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Old 01-06-2017, 02:35 PM
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TheBandit TheBandit is offline
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Join Date: Jan 2006
Location: Ventura County CA
Posts: 4,654
I finally had a chance last night to get the car off the ground and crawl underneath to check into my transmission woes. Here's a recap of my issue:

Originally Posted by TheBandit View Post
With the car up to higher speeds, I discovered the transmission (TH400) was acting up. After up shifting to 3rd, it started cycling back and forth between 2nd and 3rd at light throttle. If I gave it a little more throttle it seemed to stay in 3rd, but in light throttle it was acting up. I tried playing with the vacuum modulator adjustment and replaced the vacuum line with some even thicker fuel line just to make sure it wasn't collapsing, but observed no change in behavior. The kickdown switch is completely disconnected. I checked the shifter cable adjustment and it was spot on. I talked to the tuner and we decided if we couldn't get it to hold 3rd, we could do the tune in 2nd. Theoretically it should not affect the tune, but would show lower numbers due to 2nd gear being a bit less efficient.


With the car strapped down, we found the transmission would also downshift 3rd to 2nd under hard throttle. It would not hold 3rd unless it was at part load/part throttle - at light throttle it would cycle back and forth 2-3-2-3-2-3 and at hard throttle it would just downshift and stay in 2nd. We checked to make sure the kickdown was still disconnected (it was), we disconnected the vacuum modulator, and we tried starting at different rpms to see if it would hold 3rd, but no luck. So we had to use 2nd gear for WOT tuning.
Since the issue seemed to be engine load dependent, I figured I should inspect the vacuum modulator valve to see if the vacuum diaphragm was working right and look at the valve rod for signs of damage. The diaphragm/spring system seemed to be working just fine. The vacuum side was filled with accumulated engine oil, but it still responded to vacuum and seemed to hold/seal just fine.

So I turned my attention to the valve itself, which is a rod-shaped piece that slides into the transmission. This is a sort of spool valve that is moved outward by governor pressure and inward by a spring in the vacuum modulator. After much research, here is how I understand the operation of this modulator valve: as speed builds, the governor pressure increases, moving the spool outward and causing a 2-3 upshift. A spring in the vacuum modulator acts against the valve this to delay the upshift until enough governor pressure /speed is achieved. At lighter engine loads, vacuum acts to relieve the spring, allowing the governor to more easily move the valve and make the shift happen at lower speeds.

Attempting to use a magnet to pull it out, I found this modulator valve got stuck about 5/16" from the bottom of it's travel. I grabbed some pliers and found it took a considerable force to get it out - it was badly hung up in there, with a good 1/4" of travel feeling like a press fit. I think this is what caused my issue and may have contributed to a long-standing issue I've had with the transmission refusing to 2-3 up-shift under WOT. My guess is the valve has never moved out beyond this point as it was design to and this new engine / vacuum combination has been sticking and un-sticking the valve, resulting in the weird 2-3-2-3 back and forth shifting. That is my theory at least - I haven't had a chance yet to drive the car to test it out.

The fix was pretty easy. I cleaned out the bore with a pipe cleaner brush, then I worked the offending surface on the valve with emery cloth until it moved freely in the bore.

The photo below shows a shiny area where the valve was hanging up.

In the next photo, I've cleaned up the surface with emery cloth until it slid freely in the bore of the transmission.

Hope to get the car off jackstands tonight and give it a test drive.

Ongoing 70 Nova build:

Last edited by TheBandit; 01-06-2017 at 03:15 PM.
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