TH400 won't reliably go into passing gear...

[JCampbell]

'88 3/4 ton 4x4 Suburban. TBI 350/TH400.

Truck won't reliably go into passing gear.

1. Where is the kickdown switch located?
2. How can I test it to make sure it's the problem?

[redneckchevy]

I can tell you were it is located but not how to test it .

The TH400 has an electic kick down swith that bolts to the gas pedal.

[James McClure]

Jason, as stated the forced downshift switch is on the gas pedal. To test it, disconnect it and hook up an ohm meter. At rest it should show open, when you mash the gas it should show "0". There is a solinoid in the trans that dumps fluid and makes the shift. It is a powered unit, so one side of the switch should be hot with the ign on. Check switch adjustment and wireing also. jim

[79gmcc15]

If you don't want to deal with it. Ive heard you can hook it up to a toggle switch. and flip it on when you need it.

[JCampbell]

10-4. Thank you, James. I looked for the switch on the pedal, but didn't see it. I'll take a closer look this weekend.

[JCampbell]

Negatory. No switch on the pedal. Maybe something mounted on/near the throttle body?

[westofb]

My switch (75 truck) was on the upper part of the throttle pedal bracket, where the arm of the pedal near the throttle cable pushed on the switch. I had same problem, had to modify how it mounted on the bracket to get it to contact the arm and cause the trans to down shift. TO test the switch, with the key in the on postion, you can manually push the switch and hear the soleniod in the trans click when you move the switch. Yours being that new with TBI, it may be controlled by the computer using references from the VSS (vehicle speed sensor) and the TPS (throttle position sensor). I am not sure exactly how it works, but if you have no switch on the pedal, that would be my guess.

[redneckchevy]

Do you have an aftermarket kit that bolts to the throttle cable braket just like a 700r4 does?

[sierra4x4]

I think that with TBI the computer controls the down shift and their should be a relay beside your fuel pump relay the is for this down shift.

[JCampbell]

Roger that. That's kind of the conclusion that I was comming to...electronically controlled.

[JCampbell]

Back from the dead.

Still won’t go into passing gear reliably. Sometimes it will, sometimes I have to do it manually. I’d say it works correctly about 10% of the time.

Replaced the relay to no avail.

Please advise.

[Firebirdjones]

The older ones are so simple to diagnose, either the gas pedal switch or the switch on the valve body.

Since yours is TBI, (computer) I believe a couple of things have to happen for passing gear. Your VSS and your TPS both have to agree on speed and throttle position according to what the computer chip is programmed for, before passing gear will engage, and from there the solenoid on the valve body.

My guess is,,,,since you complain of no other drivability issues that would lead me to believe you have a VSS or TPS problem, then it's probably the valve body switch.

Is it time for a transmission fluid and filter change???

[JCampbell]

Originally Posted by Firebirdjones

Is it time for a transmission fluid and filter change???

Probably.

[Firebirdjones]

Before you do, you can crawl under the truck, remove the plug for the kick down solenoid (goes right through the case just above the trans pan driver side)

You can then hit the solenoid with 12 volts (long wire from battery or something) and if it's working, you'll hear that solenoid clicking. Maybe spend some time doing it and see if it's consistent or try to dupplicate a problem with it.

If you still aren't sure, I'd replace the solenoid during the trans service anyway because they are cheap to buy, and it's better to do it while you are servicing the trans rather than dropping the pan again later....

[JCampbell]

Cool. I'll take a peek. Thanks.

[poppy510]

Hey first post here. I just fabricated a nice bracket to couple my 69 kickdown to a holley 3310....no kickdown. I tested the switch, and found that the switch has intermittent continuity through its stroke. Dont know if this is your problem, but throw an ohmeter on yours and verify the switch is "closed" whenever it is depressed. test it through the whole range.

Now back to mine, I believe it works well enough to signal the th400 to kick. My next investigation is to check the wiring, and then check the solenoid. Does the switch care which terminal is connected to ignition power? I dont hear the solenoid clicking, so Ill take a look at that too.

[Firebirdjones]

Remember also these switches have an adjustment too (I know the gas pedal mounted switches do)

There is a ratcheting mechanism inside the switch which needs to be pulled up fully. Then install the switch and mash the gas pedal to the floor, you will hear the switch ratchet moving. Once the pedal reaches the floor, the switch is adjusted to that exact range of motion or distance. If you have a floor mat in the way it will affect adjustment.

Alot of times people will crawl under there and work the switch by hand, and this scews the adjustment. Or the switch was extended further than usual when a large mat isn't in the way Then the next time you press the pedal to the floor it's not triggering the switch because the pedal generally isn't capable of pulling that switch the entire length of movement like you can do with your hands. In this case you'll never get passing gear until the internal ratchet of the switch is readjusted.

[poppy510]

the carb mounted switch does not seem to have a ratcheting mechanism.

I did trace 12v all the way down to the solenoid. Still no click. I guess my next step is to remove it and test or replace. does it slip into the trans housing with an o-ring seal, or is it threaded? not obvious how it comes out. Here is a terrible, rotated pic of the plastic solenoid. sorry.

[Firebirdjones]

Originally Posted by poppy510

the carb mounted switch does not seem to have a ratcheting mechanism.

I did trace 12v all the way down to the solenoid. Still no click. I guess my next step is to remove it and test or replace. does it slip into the trans housing with an o-ring seal, or is it threaded? not obvious how it comes out. Here is a terrible, rotated pic of the plastic solenoid. sorry.

I didn't remember the carb mounted switches having adjustment (all the cars I have left are floor mounted) which is why I mentioned I new the gas pedal mounted switches "do" have an adjustment. Just thought I'd toss that out there in hopes of possibly helping.

[Firebirdjones]

Originally Posted by poppy510

the carb mounted switch does not seem to have a ratcheting mechanism.

I did trace 12v all the way down to the solenoid. Still no click. I guess my next step is to remove it and test or replace. does it slip into the trans housing with an o-ring seal, or is it threaded? not obvious how it comes out. Here is a terrible, rotated pic of the plastic solenoid. sorry.

Oh sorry, about the other question.

That is simply a plastic terminal that passes through the case with an 0-ring seal. Generally nothing wrong with those, you won't be replacing that.

Once you drop the pan you'll see on the other end of that plastic piece there is another terminal and a wire (usually white) that wire is going to the actual solenoid that is bolted to the trans right next to the valve body. There are two styles of these switches (both still available) and they are held with 2 bolts. Underneath is a small thin steel gasket.

A new switch should come with a new steel gasket. Very easy to replace once you get in there, this will all make sense.

If you are getting power at the orange wire and there is no clicking when it's plugged in, it certainly sounds like a solenoid issue.

[poppy510]

Thanks a bunch FB....Ill report the results!

[chengny]

If you are getting power at the orange wire and there is no clicking when it's plugged in, it certainly sounds like a solenoid issue.

What about the opposite?

Sorry to jump the thread but, I have never been able to get a downshift since we've had this truck. The electrical side of the system is good and it shuttles the solenoid, but the TH400 will not downshift.

The kickdown/detent switch checks out okay, the orange wire has continuity and the solenoid produces an audible click when the switch is closed. It can be heard from inside the cab if the engine is off.

The transmission shifts normally otherwise. All the ATF has been renewed (dumped the torque converter during a recent engine change). New filter (pick-up tube and o-rings are tight). Modulator holds vacuum and tubing/rubber to manifold is tight.

It would be nice to get kickdown but it's not worth a transmisson change or rebuild.

Any ideas?

[NorthwestGrudgeRacer]

I would just pull the VB and clean it and replace the KD valve. You dont have to pull the trans for that. hope that helps.

heres a pic

[chengny]

Thanks,

I'm thinking that the detent valve is shuttling correctly - only because the solenoid that shifts it responds with a definite snap. I may be wrong and will get into the valve body if it comes to that.

But, after looking at the image you provided, I remembered that the detent valve does not work alone. It is dependent on the modulator valve - specifically that the mod vlv must be able to stroke fully.

While I have checked the actuating portion of the modulator for stroke and that it senses engine load (intake manifold vacuum) via the tubing, I neglected to inspect the actual regulating part. If it is binding up, the pressure from the detent valve goes no further than the mod vlv:



And I also read this that I had saved:

TH400 has two downshift mechanisms, vacuum and electric. The electric is the "passing gear" which kicks in near full throttle. I believe that the electric is dependent on the vacuum to work.

First I will tell you to always check the simple things first. Is the vacuum line kinked or flattened? Are the hoses connecting the line to the modulator and the intake manifold tap in good shape and tight fitting?

Manual says to first check vacuum modulator and lines for leaks, also check for a stuck modulator valve. I have also seen the tap on the manifold plugged with carbon on older vehicles. An easy way to check this is to pull the hose off the tap fitting and check for vacuum with the engine running.

Next step is to remove pan and valve body. Check for stuck 3-2 valvetrain by shaking valve body. Valves should move under their own weight, also check for broken spring.

Next is to remove pan and disassemble pressure regulator and check for worn or stuck parts.

[poppy510]

For all you considering NOT repairing a non-functional KD system and manually downshifting instead, I encourage you to reconsider.

I pulled the solenoid, cleaned and replaced, and the system (modulator plus electric) are now functioning together as designed. I am now enjoying a a good old fashioned tire roasting at low speeds upon kickdown.

The response and shift points of the properly functioning system are much improved over manually shifting. Thanks to all on the thread for the help!