71 TH400 Transmission Problems

[Seth G.]

I recently replaced my radiator on my small block 350 with a TH400 automatic and had to disconnect and reconnect the transmission lines. When I drove my car to the shop everything was fine. When the job was completed, my transmission was slipping and would not upshift. This is low milage car so I think I may have a vacuum problem. The first thing I checked was the fluid which reads full when measured with engine up to temp and running. I checked for vacuum leaks but could find none. I then removed and cleaned the transmission vacuum module to no avail. When cold, the transmission slips badly until the fluid warms up. I presume the car has its original TH400. I was going to replace the vacuum module. Does anyone know if I should be looking elsewhere? Thank you.

[Terry M.]

Re: 71 TH400 Transmission Problems

I recently replaced my radiator on my small block 350 with a TH400 automatic and had to disconnect and reconnect the transmission lines. When I drove my car to the shop everything was fine. When the job was completed, my transmission was slipping and would not upshift. This is low milage car so I think I may have a vacuum problem. The first thing I checked was the fluid which reads full when measured with engine up to temp and running. I checked for vacuum leaks but could find none. I then removed and cleaned the transmission vacuum module to no avail. When cold, the transmission slips badly until the fluid warms up. I presume the car has its original TH400. I was going to replace the vacuum module. Does anyone know if I should be looking elsewhere? Thank you.

Has the transmission filter been changed lately? It might be time to do that.

[Ray G.]

Re: 71 TH400 Transmission Problems

Hello;
Occasionally transmission coolers in radiators will leak antifreeze/coolant into the transmission when the cooling system builds more pressure than the trans. cooler lines. which is about 15 PSI.
Antifreeze/water will destroy the transmission in several ways.
Antifreeze/coolant will settle to the bottom of the transmission pan and may not show on the dipstick.
Do NOT drive the car until it is diagnosed.
Drop the transmission pan and look for water/coolant. When enough coolant enters the trans. to allow the moving parts to touch the liquids the trans fluid will appear milky and foamy.
If there is a drain plug on the transmission just loosening will allow any coolant to seep out. Coolant is lower viscosity than oil.
If diagnosed early it can be repaired by flushing the trans. and replacing the trans. cooler in the radiator.
Good luck.
Ray

[Seth G.]

Re: 71 TH400 Transmission Problems

The filter was changed in 2011 with only 250 miles since. Unless it is defective, it would not need changing but I suppose you never know. Thanks

[Edward J.]

Re: 71 TH400 Transmission Problems

Seth, the first step would be to check the filter and its seal to make sure that the filters retainer did not come loose On the 400 trans there is only a shouldered bolt that retains the filter just make sure its in place. since you had no warning that would be the first place to look. and note the pan when you drop it down, any metal pieces laying around ?? don't replaced the modulator it won't fix the problem your having. sounds like lack of pressure, or applied pressure to the clutch or band

[Steven B.]

Re: 71 TH400 Transmission Problems

You might want to also check the governor spool valve. If there is the tiniest bit of foreign material in the body, the valve won't function correctly. Also, if your vacuum line to the modulator is split, you might not hear or notice it. If you can provide a source of vacuum, try pulling it down at the manifold end of the vacuum line and see if it holds. I had a transmission shop misdiagnose a TH350 with a shifting problem, and it turned out to be a small crack in the vacuum line just ahead of the module.


Steve.

[Mark E.]

Re: 71 TH400 Transmission Problems

You're saying the tranny performed fine before the radiator was replaced? In situations like this, I ask: What's changed?

Double check everything the radiator shop did. They had to add fluid after the radiator was replaced... what type of fluid did they use? Are you sure the fluid level is correct? Is a coolant line pinched? Is the new radiator leaking coolant into the transmission? Did they abuse the car while "testing" it? Did they forget to add transmission fluid when they first test drove it?

[Seth G.]

Re: 71 TH400 Transmission Problems

Thanks. I spoke to a transmission shop and they talked about the retainer and the possibility the oil pick up tube and/or retainer could be loose. Appreciate your help.

[Seth G.]

Re: 71 TH400 Transmission Problems

Thanks Steve, good thought.

[Seth G.]

Re: 71 TH400 Transmission Problems

They did not add fluid as the level was full when measured with engine running and warm. I later thought it could be down a pint and added Dexron/Mercon III fluid to the fill level, again after measuring with engine running and warm and shifting with brake on through the gears. After driving it, still had problem and fluid then appeared overfilled so evacuated the pint I had added....

[Mark E.]

Re: 71 TH400 Transmission Problems

[QUOTE=Seth Gordon (11084);752654]They did not add fluid as the level was full when measured with engine running and warm./QUOTE]

I guess I don't understand what the shop did to your car. How could the transmission fluid level not be low after replacing the radiator with its transmission fluid tank?

[Seth G.]

Re: 71 TH400 Transmission Problems

I agree, does not seem right. However,after the new Radiator was installed and transmission lines hooked up, the fluid level read "full" with the engine running at temp. I don't understand it either but I measured the fluid myself afterwards as well.

[Ray G.]

Re: 71 TH400 Transmission Problems

Hello;
Occasionally transmission coolers in radiators will leak antifreeze/coolant into the transmission when the cooling system builds more pressure than the trans. cooler lines. which is about 15 PSI.
Antifreeze/water will destroy the transmission in several ways.
Antifreeze/coolant will settle to the bottom of the transmission pan and may not show on the dipstick.
Do NOT drive the car until it is diagnosed.
Drop the transmission pan and look for water/coolant. When enough coolant enters the trans. to allow the moving parts to touch the liquids the trans fluid will appear milky and foamy.
If there is a drain plug on the transmission just loosening will allow any coolant to seep out. Coolant is lower viscosity than oil.
If diagnosed early it can be repaired by flushing the trans. and replacing the trans. cooler in the radiator.
Good luck.
Ray

Sorry, I was thinking you had a radiator with an internal transmission cooler .

[Joe L.]

Re: 71 TH400 Transmission Problems

Sorry, I was thinking you had a radiator with an internal transmission cooler .

Ray------


That's what he should have. All 1968+ Corvettes with THM-400 used a radiator with an internal transmission oil cooler.

[Seth G.]

Re: 71 TH400 Transmission Problems

Thanks, this is the first place I'm going to start. If that is not it, will look at replacing modulator next if Governor spool valve good. Appreciate your help, thank you.