Kickdown going back to third too soon

[jangell]

My auto's kickdown seems to be reseting too early. By "reseting", I mean that it does kick down, but only until about 3000 RPM before it goes back into third. It used to go up to around 5000 RPM or soemthing. I'm not quite sure what changed.

I noticed that trying to manually drop to 2 doesn't seem to do anything. It's like the transmission really wants to be in third. This makes it hard to get up to speed on the highway or pass other cars.

To be clear, it shifts properly from 1 to 2 to 3. It's just that trying to manually drop to 2 or using the kickdown seems to only work in a narrow range before reseting to 3. I'm determining this by the sound of the motor; I don't have any test lights set up that I can see in the cabin, although I should probably rig that up some day.

Does anyone know how the transmission decides to stop kicking down? I couldn't find anything in the workshop manual about it.

Thanks!

-- Joe

[jangell]

Answering my own thread, this would up being pretty simple: the throttle cable wasn't adjusted correctly. I didn't realize the throttle cable had anything to do with the kick down, but apparently, it does. The car feels a little more responsive in general now, so it may have been shifting a bit early as well, but it wasn't far enough off to think that something was really wrong.

Anyway, all good now. If anyone else has this kind of problem, be sure your governor's throttle cable doesn't have any slack in it.

-- Joe

[David T]

Answering my own thread, this would up being pretty simple: the throttle cable wasn't adjusted correctly. I didn't realize the throttle cable had anything to do with the kick down, but apparently, it does. The car feels a little more responsive in general now, so it may have been shifting a bit early as well, but it wasn't far enough off to think that something was really wrong.

Anyway, all good now. If anyone else has this kind of problem, be sure your governor's throttle cable doesn't have any slack in it.

-- Joe

Glad you found the problem quickly. As a general rule whenever you have a problem with the automatic transmission the first step is always check ALL of the external adjustments. That includes a fluid check (the trans AND the final drive), inspecting the wiring harness to the shift computer (they can be damaged by the hot exhaust pipes) and all of the shift linkage for play and adjustment. Once in a while you see where someone who didn't know how to shift into reverse and they force the shifter and mess it all up. Also motor mounts, if they are bad, can mess up the shifter adjustments. As you see, the throttle cable to the shift computer is important. I also see the vacuum line to the pressure regulator disconnected often. That is to reduce the pressure a bit to make the shifts softer.

[jangell]

That's good to know about the vacuum line -- I always wondered what that did. The auto trans is missing a good list of troubleshooting steps and "what happens if you unplug this" information that can make some of this stuff hard to diagnose.

-- Joe

[David T]

That's good to know about the vacuum line -- I always wondered what that did. The auto trans is missing a good list of troubleshooting steps and "what happens if you unplug this" information that can make some of this stuff hard to diagnose.

-- Joe

You have to remember the Workshop Manual was written for technicians that already have a good working knowledge of automobiles. It does not have every step or explain itself in great detail.

[WHO1DMC]

You have to remember the Workshop Manual was written for technicians that already have a good working knowledge of automobiles. It does not have every step or explain itself in great detail.

Well the workshop manual was kind of a rush job. The POGs at the QAC centers were writing and doing the illustrations for the manual at the same time trying to get new cars ready for delivery. The parts of the manual they really didn't do were the engine and transmission main components. Those manuals they got from the supplier. But with any car, they weren't supposed to be worked on by the owner. Only your friendly trained technician.

So after 40+ years if they missed some obscure details, there's no one to complain too.




Dave B.

[jangell]

Right, the workshop manuals were meant for trained mechanics, which is fine and to be expected. The rest of us just have to learn how to decode them. :)

I went to DCS this year and there was a great talk by a couple of the POGs (Jerry and Chris) discussing how they created the Technical Manual specifically to get mechanics up to speed because the DeLorean was so unusual.

At the same time, I totally understand that the engine and transmission were known quantities, and so there was no need to create new manuals for them.

Incidentally, there is a troubleshooting section for the automatic transmission in the Workshop Manual (G:04:03) -- it just has no mention of kick-down-related issues.

I'm not really complaining about the omissions per se -- it can just be hard to find information on some things in general, trained or not. Not much to be done about it beyond asking if anyone else knows, which is what makes communities like this so great. Or, do what the POGs did when they wrote the Technical Manual -- start disabling things and make note of what isn't working anymore.

-- Joe

[Ron]

incidentally, there is a troubleshooting section for the automatic transmission in the workshop manual (g:04:03) -- it just has no mention of kick-down-related issues.
FWIW:
"INCORRECT GEAR CHANGING SPEEDS" 4, 5, ...
4) Accelerator Controls
5) Governor Cable

[jangell]

I thought that meant "the shift points feel wrong", not "the kick-down isn't working properly", but I can see how that would apply. I was thinking too narrowly.

And the shift points were wrong -- they just weren't wrong enough for me to notice. That's the problem with taking 2 months to do some repairs and then forgetting what the car's supposed to feel like.


To be honest, I'm slightly confused about how the kick-down actually works. I know the microswitch activates the kick-down, but I also know that something else in the governor overrides it. I suck at reading circuit schematics and figuring out what is actually does from different inputs, but I have been tempted to try loading the governor schematic into a simulator like LTSpice and start feeding it signals and seeing what it outputs. I've never used a simulator like that before, so I'm not entirely sure it would work, but it seems like it should.

-- Joe

[David T]

A trained technician, when he gets a car, has to try to understand what the actual complaint is. Then he has to verify it hoping it isn't an intermittent. Now he must diagnose it. All of these steps are at least as important as being able to fix the problem. Once it is fixed the Tech should test the function to verify if the problem has been found and fixed. As for the manual, it cannot have every failure mode or how to repair everything. That is why it explains some things so a Tech can understand how it is supposed to work and then figure out what is not working and how to fix it. Finally the manual was not written to work on a 40 year old car considering how the parts would age. This forum tries to bring the knowledge of all of the people who work on them and have seen all sorts of failure modes and can offer advice on what to do. For most people and even most Techs, the automatic transmission is a "Black Box" and they do not fully understand how it functions. That is what the diagnosis charts are for.

[Ron]

Maybe this will help...

Basically, shifting is done using two solenoids (to open and close two valves). They are controlled by the "computer governor" (GC), which receives input from a signal generator (for output shaft RPM input), a multifunction switch (for operator selected gear input), a "governor cable" (for operator selected acceleration level input), a micro switch (for operator selected downshift request input).
Let's go for a ride...
When you are sitting in gear, both solenoids are ON (1st gear).
Floor it!...
As you continue to accelerate, the generator outputs more voltage. The GC sees the rise in voltage and turns off solenoid A (now 2nd gear). The load on the engine raises so the RPMs and the generator's output voltage drop back down. Continuing to accelerate, the GC sees another rise in RPMs and turns off solenoid B (now 3rd gear).
Cruising...
The generator's input is combined with the governor cable's input to determine the point to turn a solenoid on/off (engine load versus demand).
Passing...
Putting the pedal to the floor closes the kickdown microswitch (request for it to override the GC). The GC will turn a solenoid ON (B then A), iff it is safe to do so.

If you haven't, check out:
https://dmctalk.org/showthread.php?350-Automatic-Transmission-Governor
70012

[jangell]

Thanks -- that does help.

That governor thread is extremely valuable -- I've gone over it a number of times in the past. I think my confusion comes from this part of the description about how the kick-down works:


"It basically provides a path to ground for the solenoids, bypassing the voltage comparing Op-Amp part of the circuit, and forcing the car into a lower gear. Once you let up off the gas, the kick down opens up, and the solenoids are once again controlled by the comparator Op-Amps."

That suggested to me that the comparators (and thus the generator and throttle position) weren't considered when the kick-down is engaged, but that's clearly not true.

Really, it's just that I suck at reading logic flow in circuit diagrams.

-- Joe

[Ron]

I think you may be missing that the solenoids are wired hot (12V) at all times. (They are ground controlled -- Pull the fuse and it is in 3rd!)
...The GC grounds a solenoid to turn it on, forcing a LOWER gear. The microswitch's input (ground) is fed into the GC circuit's where it will make the GC ground a solenoid. It's signal is on the Green wire that taps between D2 and D3 in the schematic.

[jangell]

Ah, you're right! I play with microcontrollers enough to know that it's easier to switch ground with a MOSFET than 12v (I know this isn't quite the same, but still, I'm still used to switching ground). It's even mentioned in that thread -- I should have re-read it more thoroughly last time. That explains it. Thanks!

-- Joe

[Ron]

YW!

[David T]

Maybe this will help...

Basically, shifting is done using two solenoids (to open and close two valves). They are controlled by the "computer governor" (GC), which receives input from a signal generator (for output shaft RPM input), a multifunction switch (for operator selected gear input), a "governor cable" (for operator selected acceleration level input), a micro switch (for operator selected downshift request input).
Let's go for a ride...
When you are sitting in gear, both solenoids are ON (1st gear).
Floor it!...
As you continue to accelerate, the generator outputs more voltage. The GC sees the rise in voltage and turns off solenoid A (now 2nd gear). The load on the engine raises so the RPMs and the generator's output voltage drop back down. Continuing to accelerate, the GC sees another rise in RPMs and turns off solenoid B (now 3rd gear).
Cruising...
The generator's input is combined with the governor cable's input to determine the point to turn a solenoid on/off (engine load versus demand).
Passing...
Putting the pedal to the floor closes the kickdown microswitch (request for it to override the GC). The GC will turn a solenoid ON (B then A), iff it is safe to do so.

If you haven't, check out:
https://dmctalk.org/showthread.php?350-Automatic-Transmission-Governor
70012

While this explanation is mostly correct it is not that simple. The valve body also controls the shifts too. For instance it will not allow upshifts or downshifts unless you are within the correct RPM range for the shift. The solenoids do tell the valve body what gear to be in but the valve body has the final control. If the shift hold is working the shift computer can command the transmission to go into 3rd but the shift lever can limit your upshifts to 2nd or 1st. Anyway if you are not shifting correctly, chances are the shift computer (GC) is not functioning correctly. Most of the time it is the shift computer but before you say that, you have to inspect the wiring harness. If it has not been secured properly away from the exhaust pipes it gets melted and can cause all kinds of weird problems. You also should go over every external adjustment too. Unlike most cars the throttle cable does not activate the kick-down. The micro switch on the throttle spool does that. That is why there are 2 switches there at WOT (Wide Open Throttle) on automatics. 1 for the kick-down and the other to force the Lamda system to full duty cycle for more fuel.