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I'm thinking I need to replace the governor for the TH400 in my '72 Supreme, but while doing my research, it seems they're not all the same, and one article I read said something about calibrating it before installation.
Yes, it is true. The shift points on a TH 400 in an 88 or a 98 will be different than those in a TH400 in a W-30 or a 442.
So why do you feel the need to replace the governor? Are the full-throttle shift points not appropriate?
Full throttle shift points are fine, but a while back, I had an issue with no upshift from first at normal throttle. I posted the question here and since then it's been fine. Then last weekend while leaving a show, it happened again. This time, I simply shifted into park then back to drive while at idle, and had no other issues on the way home. So at this point I'm just exploring the possibility of a faulty governor.
Check the condition of the vacuum hose and line from the intake manifold to the vacuum modulator. An intermittent problem isn't likely to be vacuum related but it could be and is easy to check.
The rubber hose connections, that are located on either end of the metal vacuum pipe that connects engine manifold vacuum to the transmission vacuum modulator, can split lengthwise. You have to check the rubber hose connections closely - that means remove and examine them.
The rubber hose connections, that are located on either end of the metal vacuum pipe that connects engine manifold vacuum to the transmission vacuum modulator, can split lengthwise. You have to check the rubber hose connections closely - that means remove and examine them.
I just have a rubber hose from the modulator to a metal fitting at the intake manifold, and wouldn't it be a constant issue if it were the hose?
Check the easy things first. Removing the transmission governor (some of us doubt that's what's causing your trouble) is lots more work than removing the modulator hose to examine it.
I have seen a few examples of a split/deteriorated rubber hose connection at the modulator. This condition causes problems - sometimes.
Hopefully you'll find a rubber hose that is past it's useful service life because that's an easy and inexpensive repair.
We're all here to help and share our experience.
I just have a rubber hose from the modulator to a metal fitting at the intake manifold, and wouldn't it be a constant issue if it were the hose?
Dave - I'm not certain how to interpret your statement. The modulator pipe is ~4 ft. in length. You need the entire length of modulator pipe. If you don't have the entire length of steel pipe you have a significantly long hose suspect to cracking, abrasion, pinhole leaks, tearing, etc.
Norm (THX1138)
Dave - I'm not certain how to interpret your statement. The modulator pipe is ~4 ft. in length. You need the entire length of modulator pipe. If you don't have the entire length of steel pipe you have a significantly long hose suspect to cracking, abrasion, pinhole leaks, tearing, etc.
Norm (THX1138)
I'll check it tomorrow. I don't have a metal tube from the modulator, it's just a run of vacuum hose that goes up to a t connection then onto a fitting that screws onto the back of the intake behind the carb.
Dave - (1) Your ATM (Automatic Transmission Modulator) routing is incorrect if it does not contain the full steel metal ATM pipe; (2) There should exist no "T" in the ATM vacuum routing (that I am aware of). The ATM should be routed directly/dedicated to a single intake manifold vacuum port. Your intake manifold vacuum port may vary from my location (1971 350cid) - image below demonstrates my ATM routing from the modulator pipe>rubber vacuum hose>intake manifold port. I would suggest the first thing you need is the correct ATM modulator steel pipe (as demonstrated in the 1972 PIM, above). NOTE: Yes, there exists a vertical loop in that ATM modulator pipe which functions as a thermal expansion loop. It does not wrap around or fasten to anything - it is a thermal expansion loop - don't spend one second worrying about it. This is not to say you aren't in need of a new ATM (as suggested in Post #4) - they do become faulty and require replacement. There also exists in the CSM the correct installation/adjustment of the ATM which you should examine when you remove the jerry-rigged rubber hose and install the proper ATM steel modulator pipe.
Dave - Review your 1972 CSM which demonstrates routing based upon several configurations if you have the TCS/DVCS (Transmission Controlled Spark/Distributor Vacuum Control Switch - ["Christmas Tree"]) installed & functioning. I note in the 1971 CSM there are in fact two configurations w/ a "T" which connect to the ATM (posting an image of one of them). Review your 1972 CSM illustrations for correctness - I don't own a 1972 CSM. I deleted my TCS/DVCS therefore my ATM is direct to a single intake manifold port.
Last edited by Vintage Chief; Sep 1, 2024 at 07:41 PM.
If your modulator line is vaccum hose, it’s entirely possible that vacuum is causing the hose to pinch closed, especially when it gets hot. It’s not difficult to carefully bend some brake line to make replacement modulator line. A couple of inches of rubber hose at the ends for the connections is all you need.
To answer your original question, no, not all 400 governors are the same. There are dozens, probably hundreds of different part numbers, and probably thousands of different weight and spring combinations.
Basically, the heavier the weights, the earlier the shift points. The springs can be used to fine tune the shift points. The heavier the springs, the closer the shift points become.
If you want to experiment with the govenor, I’d suggest starting with a spare. Install it, and carefully note the shift points at WOT. If you want to go further, get a governor calibration kit. TCI, B&M, TransGo, Superior, or other venders carry them. Follow the instructions and take detailed notes. Change one weight or spring at a time.
If your modulator line is vaccum hose, it’s entirely possible that vacuum is causing the hose to pinch closed, especially when it gets hot. It’s not difficult to carefully bend some brake line to make replacement modulator line. A couple of inches of rubber hose at the ends for the connections is all you need.
I dunno...I drove it back and forth to work yesterday and it was fine. I'm probably gonna take your advice and put a steel line on.
In the diagram above, it shows the vacuum connection for the modulator coming off at about the 2 o'clock position. My modulator vacuum connection comes straight out.
This would seem to indicate that I have an aftermarket generic modulator. If that is the case, I have two choices. Either A) Use a 90 deg elbow to connect the vacuum tube, or B) get the correct modulator.
Edit:. After looking for a replacement modulator, it appears that they all have the connection pointing straight out like mine. Looks like I'll be using an elbow.
Understood Joe, but the only source for this style is ebay, and the cheapest one I saw was for a '77 Pontiac and 50 bucks was the price. Besides, if the one I have now is correctly set for my transmission, then how would I know the one for the Pontiac would work?
Unfortunately I'm not aware of any source for repro large modulator, so the stupid-money ones on ebay are the only option if you want to look correct. They are NOT all the same. The vacuum vs stroke curve is different and there are a lot of different P/Ns of the original modulators as a result. Most folks just run the small aftermarket ones. Most of those are adjustable.
Unfortunately I'm not aware of any source for repro large modulator, so the stupid-money ones on ebay are the only option if you want to look correct. They are NOT all the same. The vacuum vs stroke curve is different and there are a lot of different P/Ns of the original modulators as a result. Most folks just run the small aftermarket ones. Most of those are adjustable.
So since I have a shift kit and a 2800 stall converter, I should probably just keep the one I have and use an elbow, agree?
It’s not difficult to carefully bend some brake line to make replacement modulator line. A couple of inches of rubber hose at the ends for the connections is all you need.
The factory modulator line has a built in restriction in it via a crimp at the end. Although brake line will work it isn't quite what the factory did.
NOTE: Yes, there exists a vertical loop in that ATM modulator pipe which functions as a thermal expansion loop. It does not wrap around or fasten to anything - it is a thermal expansion loop - don't spend one second worrying about it.
Final installation...and I was able to get rid of the "t" connection that supplied vacuum to the modulator when there was just a hose. Now I just have a straight run to the intake. Bonus!
Question... Do the rubber end pieces fit over the "barbs" on the steel line, or "up to" the barbs? I have mine up to the barbs. Also, I connected my line to carb vacuum instead of manifold vacuum. Does it make a difference?
Question... Do the rubber end pieces fit over the "barbs" on the steel line, or "up to" the barbs? I have mine up to the barbs. Also, I connected my line to carb vacuum instead of manifold vacuum. Does it make a difference?
I've always installed the vacuum hose up to the enlarged flange (barb as you're calling it). On most carburetors you have both a ported vacuum port and a manifold vacuum port (or several). Bottom line is the modulator line should be connected to manifold vacuum.
I've always installed the vacuum hose up to the enlarged flange (barb as you're calling it). On most carburetors you have both a ported vacuum port and a manifold vacuum port (or several). Bottom line is the modulator line should be connected to manifold vacuum.
It's hooked up to the port on the carb below the throttle blades.
Bottom line is the transmission needs to "feel" engine load. This is accomplished via connection to manifold vacuum not ported vacuum. If you're connected below the throttle plates then you're connected to manifold vacuum.
Bottom line is the transmission needs to "feel" engine load. This is accomplished via connection to manifold vacuum not ported vacuum. If you're connected below the throttle plates then you're connected to manifold vacuum.
That's what I thought Norm...and just to clarify, it's the port just to the left and slightly above the port for the pcv hose...not the timed spark advance port.
Double-check your carburetor manual - I think you have an Edelbrock? I'm not good w/ Edelbrock carburetors. So, I'm not 100% on how a Edelbrock (or any of their models) is/are configured. If it's below the throttle body on a Rochester Quadrajet, it's full manifold vacuum. Double check to be sure, but the modulator line wants full manifold vacuum.
Double-check your carburetor manual - I think you have an Edelbrock? I'm not good w/ Edelbrock carburetors. So, I'm not 100% on how a Edelbrock (or any of their models) is/are configured. If it's below the throttle body on a Rochester Quadrajet, it's full manifold vacuum. Double check to be sure, but the modulator line wants full manifold vacuum.
It's a Holley 770 Street Avenger...hence the diagram I provided above.
