TH400 kickdown

[Laderhosen]

I have a 1982 Chevy C20 Scottsdale with a 350 into a TH400 transmission, the problem I've got is getting a full kick down.

The truck downshifts from 3rd to 2nd, but going down to 1st is when it has a problem. I was going up a very steep hill the other day and had to manually downshift using the column stalk.

I've been told it could be a problem with the kickdown switch, I've inspected this and the gas pedal is striking the switch.

Any suggestions.....

[ccz145a]

I am pretty sure the switch solenoid only works on 3 to 2 downshifts.

Anyone else?

[JJSZABO]

I am pretty sure the switch solenoid only works on 3 to 2 downshifts.

Anyone else?

+1  I've been told the same thing and only when doing 40mph or more etc......

[VileZambonie]

You will never see a 3-1 downshift without the detent solenoid unless you do it manually. It's real simple. 12v switch on the gas pedal brings 12v to the solenoid and the solenoid is case grounded. so get a test light and see if you have 12v to the solenoid when the pedal is to the metal.

[Laderhosen]

I rang a transmission place today and they said to get a test light and check if the wire's receiving current when the kickdown switch is triggered. Anyone got any diagrams or photos of the where the wire goes into the transmission???

Basically I was in third, and turned into this REALLY steep hill. I heard it drop down to 2nd as I went into start of the gradient. I'd given it a little extra gas but not enough to tigger the kickdown switch. As I went up the hill it began to slow, probably to 10 mph. When I really floored it, it didn't change from 2nd to 1st.

So does the kickdown switch not allow for a kick down from 2nd to 1st?

[zieg85]

My 71 never went into 1 either.  I had a 4.10 gear but I always had to use the shifter if I wanted it in first.  My buddies dad worked at the Chevy dealer at the time and said it was normal for a TH400. 

[ccz145a]

it is the only wire that enters the left (driver's) side on the housing, if that helps. no pic though.

[Laderhosen]

it is the only wire that enters the left (driver's) side on the housing, if that helps. no pic though.

Thanks, does it go in near the front of the box, near to the bell housing???

Does the wire have a metal collar type fitting??

[VileZambonie]

It's a single blade spade type terminal above the pan. Yes it will detent 1st gear.

[JJSZABO]

Vile, could you explain this.

It's a single blade spade type terminal above the pan. Yes it will detent 1st gear.

I was told by our local transmission shop (where I had my TH400 rebuilt)  that the kickdown switch was for 3 to 2 downshift only when you where going faster that say 40mph ie... when you were trying to merge or pass.

[VileZambonie]

Well they need to get with it. The TH400 is an old transmission and can do 2-1, 3-1 & 3-2 downshifts automatically. They should know this.

[JJSZABO]

But not by the switch at the accelerator pedal correct?  The switch at the pedal will only kickdown from 3 to 2?  Is this the same as detent?  I need to research this with my shop manual.......

[JJSZABO]

OK - this is from the 1986 shop manual:

Detent (downshift) switch

Checking detent switch opperation

1.  Check the transmission oil level and correct as required.
2.  Be sure the engine is operating properly and the brakes are not dragging.
3.  Check the wire connection at both ends.
4.  Adjust the switch.
5.  Road test the vehicle.  A dentent 3-2 downshift is possible at 73-68 mph by depressing the accelerator fully, and a 2-1 downshift can also be accomplished at 32-28 mph.

[Laderhosen]

Interesting. I'll go out and do some tests, using the sat nav to get accurate speed cos my speedo's out by 5mph!! 

[VileZambonie]

The detent solenoid, modulator pressure, line pressure and governor pressure all dictate where it will shift, when it will shift and it will delay an upshift or force a downshift. Without the detent solenoid working you are strictly relying on governor and modulator pressure and will most likely never see an automatic 3-1 downshift. If the detent solenoid is working and is adjusted correctly you better believe it will do a 3-1 downshift!


The factory settings are under 50 mph 3-1 downshift and 3-2 downshifts under 75 mph in drive range.

DETENT VALVE - Affects regulated modulator pressure tending to hold the 1-2 shift and 2-3 shift valves in the downshift position and provides areas for modulator and detent regulated pressuresfor detent 2-1, 3-1, and 3-2 downshifts.

Trust me, my first transmission was a TH400 and I've always had one since I've had vehicles. I've built a lot of them too. If yours won't 3-1 downshift check out your switch adjustment.