I've been fighting the issue for about a week now and I'm completely out of ideas on how to resolve it, short of taking it into a shop. As the title implies, the TH400 in my '75 Regency has taken a dump. It stopped up-shifting this past Monday and had thought the vacuum modulator had gone out. Well, it turns out that wasn't the issue as it persisted after replacing the modulator, the rubber line that ran from it to the steel line and changing the filter and fluid. I was still under the assumption it was vacuum leak, so it checked the lines up by the front of the carb and found that the one that ran from the carb to the "tree" all of the lines merged into had a pin-hole in it, so I replaced it (and the "tree" after it broke trying to remove the line) and made sure everything held vacuum afterwords, which it did. While the engine did idle better, the "whistling" noise still persisted and the transmission would only shift into second (albeit with a lot of hesitation) at 50MPH.

At this point I have no idea where to go from here (aside from a transmission shop) and I wanted to check and see if anyone here had any other recommendations of things to check before I take it in.

 

The whistling noise is a strong indicator of a vacuum leak which as you are aware is going to cause delayed shift points. Keep searching for vacuum leaks, see if a local shop will do a smoke test for you.

A general repair shop would be my first choice vs a transmission shop at this juncture.
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Good luck!

 

Alright, I haven't been able locate any more vacuum leaks and honestly I doubt I have any as idle is smooth and I would assume anything leaking enough vacuum to stop the transmission from shifting would also cause rough idle or stalling. I still may hit it with a vacuum gauge when I get the chance, though. The "whistling" sound I'm hearing seems to be coming from the front of the engine, so I'm inclined to say it's one of the accessories making the noise at a higher RPMs. I'm honestly unsure if it is even shifting into second at this point (I certainly haven't been able to feel it shift into second). It definitely tries to up-shift around 50MPH but doesn't seem to be able to fully make the gear change and just slips back into the previous gear. It will engine brake as expected when shifting from Super down to Low, but does not when shifting down to Super from Drive.

On another note, I decided to siphon some of the trans fluid out to see how it looked considering I changed roughly half of it yesterday and rather than being the dark / dirty red I expected, it was almost pitch black after siphoning it into a bottle and was dark "burnt orange" / dark brown in color when spread onto a white cloth and smelt burnt. As much as I rather it not be the case, everything seems to slowly be aligning towards a mechanical failure in the transmission.

 

The TH400 had an odd color lifetime fluid in '75 IIRC, it may still be in there. No upshift can also be a sticking or bad governor. Yes at the end of the day it may need a rebuild. I'd drop the pan and have a look, if there isn't a lot of metal and/or clutch material then replace the filter and fluid, remove clean and inspect the governor including the plastic gear on the governor.

Confirming good vacuum to the modulator is the place to start.

Good luck!!!

 

Been thinking that "Lifetime Fluid" may have been a 100k interval...it's been a while.

 

I’m an all or nothing kinda guy. I would connect the transmission vacuum modulator directly to a manifold vacuum source with a new rubber hose to ensure the transmission will shift properly.

 

^^^A good idea but check that the engine is producing good vacuum via a gauge.

 

Check the steel vacuum modulator line for a crack or hole

 

When you have the transmission pan off next time......perhaps the 1-2 shift valve is sticking and not shuttling ? Clean it.

 

It sounds like your transmission filter is pulling air. Pull the pan and check the tube and tube orings

 

If the fluid is black, you have issues.

 

X2
If your fluid is black and smells burnt, your trans is toast. Quadruple checking your vacuum signal is grasping at straws.

 

You can take it to a transmission shop, but your ATF fluid is completely oxidized, lost all its viscosity and likely you have a lot of air in the system. The very first thing (in any attempt to salvage) is to perform a 100% complete transmission flush. Do it the first time, run it 30 miles, then do one additional 100% complete transmission flush. Change filters both times. Good luck.

EDIT: I should qualify my statement. You cannot flush the entire transmission of fluid since the majority remains in the torque converter. I'm speaking in hopes the transmission can be salvaged from more expensive and extensive repairs. This is why I suggested to change the fluid and the filter TWICE. Very important to change the filter twice since more sludgy oxidized ATF fluid is going to begin to migrate through the system. Again, good luck.

 

Why, putting new fluid in a transmission that needs an overhaul is a waste.

 

It’s time to pull the pan. If there is a ton of debris in the pan, and burnt fluid, it’s time to accept reality that it’s cooked.

If you accelerate up to about 30-35 mph and let off the throttle, the trans should make a 1-3 upshift. If so, once the trans is in high gear pull the lever into 2nd gear. It should drop into second, if not, most likely the 2nd gear is burnt. Pull the shifter into 1st gear, it should go into 1st gear.

if it needs rebuilt, make sure the builder throws away the stupid plastic accumulator pistons that GM started to use around the time the car was built. The piston break, causing cross leaks and sloppy shifts. Go back to the old school aluminum pistons