Hi My ST-400 suddenly stop upshifting, I think it is the vacuum modulator? Can I clean it with some sort of cleaning like brake clean or are there something in it that can break? Or can it be some other things than the vacuum modulator? Then I wonder if it are better with a 1/4" pipe instead for the stock pipe size?
Start by buying a new one. Standard makes one. There are two types. Big like original or small. They both work. I don’t like the small as vacuum fitting is in different position so you need a connector hose. The big type may have been discontinued. There are rubber seals inside that go bad. Very common. Another thought is the governor. Don’t mess with your original pipe. Try the simple stuff like this first before going to a transmission shop and they tell you that you need a rebuild. But you might.
Also check the fitting at modulator and intake. There are rubber hoses that go over fittings that can rot and fitting can pop off.
No vacuum to the modulator will cause no upshift. Start by checking vacuum line at both ends. Be a mechanic, not a parts changer.
I had one years ago, and it had clogged up with fuel/oil deposits and it couldn't get a vac signal is
Look for leakage of ATF at the modulator nipple. ANY ATF means the modulator diaphragm is torn. Do not confuse water/fuel condensation for ATF. Connect the modulator nipple to a vacuum pump to verify that it holds vacuum. Assure that the vacuum tube actually has vacuum at the lower end, and seals properly to the modulator nipple. I suppose it's possible that the modulator valve in the trans is seized. Never actually seen that happen, though. Wild Guess: The problem is the governor, not the modulator. Stripped teeth on the gear, jammed plunger, etc.
Agree . Looking for leaky nipple is the fastest way. If you have a leaky nipple you can tell by flooring it down the road. I t will suck up transmission fluid into the intake. ATF makes tons of smoke. Then again probably eaiser to dive under there and pull the hose off the modulator and see for leaky nipple.
