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Roy, import/domestic specialest
Category: Transmissions
Satisfied Customers: 5293
Experience:  ASE master tech for 40 years
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Cadillac Deville: looking at a GM HT400 automatic transmission

Customer Question

looking at a GM HT400 automatic transmission that will not shirt out of 1st gear. Car has been sitting all winter. Checked vacuum at the carb. Next going to track vacuum back to transmission. No experience w/ modulator on these.
Submitted: 1 year ago.
Category: Transmissions
Customer: replied 1 year ago.
I have now replaced the modulator and checked the vacuum line back from the engine. Still stuck in 1st gear. (goes into park, neutral and reverse).
Expert:  Roy replied 1 year ago.
good morning my name is ***** ***** I will assist
did you make sure the little rod behind the modualtor is in correctly?? did trans oil drip out the vacuum port when you removed the valve??
other possible causes are valve body issue with valves sticking. changing the trans oil/filter may or may not help. you may want to try adding a vavle body modifier to see if will help with the movement of the valves. the other possibility is the governor. it is on the drivers side toward the back. it has cover that you can remove. it is round about 3 inches or so. remove it and then you can remove the governor. inspect the plastic gear for damage or dishing in the middle. it was a common failure for not shifting back in the day.
Customer: replied 1 year ago.
Not the governor either, I give up - taken car to Aamco yesterday. They say is an internal problem and requires a rebuild. SMH.
Expert:  Roy replied 1 year ago.
sorry to hear that. we tried with the basics