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TH400 wont downshift to L2

K

KeyBlazer

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My 87 Burb with 454 and a th400 never downshifted to L2 on a hill.... in fact it was like you put it in N and it would run away!!
I was told the band was burnt out as the PO had done a lot of towing.

Rebuilt the motor and swapped in a trans that I was "told" was good.
The new trans shifts fine, if not as sharp as the original one, as I think it had a shift kit.
Anyway, the new trans will not go into L2 either! It shifts smooth, and willl go into L1 when the speed is right.
Just bad luck, or did I miss something?

Also the original trans had HD cast into the housing... does that mean anything?
 
TH400 doesn't have a cable. Passing gear/kickdown is controlled by a switch on the gas pedal.
 
johnnychimpo said:
Or one was never hooked up or installed.
Click to expand...

x2. Look at the top of the gas pedal. When the pedal is pressed, it should activate an elec switch. This runs to a plug on the drivers side of the tranny. The vacum actuator SHOULD kick it down, just not nearly as fast as the elec. switch. Another thing to do it pull the vacum hose (Then attach a new / clean hose) to the actuator and suck in, if its free flowing, then its bad, you cant suck in then its good.
 
Maybe I didnt explain it right...
The trans shift fine and downshift to 2 on demand, but when I pull the shift to L2 like on a hill to get some engine braking, it stays in 3 regardless of my speed. If I put it into L1, it will down shift at about 15mph.
 
Last edited:
To me it sounds like yes the front band is burnt out then. The guy who I get to build my racing transmission does not install the front band on them for durability sakes. And with no front band it does exactly what you are saying yours is doing. You can be driving down the road in 3rd downshift to 2nd but it feels like it goes into N.
 
Silly question...
In the Haynes manual Emmisions section in the picture of the "Typical TBI V8" engine bay, there is a reference to a "Downshift Relay, TH400 only".
My Sub has this relay, and out of it there comes a wire that has a femal spade connector that is not connected to anything.
What does this relay control?
 
Here is a photo of the Relay..its the right one in this pic.
They are on the Pass side firewall next to the MAP sensor.
You can see the terminal that is not connected to anything..

RelayE.jpg


What does this relay do?
 
That is your fuel pump relay, the red wire that is not connected is so you can put 12 volts to force the fuel pump to work effectively bypassing the relay for test purposes.
 
I'd like to know more about this TH400 downshift relay. The TH400 IS electrically downshifted, but I have never heard of a downshift relay.

Anyone got a service manual of the late 80's that talks about this? Not necessarily believing haynes, but they simply copy data out of the service manuals, so it may indeed be true.

But I don't believe the kickdown solenoid will have anything to do with a "runaway" second gear condition.
 
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