TH400 Detent Switch

[78 Chevyrado]

I've read about adjusting the switch by the gas pedal and all that, what I'm curious of, is how does the switch know the pedal is at WOT?

You adjust the pedal by flooring the gas and then the switch stays where it was at WOT.  since the switch doesn't move back and forth with the gas pedal arm like, say a modern TPS does with the throttle valves, how does it know its at WOT?  does it sense the pedal is arm is close to it?   If the switch doesn't freely move back and forth WITH the gas pedal arm, how does it know?   I totally don't understand how this switch system is supposed to work   I understand modulators, just not this weirdness.

If it IS supposed to move back and forth freely like a TPS does, I need a new switch. 

[78 Chevyrado]

And something does need adjustment.   Over about 45MPH it doesn't downshift, period, unless I do it manually.  I think I did put a new modulator on the truck like 7 or 8 years ago as granddad had knocked the other one off on something in the woods.  

Whats the difference between the Red, blue and green striped modulator?  IIRC the one I have now is a Green stripe, but its been quite a while.

[VileZambonie]

The switch is adjustable. When the switch contacts close it enegizes the solenoid in the trans giving you the detent.

[HAULIN IT]

If it IS supposed to move back and forth freely like a TPS does, I need a new switch. 

You need a new switch...The "pin" on the switch is spring loaded toward the firewall & should ride with the pedal, as it reaches very near WOT, the contact is made & the 12v goes to the transmission. When the pedal is lifted, the "pin" slides back toward the firewall ready for the next time. Lorne

[78 Chevyrado]

I figured it out.  Asked granddad about it too.  He said he always had to downshift manually when hauling his trailer, as when you floored it it didn't do it on it's own.  he said it was like that since it was new and thought that's how it should have been, maybe something odd about 3/4 and 1 tons trucks.  I didn't give it much thought and thought maybe it was a TH400 peculiarity...  

I took the switch apart to see how it worked, cleaned it out (wasn't bad) and put more grease on the contacts, and bent the moving contacts a bit for more spring pressure.  I verified the switch was getting power and it was.

What was confusing me was that throughout the pedals travel, it never even touched the switch to move it at all except when adjusting it.  once it moved the switch to where the detent is after adjusting it, it would almost touch it but wouldn't go far enough to reach the switch and move it to make contact.

EDIT:  I wonder if the edelbrock carb has less travel than the stock q-jet did.  What i mean is since I swapped carbs, maybe that caused the thing not to travel enough to reach it...    On a side note when i got the switch out, there wasn't any evidence the switch had been touched by the gas pedal arm, so maybe it never did reach it?

It's was off A LOT!    pics are coming...

So...   I put some fuel line on the switch to give it more reach, nothing yet.  I put a split piece of the same hose over the first and it almost worked, It worked if you pressed the pedal way to far past where the carb stops.   then I put a piece of 1" hose over all of it and now it's working properly.  It kicks in in the spot I think it should, and when you floor it from a stop it now up-shifts around 45MPH and about 68MPH like it should.  

What was confusing me was that the switch isnt adjustable in how it bolts on the bracket, and they couldn't have mounted it in such a way that it didn't work right from the factory and granddad never really messed with the truck so I'm real sure its all stock.    That it works properly now and the switch was right and good tells me something wasn't right when it was new.  maybe the bracket geometry is off on this one.
It works great now though, I'll put up a pic in a little while.

[ccz145a]

If I remember, it is almost self adjusting... you pull it back and let it snap. It will end up where it needs to be. Not sure why your's is not right.

I still need to hook mine back up after my body swap.

[VileZambonie]

Hmmm, something isn't right. I'll wait to see your pix.

[78 Chevyrado]

The pic



The carpets not stopping the pedal from full travel, so all i can figure is the edelbrock must not have as much travel as a Q-jet...

The rubber hose on the switch puts the lever right where it should be, that's how far off it was from pushing the switch enough to make contact.  Inside of the switch it looked brand new, with almost no wear on it, except where I been playing with it from time to time.

[HAULIN IT]

Kenny, That switch looks like mine...except for all the rubber hose
It does not touch at idle, just starts moving about half throttle.
 I would not be supprised if the Edelbrock carb is what altered the "total travel" of the pedal. Lorne

[VileZambonie]

Push detent switch plunger fully forward. This will preset the switch.
Detent switch will automatically adjust when accelerator pedal is depressed to wide open throttle position.

[78 Chevyrado]

When you pull the switch forward, and then adjust it with the pedal, there's a detent there that keeps the switch from going all the way forward like when you put it there manually.  

When the switch is in the middle position, not being touched by the pedal arm, the gas pedal arm will just barely touch it, but won't move it.  When the pedal is in the position in the pic above, but without the rubber hoses, the pedal arm just barely touches it, but not enough to reach the contacts in the switch.  I used an ohm meter just to tell exactly where the switch makes contact and compared to the pedal arms reach it was off the amount those hoses make up.  It's still working good, I been using it a fair bit since it works now...

I'll see about getting a pic like above, but without the rubber hoses and with the gas pedal floored.  If somethings not right (and its obviously not)  I'd at least like to know why..  

[VileZambonie]

I don't think you've adjusted the switch right. Did you move the plunger all the way?

[78 Chevyrado]

Yeah I pushed the plunger all the way forward and it stayed there, and then to set it I just mashed the gas to the floor.

I also set it like that with the rubber hose spacer too.