Discussion Starter · #28 ·
I did my hydraulic lifter 1/4 turn looser adjustment and I'm now at 1/2 of a turn preload. I think I found a lifter that was partially collapsing or compressing too easily.

I took it for a drive and it did seem that the tranny was more willing to shift without me taking my foot off the gas, but still nowhere near right. So I installed the purple stripe 12-14 Hg adjustment range one and screwed in it's adjustment screw 2 turns. I should have then been close to 12 Hg.

Hooray! My tranny shifted without me taking my foot off the pedal! Although good, it was shifting a little too early and I came home, parked on the lift, raised it, and unscrewed it one turn. I should be close to 13 hgs, but who knows. When I do another test drive I'll find out if I've got it just right.

The Conclusion:
Adjustable auto tranny modulators work! It fixed my problem or at least the symptom of my problem. Good enough for now. Now I can just focus on the engine FI tuning without the tranny complicating matters.

🤩
 
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