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Discussion Starter · #1 ·
Hey guys - I have a 68 GTO, TH400 transmission.
Car runs perfectly for the first 20 minutes when it is initially somewhat cold.

However, once the car gets to operating temperature, the transmission has issues.

I cannot quite explain it but it seems like the car is shifting from 1st to 2nd gear, but the car revs very very high but not enough power is going to the wheels. So I cannot get over 20-25 MPH without redlining. (Once again, this only happens when the car is hot).

I tried manually shifting but the same issue continues.

This is what I have done:
Replaced Vaccum Modulator
Checked if appropriate vacuum is going to modulator
Checked fluid levels

Any help would be appreciated. Thanks,
 

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I had the same problem with the wife's Cadillac. Once the engine warmed up, the transmission started slipping.

The Cause; The engine warm up was coincidental as the transmission clutches were found to be badly worn when the transmission was removed.
Within a couple miles of starting, the transmission would begin to slip. The debris from the clutch slippage would start packing the oil pick up screen which cut off the oil supply which caused more slipping.
When the engine shut down, the debris fell back into the pan. The cycle would repeat once the engine was started up again.

Not saying this is your problem, but you may want to drop the transmission pan to check for metallic sludge plugging up the internal filter.
 

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I had the same problem with the wife's Cadillac. Once the engine warmed up, the transmission started slipping.

The Cause; The engine warm up was coincidental as the transmission clutches were found to be badly worn when the transmission was removed.
Within a couple miles of starting, the transmission would begin to slip. The debris from the clutch slippage would start packing the oil pick up screen which cut off the oil supply which caused more slipping.
When the engine shut down, the debris fell back into the pan. The cycle would repeat once the engine was started up again.

Not saying this is your problem, but you may want to drop the transmission pan to check for metallic sludge plugging up the internal filter.
I told him...Help is on the way.😊😉👍
 

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I've lost count of the times that my friends and I have started to encounter different shifting problems with our T400 transmissions. Most transmissions returned to normal operation with a fluid and filter change. Might be debris, might be moisture, but it's amazing how many problems go away with new fluid and filter.
 

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Discussion Starter · #13 ·
Thanks everyone.
Next step is checking out the governor for me as it is still not shifting.

2 questions:
1. I have heard of the kickdown solenoid being stuck in downshift. How do I unhook it to see if this will fix the issue?
2. How much vacuum should I be getting to the modulator?
 

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the kickdown solenoid is the one wire on the trans. you will see what im talking about when you look. just pull the plug. maybe check the modulator line see if it is clogged. i dont know the exact vacuum. i can tell you its a full vacuum. if the solenoid is stuck im not sure unplugging will make a difference. the wire may be grounded somewhere.
 
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